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Università degli Studi della Basilicata Dottorato di Ricerca in Rischio Sismico, Ingegneria Strutturale e Geotecnica INFLUENCE OF PORE FLUID COMPOSITION ON CLAY BEHAVIOUR AND CHEMO-MECHANICAL STUDY OF A CLAYEY LANDSLIDE Settore Scientifico-Disciplinare ICAR/07 Coordinatrice del Dottorato Prof.ssa Caterina Di Maio Tutor Prof.ssa Caterina Di Maio Dottorando Dott. Gianvito Scaringi A.A. 2014/2015, Ciclo XXVIII

Scaringi Gianvito - PhD Thesis

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Università degli Studi della Basilicata

Dottorato di Ricerca in

Rischio Sismico, Ingegneria Strutturale e Geotecnica

INFLUENCE OF PORE FLUID COMPOSITION

ON CLAY BEHAVIOUR AND CHEMO-MECHANICAL

STUDY OF A CLAYEY LANDSLIDE

Settore Scientifico-Disciplinare

ICAR/07

Coordinatrice del Dottorato

Prof.ssa Caterina Di Maio

Tutor

Prof.ssa Caterina Di Maio

Dottorando

Dott. Gianvito Scaringi

A.A. 2014/2015, Ciclo XXVIII

To my family

ACKNOWLEDGEMENTS

I would like to thank first and foremost my advisor, Prof. Caterina Di Maio. Her

advice, guidance, support and inspiration were fundamental throughout my

undergraduate and graduate studies and in each achievement of this research.

Thanks are also due to Prof. Roberto Vassallo for his precious advices, his critical

point of view and his constant support.

I would also like to thank Dr. Angela Perrone and Dr. Enzo Rizzo of the CNR-IMAA

Institute for kindly lending their testing equipment and for helping me in the

interpretation of the test results. Thanks are also due to Prof. Paolo Simonini and Prof.

Simonetta Cola of the University of Padova for the X-ray tomography on laboratory

specimens, to Prof. Salvatore Masi and Mr. Domenico Molfese for the ICP-AES

analyses of fluid samples and to Mr. Alessandro Laurita for the ESEM micrographs.

Special thanks are due to the technical staff, to the undergraduate and graduate

temporary members of the geotechnical research unit and to my doctoral colleagues,

with whom I had the pleasure to collaborate and with whom I shared a piece of my

scientific and personal growth. Last, but not least, I wish to thank my better half, my

family and my friends for their continuous support and encouragement.

SUMMARY

Abstract .................................................................................................................................. 1

1 Introduction .................................................................................................................... 2

2 Influence of pore fluid composition on clay behaviour ................................................. 5

2.1 State of the Art ........................................................................................................ 6

2.2 Experimental results relative to the Costa della Gaveta soil ............................... 23

2.2.1 Residual shear strength .................................................................................. 23

2.2.2 Observation of the shear surface .................................................................... 36

3 Influence of pore fluid composition on creep behaviour ............................................. 42

3.1 Shear creep: a brief overview of the phenomenon ............................................... 43

3.2 Experimental results relative to the Costa della Gaveta soil ............................... 49

3.2.1 Stress-controlled shear tests on the Costa della Gaveta soil ......................... 49

3.3 Experimental results relative to other clays ......................................................... 58

3.3.1 Stress-controlled shear tests on bentonite ...................................................... 58

3.3.2 Water content and pore ion concentration at the end of the tests .................. 77

3.4 Modelization of ion diffusion and strength reduction .......................................... 80

3.5 Discussion ............................................................................................................. 89

4 Pore fluid composition in clays of marine origin ........................................................ 91

4.1 Data from Literature .............................................................................................. 92

4.2 Pore fluid composition at Costa della Gaveta ................................................... 103

4.3 Electrical resistivity of the system solid skeleton – pore fluid ........................... 113

5 Conclusion ................................................................................................................. 120

References .......................................................................................................................... 122

1

ABSTRACT

This work reports on experimental results aimed at evaluating the influence of pore

solution composition on some aspects of clay behaviour. Besides some pure clays, the soil

of Costa della Gaveta hill (Potenza, Italy) has been analysed trying to understand the

implications of test results on the behaviour of the landslides there occurring.

Several shear tests have been carried out, both under controlled rate of displacement, to

evaluate the influence of pore fluid composition on the residual shear strength, and under

constant shear stresses, to evaluate the rheological behaviour of the soil along a slip surface

in residual condition when subjected to changes in pore fluid composition. The

composition of the pore fluid is shown to affect the residual shear strength of the tested soil

noticeably. The tests carried out under constant shear stresses showed that a pore solution

concentration decrease can produce an increase in displacement rate on a pre-existing slip

surface with a pattern typical of tertiary creep.

The natural pore fluid composition of the Costa della Gaveta soil was evaluated on a large

number of samples, both by chemical and by electrical analyses. Some preliminary

evaluations of the electrical resistivity of the system solid skeleton – pore fluid were made

as well. The natural pore fluid is shown to be a composite ion solution, in which Na+ is the

most abundant cation. Its concentration decreases noticeably from the depth towards the

ground surface, from values close to that of seawater to negligible values. The

concentration range evaluated in situ corresponds to the range in which the greatest

gradients in the residual friction angle have been evaluated.

2

1 INTRODUCTION

The composition of the pore fluid affects the mechanical behaviour of clays significantly

(e.g. Bolt, 1956; Kenney, 1967; Mesri and Olson, 1971; Mitchell et al., 1973; Sridharan

and Ventakappa Rao, 1973; Di Maio, 1996a, 1998). Several studies, in particular, showed

the great influence that the pore fluid composition exerts on the residual shear strength

(among others: Kenney, 1967; Chattopadhyay, 1972; Sridharan and Ventakappa Rao,

1979; Sridharan, 1991; Di Maio and Fenelli, 1994; Di Maio, 1996b; Anson and Hawkins,

1998).

The residual shear strength is the minimum strength that a soil can exhibit, under a definite

normal stress, after large displacements along a regular slip surface (e.g. Skempton, 1985).

Its evaluation is thus very important in engineering problems concerning slope stability and

in predicting landslide movements. Changes in the available strength due to pore pressure

variations induced by changing hydraulic boundary conditions are generally accounted for

in such problems, while the influence of pore fluid composition is often neglected,

although its effects can be dramatic.

The composition of the pore fluid of clays in nature can vary, in space and in time, due to

different natural and anthropic processes (e.g. Bjerrum, 1954; Rosenqvist, 1955; Quigley et

al., 1983; Pearson et al., 2003; Torres et al., 2011). The mechanical properties can thus

change and, consequently, affect soil stability and landslide movements, as shown, for

instance, by Gregersen (1981), Moore and Brundsen (1996), Geertsema and Torrance

(2005), Zhang et al. (2009) and Zhang et al. (2013).

This work reports on experimental results aimed at characterising the natural pore fluid

composition in a clayey slope affected by landslides, and at evaluating the influence of

pore fluid composition on the residual shear strength and on the rheological behaviour of

1. Introduction

3

the soil along the slip surface. To this aim, the case study of the Costa della Gaveta slope

(Di Maio et al., 2010, 2011, 2012, 2013), located in the Southern Italian Apennines, was

considered. Costa della Gaveta hill is formed by a marine origin clay formation, locally

known as the Varicoloured Clays. The hill is affected by several different landslides.

The homonymous Costa della Gaveta landslide, a very slow earthflow in steady state

motion (Hungr et al., 2014) involves a volume of 6 million cubic metres soil, with

displacements concentrated in a narrow shear zone in the residual condition, which reaches

a depth of about 40 m (Di Maio et al., 2010). Several aspects of the landslide behaviour

have been studied, such as: the response of pore pressures to rainfall and their effects on

landslide displacements, the time trend of displacements on the shear surface and of

deformations in the landslide body, and the possible triggering factors (Di Maio et al.

2010; Vassallo et al. 2012; Di Maio et al. 2013; Vassallo et al., 2015a). More recently, the

research has also been focused on the characterisation of the natural pore fluid composition

and on its role in the mechanical behaviour of the soil (Di Maio et al., 2015a, 2015b; Di

Maio and Scaringi, 2015).

The Varco d’Izzo landslide, located a few hundred metres East of the Costa della Gaveta

landslide, is a wider – more than 1 km large – and complex landslide system whose

movements cause severe damage to houses and infrastructures, with very different rates of

displacement from site to site (Di Maio et al., 2012). An earthflow within the landslide

system also affects a 200 m long railway tunnel. The interaction between this latter and the

landslide body is currently under study (Vassallo et al., 2015b). The area is being

monitored through several inclinometers, GPS stations and piezometers (Di Maio et al.,

2011, 2012; Calcaterra et al., 2012).

The results of laboratory tests, for the evaluation of the residual shear strength of the Costa

della Gaveta material with different pore solutions, are reported in Chapter 2. Several

direct and ring shear tests were carried out on reconstituted specimens in absence of

chemical gradients between the pore fluid and the cell fluid. Some other tests were carried

out in order to evaluate the behaviour of the soil when subjected to a decrease or to an

increase in pore fluid ion concentration. Some first observations by means of X-ray

tomography and ESEM microscopy have been performed after the shear tests to

characterise the soil along the slip surface.

1. Introduction

4

In Chapter 3 the influence of pore fluid composition on creep behaviour is studied by

means of stress-controlled tests on pre-sheared specimens of the Costa della Gaveta soil.

Tests results relative to a sodium bentonite are also reported in order to attempt a

generalisation of the results. During the course of the tests, the specimens were exposed to

distilled water in order to simulate a process of pore ion concentration decrease. Often, the

specimens were analysed after the tests in order to determine water content and ion

concentration profiles along the specimen’s height.

In Chapter 4, the results relative to the experimental evaluation of the natural pore fluid

composition of the Costa della Gaveta soil are reported. Both chemical and electrical

analyses have been carried out on the pore fluid and some first electrical resistivity

measurements were performed on many undisturbed specimens, reconstituted specimens

and slurries. In situ electrical resistivity tomographies were also carried out.

5

2 INFLUENCE OF PORE FLUID

COMPOSITION ON CLAY BEHAVIOUR

The residual shear strength is the minimum strength that a soil can exhibit under a given

normal stress. It is generally the available shear strength on the slip surface of active

landslides which have experienced large displacements along a regular slip surface

(Skempton, 1985). A reliable evaluation of the residual shear strength is thus essential in

stability analyses and for predicting landslide displacements.

The first part of this Chapter is a review of some of the main studies on the influence of

pore fluid composition on the residual shear strength. Then, the Chapter reports on the

results of a number of laboratory tests carried out in this work to investigate the influence

of pore fluid composition on the residual shear strength of the Costa della Gaveta soil. To

this aim, several direct and ring shear tests were performed.

The residual shear strength of a soil is greatly influenced by the mineralogy of its clay

components. Such influence is here analysed by comparing the results of tests carried out

on different clays.

The Chapter also reports on the results of ESEM and X-ray observations of sheared

specimens, carried out in order to observe the soil fabric in the shear zone and on the slip

surface.

2. Influence of pore fluid composition on clay behaviour

6

2.1 STATE OF THE ART

The chemical composition of the pore fluid influences several aspects of the mechanical

behaviour of clays, such as volume change, hydraulic conductivity, swelling pressure,

osmotic efficiency and shear strength.

Experimental results regarding, in particular, the residual shear strength were reported,

among others, by Kenney (1967), Ventakappa Rao (1972), Balasubramonian (1972),

Chattopadhyay (1972). In the following years, different Authors (e.g. Sridharan and

Ventakappa Rao, 1979; Moore, 1991; Di Maio and Fenelli, 1994; Di Maio, 1996a,b;

Anson and Hawkins, 1998; Tiwari et al., 2005) pointed out the influence of pore fluid

composition and ion concentration on the residual shear strength of different clays. The

Authors also gave interpretations of their results, attempting to consider them in a unique

framework which could be suitable for different clays and/or be able to explain also the

influence on other mechanical aspects comprehensively.

Sridharan and Ventakappa Rao (1979) investigated the drained shear strength of kaolinitic

and montmorillonitic clays prepared with different pore fluids (i.e. distilled water and

various organic fluids). Figure 2.1, for example, shows the results relative to specimens of

compacted kaolinite: the influence of the used fluid is evident. The Authors interpreted the

results as a function of the dielectric constant of the pore fluid and observed that both for

kaolinitic and for montmorillonitic clays the shear strength seemed to decrease when the

dielectric constant increased, as shown by Figure 2.2. Furthermore, the results were found

consistent with a modified effective stress concept accounting for electrical attractive and

repulsive interparticle forces (among others: Bolt, 1956; Lambe, 1960; Sridharan, 1968;

Sridharan and Ventakappa Rao, 1973).

In order to evaluate the influence of pore fluid composition on the residual shear strength,

Chatterji and Morgestern (1989) performed shear tests on specimens of Na-

montmorillonite prepared with a concentrated (33.6 g/l) NaCl solution and subsequently

leached with distilled water. Similarly to Sridharan and Ventakappa Rao (1979), they

interpreted the results in terms of a modified effective stress concept accounting, in

2. Influence of pore fluid composition on clay behaviour

7

particular, for the repulsion force in the diffuse double layer (DDL; Gouy, 1910; Chapman,

1913). The Authors showed that, by this concept, it is possible to find a unique value of

residual friction angle which is independent of pore fluid salinity, as shown by Figure 2.3.

The Authors also reported that, for clays such as kaolinite, being the DDL repulsion forces

lower, the residual shear strength does not appear to be influenced by pore fluid

composition significantly.

Figure 2.1 Drained shear strength of statically compacted kaolinite prepared with different

fluids (Sridharan and Ventakappa Rao, 1979).

2. Influence of pore fluid composition on clay behaviour

8

Figure 2.2 Shear strength normalised with respect to the normal pressure against the

dielectric constant of the used pore fluid for specimens of kaolinite (left) and

montmorillonite (right) (Sridharan and Ventakappa Rao, 1979).

Figure 2.3 Residual shear strength against true effective stress in the formulation by

Chatterji and Morgernstern (1989) for specimens of sodium montmorillonite prepared with

a concentrated NaCl solution, before and after leaching with distilled water.

Some decades earlier, the DDL concept had been used by Bolt (1956) to predict the

volume change behaviour of clays. The Author interpreted the compression behaviour of

montmorillonite and illite in salt solutions at different concentrations and provided a

relation between the void ratio, e, and the swelling pressure, p. The two quantities were

related to the specific surface of the clay, the interparticle distance, the ion concentration at

mid-plane between two particles and the ion concentration in the bulk solution.

Subsequently, Mitchell (1960) investigated the volume change behaviour of Na-kaolinite,

Na-illite and Na-montmorillonite. He concluded that the DDL theory is not applicable to

2. Influence of pore fluid composition on clay behaviour

9

all clays, but only to those containing clay particles of diameter smaller than 0.2-1.0 µm. A

detailed study on the applicability of the DDL theory was also conducted by Sridharan and

Jayadeva (1982), who showed that the e – log p relation is primarily controlled by the

specific surface of the clay. Furthermore, they evaluated that the contribution of the Van

der Waals attractive forces is negligible if compared to the repulsion forces caused by the

interacting diffuse double layers in the range of pressures in engineering practice. The

DDL concept was also used by Olson and Mesri (1970) and Mesri and Olson (1971), and

proved to work satisfactorily in interpreting the consolidation curves of artificially

sedimented Na-montmorillonite, consolidated in water or in solutions of NaCl at different

concentrations (Figure 2.4). They showed the remarkable difference of void ratio against

the normal effective stress for specimens saturated with different fluids and also noticed

that the clay, when prepared with some organic fluids, exhibited much lower void ratios

and much higher hydraulic conductivities (4-6 orders of magnitude!) than when prepared

with water.

Figure 2.4 Void ratio against normal applied stress for specimens of Na-montmorillonite

saturated with NaCl solutions at different concentrations (Mesri and Olson, 1971).

2. Influence of pore fluid composition on clay behaviour

10

The link between the DDL concept and the dielectric constant of the pore fluid in

explaining the mechanical behaviour of clays was shown with respect to the volume

change behaviour by Sridharan and Ventakappa Rao (1973). The Authors recognised two

mechanisms related to the clays’ microstructure, i.e. the shearing resistance at the contact

points, on which shear displacements and/or sliding between particles depend, and the

long-range electrical repulsive forces, on which the DDL behaviour depends. The former

mechanism was found to prevail in kaolinite, while the latter in montmorillonite. Chen et

al. (2000) observed that the compression index of kaolinite changes with the dielectric

constant of the organic fluids in a way similar to the Hamaker constant, on which the

attractive van der Waals forces depend and shows a minimum at D = 24. Similar results

were found by Moore and Mitchell (1974). Calvello et al. (2005) reported evidence of the

dependence of the compression index, coefficient of consolidation and hydraulic

conductivity on the pore fluid dielectric constant also for smectitic clays (Figure 2.5).

However, the relations between clay properties and dielectric constant appeared different

than those found for kaolinite, thus possibly highlighting the different mechanisms

controlling the compressibility of the two clays.

Di Maio (2004a) and Calvello et al. (2005) analysed the residual shear strength of different

smectitic soils prepared with water, salt solutions or organic fluid in terms of the dielectric

constant of the pore fluid. They found that residual strength decreases with the dielectric

constant increasing up to D = 80 (Figure 2.6). It is worth noting that a non-polar organic

fluids, such as cyclohexane, with very low dielectric constant, produced the same

behaviour as that of dry specimens.

2. Influence of pore fluid composition on clay behaviour

11

Figure 2.5 Compression index, Cc, normalized with respect to that of materials

reconstituted with distilled water, against pore fluid static dielectric constant, D, for Na-

montmorillonite (Calvello et al., 2005).

Figure 2.6 Residual friction coefficient τr/σ’n against the pore fluid static dielectric

constant D for different smectitic soils (Calvello et al., 2005).

2. Influence of pore fluid composition on clay behaviour

12

Furthermore, Di Maio et al. (2004) performed a large number of oedometer tests on

different natural soils containing smectite, illite and kaolinite and on some of their

mixtures. The materials were reconstituted with – and submerged in – water, salt solutions

or organic fluids. The Authors found a good agreement between the intrinsic compression

index against the void ratio at the liquid limit and the regression line found by Burland

(1990), both for soils prepared with water and for soils prepared with salt solutions (Figure

2.7). According to the Authors, this suggests that the liquid limit (which is a measure of the

soil strength under standardised conditions) can be a reference state to predict the

compression behaviour, in the range of validity of the relation, also with pore fluids

different from water.

Figure 2.7 Intrinsic compression index Cc* against void ratio eL at liquid limit. For each

materials the values of Cc* obtained with different pore solutions are reported (Di Maio et

al., 2004).

Di Maio and Fenelli (1994) published the result of direct shear tests carried out on a

sodium bentonite reconstituted with distilled water and sheared to the residual condition

while in a bath of distilled water. The specimen was subsequently exposed to a

concentrated NaCl solution. This caused a progressive and noticeable increase in the shear

strength (Figure 2.8). Subsequent re-exposure to water produced a progressive shear

strength decrease down to the value attained before exposure to the salt solution. The

effects on the residual sear strength of the exposure to NaCl solutions of sodium bentonite

2. Influence of pore fluid composition on clay behaviour

13

are thus reversible. The test was repeated on a specimen of kaolin, which did not exhibit

any strength variations. Tests conducted on mixtures of bentonite and kaolin showed that

the strength variation due to the exposure to salt solution is remarkable for bentonite

contents as low as 25% in dry weight, under the investigated normal stress, meaning that

such a percentage is able to control the residual shear strength of the mixture.

Di Maio and Fenelli (1997), performing several compression tests with exposure to

different fluids on specimens of natural soils containing different clay minerals, showed

that the influence of pore fluid composition is very significant for soils containing smectite.

The Authors thus stressed the importance of using the appropriate pore fluid when

evaluating the possible mechanical behaviour in situ. In fact, if a specimen of a soil whose

natural pore fluid is a salt solution is tested in a bath of distilled water, it can exhibit a

behaviour which can differ significantly from that in situ, due to possible transient

phenomena (e.g. ion diffusion, osmotic water flow) occurring in the course of the test.

Di Maio (1996a) showed the remarkable effects of the exposure of a sodium bentonite to a

fluid different from its pore fluid and Di Maio (1996b) showed similar effects for several

natural soils containing montmorillonite. Among the results of the direct shear tests, Di

Maio (1996a) reported those relative to two specimens (see Figure 2.9), one reconstituted

with a concentrated NaCl solution and sheared to the residual condition while submerged

in the same solution (specimen 1a) and another reconstituted with water and sheared to the

residual condition while submerged in water (specimen 1b). Their residual shear strength

resulted very different: τr/σ’n ≈ 0.1 in water and τr/σ’n ≈ 0.3 in salt solution. Specimen 1b,

initially in water, was then exposed to the salt solution, showing a progressive strength

increase. Conversely, specimen 1a, initially in salt solution, was exposed to water, showing

a progressive strength decrease. At the end of the process, the specimen exposed to water

had reached the same strength as that reconstituted with – and submerged in – water, while

the specimen exposed to the salt solution had reached the same strength as that

reconstituted with – and submerged in – the salt solution. This was considered a further

confirmation of the reversibility of the effects of NaCl solutions on sodium bentonite, this

time proved also on a specimen reconstituted with the salt solution.

2. Influence of pore fluid composition on clay behaviour

14

Figure 2.8 Shear trends of bentonite, sheared in water and then exposed to NaCl solution

and finally to water again (Di Maio and Fenelli, 1994).

2. Influence of pore fluid composition on clay behaviour

15

τ/ σa

sh

ea

r dis

pla

cem

en

ts (m

m)

Figure 2.9 Shear trends of bentonite specimens first mixed or exposed to saturated NaCl

solution, and then to water (Di Maio, 1996a).

2. Influence of pore fluid composition on clay behaviour

16

A different behaviour was observed with the exposure of water saturated Na-bentonite to

CaCl2 and KCl solutions. Both solutions produced a progressive residual shear strength

increase, but the subsequent re-exposure to water did not cause but a negligible shear

strength decrease. Di Maio (1996a) showed that the irreversibility is exhibited also in terms

of volume changes. During the course of oedometer tests, the Author showed in fact that if

a specimen of sodium bentonite reconstituted with water is exposed to a NaCl solution, it

exhibits a volume decrease under constant Terzaghi’s effective stresses. If, afterwards, the

specimen is re-exposed to water, it undergoes a volume increase (Figure 2.10), the

magnitude of volume changes depending on Terzaghi’s effective stresses. On the contrary,

the effect of the exposure to CaCl2 solutions were non-reversible upon re-exposure to

distilled water (Figure 2.11). Similarly, irreversibility was observed after exposure to KCl

solutions. This was attributed to ion-exchange which probably transformed the Na-

montmorillonite into K-montmorillonite or Ca-montmorillonite, which are characterised by

smaller double layers. Di Maio (1998) showed the possibility of reversing the exchange

reaction by re-exposing the specimens to concentrated NaCl solutions and then to water

(Figure 2.11), but discussed that such process is unlikely to occur in nature, thus

introducing a possible long lasting chemical treatment to improve the mechanical

characteristics of the clay.

Regarding the influence of pore fluid ion concentration, Di Maio (1996a) showed that most

of variations in the residual shear strength of sodium bentonite with respect to NaCl

solutions occur in the range 0-1 mol/l, while the residual shear strength does not change

significantly for concentrations from 1 mol/l to saturation. The same trend was observed on

the liquid limit against NaCl concentration, i.e. wL decreases noticeably from water to 1

mol/l NaCl solution, while does not vary much for higher concentrations. Such dependence

of the residual shear strength on the solution concentration was confirmed by Di Maio

(2004a) on several natural soils containing montmorillonite (Figure 2.12).

2. Influence of pore fluid composition on clay behaviour

17

Figure 2.10 Consolidation produced by exposure to NaCl solution and swelling caused by

exposure to water under two different normal stresses (Di Maio, 1996a).

Figure 2.11 Volume change due to mechanical consolidation and exposure to NaCl

solution, CaCl2 solution and water (Di Maio, 1998).

2. Influence of pore fluid composition on clay behaviour

18

Figure 2.12 Residual shear strength against NaCl solution molarity for different clay soils

under σ’n = 200 kPa (Di Maio, 2004a).

Xu et al. (2014) have recently proposed a new definition for the effective stress which, in

particular, was used to interpret the volume change behaviour of smectitic clays. They

assumed that the clay surface has a fractal dimension, D. A modified effective stress pe was

defined, which takes into account this fractal dimension. By means of this concept, they

found a unique relation between the void ratio e and pe which is insensitive to pore fluid

composition and applied such relation to different smectitic soils. Figure 2.13 shows the

void ratio against such modified effective stress for two soils: the Bisaccia clay (data from

Calvello et al., 2005) and the Ponza bentonite (data from Di Maio et al., 2004) with water

and different NaCl solutions. The e-pe relation predicted by the model, represented by the

solid lines in the figure, seems to agree with the experimental data for concentrations up to

saturation. However, this relation does not prove satisfactory in predicting the residual

shear strength at NaCl concentrations higher than 1 mol/l, as shown by Figure 2.14. This

suggests that, at high concentrations, the shear resistance is limited by other phenomena

rather than electrostatic forces of the DDL.

2. Influence of pore fluid composition on clay behaviour

19

Figure 2.13 Void ratio against modified effective stress for two smectite rich clays with

different pore fluids (Xu et al., 2014).

0 1000 2000 3000pe (kPa)

water0.1 M NaCl0.6 M NaClsaturated NaCl

Bisaccia clay

0

20

40

60

80

100

120

0 1000 2000 3000

τr(k

Pa)

pe (kPa)

water0.2 M NaCl0.5 M NaCl1M NaClsaturated NaCl

Ponza bentonite

Figure 2.14 Residual shear strength against the modified effective stress defined by Xu et

al. (2014) for the Ponza bentonite and the Bisaccia clay (data from: Di Maio, 2004a; Di

Maio et al., 2004; Calvello et al., 2005).

Di Maio and Onorati (2000) showed that the pore fluid composition has a remarkable

influence also on the shear strength determined by means of triaxial tests. The Authors

performed CiU triaxial tests on normally consolidated (see Figure 2.15) and

overconsolidated specimens of the montmorillonitic Bisaccia clay. Important effects of

pore fluid composition were noticed, more recently, by Zhang et al. (2013) on the

undrained shear strength, by Siddiqua et al. (2014) on the stress-strain behaviour during

triaxial tests and by Gratchev and Sassa (2013) on the cyclic shear behaviour. The latter

Authors performed also some tests by using pore fluids characterised by different values of

pH.

2. Influence of pore fluid composition on clay behaviour

20

0

100

200

300

400

0 200 400 600p' (kPa)

q (k

Pa)

distilledwater

1 M NaClsolution

0 200 400 600

0

σ' (kPa)

τ (k

Pa)

70

140

210

350280 1 M NaCl

solutiondistilledwater

420

Figure 2.15 CU triaxial tests on normally consolidated specimens of the Bisaccia clay (Di

Maio and Onorati, 2000).

As for the influence of pH on the mechanical behaviour of clays, Suarez et al. (1984)

showed the effects on hydraulic conductivity and clay structure. The effect of pH is

particularly important in practice when contaminated soils, e.g. by acid leachate, are

considered. Also Palomino and Santamarina (2005) investigated the effect of pH on clay

structure. They produced a fabric map for kaolinite as a function of pore solution

concentration and pH, highlighting the changes in particle arrangement and surface charge.

Gajo and Maines (2007) showed that acid solutions influence both the volume change

behaviour and the residual shear strength of sodium bentonite. In particular, the residual

shear strength evaluated in acid solutions is higher than that in water (Figure 2.16). The

effects of exposure to an acid solution (i.e. to H+ cation) are similar to those of other

cations different from Na+. They do not appear reversible by re-exposing the specimens to

water, like those of calcium and potassium chloride, but can be reversed by exposing the

clay to a basic solution. The results of the shear tests, as well as those relative to

compression tests, were interpreted by the Authors with the concepts of cation exchange on

permanently charged surface sites and of acid-base reactions on variably charged sites.

According to the Authors, some aspects of the chemo-mechanical interaction of active

clays subjected to pH variations of the pore fluid can actually be roughly described without

considering the acid–base reactions, whereas the effects of exposure first to an inorganic

acid and then to bases or salts cannot be understood without taking the role of acid–base

reactions at the clay edges into account.

2. Influence of pore fluid composition on clay behaviour

21

Figure 2.16 Residual shear strength as a function of normal effective stress on shear plane

raised to power of -1/3 (Gajo and Maines, 2007).

Wahid et al. (2011a,b) showed that the mechanical behaviour of kaolin is influenced by pH

much more than by pore fluid salinity. This was attributed to the major role played by the

variably charged sites, which affects edge-to-face particle interaction and can thus produce

irreversible strains. Additional examples of the influence of pH, with respect to the

compressibility of natural clays are reported, for example, by Gratchev and Towhata

(2011, 2015) for different clay formations in Japan containing different amounts of

smectite, illite, chlorite and kaolinite. Finally Zhao et al. (2011) reported that, in addition,

acid solutions could influence the residual shear strength of clays by changing the clay type

(from illite to smectite to kaolinite).

The influence of pore fluid composition on the residual shear strength has a practical

importance in slope stability, since can play a major role in the reactivation and

movements of landslides in clay soils. Furthermore, as pointed out by Di Maio et al.

(2015a) and similarly to what already suggested by Di Maio and Fenelli (1997), the

evaluation of the available residual shear strength along slip surfaces in clay soils should

be done taking into account also the natural pore fluid composition, i.e. by considering the

2. Influence of pore fluid composition on clay behaviour

22

soil as a solid skeleton – pore fluid system governed by a chemo-mechanical coupling. As

a matter of fact, the Authors showed that the use of distilled water as pore fluid and cell

fluid during the tests can lead to an estimation of a value of residual shear strength which is

different from that available in situ. Furthermore, the use of a unique value of residual

friction angle in stability analyses may be misleading even in soils which are

“homogeneous”, if the pore fluid composition is not homogeneous.

2. Influence of pore fluid composition on clay behaviour

23

2.2 EXPERIMENTAL RESULTS RELATIVE

TO THE COSTA DELLA GAVETA SOIL

2.2.1 Residual shear strength

The residual shear strength was evaluated in the course of displacement-controlled shear

tests by means of different apparatuses: the Casagrande and the reversal direct shear, and

the Bishop and the Bromhead ring shear. The tests were usually performed at v = 0.005

mm/min in the Casagrande, reversal and Bishop apparatuses and at v = 0.018 mm/min in

the Bromhead apparatus, which is the lowest displacement rate that the machine in use

allows.

Since the object of the study is the residual state, which is independent of initial conditions

and stress history, the specimens were prepared by hydrating the powdered, oven-dried,

material (fraction finer than 0.425 mm) at water contents generally lower than the liquid

limit relative to the material hydrated with the used fluid. This was done in order to reduce

the volume decrease due to consolidation and the consolidation time as well.

In some cases, the specimens tested in the Casagrande, reversal and Bishop devices were

cut manually, both before and during the course of the tests, to ensure the flatness of the

shear surface and to reduce the time required to achieve the residual state.

In order to investigate the effect of the pore fluid composition, two groups of tests were

conducted: 1. some specimens were reconstituted with salt solutions at different

concentration and tested in a bath of the same solution, that is, in absence of chemical

gradients; 2. some specimens, pre-sheared to the residual condition, were exposed to a

fluid different from the pore fluid by replacing the cell fluid, i.e. the tests were carried out

in presence of chemical gradients.

The tests were performed on several specimens of the Costa della Gaveta soil. The

material was extracted from different boreholes, whose locations are indicated in Figure

2.17. For comparison, some tests were conducted also on specimens of a sodium bentonite

and of a kaolin.

2. Influence of pore fluid composition on clay behaviour

24

N

Potenza

Costa della Gaveta

landslide

Varco d’Izzo

landslide

Ii: inclinometer casings

Pi, Si, TP, CP: boreholes with piezometers

TM, TV: boreholes with tensiometers

Ki: boreholes

centre of ERT2

0 250 500 m

I11

S11

S9

S5

I5

S4

I4I3

S3

I2

S2

S1

I1

S8I8 I7

S7

I10

I9

I12

P12 S10

I6

S6

Figure 2.17 Portion of the Costa della Gaveta slope with location of the boreholes.

2. Influence of pore fluid composition on clay behaviour

25

Some properties of the tested materials are reported in Table 2.1. The Costa della Gaveta

soil is characterised, in general, by high clay fraction. The clay minerals are abundant and,

among them, illite-muscovite, kaolinite and smectite were found (Summa, 2006). The

chosen bentonite, provided by Laviosa Minerals SpA, Livorno, Italy, is mainly composed

of sodium montmorillonite and exhibits characteristics very similar to those of the Ponza

bentonite, which was used in past experimentations extensively (e.g. Di Maio, 1996a;

Calvello et al., 2005) and was the reference soil for constitutive modelling (e.g. Gajo and

Loret, 2003). The used kaolin is mainly composed of kaolinite and is sold by Imerys Ltd,

UK, under the trademark Speswhite.

Material

Borehole-

Sample Depth (m)

c.f.

(%)

γγγγs

(g/cm3)

wL

(%)

wP

(%)

IP

(%)

A

Costa della

Gaveta soil

S7-CD2 28.0 - 29.6 52 2.67 65.2 26.2 39.2 0.75

S9-MIX 23.5 – 24.8 45 - 55.9 - - -

S9-A 24.0 – 24.8 48 2.66 64.3 - - -

S9-CD18 24.8 – 25.0 46 - 51.8 - - 0.52

S9-B 25.2 - 27.2 36 2.65 53.9 - - -

I9b-CD9bis 8.3 - 8.6 35 2.58 55.6 - - -

I9b-CD12 11.5- 11.7 - - 61.0 - - -

I9b-A 11.7 - 12.4 - - 64.9 - - -

I9b-CD12 11.5 - 11.7 - - 60.9 - - -

I9c-CD18 4.00 - 4.35 33 2.67 77.8 28.6 49.2 1.49

S10-CD20 9.3 – 9.5 47 - 65.4 - - 0.52

I15-CD6 18.3 60 2.52 123 46.9 76.1 1.27

Bentonite - - 82 2.75 324 44.8 279.2 3.4

Kaolin - - 75 2.60 66.8 32.9 33.9 0.45

Table 2.1 Physical properties and Atterberg limits of the tested soils.

In order to get some preliminary information on the influence of pore fluid composition on

the behaviour of the tested soils, their liquid and plastic limits were evaluated by hydrating

the materials both with distilled water and with various salt solutions at different

concentrations. The results are shown in Figure 2.18 against the molarity of the used

solution. It can be seen that the liquid limit of the Costa della Gaveta soil does not vary

with the pore solution concentration significantly. Only one sample (I15-CD6),

2. Influence of pore fluid composition on clay behaviour

26

characterised by a liquid limit in water sensibly higher than that of the others, shows to be

significantly influenced by the used fluid, probably because of a different clay mineralogy.

The liquid limits in NaCl and in KCl solutions seem consistent to one another. The liquid

limit of the tested bentonite is influenced by the used fluid noticeably. The values decrease

noticeably in the range 0-1 mol/l, independently of the used solution, while much smaller

variations are seen at higher concentrations. Only small effects of pore solution

concentration are evaluated for the tested kaolin.

0

50

100

150

200

250

300

350

0 1 2 3 4 5 6

wL

(%)

solution molarity (M)

NaCl

KCl

CaCl2.6H2O

MgCl2.6H2O

NaClKClCaCl2⋅6H2OMgCl2⋅6H2O

wP NaCl

Bentonite

0

50

100

150

200

250

300

350

0 1 2 3 4 5 6

wL

(%)

solution molarity (M)

S9-A I9b-AI9b-CD12I15-CD6

Costa della GavetaNaCl solutions

0 1 2 3 4 5 6solution molarity (M)

S9-A I9b-AI9b-CD9bisI9c-CD18S7-CD2

Costa della GavetaKCl solutions

0 1 2 3 4 5 6solution molarity (M)

NaCl

KCl

wP

Kaolin

Figure 2.18 Liquid limit, wL, of the tested materials in water and salt solutions at different

concentrations. Some determinations of the plastic limit, wP are indicated as well.

While the influence of pore solution concentration on the liquid limit seems small, the

influence on the residual shear strength is noticeable. Figure 2.19, for instance, shows the

results of shear tests carried out, in the Bromhead apparatus, on the same material prepared

with water, with 0.2 M NaCl solution and with 2 M NaCl solution. The residual friction

coefficient τr/σ’n of the material varies between less than 0.2 in water and about 0.3 in the

2. Influence of pore fluid composition on clay behaviour

27

concentrated salt solution, which corresponds to a variation in the residual friction angle

ϕ’r from about 10° to about 16°. The use of a relatively less concentrated solution (0.2 M

NaCl) produces a strength increase, with respect to the strength obtained in water, which is

already significant.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 20 40 60 80

τ/σ

' n

horizontal displacement (mm)

Costa della Gaveta (S9, 24-25 m, σ'n = 150-200 kPa)Bromhead apparatus (v = 0.018 mm/min)

2 M NaCl0.2 M NaCl

water

Figure 2.19 Friction coefficient against the horizontal displacement for specimens of the

Costa della Gaveta soil tested in water and in NaCl solutions.

Several other shear tests were carried out on many specimens of the Costa della Gaveta

soil. Figure 2.20 shows the results comprehensively in terms of the residual shear strength

against the normal applied stress. In particular, Figure 2.20a refers to specimens

reconstituted with distilled water and tested in a bath of distilled water as well. The results

are compared to those previously obtained on other specimens of the Costa della Gaveta

soil (Di Maio et al., 2010, 2013). The results, which seem consistent to one another, lie

between two lines through the origin corresponding to ϕ’r = 8° and ϕ’r = 10°. The effect of

the testing apparatus seems negligible, as well as that of the normal stress for σ’n > 100

kPa.

In Figure 2.20b the results relative to specimens reconstituted with – and submerged in –

solutions of NaCl at different concentrations (tests without chemical gradients) are added

to those shown in Figure 2.20a. It can be seen that the residual shear strength of all

specimens in salt solutions is significantly higher than that of specimens in water. The

2. Influence of pore fluid composition on clay behaviour

28

experimental points can be interpreted in terms of residual friction angles ranging between

13° and 20°, that is up to twice those evaluated in water.

other specimens

S9-A (Bishop)

S9-A (Bromhead)

S9-B (Bromhead)

S9-B (Casagrande)

S9-MIX (Bromhead)

S9-MIX (Casagrande)

S7-CD2 (Casagrande)

0

20

40

60

80

100

0 100 200 300 400 500

τ r(k

Pa)

σ'n (kPa)

Tests in distilled water

ϕ'r = 8°

ϕ'r = 10°

0

20

40

60

80

100

0 100 200 300 400 500

τ r(k

Pa)

σ'n (kPa)

ϕ'r = 8.5°

ϕ'r = 13°ϕ'r = 20°Tests in salt

solutions

a)

b)tests in water

0.2 M NaCl

0.5 M NaCl

1 M NaCl

2 M NaCl

5 M NaCl

Saturated Solution NaCl

Figure 2.20 Residual shear strength against normal effective stress of specimens of Costa

della Gaveta soil: a) specimens tested in distilled water; the results are compared to those

of other specimens from different samples (data from Di Maio et al., 2010; 2013); b) tests

in NaCl solutions at various concentrations, compared to those obtained in water.

In Figure 2.21 the values of residual shear strength relative to specimens of kaolin (a) and

bentonite (b) reconstituted with – and submerged in – water or 1 M NaCl solution are

plotted against the normal applied stress. These specimens were tested in different

apparatuses, without observing significant influence of the testing device on the results. A

noticeable difference between the residual shear strength in water and in solution can be

2. Influence of pore fluid composition on clay behaviour

29

seen for the used bentonite: a residual friction angle ϕ’r = 5° can be evaluated in water,

while ϕ’r = 17° can be evaluated in the 1 M NaCl solution. For the tested kaolin, the same

value of residual friction angle, ϕ’r = 13°, was evaluated in different apparatuses, under

different normal stresses, both on specimens in water and in 1 M NaCl solution.

0

20

40

60

80

100

0 100 200 300 400 500

τ r(k

Pa)

σ'n (kPa)

Casagrande Reversal Bromhead

Kaolin

1M NaClCasagrande distilled water

ϕ'r ≈ 13°

0

20

40

60

80

100

0 100 200 300 400 500

τ r(k

Pa)

σ'n (kPa)

Casagrande Bishop Bromhead

ϕ'r = 17°

ϕ'r = 5°

1M NaCl

distilled water

Bentonite

a)

b)

Figure 2.21 Residual shear strength of kaolin (a) and bentonite (b) in water and 1M NaCl

solution evaluated by means of different apparatuses.

Figure 2.22 shows the residual friction angle ϕ’r against the NaCl concentration in the pore

fluid of several specimens of the Costa della Gaveta soil, tested under similar normal

stresses. The residual shear strength of two undisturbed specimens taken close to the shear

surface in borehole K1bis (8.3 and 8.4 m) is also shown. A residual friction angle of 12°

was evaluated on both specimens. The pore ion concentration was evaluated on the

2. Influence of pore fluid composition on clay behaviour

30

material from the same undisturbed sample. Subsequently, the specimens were sheared

further and exposed to distilled water, allowing ion diffusion outward from the pores. This

caused a decrease in the residual friction angle from 12° to 9.8°, suggesting that the

available residual strength on the slip surface of the landslide can decrease further as an

effect of ion concentration decrease.

The figure shows that the Costa della Gaveta soil exhibits a noticeable shear strength

increase with increasing NaCl concentration. The experimental points relative to the

undisturbed specimens lie on the same curve as that of the reconstituted specimens. The

relation between ϕ’r and pore solution molarity is not linear, with higher gradients at lower

concentrations. In particular, most of the strength variations are achieved within the range

0 – 1 mol/l.

5

10

15

20

0 1 2 3 4 5 6

resi

dual

fric

tion

ang

le,

ϕ' r

NaCl molarity, M

K1bis undisturbed 100 kPa

S9-A reconstituted 150-175 kPa

S9B reconstituted 150-225 kPa

S9-MIX reconstituted 204 kPa

undisturbed K1bis specimensclose to the slip surface

K1bis after exposure to water

Figure 2.22 Residual friction angle against NaCl concentration in the pore solution for

reconstituted and undisturbed specimens of the Costa della Gaveta soil (mod. from Di

Maio et al., 2015c).

The results relative to the Costa della Gaveta soil, those relative to bentonite and those

obtained by Di Maio (2004a) on several soils are compared in Figure 2.23 in terms of

residual friction angle against NaCl concentration. The experimentation carried out by Di

Maio (2004a) was conducted under σ’n = 200 kPa, a value comparable to the normal

stresses applied during the tests shown in this section. The trend of residual shear strength

2. Influence of pore fluid composition on clay behaviour

31

increase with concentration has practically the same shape for all materials, although the

magnitude of the effect of pore solution molarity is different. The highest dependence on

NaCl concentration is shown by the bentonite, whose ϕ’r ranges from 5° in water to more

than 20° in the 3 mol/l NaCl solution. The Ponza bentonite is mainly smectitic, Bisaccia

and Gela clays also contain relevant percentages of smectite (Di Maio, 2004a), which

probably control their behaviour.

0

5

10

15

20

0 1 2 3 4 5 6

ϕ' r

(°)

NaCl concentration (mol/l)

Costa della Gaveta soilBisaccia clay (Di Maio, 2004a)Gela clay (Di Maio, 2004a)Ponza bentonite (Di Maio, 2004a)Commercial bentonite

Figure 2.23 Residual friction angle against NaCl concentration in the pore fluid of

specimens of different clays.

Some of the specimens pre-sheared to the residual condition were subsequently exposed to

a different fluid and sheared further. In particular, some specimens initially in distilled

water were exposed to a concentrated salt solution.

Figure 2.24 shows the case of a specimen of Costa della Gaveta material which was

exposed to 1 mol/l solution of KCl. The exposure produced a gradual but noticeable shear

strength increase up to a value of residual shear strength triple than that attained in water.

On the subsequent re-exposure to distilled water, the shear strength exhibited only a

negligible decrease, thus suggesting that ion exchange had taken place. No effects were

seen on the volume change of the specimen.

2. Influence of pore fluid composition on clay behaviour

32

0

20

40

60

80

100

τ r(k

Pa)

S7CD2 - σ'n = 155 kPaexposure to 1M KCl exposure to distilled water

manual cutmanual cut

-0.05

0.00

0.05

0 50 100 150 200 250

heig

ht v

aria

tion

(mm

)

horizontal displacement (mm) Figure 2.24 Shear strength and height variation of a specimen, reconstituted with – and

submerged in – distilled water, pre-sheared to the residual condition and then exposed to 1

M KCl solution and, subsequently, to distilled water.

Some other specimens were exposed to 1 M NaCl solution, which caused a significant

shear strength increase, although of lower magnitude than with KCl, to values consistent

with those obtained on specimens reconstituted with – and submerged in – 1 M NaCl

solution.

One specimen was prepared with the soil extracted from borehole S9 at a depth of about 26

m (close to the slip surface), reconstituted with distilled water and pre-sheared to the

residual condition in a bath of distilled water. During the course of the test, the specimen

was exposed to a composite “natural” solution, i.e. a solution prepared using NaCl, KCl,

MgCl2 and CaCl2 in proportions such that the cations Na+, K+, Mg2+ and Ca2+ would have

the same concentrations as those evaluated in the natural pore solution of the same sample:

0.372 M Na+, 0.017 M K+, 0.092 M Ca2+, 0.045 M Mg2+. The exposure caused a gradual

but significant shear strength increase (Figure 2.25), corresponding to a residual friction

2. Influence of pore fluid composition on clay behaviour

33

angle increase from 7° to 13°, without significant volume changes. Figure 2.26 shows the

residual shear strength evaluated on the specimen, against the normal stress, during

different phases of the test. Since the beginning of the test, the cell water was frequently

replaced with distilled water. The values of the residual shear strength in this phase are

indicated in the figure by points 1-4. It can be seen that, probably as an effect of the

continuous exposure to water, the ions already in the pores diffused away, thus the residual

friction angle decreased. At point 4 the specimen was exposed to the “natural” solution

which caused the strength increase (to point 5) shown in Figure 2.25. The specimen was

then loaded (point 6), confirming the same value of the residual friction angle..

0

10

20

30

40

50

τ r(k

Pa)

S9B - σ'n = 151 kPa

exposure to "natural solution"

-0.05

0.00

0.05

0 10 20 30 40 50 60 70 80 90 100

heig

ht v

aria

tion

(mm

)

horizontal displacement (mm)

Figure 2.25 Shear strength of a specimen, reconstituted with – and exposed to – distilled

water, pre-sheared to the residual state and then exposed to the “natural solution”.

2. Influence of pore fluid composition on clay behaviour

34

0

10

20

30

40

50

60

0 50 100 150 200 250

ττ ττr(k

Pa

)

σσσσ'n (kPa)

S9B, Casagrande apparatus

exposure to natural solution

esposure to water

1

2

3

4

5

6

Figure 2.26 Residual shear strength history against normal effective stress of the specimen

of S9B material.

The effects of exposure of pre-sheared specimens to fluids different from the pore fluid

were evaluated also on some specimens of bentonite for comparison.

A specimen was prepared by mixing the material with 1 mol/l NaCl solution. The

specimen was first sheared to the residual state while immersed in the same solution. The

residual shear strength was found consistent with the values reported in Figure 2.21b. The

cell fluid was then replaced by distilled water, which was renewed frequently to keep the

chemical gradient between the pore fluid and the cell fluid as high as possible, and the

specimen was sheared further. The shear strength, shown in Figure 2.27a against time,

gradually decreased and became finally equal to that of specimens prepared with water and

sheared while immersed in water (corresponding to ϕ’r ≈ 5°, as in Figure 2.21b). Figure

2.27b shows the height variations undergone by the specimen. Although the shear box is

not suitable to evaluate the volume change behaviour, it can be seen that significant

swelling started to occur after about 40 days of continuous exposure to water, that is when

the strength had already decreased noticeably.

2. Influence of pore fluid composition on clay behaviour

35

0

10

20

30

40

50

60sh

ear s

tren

gth

, τ(k

Pa)

v = 0.0025 mm/min

manual cut

manual cut

manual cut

commercial bentoniteCasagrandeapparatus

σ'n = 150 kPa

τr in 1 M NaCl

τr in water

-10

1

2

3

45

6

7

hei

ght

var

iati

ons

(mm

)

0.0

0.2

0.4

0.6

0.8

1.0

0 10 20 30 40 50 60 70 80

NaC

l in

the

por

e fl

uid

(m

ol/l

)

time since exposure to water (days)

average concentration in the specimen evaluated after the test

0.00

0.01

0.02

NaC

l in

the

cell

flu

id

(mo

l/l)

a)

b)

c)

d)

Figure 2.27 Exposure to distilled water of a specimen of bentonite reconstituted with 1 M

NaCl solution and sheared until the residual state while immersed in 1 M NaCl solution:

shear strength (a), height variations (b), NaCl concentration in the cell fluid before each

water renewal (c), and estimated average concentration in the pore fluid (d).

2. Influence of pore fluid composition on clay behaviour

36

Before each water renewal, the Na+ concentration of the cell water was measured by means

of an ion-selective electrode to evaluate the possible ion diffusion. The values are plotted

in Figure 2.27c. Being the cell water and the pore water volumes known, it is possible to

estimate how the average NaCl concentration in the pores decreased during the process of

exposure to water (Figure 2.27d). In order to check whether the obtained curve of

concentration versus time was reliable, at the end of the test the specimen was oven-dried

to determine its water content and subsequently powdered and mixed with a known

amount of distilled water. Settlement of the suspension was allowed and the sodium

concentration of the supernatant fluid was measured. Under the hypothesis that all the ions

in the pore fluid were dispersed in the solution, the sodium concentration of the former

could be estimated. The result is represented by the red hollow marker in Figure 2.27d. The

value is consistent with the final concentration evaluated by means of measurements of

Na+ in the cell fluid.

2.2.2 Observation of the shear surface

In order to estimate soil parameters such as viscosity, it is important to evaluate the

thickness of the soil portion affected by shearing deformations. To this aim, and to

understand if the shear zone is characterised by different properties, some analyses have

been carried out by different techniques.

A specimen of Costa della Gaveta soil (S9B), reconstituted with distilled water, was

sheared in a bath of distilled water in the Bromhead apparatus. After the test, the specimen

was analysed by means of an environmental scanning electron microscope (ESEM) in

order to examine the material along the slip surface.

Figure 2.28 shows a ESEM micrograph of the investigated specimen. The figure refers to a

vertical cross section, in which the shear surface is located at the bottom. Close to the

surface, a zone in which the particle aggregates appear well aligned can be seen. The

thickness of this zone can be estimated in about 200 µm. However, a particle alignment in

the direction of shearing can be seen also on the top of the image, while on the left side a

band of particles with similar inclination can be seen. This suggests that all the area shown

in the micrograph, which has a thickness of about 1 mm, can be part of the shear band

2. Influence of pore fluid composition on clay behaviour

37

whose thickness has been estimated to be about 1.5 mm for each half of a specimen tested

in the Casagrande apparatus (Di Maio et al., 2013).

Some additional micrographs, taken with different magnifications, are shown in Figure

2.29. It can be seen that the material is mostly constituted by platy particles arranged in

stacks with a preferential direction. The thickness of the stacks is in the order of several

microns, while the thickness of the single foils seems much lower than 1 µm.

aligned aggregates

shear surface

Figure 2.28 ESEM micrograph of the shear zone of a specimen of Costa della Gaveta soil

tested in the Bromhead apparatus.

2. Influence of pore fluid composition on clay behaviour

38

Figure 2.29 ESEM micrographs with increasing magnification of the shear zone of a

specimen of Costa della Gaveta soil tested in the Bromhead apparatus

A second specimen of the same material, tested in the Casagrande apparatus, was

submitted to three dimensional X-ray tomography at the University of Padova, Italy.

The technique allows for the investigation of the whole specimen’s volume, overcoming

the limitation of the microscopy, by means of which only the surface can be studied. The

technique is similar to the X-ray analyses for medical purposes, it is non-invasive and does

not cause sample disturbance.

The instrument provides a 3D image made of “voxels” (i.e. 3D pixels) whose values can be

interpreted as a mean local density when the voxels are significantly larger than the grain

size. Alternatively, the single grains can be delineated when the voxels are significantly

smaller than them (Viggiani et al., 2015).

2. Influence of pore fluid composition on clay behaviour

39

Some promising results regarding the use of this technique for geotechnical purposes have

been published, for instance, by Lenoir et al. (2007), Andò et al. (2011) and Viggiani et al.

(2015), who used the 3D X-ray tomography to reveal processes in soils such as strain

localisation, deformations due to volume removal, ice formation and desiccation cracks.

The tomography shown in this work was carried out by means of the Skyscan1172

instrument (Bruker microCT), equipped with a 11 Mp camera. The resulting voxel size

was 4.77 µm. The investigated specimen is a small portion of the shear specimen of about

6 mm side, sampled close to the slip surface. Since the observations were made some days

after the specimen was extracted, some drying of the material took place.

Figure 2.30 shows an example of 3D view of the shear surface and vertical cross sections

of the investigated specimen (the slip surface is located on the top). The shades of grey

show the different relative density of the material, which can possibly depend both on non-

homogeneity of the soil composition and of the water content. Lighter (i.e. relatively

denser) zones are possibly constituted by coarse grains or clay aggregates with relatively

lower water content. It can be seen that in the zone close to the slip surface the denser

zones are less abundant. About 1 mm below the slip surface, a zone characterised by lower

density, or even a void, can be seen. It is possible that this discontinuity was caused by

different shrinkage, due to drying, of the material close to the slip surface with respect to

the rest of the specimen, possibly because of different water contents resulting after

shearing.

Some statistical analyses have been carried out on the results of the X-ray tomography.

Figure 2.31a shows how the mean value of the relative density (in arbitrary units) varies in

the vertical direction. It can be seen that in most of the specimen’s volume the density

remains quite constant. However, it decreases towards the top, that is close to the shear

surface. Most of the decrease occurs in a zone about 1 mm thick., which corresponds to the

zone above the discontinuity seen in Figure 2.30. In Figure 2.31b the density distribution in

two horizontal sections of the specimen is plotted. The difference between the curves

relative to the shear zone and to the rest of the specimen is evident.

2. Influence of pore fluid composition on clay behaviour

40

1 mm

1 mm

1 mm

Figure 2.30 3D view of the shear surface and vertical sections of the specimen of the Costa

della Gaveta soil seen by X-ray tomography.

2. Influence of pore fluid composition on clay behaviour

41

0

20000

40000

60000

80000

100000

120000

20 30 40 50 60 70 80 90 100 110fr

eque

ncy

class of density

lower density

higherdensity

h = 4.5 mmh = 0.2 mm

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

45 46 47 48 49 50 51

heig

ht (m

m)

class of density

top

bottom

mean density

position of the discontinuity

b)

a)

Figure 2.31 Variation of the mean relative density (arbitrary units) in the vertical direction

(a) and relative density distribution against frequency for two horizontal cross section of

the specimen.

42

3 INFLUENCE OF PORE FLUID

COMPOSITION ON CREEP BEHAVIOUR

This Chapter reports on the results of laboratory tests aimed at evaluating the mechanical

behaviour of the material along a pre-existing slip surface in the residual condition when

the specimen is subjected to changes in the pore fluid composition. The shear creep

behaviour and the chemically-induced displacement evolution were investigated by means

of shear tests under constant applied shear stresses in modified Casagrande and Bishop

apparatuses.

In the first paragraph, interpretations and modelization of creep phenomena reported in the

technical literature are reviewed and commented. Subsequently, the results of stress-

controlled shear tests on the Costa della Gaveta soil and on specimens of bentonite are

reported. Finally, the description of a simplified modelization of ion diffusion and shear

strength variation, which was helpful in the test interpretation, is presented. The main

results contained in this Chapter have been published by Di Maio and Scaringi (2015) and

Di Maio et al. (2015a).

3. Influence of pore fluid composition on creep behaviour

43

3.1 SHEAR CREEP: A BRIEF OVERVIEW OF THE

PHENOMENON

Creep is defined as the progressive, irrecoverable deformation of a soil element under a

state of constant effective stress (Kwok and Bolton, 2010). An increase in the deviatoric

stress level can result in a deformation response characterised by three successive phases

which are named primary, secondary and tertiary creep, characterised by decreasing,

constant and increasing strain rate respectively (Figure 3.1). The actual strain pattern is

hypothesised to depend on the type of soil, stress level and stress history (Singh and

Mitchell, 1968; Tavenas et al., 1978; Augustesen et al., 2004; Le et al., 2012).

Figure 3.1 Definition of creep stages according: strain versus time (a) and log(strain rate)

versus log(time) (b) (Augustesen et al., 2004).

Failure of cemented bonds or increase in the ratio of tangential to normal forces at the

interparticle contacts are among the processes which can lead to creep rupture for loss of

strength, in drained conditions and in the absence of chemical changes (Kuhn, 1987; Kuhn

and Mitchell, 1993; Mitchell and Soga, 2005; Kwok and Bolton, 2010).

The magnitude of creep strains increases with increasing plasticity, activity and water

content of the soil. The most active clays usually exhibit the greatest time-dependent

response because the smaller the particle size, the greater is the specific surface, and the

greater the water adsorption (Mitchell and Soga, 2005).

3. Influence of pore fluid composition on creep behaviour

44

Most soils have a characteristic relationship between strain rate and time. This was shown,

for instance, by Bishop (1966) for drained triaxial compression creep of London clay and

by Murayama and Shibata (1958) for undrained triaxial compression creep of soft Osaka

clay.

Pore pressures may change during creep according to the volume change tendency of the

soil and to the possibility of drainage during the deformation process (Mitchell and Soga,

2005).

The theoretical shape of the curve of creep strain against time (Figure 3.1) may not exist at

all, as discussed by Ter-Stepanian (1992) who observed that a “jump-like structure

reorganization” may occur, reflecting a stochastic character for the deformation. This

behaviour was observed during a shear creep test on an undisturbed specimen of

overconsolidated clay.

Ter-Stepanian (1992) suggests the existence of four levels of deformation, two of them

concerning the deformation of matter and two of them the deformation of

particles/aggregates. In particular, regarding the matter, the Author focuses on (1) a

molecular level, which consists of displacement of particles by surmounting energy

barriers, and (2) on mutual displacement of particles as a result of bond failures, but

without rearrangement. With respect to the particle/aggregate deformation, the Author

points out (3) a structural level of soil deformation involving mutual rearrangement of

particles, and (4) deformations at the aggregate level.

Deformations at levels (3) and (4) should not be uniform due to the particulate nature of

soils and should proceed through a series of structural readjustments corresponding to the

relative movement of particles with respect to each other, thus leading to an irregular

sequence of deformations. Regarding the effects of particle rearrangement, Kuhn (1987)

developed a discrete element model that considers “visco-frictional” sliding at interparticle

contacts. Subsequently, Kuhn and Mitchell (1993) performed numerical analyses using a

discrete element model, obtaining a discontinuous creep behaviour comparable to that

observed on several soils.

In order to investigate deformations at levels (1) and (2), creep phenomena can be studied

as a rate process by means of the theory of absolute reaction rates (Eyring, 1936; Glasstone

3. Influence of pore fluid composition on creep behaviour

45

et al., 1941), which is based on statistical mechanics. An adaptation of the theory to the

study of soil behaviour can be found, among others, in Feda (1989, 1992) and in Kuhn and

Mitchell (1993). The concept is that atoms, molecules and/or particles involved in a

deformation process (termed “flow units”) are constrained from relative movement by

energy barriers which separate adjacent equilibrium positions. In order to produce a

displacement, the flow unit must overcome the barrier by acquiring a surplus of potential

energy, termed the “activation energy”, ∆F. The potential energy of the flow unit after the

displacement may be lower than, equal to, or higher than the potential energy before the

displacement, thus defining conditions of increased stability, steady-state or decreased

stability respectively.

The activation energy may be provided by thermal energy or by an applied potential. If this

latter is not directional, flow units can surmount the energy barrier with equal probability

in all directions, therefore no macroscopic deformation is produced. On the contrary, if a

directional potential, such as gravity or a shear stress, is applied, than the barrier heights

are not equal in all directions, but lower in the direction of shearing and higher in the

opposite direction. Consequently, the barriers are most probably crossed in the direction of

shearing, thus producing a macroscopic deformation. A schematic representation of the

effect of a shear force on the activation energy required for deformation is shown by

Figure 3.2 (Mitchell and Soga, 2005).

Figure 3.2 Schematic representation of energy barriers in rate process theory in absence

and in presence of a directional potential (Mitchell and Soga, 2005).

3. Influence of pore fluid composition on creep behaviour

46

Mitchell et al. (1968) showed that the rate of macroscopic deformation resulting from the

application of a directional potential, such as a shear force, can be expressed as a function

of the applied potential and of thermodynamic parameters, as in Figure 3.3. However, the

equation obtained by the Authors, since it is referred to deformations at levels (1) and (2)

only, does not account for structural changes. Therefore, if shear stress and thermodynamic

parameters (e.g. temperature) do not vary, than the strain rate remains constant, i.e. a

secondary creep is produced. In order to generalise their result, the Authors introduced a

parameter (termed X in Figure 3.3, and further defined by Ter-Stepanian, 1975) which can

be both structure and time dependent, so that primary and tertiary creep due to

deformations at level (3) and (4) could be included in the model.

Figure 3.3 Strain rate as a function of an applied directional potential according to the

rate process theory (Mitchell and Soga, 2005).

Notwithstanding this limitation, the equation was used by Kuhn and Mitchell (1993) as

part of the particle contact law in their discrete element modelling, and by Puzrin and

Houlsby (2003) as an internal function of a thermo-mechanically based model, deriving a

rate-dependent constitutive model for soil. Mitchell and Soga (2005) reported that the real

behaviour of many systems is substantially consistent with the statistical mechanics

formulation of the rate process theory. Different parts of the formulation have been tested

separately by Mitchell et al. (1968), giving results according to predictions.

Different Authors, among whom Mitchell et al. (1968), provided some ranges of activation

energy for soil creep. Mitchell and Soga (2005), following Andersland and Douglas

(1970), concluded that variations in water content (including complete drying), adsorbed

cation type, consolidation pressure, void ratio, and pore fluid have no significant effect on

the required activation energy. As a consequence, variations in strain rate in the absence of

structural rearrangements would not be due to changes in the activation energy but only to

changes in the number of bonds. However, this does not seem reasonable in phyllosilicates

with face-to-face orientation, which are kept together by electrostatic forces. In order to

preserve electroneutrality, the total charge of the adsorbed cations cannot change and,

3. Influence of pore fluid composition on creep behaviour

47

therefore, the number of interparticle weak bonds will remain constant. On the contrary, it

must be considered that an increase in the double layer thickness, due to a decrease in ion

concentration or to an increase in the dielectric constant of the pore fluid, could weaken the

bonds and reduce the activation energy required to break them.

Additional considerations by Mitchell and Soga (2005) are the following: 1. the number of

bonds is directly proportional to effective consolidation pressure for normally consolidated

clays; 2. overconsolidation leads to more bonds than in normally consolidated clay at the

same effective consolidation pressure.

In fact, the validity of the conclusions drawn by Andersland and Douglas (1970) relies

upon the existence of solid-to-solid contacts between clay particles. Evidence of this have

been provided for some cases, for instance, by Matsui et al. (1977, 1980) by means of

photomicrographs, and by Koerner et al. (1977) by means of acoustic emissions. However,

this may not be valid in the case of smectites, especially in the residual condition. Normal

effective stresses and shear stresses can be transmitted only at interparticle contacts in most

soils. Pure sodium montmorillonite may be an exception (Mitchell and Soga, 2005) since a

relevant part of the normal stress can be carried by physicochemical forces of interaction.

Deformation at large strain can approach a steady-state condition in which there is little

further structural change with time (this is the case of residual state). This means that,

following Ter-Stepanian (1992), creep strains are due only to level 1 and level 2

deformations (rearrangement of matter). The governing equations of the rate process

theory may be rewritten in a form which is similar to the Coulomb equation for strength

(see Mitchell and Soga, 2005) which states that both cohesion and friction depend on the

number of bonds times the bond strength, and that the values of c and ϕ should depend on

the rate of deformation and the temperature. As a consequence, in the absence of structural

rearrangements, the shearing resistance should increase linearly with the logarithm of the

strain rate. Karlsson (1963) gave experimental evidence of this by means of vane tests on

different remoulded clays subjected to shear at different rates. The rate effect on the

residual shear strength may follow the same law provided that no changes in the shearing

mode occur (see Lupini et al., 1981, and Tika et al., 1996). Conversely, transition from

laminar to turbulent shearing mode, which involves particle rearrangement, should result in

a different strength – rate relationship.

3. Influence of pore fluid composition on creep behaviour

48

A possible volumetric-deviatoric creep coupling may occur, as highlighted by Mitchell and

Soga (2005). This implies that a rapid application of a stress or a strain can result in rapid

change of pore water pressure in a saturated soil under undrained conditions. The rapid

application of a shear stress on clay specimens, i.e. characterised by very low hydraulic

conductivity, may result in pore fluid pressure excess. The dissipation of pore pressure

excess produces an increase in the effective normal stress, which may result in a creep

phase characterised by a decreasing strain rate, i.e. can appear as primary creep.

Furthermore, when a shear creep test is performed, the necessary time for primary

consolidation of the specimen is waited before applying the shear force but, for the entire

duration of the test, volumetric creep takes place. Consequently, the shear strength of the

material may increase due to the formation of additional bonds and/or to the strengthening

of existing bonds, as proved by Nakagawa et al. (1995).

Mitchell and Soga (2005) reported four possible causes of strength loss which lead to

failure under shear creep: (1) failure of cementation bonds, if a significant portion of the

strength of a soil is due to cementation; (2) in the absence of chemical or mineralogical

changes the strength depends on effective stresses: if creep causes changes in effective

stresses, then strength changes will also occur; (3) in almost all soils, shear causes changes

in pore pressure during undrained deformation and changes in water content during drained

deformation); (4) water content changes cause strength changes.

Besides these reasons, also chemical changes, such as pore fluid composition variation in

certain types of soil, can cause shear strength changes and, consequently, it can be

reasonable to expect that they can produce creep failure.

3. Influence of pore fluid composition on creep behaviour

49

3.2 EXPERIMENTAL RESULTS

RELATIVE TO THE COSTA DELLA GAVETA SOIL

The chemical composition of the pore fluid affects the mechanical behaviour of clays

noticeably. The influence of pore fluid composition on the residual shear strength of the

Costa della Gaveta soil, determined by displacement-controlled tests, was shown in

section 2.2.1. The following paragraph shows the results relative to stress-controlled tests.

3.2.1 Stress-controlled shear tests on the Costa della Gaveta soil

In order to investigate the rheological behaviour of the soil along a pre-existing shear

surface, direct and ring shear tests were carried out under constant shear forces or stresses

(“force-controlled” or “stress-controlled” tests).

To perform such type of tests, the Casagrande and the Bishop apparatuses were modified

(Figure 3.4) in order to convert vertical forces, applied by means of dead loads, into

horizontal forces acting on the upper box or upper ring respectively (Di Maio et al., 2013,

2015a; Di Maio and Scaringi, 2015). During shearing in the ring shear device the contact

area does not change, thus constant forces correspond to constant average shear stresses,

i.e. the test is properly a “stress-controlled” test. On the contrary, small area variations

occur in the Casagrande direct shear and the test can be considered only “force-controlled”.

However, the small area variations during the test (< 2%) have been accounted for in the

interpretation of the results.

3. Influence of pore fluid composition on creep behaviour

50

a) b)

load cell load cell

Figure 3.4 Schematic representation of the direct shear apparatus modified to perform

force-controlled tests (a). Picture of the Bishop ring shear modified to perform stress-

controlled tests (b).

The tests reported in this section were carried out on specimens of the Costa della Gaveta

soil. Subsequently, further tests were performed on specimens of sodium bentonite in order

to compare the obtained results to those relative to a pure clay and to see whether they

have more general validity. The results of these latter tests are reported in section 3.3.

The adopted test procedure was the following:

1. the specimens were prepared by mixing the powdered material with a concentrated salt

solution (1 M NaCl) and were sheared until the residual state was attained while immersed

in the same solution (displacement-controlled phase without chemical gradients);

2. the apparatuses were modified as in Figure 3.4 to perform the force/stress-controlled

tests (force/stress-controlled phase, or creep phase);

3. at the end of this phase, the original configuration of the apparatuses was restored to

perform additional displacement-controlled shearing to verify the available shear strength.

For sake of simplicity the test phases will be referred to as first, second, and third test

phases respectively. Table 3.1 summarises the test phases, the parameters which were

monitored and the used instruments. The table also reports the fluid in which the

specimens were submerged during each phase.

3. Influence of pore fluid composition on creep behaviour

51

Phase Test mode Cell fluid Measured quantities and instruments

1 displacement-

controlled shear

test

water or salt solutions

at different

concentrations

horizontal displacements (LVDT),

shear strength (load cell), height

variations (LVDT)

2a force-controlled

or stress-

controlled shear

test

same as in phase 1 horizontal displacements (LVDT),

shear strength (load cell), height

variations (LVDT)

2b force-controlled

or stress-

controlled shear

test

distilled water

(frequently renewed)

horizontal displacements, height

variations, cell fluid electrical

conductivity (4-electrode conductivity

probe) and/or Na+ concentration (ion-

selective electrode)

3 displacement-

controlled shear

test

distilled water

(frequently renewed)

horizontal displacements (LVDT),

shear strength (load cell), height

variations (LVDT), cell fluid electrical

conductivity (4-electrode conductivity

probe) and/or Na+ concentration (ion-

selective electrode)

Table 3.1 Test phases, measured parameters and devices.

The first phase is similar to those described in Chapter 2. Each tested material was sheared

to the residual under displacement rate condition and the residual strength was determined

both with distilled water and 1 M NaCl solution as pore and cell fluid, in absence of

chemical gradients and in drained conditions.

In the second phase all the specimens prepared with and immersed in 1 M NaCl solution,

were subjected to an average horizontal shear stress lower than the residual strength

obtained, under the same normal stress, with the salt solution and higher than the residual

strength obtained for the same material with distilled water (Figure 3.5, Table 3.2).

The application of the horizontal force caused very small horizontal displacements with

decreasing rate (Figure 3.6a). This process occurred under constant effective stresses, i.e. it

3. Influence of pore fluid composition on creep behaviour

52

is a primary creep (Augustesen et al., 2004). Subsequently (time = 0 in Figure 3.6) the cell

solution was replaced by distilled water, which was frequently renewed (usually twice a

day) to keep the chemical gradient between the pore fluid and the cell fluid as high as

possible.

0

10

20

30

40

50

60

70τ r

(kP

a)

τr in 1M NaCl

τr in dist. water

τ applied

S9A

Costa della Gaveta

0

10

20

30

40

50

60

70

0 100 200 300 400

τ r(k

Pa)

σ'n (kPa)

τr in 1M NaCl

τr in dist. water

τ applied B2P1

Costa della Gaveta

a)

b)

Figure 3.5 Test conditions of the specimens of the Costa della Gaveta soil submitted to

stress/force-controlled shear tests.

Spec.

Borehole

- Sample

Shear

apparatus

σσσσ’n

(kPa)

ττττr in 1 M NaCl

solution (kPa)

Applied ττττ

(kPa)

ττττr in water

(kPa)

P1 S9-MIX Casagrande 204 55 45.0 35

S9A S9-A Casagrande 253 50 44.3 36

B2 S9-MIX Bishop 205 55 49.8 35

Table 3.2 Test conditions of the specimens of Costa della Gaveta soil submitted to

stress/force-controlled shear tests.

3. Influence of pore fluid composition on creep behaviour

53

0.0

0.5

1.0

1.5

2.0

2.5

3.0

-20 0 20 40 60 80

hori

zont

al d

ispl

acem

ent

(mm

)

1M NaCl solution

distilled water

S9A

B2

P1

0

50

100

150

-20 0 20 40 60 80

dis

plac

emen

t rat

e (µ

m/d

ay)

S9AB2P1

-0.1

0.0

0.1

-20 0 20 40 60 80

heig

ht v

aria

tion

(m

m)

time (days)

S9A

B2

P1

a)

b)

c)

exposure to water

exposure to water

Figure 3.6 Effect of exposure to distilled water of specimens Costa della Gaveta soil

reconstituted with 1 M NaCl solution, previously immersed in the same solution and then

(time=0) exposed to distilled water: horizontal displacement, displacement rate and height

variation against time.

3. Influence of pore fluid composition on creep behaviour

54

As a consequence of exposure to distilled water, the displacement rate increased (Figure

3.6b) with a non linear trend, with a pattern similar to that of secondary and then tertiary

creep, until “failure”. This term seems inaccurate since the specimens are subjected to

shearing along a pre-existing shear surface in residual condition. The term “failure” is used

here to indicate a dramatic increase in the displacement rate.

More in detail, the displacement rate of the specimens of the Costa della Gaveta soil

remained in the order of 1-10 µm/day for some weeks (Figure 3.6b) Afterwards, specimens

S9A and P1 experienced a sudden rate increase, while B2, tested in the Bishop apparatus,

underwent a progressive and more regular displacement rate increase. The causes of such

different patterns probably depend on the different machines as well as sub-experimental

differences. Figure 3.6c shows that all the specimens, when exposed to water, exhibited

some tendency to swell. Specimen B2 underwent a noticeable height decrease due to soil

loss from the gap between the box halves, possibly due to the loss of strength of the

material in contact with water.

In order to understand better the volume change behaviour of the Costa della Gaveta soil

with different pore fluids, several specimens were submitted to oedometer tests. For

instance, Figure 3.7 shows that a specimen reconstituted with 1 M NaCl solution - and

exposed to water in the course of the test - exhibits a noticeable tendency to swell.

0.0

0.1

0.2

0.3

0.4

0.1 1 10 100

heig

ht v

aria

tion

(mm

)

time (days)

S9B (σ'n = 150 kPa)

Figure 3.7 Effect of the exposure to distilled water during the course of an oedometer test,

on a specimen of Costa della Gaveta soil, initially in equilibrium with 1 M NaCl solution.

3. Influence of pore fluid composition on creep behaviour

55

Soon after failure, in order to evaluate the available shear strength at the end of the stress-

controlled phase, the apparatuses were turned to the displacement-controlled mode and the

specimens were sheared further (third test phase).

Figure 3.8, Figure 3.9 and Figure 3.10 plot the shear strength available after failure for

specimens S9A, P1 and B2 respectively. The curves are compared to the applied shear

stress during the second phase and to the shear strength exhibited by the specimens during

the first test phase, while immersed in 1 M NaCl solution. It can be seen that the available

shear strength in the third phase is much lower than that of the material in the NaCl

solution and close to the shear stress applied during the second test phase.

The height variations undergone by the specimens are shown as well. It can be seen that

S9A and P1, after the creep phase, continued to swell, while the height of specimen B2

continued to decrease due to soil extrusion. On the contrary, before the creep phase, that is

while the specimens were submerged in the NaCl solution, the height variations had

become practically negligible.

0

20

40

60

80

τ(k

Pa)

τ in 1M NaClapplied shear stress

τafter failure

S9A

-0.10

-0.05

0.00

0.05

0.10

0 2 4 6 8 10

heig

ht v

aria

tion

(m

m)

horizontal displacement (mm)

Figure 3.8 Shear strength and height variation against the horizontal displacement in 1M

NaCl solution (before the creep test), applied shear stress during the creep phase with

exposure to distilled water, shear strength and height variation after creep failure for

specimen S9A.

3. Influence of pore fluid composition on creep behaviour

56

0

20

40

60

80

τ(k

Pa)

τ in 1M NaCl

applied shear stressτ after failure

P1

-0.02

-0.01

0.00

0.01

0.02

0 5 10 15 20 25 30 35 40

heig

ht v

aria

tion

(m

m)

horizontal displacement (mm)

Figure 3.9 Shear strength and height variation against the horizontal displacement in 1M

NaCl solution (before the creep test), applied shear stress during the creep phase with

exposure to distilled water, shear strength and height variation after creep failure for

specimen P1.

0

20

40

60

80

τ(k

Pa)

τ in 1M NaCl

applied shear stress

τ after failure

B2

-1.5

-1.0

-0.5

0.0

0 5 10 15 20 25 30 35 40

heig

ht v

aria

tion

(m

m)

horizontal displacement (mm)

Figure 3.10 Shear strength and height variation against the horizontal displacement in 1M

NaCl solution (before the creep test), applied shear stress during the creep phase with

exposure to distilled water, shear strength and height variation after creep failure for

specimen B2.

3. Influence of pore fluid composition on creep behaviour

57

During exposure to distilled water of specimens P1 and B2, both in the second and in the

third test phases, the electrical conductivity of the cell fluid was often measured before

water renewal because its values allow an estimation of the amount of salt diffused

outward from the specimen’s pores in the time period between consecutive fluid renewals.

The values of conductivity are plotted in Figure 3.11 against the time since the beginning

of exposure to water. Unfortunately, the conductivity was not measured during the first

days, therefore an estimation of the cumulative amount of salt diffused, and thus of the

average NaCl concentration in the pore fluid could not be made. However, it can be

noticed that the values of electrical conductivity generally decreased with time for both

specimens. Significantly higher values were recorded for P1 when the water renewal were

not performed twice a day but less frequently.

0

500

1000

1500

2000

0 10 20 30 40 50 60 70 80 90

elec

tric

al c

ondu

ctiv

ity

(µS

/cm

)

time (days)

P1

B2

Figure 3.11 Electrical conductivity - since first exposure to distilled water - of the cell fluid

of specimens P1 and B2 measured before water renewal

3. Influence of pore fluid composition on creep behaviour

58

3.3 EXPERIMENTAL RESULTS

RELATIVE TO OTHER CLAYS

The experimentation carried out on the Costa della Gaveta soil was repeated for other

materials. The following paragraph reports on the results relative to a sodium bentonite.

The testing procedure was the same as that used for the Costa della Gaveta soil.

3.3.1 Stress-controlled shear tests on bentonite

The specimens were reconstituted with 1 M NaCl solution and first sheared to the residual

state under constant rate of displacement (v = 0.005 mm/min) and under different normal

stresses in the range 75 kPa < σ’n < 300 kPa. The attained values of residual shear strength,

as well as those evaluated on the same material reconstituted with – and submerged in –

distilled water, are plotted in Figure 3.12 and reported in Table 3.3 together with the

indication of the used apparatus and the test conditions. Specimens B1, C1 and L13, and

specimens C2 and C4, were submitted to the same stress conditions in different

apparatuses in order to check data reproducibility. Specimens L8 and L9, sheared to the

residual condition under the same vertical stress (σ’n = 150 kPa) as specimen C1, were

submitted to different shear stresses during the creep phase. Specimen L11 was prepared

with a 0.6 M NaCl solution instead of a 1 M NaCl solution.

3. Influence of pore fluid composition on creep behaviour

59

0

20

40

60

80

100

0 100 200 300 400

τ r(k

Pa)

σ'n (kPa)

τr in 1M NaCl

τr in dist. water

τ applied C3

C2, C4

L9C1, B1, L11, L13

L8

bentonite

L5

Figure 3.12 Test conditions of the specimens of bentonite submitted to stress/force-

controlled shear tests.

Spec. Material

Shear

apparatus

σσσσ’n

(kPa)

ττττr in 1 M NaCl

solution (kPa)

Applied ττττ

(kPa)

ττττr in water

(kPa)

B1 bentonite Bishop 150 46 29.3 12.4

C1 bentonite Casagrande 150 46 29.3 12.4

C2 bentonite Casagrande 205 60 39.8 16.9

C3 bentonite Casagrande 287 88 55.7 23.6

C4 bentonite Casagrande 205 63 39.8 16.9

L5 bentonite Casagrande 75 23 14.7 6.2

L8 bentonite Casagrande 150 46 21.2 12.4

L9 bentonite Casagrande 150 46 37.5 12.4

L11 bentonite Bishop 150 40 (0.6 M NaCl) 29.3 12.4

L13 bentonite Casagrande 150 46 29.3 12.4

Table 3.3 Test conditions of the specimens of bentonite submitted to stress/force-controlled

shear tests.

3. Influence of pore fluid composition on creep behaviour

60

Figure 3.13a shows the results, in terms of horizontal displacements against time, relative

to the specimens which underwent all the three test phases without technical problems.

These curves, therefore, are of easier interpretation and will be discussed in more detail.

The results relative to all the performed tests are shown in Figure 3.14.

Similar to the specimens of the Costa della Gaveta soil, in the second phase (see Figure

3.6) the application of a shear stress lower than the residual shear strength attained in the

solution cause only small displacements with decreasing rate, which became negligible or

even null within a couple of weeks. Subsequently, in the third phase (time t=0 in Figure

3.13), the cell fluid was replaced by distilled water, which was renewed frequently –

usually twice a day – to remove the ions diffusing outward from the specimens’ pores and

to keep the concentration gradient between the pore fluid and the cell fluid as high as

possible.

The displacement rate (Figure 3.13b) increased significantly soon after the exposure to

water. Within a few days the displacement rate reached values of about 50 µm/day.

Subsequently, it remained roughly constant for some time, resembling secondary creep.

This phase had a longer duration for specimen C3, sheared under a normal stress higher

than that of specimens C2 and C1. Furthermore, the higher the displacement rate in this

phase, the lower the normal stress. Finally, after 15-25 days of continuous exposure to

water, the displacement rate increased more rapidly to values typical of failure.

The specimens of the Costa della Gaveta soil tested in the Casagrande apparatus

experienced sudden failure, while the displacement rate increased progressively in the

Bishop apparatus. On the contrary, all specimens of bentonite experienced a progressive

increase of the displacement rate, until failure, independently of the used apparatus.

Figure 3.13a shows that the two specimens submitted to the same stress conditions in

different apparatuses (B1 and C1) exhibited a very similar behaviour in terms of

displacements against time. This can be considered a validation of the tests carried out by

means of the Casagrande apparatus, which are inevitably less accurate than those in the

Bishop apparatus.

3. Influence of pore fluid composition on creep behaviour

61

0.0

0.5

1.0

1.5

2.0

2.5

3.0

-20 -10 0 10 20 30

hori

zont

al d

ispl

acem

ent (

mm

) 1M NaCl solution

distilled water

C3

C2

B1C1

0

100

200

300

400

500

-20 -10 0 10 20 30

disp

lace

men

t rat

e (µ

m/d

ay) C3B1

C2C1

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

-20 -10 0 10 20 30

heig

ht v

aria

tion

(mm

)

time (days)

C3

B1

C2

C1

a)

b)

c)

exposure to water

exposure to water

exposure to water

Figure 3.13 Effects of exposure to distilled water of bentonite reconstituted with 1 M NaCl

solution and subjected to stress-controlled tests: horizontal displacement, displacement

rate and height variations against time.

3. Influence of pore fluid composition on creep behaviour

62

Due to the exposure to water, all specimens exhibited swelling (Figure 3.13c), of

increasing magnitude with normal stress decreasing. Furthermore, the specimen tested in

the Bishop apparatus (B1) underwent more significant swelling than the specimen tested in

the Casagrande shear box (C1) under the same normal stress.

Besides the tests shown in Figure 3.13, additional tests were performed, during which

different technical difficulties arose. Typically, significant oxidation of the metallic

components in contact with the salt solution for long periods of time. Although the cell and

the components, where possible, were periodically cleaned, in some cases these

phenomena were found anyway responsible of additional friction between the two half-

boxes or within them, thus slowing down the displacements, impeding the correct

application of the normal loads and preventing free volume changes. The results of such

tests, however, are reported in Figure 3.14 in terms of horizontal displacements,

displacement rate and height variation against time during the force/stress-controlled

phase.

Notwithstanding the technical difficulties, all specimens reached failure, although with

displacement patterns that do not seem easily correlated to the stress state. For example,

specimens C2 and C4, which were tested under the same conditions, did not exhibit the

same displacement pattern. Specimens L8, sheared by a lower force than that applied on

C1, reached failure after a longer time than that needed for C1. This is consistent with the

fact that more time was needed to produce a larger decrease in the available strength.

However, specimen L9, sheared by a force which was higher than that on C1, did not reach

failure in a time shorter than that needed for this latter. The test on specimen L5 was not

considered for further interpretation because at the normal applied stress σ’n = 75 kPa

significant swelling took place before creep failure (Figure 3.14c), increasing the gap

between the box halves noticeably and thus possibly modifying the stress state of the

material along the slip surface.

3. Influence of pore fluid composition on creep behaviour

63

0.0

0.5

1.0

1.5

2.0

2.5

3.0

-20 -10 0 10 20 30 40 50 60

hori

zont

al d

ispl

acem

ent (

mm

) 1M NaCl solution

distilled water

C3

C2

B1C1

C4

L8L9

L13

L11

L5

pore pressure transducer removed, load piston changed

0

100

200

300

400

500

-20 -10 0 10 20 30 40 50 60

disp

lace

men

t rat

e (µ

m/d

ay)

time (days)

C3B1C2

C1 C4

L8

L9

L13

L11

L5

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

-20 -10 0 10 20 30 40 50 60

heig

ht v

aria

tion

(m

m)

time (days)

C2B1

L5

C1

L13

C3

L8

C4

L11L9

a)

b)

c)

Figure 3.14 Horizontal displacements (a), displacement rate (b) and height variation (c)

against time for specimens of bentonite in the course of force/stress-controlled tests (all

tests).

3. Influence of pore fluid composition on creep behaviour

64

The displacement pattern of specimen L13 is significantly different from that of the other

specimen. In this case, a miniature pore pressure transducer which was installed inside the

specimen, close to the shear surface, may have slowed down the displacements noticeably.

In fact, after the removal of the transducer, the displacement rate increased noticeably, the

specimen exhibited swelling and it eventually reached failure within a few days.

After failure, the shear apparatuses were turned back to the displacement-controlled mode

(third test phase) and the specimens were sheared further in order to evaluate the available

shear strength. Figure 3.15, Figure 3.16, Figure 3.17 and Figure 3.18 show the results in

terms of shear strength and height variations undergone by specimens B1, C1, C2 and C3

respectively, against the cumulative horizontal displacement. The shear strength measured

during the first and the third test phases was plotted. For the second phase, the applied

shear stress is indicated.

Similarly to the specimens of the Costa della Gaveta soil, also the specimens of bentonite

exhibited, after failure, a shear strength not higher than the applied shear stress during the

creep phase, and much smaller than that exhibited while they were immersed in the 1 M

NaCl solution.

To observe the effect on shear strength of exposure to distilled water directly, the

specimens were sheared further, renewing the cell water frequently. All specimens

exhibited a continuous decrease in strength, until a minimum value, very close to that

obtained for the water-saturated specimens tested in a bath of distilled water. Furthermore,

all specimens continued to swell and, often, significant soil loss was seen from the gap

between the box halves. The test on specimen C2 (Figure 3.17) was interrupted before

reaching the minimum value of shear strength because the load piston was significantly

tilted, thus preventing the correct application of the normal load and the height variation.

3. Influence of pore fluid composition on creep behaviour

65

0

10

20

30

40

50

60

τ(k

Pa)

τr in dist. water

B1 (σ'n = 150 kPa)

τr in 1M NaClapplied shear stress

phase 1 phase 2 phase 3

exposure to distilled water

-0.5

0.0

0.5

1.0

1.5

2.0

0 20 40 60 80 100 120 140 160 180

heig

ht v

aria

tion

(mm

)

horizontal displacement (mm)

soil extrusion

Figure 3.15 Shear strength and height variation against shear displacement in the three

different test phases for specimen B1.

0

10

20

30

40

50

60

τ(k

Pa)

τr in dist. water

C1 (σ'n = 150 kPa)

τr in 1M NaClapplied shear stress

phase 1 2 phase 3

shearing under different normal stresses

exposure to distilled water

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

0 20 40 60 80 100 120 140 160 180

heig

ht v

aria

tion

(mm

)

horizontal displacement (mm)

shearing under different normal stresses

Figure 3.16 Shear strength and height variation against shear displacement in the three

different test phases for specimen C1.

3. Influence of pore fluid composition on creep behaviour

66

0

20

40

60

80

100

τ(k

Pa)

τr in dist. water

τr in 1M NaCl

C2 (σ'n = 205 kPa)

phase 1 2 phase 3

applied shear stress

exposure to distilled water

-2.0

-1.5

-1.0

-0.5

0.0

0 20 40 60 80 100 120 140 160 180

heig

ht v

aria

tion

(mm

)

horizontal displacement (mm)

soil extrusion

Figure 3.17 Shear strength and height variation against shear displacement in the three

different test phases for specimen C2.

0

20

40

60

80

100

τ(k

Pa)

τr in dist. water

τr in 1M NaCl

C3 (σ'n = 287 kPa)

phase 1 2 phase 3

applied shear stress

exposure to distilled water

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0 20 40 60 80 100 120 140 160 180

heig

ht v

aria

tion

(mm

)

horizontal displacement (mm)

soil extrusion

Figure 3.18 Shear strength and height variation against shear displacement in the three

different test phases for specimen C3.

3. Influence of pore fluid composition on creep behaviour

67

At the base of the observed strength decrease, two time dependent processes can be

hypothesised: ion diffusion from the pore fluid (initially a concentrated salt solution) to the

cell fluid (distilled water), and osmotic water flow from the cell fluid toward the pores. The

processes can take place simultaneously, and both contribute to pore fluid concentration

decrease. Given the time dependence of the processes, the shear test results, already shown

in terms of horizontal displacements, should conveniently be analysed against time.

Figure 3.19a, Figure 3.20a, Figure 3.21a and Figure 3.22a, relative to specimens B1, C1,

C2 and C3 respectively, show the shear strength measured in the first and third test phases

(left axis) and the horizontal displacements measured in the second phase (right axis). The

figures also show the height variations undergone by the specimens during the three test

phases. For specimens B1 and C2 the cumulative amount of salt diffused outward from the

pores is also plotted (Figure 3.19c and Figure 3.21c respectively). This amount was

estimated by measuring the electrical conductivity in the cell fluid before each water

renewal.

After exposure to water, the shear strength decreased from point A to B to C. The way by

which it decreased from B to C has been directly evaluated and is shown by the figures. To

have an idea of how the strength decreased from A to B during exposure to water, it is

useful to compare the displacement and strength history of one of the specimens (e.g. C3,

Figure 3.22) to that of a similar material, Ponza bentonite (P in Figure 3.23), which, in

analogous test conditions, was sheared at constant displacement rate soon after exposure to

distilled water. Figure 3.23 shows that in A the two materials exhibited very similar values

of strength. From B to C, the observed strength patterns of the two specimens are very

similar too. It is reasonable to hypothesise that also from A to B the shear strength of C3

decreased similarly to that of P.

In order to evaluate whether and how ion and water diffusion occurred in the different test

phases, both the electrical conductivity of the cell solution and the specimens’ height were

monitored. In a saturated material, considering water incompressible, water diffusion

toward the specimens’ pores can actually happen only with a volume increase of the

specimen, therefore height variations are useful for its evaluation. However, a tendency of

water to diffuse can exist anyway also in absence of volume changes (e.g. in case they are

not allowed), producing pore pressure increase (i.e. osmotic pressure). This, in turn, would

3. Influence of pore fluid composition on creep behaviour

68

result in a decrease in the effective stresses and, consequently, in a decrease in the

available strength.

To convert electrical conductivity into concentrations, the conductivity of NaCl solution at

various concentrations in the range 0.001 M ≤ c ≤ 0.5 M was evaluated by means of a 4-

electrode conductivity cell, finding the following empirical relation: c = 4⋅10-6⋅κ1.1, in

which the salt concentration c is expressed in molarity M and the unit of electrical

conductivity, κ, is µS/cm. The values of κ are referred to 20°C. The relation provides

values consistent with those of the technical literature (e.g. Christian, 1994; Dominijanni et

al., 2013; Haynes, 2014).

By using the empirical relation, and under the hypothesis that NaCl is the only diffusing

salt, the cell solution concentration and the amount of NaCl diffused from the pores of the

specimen into the cell solution were evaluated (Figure 3.19c and Figure 3.21c).

The reliability of the interpretation has been verified on specimen O11, exposed to distilled

water in the course of an oedometer test. The comparison between the concentrations

derived from the empirical relation and those determined by a Na+ selective electrode is

reported in Figure 3.24. The figure compares the electrical conductivity of the cell fluid,

the amount of salt diffused from the pores and the height variation undergone by

specimens B1 and C2 against time. The maximum amount of salt expected to diffuse in the

cell, which is equal to the initial salt content of the pore solution is reported as well. The

figure shows that the process of ion diffusion began soon after exposure to water and that,

at the time of failure, the diffused NaCl was beyond 50% of the total.

Figure 3.24c reports the height variations undergone by the specimens after exposure to

water. The tendency to swell of the specimens some days after exposure to water can be

observed, although swelling became significant only when most of salt had diffused

outward from the pores. This is a process to investigate further.

3. Influence of pore fluid composition on creep behaviour

69

0

1

2

3

4

0

10

20

30

40

50

60

τ(k

Pa)

shear strength τr in dist. water

B1 (σσσσ'n = 150 kPa)

shear strength τr in 1M NaCl

shear stress τ applied in phase 2

shear displacement under constant shear stress

exposure to distilled water

A

B

C

shea

r dis

plac

emen

t (m

m)

curve obtained by the model with D* = 6 ⋅ 10-10 m2/s

ph.1 phase 2 phase 3

-0.5

0.0

0.5

1.0

1.5

2.0

heig

ht v

aria

tion

(mm

)

soil extrusion

0

50

100

150

200

0 10 20 30 40 50 60 70 80 90 100

NaC

l rem

oved

(mm

ol)

time (days)

a)

b)

c)

Figure 3.19 a) shear strength (left axis) and horizontal shear displacements (right axis); b)

height variation; and c) cumulative NaCl removed against time in the three test phases for

specimen B1.

3. Influence of pore fluid composition on creep behaviour

70

0

1

2

3

4

0

10

20

30

40

50

60

τ(k

Pa)

τ applied in phase 2

τr in water

C1 (σσσσ'n = 150 kPa)

τr in 1M NaCl

shear displacement under constant shear stress

exposure to distilled water

B

ph.1 phase 2 phase 3

A

C

shea

r dis

plac

emen

t (m

m)

D* = 6·10-10 m2/s

-2.0-1.5-1.0-0.50.00.51.01.5

0 10 20 30 40 50 60 70 80 90 100

heig

ht v

aria

tion

(mm

)

time (days)

a)

b)

Figure 3.20 a) shear strength (left axis) and horizontal shear displacements (right

axis);and b) height variation against time in the three test phases for specimen C1.

3. Influence of pore fluid composition on creep behaviour

71

0

1

2

0

20

40

60

80τ

(kP

a)

τ applied in phase 2

τr in water

C2 (σσσσ'n = 205 kPa)

τr in 1M NaCl

exposure to distilled water

shear displacement under constant shear stress

ph.1 phase 2 phase 3

A

B

shea

r dis

plac

emen

t (m

m)

D* = 6·10-10 m2/s

-2.0

-1.5

-1.0

-0.5

0.0

heig

ht v

aria

tion

(mm

)

soil extrusion

0

0.2

0.4

0.6

0.8

1

0

10

20

30

40

50

0 10 20 30 40 50 60 70 80 90 100 NaC

l con

cent

ratio

n (m

ol/l

)

NaC

l rem

oved

(mm

ol)

time (days)

average concentration (deduced from NaCl removed)

concentration on the shear surface (model)

average concentration (experimental)

a)

b)

c)

Figure 3.21 a) shear strength (left axis) and horizontal shear displacements (right axis); b)

height variation; and c) cumulative NaCl removed (left axis) and concentration (right axis)

against time in the three test phases for specimen C2.

3. Influence of pore fluid composition on creep behaviour

72

0

1

2

3

0

20

40

60

80

100τ

(kPa

)

τ applied in phase 2

τr in water

C3 (σσσσ'n = 287 kPa)

τr in 1M NaClexposure to distilled water

shear displacement under constant shear stress

ph.1 phase 2 phase 3

A

B

C

shea

r dis

plac

emen

t (m

m)

D* = 6·10-10 m2/s

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0 10 20 30 40 50 60 70 80 90 100

heig

ht v

aria

tion

(mm

)

time (days)

soil extrusion

a)

b)

Figure 3.22 a) shear strength (left axis) and horizontal shear displacements (right axis);

and b) height variation against time in the three test phases for specimen C3.

0

1

2

3

0.0

0.1

0.2

0.3

0.4

0 10 20 30 40 50 60 70

τ/ σ

' n

time (days)

Ponza bentonite, Di Maio 1996a (shear strength) C3 (shear strength) C3 (displacements)

τr in 1M NaCl

τr in water

shear stressapplied in phase 2

A

shea

r dis

plac

emen

t (m

m)

B

C

manual cut

D* = 6·10-10 m2/s

exposure to distilled water

Figure 3.23 Horizontal shear displacements (right axis) and shear strength (left axis) of

specimen C3 compared to the shear strength of the Ponza bentonite (Di Maio, 1996a)

during exposure to distilled water (Di Maio and Scaringi, 2015).

3. Influence of pore fluid composition on creep behaviour

73

0

1000

2000

3000B1

C2

elec

tric

al c

ondu

ctiv

ity, κ

(µS/

cm)

1

10

100

1000

NaC

l dif

fuse

d (m

mol

)

B1 - estimd from κ

C2 - emated from κ

O11 est

O11 (Na+)

model B1

B1C2O11O11 (measured Na+)calculated with D* = 6⋅10-10 m2/s

initial NaCl in B1

initial NaCl in C2

initial NaCl in O11

= failure

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0 15 30 45 60 75

B1

C3

C1

C2 = failure

∆h

(mm

)

a)

b)

c)

time (days)

Figure 3.24 Electrical conductivity, κ, of the cell water of some specimens before each

water renewal (a), cumulative amount of NaCl diffused from the pores in the cell solution

(b), swelling of all the specimens after exposure to distilled water (c) (Di Maio and

Scaringi, 2015).

3. Influence of pore fluid composition on creep behaviour

74

Figure 3.25 reports the estimated average NaCl concentration in the pore fluid against time

for several specimens of bentonite. It can be noticed, with the exception of specimen L13,

that the curves do not show sudden slope changes. As a matter of fact, the specimens were

exposed to water both in the force/stress-controlled phase and in the subsequent

displacement-controlled phase. The regularity of the curves can be considered a proof of

the independence of ion diffusion of the shearing mode. On the contrary, the non regular

shape of the curve can be a symptom of an incorrect course of the test. In the case of

specimen L13, it can be seen that ion diffusion was, initially, much slower than other

specimens in the same stress conditions (e.g. L8). This probably happened because of the

used load piston (built specially to allow the insertion of the pore pressure transducer) and

the absence of the upper porous plate, which did not allow free drainage and diffusion. In

fact, after its removal and substitution with standard piston and plate, ion diffusion

accelerated. If to look to Figure 3.25, it is also possible to see that specimen L11, initially

in equilibrium with 0.6 M NaCl solution, experienced a slower concentration decrease than

expected on the basis of other tests and for this reason, probably, reached failure in a

longer time.

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0 10 20 30 40 50 60 70

aver

age

NaC

l con

cent

ratio

n in

the

pore

flui

d (m

ol/l)

time since the exposure to water (days)

B1 - estimated from κC2 - estimated from κL5 - estimated from κL8 - estimated from κL9 - estimated from κL11 - estimated from κL13 - measuredcreep failure

Figure 3.25 Estimated average NaCl concentration in the pore fluid for several specimens

of bentonite, during the creep phase and the subsequent displacement-controlled phase.

3. Influence of pore fluid composition on creep behaviour

75

Figure 3.26 shows the estimated average NaCl concentration in the pore fluid of the

specimens shown in Figure 3.25 against the displacement rate during the creep test. It can

be seen that most of the specimens submitted to an average shear stress intermediate

between the strength in water and that in 1M NaCl solution (B1, C2, L11) exhibited

noticeable acceleration, and thus “failure”, when the estimated concentration was around

0.4 mol/l. Actually, for such concentration, a residual friction angle of about 11° is

estimated for bentonite (Figure 2.22). As a matter of fact, the points representing the shear

stress applied to these specimens, in a σ’-τ plot, lie on a line through the origin having ϕ’ =

11°. The shear stress applied on specimen L8 was lower than that applied on B1. In fact,

the specimen reached failure when the average concentration in the pore fluid was lower.

On the contrary, probably because of technical problems, the specimen L9, submitted to a

shear stress higher than that on B1, did not experience failure while the NaCl concentration

in the pore fluid was still higher. Finally, possibly because of the noticeable swelling and

the increasing gap between the box halves, as discussed, specimen L5 reached failure only

when the NaCl concentration in the pores was significantly lower than expected.

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0.1 1 10 100 1000 10000

aver

age

conc

entr

atio

n in

the

por

e fl

uid

(mol

/l)

displacement rate (µm/day)

L5

L8

L9

L11

B1

C2

Figure 3.26 Estimated average NaCl concentration in the pore fluid against displacement

rate during force/stress-controlled tests on several specimens of bentonite.

3. Influence of pore fluid composition on creep behaviour

76

Regarding the possibility of pore pressure changes, an attempt was also made to measure

pore water pressures during exposure to distilled water of specimen L13 by means of a

miniature pore pressure transducer installed in a Casagrande device very close to the shear

surface. Figure 3.27 shows that during the monitoring period, while salt was diffusing into

the cell fluid, no significant pore pressures variations occurred, thus suggesting that the

process is drained.

0

6

12

18

24

30

-12

-6

0

6

12

18

24

30

0 5 10 15 20 25 30

cum

ulat

ive r

emov

ed sa

lt (m

mol

)

pore

pre

ssur

es (k

Pa)

time (days)

pore pressures cumulative removed salt

L13 - Bentonite, σ'n =150 kPa, Casagrande apparatus

Figure 3.27 Pore pressures on the shear surface and cumulative removed salt against time

for a specimen of bentonite, pre-sheared to the residual condition in 1 M NaCl solution,

exposed to distilled water and then submitted to a force-controlled test in the Casagrande

apparatus.

The transducer was removed because it was disturbing the course of the test, probably

because it was placed too close to the shear surface and because the load piston was not

free to slide in the vertical direction and was not allowing ion diffusion and drainage from

the top of the specimen. Figure 3.14 shows, in fact, that after the transducer was removed

and the piston changed the displacement rate increased noticeably, reaching failure in a

few days.

3. Influence of pore fluid composition on creep behaviour

77

3.3.2 Water content and pore ion concentration at the end of the tests

At the end of the tests, the specimens were removed from the shear box and some of them

were oven-dried to determine the final pore fluid concentration. The measurement was

carried out following the test procedure already described in section 4.2. Table 3.4 reports

the estimated average concentrations in the specimens at the end of the tests. The results

are in good agreement with the estimation made from the measurements of the salt diffused

in the cell fluid before each cell water renewal.

Specimen Concentration estimated from the ions diffused in the cell fluid (M)

Concentration estimated from the ions left in the pores (M)

B1 0.00 0.11

C2 0.34 0.30

L5 0.15 0.11

L8 0.28 0.31

L9 0.41 0.53

L11 0.42 0.41

L13 0.24 0.25

Table 3.4 Estimated NaCl concentrations in several specimens at the end of the tests.

In some cases, the specimens were cut in slices and submitted to chemical analyses

separately in order to evaluate possible variations along the height. Figure 3.28 shows the

water content and the Na+ concentration evaluated on horizontal slices of specimens B1,

C2, L5, L8, L9 and L11. The position of the shear surface and the initial water content of

the specimens before the exposure to water are also shown.

All specimens exhibited a water content higher than the initial one, as a consequence of

swelling. In specimen B1, tested in the Bishop apparatus, the water content close to the

shear surface is significantly higher than that above and below it. The result was obtained

along two opposite verticals of the specimen. On the contrary, the Na+ concentration does

not seem to vary significantly. In specimen L11, instead, the Na+ concentration close to the

shear surface seems lower than elsewhere, possibly because of the direct contact with the

cell water. In this specimen, the material submitted to the analyses was taken close to the

outer ring, in the core and close to the inner ring.

3. Influence of pore fluid composition on creep behaviour

78

0

2

4

6

8

10

12

14

16

0.3 0.6 0.9 1.2 1.5

z (m

m)

w0.00 0.25 0.50

Na+ (mol/l)

vertical 1

vertical 2

specimen B1, σ'n = 150 kPa (Bishop)

shear surface

0

3

6

9

12

15

18

21

24

27

0.3 0.6 0.9 1.2 1.5

z (m

m)

w0.00 0.25 0.50

Na+ (mol/l)

specimen C2, σ'n = 205 kPa (Casagrande)

shear surface

0

4

8

12

16

20

24

0.3 0.6 0.9 1.2 1.5

z (m

m)

w0.00 0.25 0.50

Na+ (mol/l)

specimen L5, σ'n = 75 kPa (Casagrande)

shear surface

0

5

10

15

20

25

0.3 0.6 0.9 1.2 1.5

z (m

m)

w

vertical 1 (outer)vertical 2 (inner)vertical 3 (outer)

0.00 0.25 0.50Na+ (mol/l)

specimen L8, σ'n = 150 kPa (Casagrande)

shear surface

0

5

10

15

20

25

0.3 0.6 0.9 1.2 1.5

z (m

m)

w

vertical 1 (outer)vertical 2 (inner)vertical 3 (outer)

0.20 0.45 0.70Na+ (mol/l)

specimen L9, σ'n = 150 kPa (Casagrande)

shear surface

0

5

10

15

20

0.3 0.6 0.9 1.2 1.5

z (m

m)

w

vertical 1 (external)vertical 2 (core)vertical 3 (internal)

0.20 0.45 0.70Na+ (mol/l)

specimen L11, σ'n = 150 kPa (Bishop)

shear surface

initi

al w

initi

al w

initi

al w

initi

al w

initi

al w

initi

al w

Figure 3.28 Water content and sodium concentration evaluated in several specimens of

bentonite after the end of the shear tests.

3. Influence of pore fluid composition on creep behaviour

79

The specimens tested in the Casagrande apparatus, with the exception of L5, compressed

under a low normal stress (75 kPa), showed a lower water content increase from the initial

one than the specimen B1, tested in the Bishop apparatus. The water content does not seem

to vary significantly in the vertical direction; the sodium concentration in the pore fluid

seem to increase towards the top of the specimen, where it is in contact with the upper

porous stone. It is possible that the top drainage is not as effective as the bottom one,

possibly also because the top plate can be sometimes not submerged by water during the

test. Furthermore, in specimens L8 and L9, in which the concentrations were evaluated

along few verticals, it can be seen that they are higher in the core of the specimens than

towards their borders.

3. Influence of pore fluid composition on creep behaviour

80

3.4 MODELIZATION OF ION DIFFUSION

AND STRENGTH REDUCTION

The results of the stress-controlled tests, as a whole, suggest that the observed “creep

behaviour” was due to a decrease in pore ion concentration.

To a first approximated evaluation of the transient process of ion concentration decrease on

the shear surface and to understand the spatial variability within the specimen, a simplified

model was formulated using the commercial software CTRAN/W (Krahn, 2008).

The code solves the diffusion problem by the Fick law along with the continuity equations,

under the hypothesis of absence of other coupled flows. In particular, the code does not

consider volume changes due to variations in pore solution concentration. Thus, in the

present case, in the absence of hydraulic gradients, the water velocity is zero and the soil

hydraulic conductivity does not influence the process. This simplification can be accepted

because, for the tested clay, for which k < 10-10 m/s, a diffusive dominated process is

hypothesised (Shackelford, 2014).

The code was used in axisymmetric configuration, thus simulating the ring shear geometry

rigorously and with an acceptable approximation the direct shear box which has a square

horizontal section.

The scheme (Figure 3.29) considers the specimens connected to the cell water laterally, in

correspondence with the interface between the two halves of the shear box, and at the

contact with the porous stone and the porous plate. These latter are simulated as porous

media with porosities: n = 0.32 and n = 0.15 respectively.

The initial conditions are set by imposing c0 = 1 M in any point of the specimen, stones

and plates, as in the experimental conditions, and c0r = 0 in any point of the water reservoir.

The difference in concentration triggers the outward ion diffusion. Thus ion concentration

in the reservoir increases with time, and it is turned to zero to simulate the cell water

renewals of the experimentation.

3. Influence of pore fluid composition on creep behaviour

81

The parameters of the transport differential equations used by the code (Freeze and Cherry,

1979) are the hydrodynamic dispersion coefficient D and the average linear velocity of the

pore fluid v. In our simulation, the latter parameter is null and thus the hydrodynamic

dispersion coefficient D is equal to the diffusion coefficient D*. The values of D* in the

pore solution, in the cell water and in the porous stone/plate water must be assigned

(Figure 3.29).

The effective diffusion coefficient D* = 6·10-10 m2/s was assumed for the pore solution.

Such value is very close to those reported in the literature for similar cases (Gajo and

Loret, 2004; Dominijanni et al., 2013; Shackelford, 2014). With such value, the calculated

amount of salt diffused in the cell solution compares satisfactory with the experimental

results (Figure 3.24b for specimens B1 and C2), particularly during the stress-controlled

phase. Figure 3.30 shows the influence of the choice of D* on the cumulated amount of

salt diffused outwards from the pores for the case of specimen B1.

In the cell water and in the pores of stones/plates, D* = 1.5·10-9 m2/s was assumed. The

value refers to dissolved NaCl at a temperature of about 20°C, in free water at infinite

dilution (Li and Gregory, 1974). Such hypothesis has been considered acceptable because

the concentration never exceeded 0.06 M in the cell water, and was equal to 1 M in the

pores of stones/plates only at the beginning of the test. For 1 M, a maximum variation of

D* of 8% is expected (Robinson and Stokes, 2002).

The inner and the outer radii of the ring shear specimen (Figure 3.29b) correspond to the

real one. The radius used to simulate the direct shear specimen, r = 3.39 cm (Figure 3.29a),

is such that the resulting horizontal section is equal to 36 cm2, as the real one. However,

this results in a smaller perimeter in direct contact with the cell water: 21.3 cm instead of

24 cm. The choice of maintaining the actual section instead of the actual perimeter was

made in order to simulate the total contaminant mass correctly.

3. Influence of pore fluid composition on creep behaviour

82

r2 = 7.62 cmr1 = 5.08 cm,

specimenc0 = 1 M

D* = 1.5 ·10-9 m2/s

b) ring shear model

r = 3.39 cm

water reservoir: c

0r = 0

D* =

1.5 ·10-9m

2/s

specimenc0 = 1 M

D* = 6 ·10-10 m2/s

porous plate c0 = 1 MD* = 1.5 ·10-9 m2/s

a) direct shear model

porous platec0 = 1 M

D* = 1.5 ·10-9 m2/s

porous stonec0 = 1 M

D* = 1.5 ·10-9 m2/s

water reservoir: c

0r = 0, D

* = 1.5 ·10

-9m

2/swat

er re

serv

oir:

c0r

= 0

, D*

= 1

.5 ·

10-9

m2 /

s

c0 = 1 MD* = 6 ·10-10 m2/s

Figure 3.29 Model of ion diffusion in the direct shear and in the Bishop ring shear

apparatuses with the indication of initial concentrations and diffusion coefficients D*.

3. Influence of pore fluid composition on creep behaviour

83

0

50

100

150

200

0 10 20 30 40 50 60 70

NaC

l dif

fuse

d (m

mol

)

time (days)

initial NaCl in B1

model:D = 8 ⋅ 10-10 m2/sD = 6 ⋅ 10-10 m2/sD = 4 ⋅ 10-10 m2/s

Figure 3.30 Comparison between the cumulative amount of NaCl diffused from the pores

of specimen B1 in the cell solution, evaluated experimentally and by the numerical model

using different diffusion coefficients.

The height of the specimen was set equal to its real one at the time of exposure to distilled

water. Due to the software limitations, the height had to be assumed constant in the

simulation. For the porous stones and plates the actual height was used as well. Similarly,

the actual box thickness was used. The spacing between the two halves of the shear box

was estimated, for the ring shear box, from the vertical movements of the upper box, which

can be directly measured. The cell water domain was shaped so that its volume would be

equal to the actual one (i.e. 500 cm3 in the ring shear and 160 cm3 in the direct shear box).

The model provides the concentration distribution of the ions in each point of the domain

at any time. Figure 3.31 reports the concentration distribution in the ring shear specimen

(B1) and in the porous stones and plates at given times. It can be seen that the shear surface

is generally characterised by lower concentrations than in the rest of the specimen due to

the direct, lateral contact with the water reservoir. In fact, outward diffusion from the

specimen’s pores occurs through the direct contact with the cell first and then also through

the porous plates and stones, which are saturated with the 1M NaCl solution at the

beginning of the exposure. Only after some days, the concentration decrease in the porous

stones and plates triggers significant diffusion also between them and the specimen.

3. Influence of pore fluid composition on creep behaviour

84

1 day 2 days

5 days 10 days

15 days 20 days

Figure 3.31 Concentration distribution within the specimen, the porous plates and stones

in the Bishop ring shear at different times. Red colour corresponds to 1 M, blue colour to

distilled water. The simulation refers to specimen B1 assuming D* = 6⋅10-10

m2/s.

3. Influence of pore fluid composition on creep behaviour

85

The distribution of ion concentration on the shear surface corresponding to D* = 6·10-10

m2/s is shown in Figure 3.32a for the case of specimen B1. The figure shows the negligible

difference with the isochrones of concentration calculated at 50 days taking into account

also the inward water flow during swelling.

Using the relation between the residual friction angle and pore solution concentration of

Figure 2.22, interpolated as in Figure 3.33 for bentonite, obtained in the absence of

chemical gradients, it is possible to hypothesise a strength distribution on the shear surface

(Figure 3.32b) and hence an average shear strength as a function of time, under the

hypothesis of a drained process. Figure 3.34 shows a good agreement between the

calculated average shear strength and the shear strength measured in the third shearing

phase for specimen B1. For comparison, the curve of the average NaCl concentration on

the shear surface is also reported. Figure 3.35 shows the average concentration profiles

along the height of specimens B1 and C2 calculated by the model at the end of the tests.

The model compares satisfactory with the experimental data.

The results of the modelization are also reported in Figure 3.19-Figure 3.23 which show

that the curves obtained for the different specimens interpret satisfactory the experimental

data during the shear test from B to C for the used commercial bentonite and from A to C

for the Ponza bentonite. It seems reasonable to assume that they also interpret the strength

decrease during the transient phase from A to B under constant shear stresses. This phase

thus appears as a fundamentally drained process in which the relation between shear

strength and average pore ion concentration on the slip surface is similar to that reported in

Figure 3.33, evaluated in the absence of chemical gradients for specimens reconstituted

with and immersed in the same solution.

3. Influence of pore fluid composition on creep behaviour

86

0

0.2

0.4

0.6

0.8

1

conc

entr

atio

n (M

)

t = 0

10

30

5 days

20

5040

t = tfailure

average concentration at t = tfailure

t = 50 days considering

swelling

0

10

20

30

40

50

50 60 70 80

τ r(k

Pa)

r (mm)

5 days

10

20

304050

applied shearstress

τr in distilled water

t = 0

t = 50 days considering swelling

t = tfailure

a)

b)

Figure 3.32 Calculated ion concentration isochrones (a) and available shear strength (b)

on the shear surface along the radius of the specimen B1 during the diffusion process,

assuming D* = 6·10-10

m2/s (Di Maio and Scaringi, 2015.

3. Influence of pore fluid composition on creep behaviour

87

0

5

10

15

20

0 1 2 3 4

ϕ' r

(°)

Molarity, M

Casagrande

Bishop

Bromhead

σ'n = 150 kPa

ϕ'r = ϕ'r(M=1) ⋅ M0.3

Figure 3.33 Residual friction angle of bentonite against molarity of the pore NaCl solution

and interpolating curve between 0.01 M and 1 M (Di Maio and Scaringi, 2015).

0

10

20

30

40

50

0.0

0.2

0.4

0.6

0.8

1.0

0 20 40 60

NaC

l con

cent

rati

on (M

)

τ (

kPa)

time (days)

NaCl cmeasured

(model)

B1 (σ'n = 150 kPa)

NaCl concentrationτ (measured)τ (model)

Figure 3.34 Calculated and measured shear strength during exposure to water of specimen

B1, and average NaCl concentration on the shear surface (Di Maio and Scaringi, 2015).

3. Influence of pore fluid composition on creep behaviour

88

0

2

4

6

8

10

12

14

16

0.00 0.25 0.50

heig

ht (m

m)

Na+ (mol/l)

vertical 1

vertical 2

model

0

3

6

9

12

15

18

21

24

27

0.00 0.25 0.50he

ight

(mm

)

Na+ (mol/l)

vertical 1

model

specimen B1σ'n = 150 kPa

(Bishop)

shear surface

shear surface

specimen C2σ'n = 205 kPa (Casagrande)

Figure 3.35 Na+ concentration profiles along the height of two specimens of bentonite at

the end of the test: comparison between measured values and result of modelization (D* =

6·10-10

m2/s).

3. Influence of pore fluid composition on creep behaviour

89

3.5 DISCUSSION

Different studies on soil creep consider that a threshold stress equal to the residual shear

strength exists for shear creep (among others: Yen, 1969; Suhaydu and Prior, 1978;

Iverson, 1985). For intact materials, some Authors (e.g. Ter-Stepanian, 1963; 1992)

assume that the creep threshold can be even higher than the residual strength. On the other

hand, all agree on the absence of tertiary creep for applied stresses lower than the residual

strength. Recently, Bhat et al. (2011) and Di Maio et al. (2013) reported experimental

results showing that, for τ < τr, only primary creep with decreasing displacement rate

occurs.

However, the residual strength can vary even under constant effective normal stress. In this

section, in particular, the case in which the variation is caused by exposure to distilled

water of a specimen reconstituted with a concentrated salt solution was shown. If the

applied stress is lower than the initial strength in the solution and higher than that in

distilled water, the difference τ-τr can decrease, leading the material to failure.

The process of pore solution concentration decrease during the creep tests described in

section 3 occurs under constant total stresses and hydraulic boundary conditions, that is

under presumed constant effective stresses. If it is so, the observed effects of such process

can meet the description of a creep phenomenon. However, different Authors discussed the

possibility that pore water pressures may change during the process of exposure to a fluid

different from the pore fluid, as an effect of osmotic processes at the scale of the specimen

(Mitchell et al., 1973; Barbour and Fredlund, 1989, Gajo and Loret, 2004; Musso et al.,

2013), as well as an effect of the diffusion of high concentration ionic front (Santamarina

and Fam, 1995).

In the case of the exposure to a concentrated salt solution of a clay initially saturated with

distilled water, which is the most studied case in the literature, several Authors (Kemper

and Rollins, 1966; Farrar and Coleman, 1967; Barbour and Fredlund, 1989; Mitchell,

1991; Malusis et al., 2003) showed that the outward osmotic water flow may be significant

only in very dense clays, characterised by low porosity, and in dilute electrolytes. On the

3. Influence of pore fluid composition on creep behaviour

90

contrary, with increasing void ratio and electrolyte concentration, ion diffusion prevails.

Dominijanni et al. (2013), while testing a sodium montmorillonite with characteristics

similar to those of the bentonite whose results are reported in this work, showed that the

osmotic efficiency ω, a measure of the extent to which the theoretical osmotic pressure

develops (Mitchell and Soga, 2005), is practically null for concentration higher than 0.1

mol/l.

The case of a clay saturated with a salt solution and exposed to distilled water was

analysed by Gajo and Loret (2003) who modelled the behaviour of the Ponza bentonite in

oedometer and compared the theoretical results in terms of volume change against time

with those obtained by Di Maio (1996a) in the laboratory. According to their model,

significant pore pressure variations associated to osmotic processes should arise. In the

case shown in this section, the direct contact of the slip surface with the cell solution

probably allowed for a quick dissipation of possible pore pressure gradients. As a matter of

fact, the trends of shear strength or displacement do not seem to reveal pore pressure

variations on the shear surface. In particular, the shear strength decrease during exposure to

distilled water could be interpreted by the relation between ion concentration and residual

shear strength found for specimens exposed to the same solution as the pore solution and

sheared in drained conditions. A first attempt to measure pore pressures by means of a

miniature transducer installed in the specimen close to the slip surface did not reveal any

pore pressure increments in the course of a stress controlled shear test on a bentonite

specimen (Figure 3.27).

At the field scale, the decrease in pore solution concentration in the material along the slip

surface of a landslide in clay soils, such as the Costa della Gaveta landslide, could result in

a decrease in the available strength, under conditions which are similar to those reproduced

in the laboratory, i.e. in the residual state and under constant normal effective stress and

applied shear stress. A process of pore solution concentration decrease could thus be a

“hidden” cause of the landslide viscous displacements, which is neglected if a merely

mechanical approach is used for the study of the landslide behaviour.

91

4 PORE FLUID COMPOSITION IN CLAYS

OF MARINE ORIGIN

This section consists of two parts. In the first one, some data on pore fluid composition

available in the technical literature are reviewed. They refer to different clay formations in

different sites of the world. The possible causes of spatial and temporal variability of the

pore fluid composition, when analysed, are reported. In the second part, the results relative

to the Costa della Gaveta soil are shown. Both chemical and electrical analyses have been

carried out for the evaluation of pore fluid composition. In addition, some preliminary

electrical analyses were performed to evaluate the electrical resistivity of soil specimens.

4. Pore fluid composition in clays of marine origin

92

4.1 DATA FROM LITERATURE

In order to get a first idea of pore fluid composition of marine origin clays, it is interesting

to analyse both data relative to clay formations which are submerged under the sea and

data relative to emerged clay formations.

Different Authors reported that the pore fluid composition of clay sediments under the

seafloor can differ from that of the overlying seawater due to several processes. The

concentration profiles determined by Comas et al. (1996) in Quaternary clay and silty clay

sediments under the seafloor of the Central Tyrrhenian Sea, show that (Figure 4.1) the

values of Na+ and Ca2+ concentrations are everywhere higher than in seawater and

increasing with depth. On the contrary, they measured concentrations of K+ decreasing

with depth, from values higher than those in seawater to lower values. While the increase

of sodium and calcium concentrations was correlated to upward ion diffusion from an

underlying Messinian brine layer, the decrease in potassium concentration was attributed to

the adsorption of potassium by clays. The pore fluid salinity of submerged clay formations

can reach saturation, as reported by Van Paassen and Gareau (2004) for the Quaternary

sediments under the Caspian Sea (Figure 4.2). The Authors attributed the high salinity to a

possible upward water flow from a deep halite deposit.

Concentrations which are significantly lower than those in seawater are possible as well. In

Quaternary silty clay sediments under the Strait of Sicily, Emeis et al. (1996) found a

decreasing total salinity with depth from 38 g/kg at the seafloor to 34 g/kg at 50 m depth

(Figure 4.3). A decrease of the same magnitude was not observed in chloride but in

sulphate concentration. This was attributed to bacterial activity and to diagenetically-

induced calcium and magnesium depletion.

4. Pore fluid composition in clays of marine origin

93

Figure 4.1 Salinity, concentrations of the main ions and pH profiles against depth below

seafloor in borehole 974B, Central Tyrrhenian Sea (Comas et al., 1996).

Figure 4.2 Pore fluid salinity against depth below seafloor for a borehole in the Caspian

Sea (Van Paassen and Gareau, 2004).

4. Pore fluid composition in clays of marine origin

94

Figure 4.3 Salinity, Cl-, K

+ and SO4

2- concentrations against depth below seafloor in

borehole 963A, Strait of Sicily (Emeis et al., 1996b).

As a consequence of the tectonic uplift, a marine-origin clay formation can be exposed to

rainwater and freshwater from confining formations. A process of salt leaching due to the

exposure to water is well known for the Quick Clays (e.g. Bjerrum, 1954, Rosenqvist,

1955). Figure 4.4 (from Helle et al., 2009) shows an example of geotechnical profile in

Quick Clays in Norway. Figure 4.5 (from Andersson-Sköld et al., 2005) shows the Na+

concentration and the electrical conductivity against depth for the pore fluid squeezed out

from the soil samples cored in a Quick Clay site in Sweden. Ion concentrations were

determined by means of ICP-AES and ICP-SMS spectrometers. Figure 4.4 shows the

noticeable salinity decrease from the depth towards the ground surface, which was

attributed to a downward freshwater flow. On the contrary, Figure 4.5 shows an increase of

sodium concentration towards the surface, which was found consistent with an upward and

lateral freshwater flow from the adjacent and underlying bedrock.

4. Pore fluid composition in clays of marine origin

95

Figure 4.4 Geotechnical profile of a borehole in the Quick Clay deposit in Smorgrav,

Norway (Helle et al., 2009).

Figure 4.5 Na+ concentration, electrical conductivity and sensitivity against depth in a

Quick Clay at Surte, Sweden (Andersson-Sköld et al., 2005).

4. Pore fluid composition in clays of marine origin

96

Similar profiles, due to the contact with more permeable formations, were found by

Aylsworth and Hunter (2004), while investigating a sensitive marine clay formation near

Ottawa (Canada) by means of in situ measurements of electrical conductivity. Pearson et

al. (2003) and Turrero et al. (2006) also reported significant concentration gradients (from

12 g/l to 2 g/l of Cl- in 150 m) within the Opalinus clay formation in Switzerland, which

were attributed to the lateral contact with a more permeable formation (Figure 4.6). In their

study, chloride concentration was measured in undisturbed samples of pore water squeezed

out by means of a downhole equipment. Quigley et al. (1983) determined the pore fluid

salinity profiles against depth from the ground surface in several boreholes in the Leda clay

formation in Canada. They found a significant salinity increase with depth in some cases,

while in other cases a decrease down to very low values (as in Figure 4.7). Rather than to

leaching, the Authors attributed such profiles to salt diffusion towards the ground surface

and towards an underlying formation characterised by low ion concentration.

Figure 4.6 Chloride concentration profile in the Opalinus clay formation (Pearson et al.,

2003).

4. Pore fluid composition in clays of marine origin

97

Figure 4.7 Geotechnical profile in the Leda clay formation (Quigley et al., 1983).

Figure 4.5 shows that sodium concentration and electrical conductivity of the pore fluid are

strongly correlated, since NaCl is the prevailing solute. In addition, pore fluid salinity can

be also correlated to the electrical resistivity of the system soil-pore fluid, as shown by

Figure 4.8 for various Quick Clay sites. This is of practical interest for pore fluid and

mechanical characterisation of the clay.

Salt leaching through the clay formation towards an aquifer can lead to a significant

salinity increase in this latter. Walraevens et al. (2007) reported the groundwater

composition of a large aquifer in the Flandres, Belgium. Field data show how, along its

flow path, the freshwater in the sands is gradually enriched of ions coming from the clay

layer (Figure 4.9). The Authors also showed that a simple transport-reaction model can

reproduce field data satisfactory.

4. Pore fluid composition in clays of marine origin

98

Figure 4.8 Electrical resistivity against salt content in Quick Clay sites (Long et al., 2012).

The study of hydrogen and oxygen isotopes can be helpful for pore fluid dating and reveal,

perhaps surprisingly, that after million years a clay formation can still preserve part of the

original pore fluid. In fact, on the basis of measurements of deuterium and oxygen-18 in

the pore water, and by means of a diffusion model, Rubel et al. (2002) inferred that the

core of the Opalinus clay formation still contains part of the original pore fluid,

notwithstanding a 10 million years long lateral exposure to fresh water (Figure 4.10). Cervi

et al. (2012), by means of isotopic analyses, could assess the presence of a deep source of

water with higher ion concentration in the Ca’ Lita landslide in Italy, in which the pore

fluid has a salinity of about 1 g/l upslope and 6 g/l downslope. De Craen et al. (2006)

reported the concentration profiles of the pore fluid squeezed out from the cored samples

of the Boom Clay in the Essen-1 borehole, Belgium. As shown in Figure 4.11, Na+ is the

main dissolved cation and its concentration increases significantly with depth, where the

formation is in contact with a salt-rich aquifer. De Craen et al. (2006) also reported the

deuterium and oxygen-18 concentration profiles and inferred that pore fluid in Essen-1 is a

mix of meteoric water and seawater (92% and 8% respectively). A further insight on the

potential of isoopic analysis in landslide studies can be found in Bogaard et al. (2007).

4. Pore fluid composition in clays of marine origin

99

Figure 4.9 Sodium concentration in the groundwater in the aquifer underlying the

Bartonian Clay formation (Belgium). The lines represent the results of the model

implemented by Walraevens et al. (2007). Figure from Walraevens et al. (2007).

Figure 4.10 Deuterium δD and Oxygen δ18O profiles in the Opalinus Clay, Switzerland

compared to concentration profiles obtained by a diffusion model (Rubel et al., 2002).

4. Pore fluid composition in clays of marine origin

100

Figure 4.11 Concentration of the main cations against depth in Essen-1 borehole, Belgium

(De Craen et al., 2006).

The composition of the pore fluid, which can change due to natural but also to anthropic

processes, can affect the mechanical behaviour of the soil and, in some cases, also slope

stability and landslide behaviour.

Deng et al. (2014) studied the behaviour of the Lianyungang clay, a Quaternary soft

marine-origin clay, mainly illitic, outcropping in the Jiangsu province, China. They

analysed the pore fluid extracted from boreholes in two sites by means of spectrometers,

finding significant concentration differences (e.g. 14 g/l and 2.81 g/l of Na+). They

observed that the site characterised by higher salinity is located in an area which emerged

more recently than that of the site which has lower salinity. The Authors performed in situ

and laboratory tests, showing the influence of pore fluid salinity on index properties, shear

strength and compressibility of the investigated clay. A similar study was conducted by

Deng et al. (2011) for two sites in the Boom Clay formation.

An example of the noticeable difference between the shear strengths of the intact Quick

Clay and of the remoulded one was shown in Figure 4.4. In fact, remoulding of a Quick

Clay can result in a complete liquefaction of the soil, with dramatic effects. A well known

case is the 5-6 million m3 large landslide in Rissa, Norway (e.g. Gregersen, 1981). Among

the others, another example of noticeable variation of pore solution concentration with

4. Pore fluid composition in clays of marine origin

101

depth in such type of clay, and its effect on the remoulded strength, was provided by

Geertsema and Torrance (2005) for the case of a landslide at the Mink Creek site in British

Columbia, Canada.

Among the anthropic processes, the pore fluid concentration decrease due to irrigation was

found to be a possible cause of soil instability by Zhang et al. (2009, 2013). The Authors

reported evidences that the desalinization caused by irrigation influenced the initiation and

movements of a number of landslides in the Chinese Loess Plateau. Wen and He (2012),

while studying the weathered illitic-smectitic red mudstone outcropping in the Lanzhou

site, China, also attributed to irrigation the pore fluid Na+ concentration decrease (from 7.5

g/l of the groundwater to 0.2-0.3 g/l of the landslide site) and correlated such decrease to a

reduction in the residual shear strength which may have caused reactivation of several

landslides, as reported also by Derbyshire (2001) and Xu et al. (2011) for different sites.

In some cases, the seasonal variability of pore fluid composition can affect slope stability

significantly. Moore and Brundsen (1996) observed seasonal fluctuation in pore fluid

concentration due to the seasonal deposition of sea-spray and salt at a mudslide toe in the

Dorset coast, England, and observed low pore solution concentrations before seasonal

reactivation or periods of high activity of the landslide. Recently, Tiwari and Ajmera

(2015) observed a decrease in fully softened shear strength due to NaCl leaching in several

landslides of coastal areas in Japan.

Finally, it is worth noting that pore fluid composition gradients can influence pore pressure

distribution and thus the effective stress state and the shear resistance of a soil. For

example, Gueutin et al. (2007) observed from 200 kPa to 600 kPa pore pressure excess

(Figure 4.12) within a low-permeability illitic-smectitic clay formation known as the

Callovo-Oxfordian formation (France). Lower pore fluid salinities were evaluated at the

top and bottom contact with more permeable formations (5 g/l, < 1 g/l respectively) with

respect to those within the formation. A regional-scale study of the pore fluid composition

of such formation is reported in Gaugher et al. (2006), who reported the Cl- concentration

profiles of the pore fluid squeezed out from several samples extracted from different

boreholes to highlight its lateral variability (Figure 4.13). Gueutin et al. (2007), by means

of a numerical model, simulated the pore pressure excess as due to osmotic pressures

generated by the concentration gradient and a non-zero osmotic efficiency. Neuzil (2000)

4. Pore fluid composition in clays of marine origin

102

and Neuzil and Provost (2009) discussed the importance and the occurrence of such

osmotic pressure in natural clay formations.

Figure 4.12 Excess head against depth from the ground surface in the Callovo-Oxfordian

formation (Gueutin et al., 2007).

Figure 4.13 Chloride concentration against depth in the pore fluid of material extracted

from different boreholes in the Callovo-Oxfordian formation (Gaucher et al., 2007).

4. Pore fluid composition in clays of marine origin

103

4.2 PORE FLUID COMPOSITION

AT COSTA DELLA GAVETA

The “water” content and the natural pore fluid composition were evaluated on several soil

samples extracted from different boreholes drilled in the Costa della Gaveta slope. The

evaluation of pore fluid composition was also carried out on the subsoil of the Varco d’Izzo

landslide (Di Maio et al., 2012), located a few hundred metres East of the Costa della

Gaveta landslide. The locations of the landslides and of the boreholes are shown in Figure

2.17 in section 2.

The water content was determined both on undisturbed samples and on partially disturbed

samples which could be considered extracted in undrained conditions. Due to the presence

of rock fragments in boreholes Ki, the material extracted from the first 5-6 m cannot be

considered adapt to such analysis.

Figure 4.14 shows the water content profiles against the depth from the ground surface

relative to boreholes I9b, I9c, I12 and Ki. The inclinometer profiles evaluated at the same

locations of I9b, I9c and I12 suggest that the landslide body is there characterised by water

contents higher than those of the stable soil. In particular, the water content decreases with

depth: in the landslide material it is about 25%, whereas it is w ≈ 15-20% in the underlying

stable soil.

In boreholes Ki, as said, the water content data are not convincing in the first 5 metres.

From 5 to 8 m depth, a layer characterised by w ≈ 30% can be identified, while in the

deeper soil the water content ranges between 15% and 20%. Actually, first inclinometer

measurements in I12b, very close to Ki boreholes, show a slip surface at about 8 m depth,

where the transition to the deeper, more consistent soil was found.

4. Pore fluid composition in clays of marine origin

104

0 0.2 0.40 0.2 0.40 0.2 0.40 0.2 0.40 0.2 0.40

5

10

15

0 0.2 0.4

dept

h (m

)

0 3 6

0

5

10

15

20

25

30

0 0.2 0.4

dept

h (m

)

0 3 6

0 0.2 0.4

0 3 6

0 0.2 0.4

0 0.2 0.40 0.2 0.40 0.2 0.40 0.2 0.40

5

10

15

0 0.2 0.4

dept

h (m

)

I12 I9b I9c

water content

cumulative horizontal displacement (cm)

water content

water content

K1 K2 K3 K4 K5 K6

K1bis K2bis K3bis K4bis K5bis

water content evaluated soon after coring

water content of specimens submitted to electrical and chemical analysesdisplacement profile

slipsurface

slipsurf.

slipsurface

slipsurface

slipsurface

slipsurface

slipsurf.

slipsurface

slipsurface

slipsurface

slipsurf.

Figure 4.14 Water content against depth from the ground surface in several boreholes and

cumulative displacement profiles (or indication of the possible slip surface) at the same

locations at Costa della Gaveta.

4. Pore fluid composition in clays of marine origin

105

The composition of the pore fluid was evaluated by means of the following procedure (Di

Maio et al., 2015a): 30 g of oven-dried and powdered soil were mixed thoroughly with

distilled water to get 500 cm3 suspensions. After sedimentation of the solids, the

supernatant solution was analysed by means of different instruments:

- an inductively coupled plasma (ICP) spectrometer for the measurement of Na+, K+,

Ca2+, Mg2+ and several other cations (I9b, I9c and I12);

- ion-selective electrodes for the measurements of Na+, K+ and Ca2+ (Ki, S3, S8, S9

and S11).

Under the assumption that all the ions found in the supernatant solution were dispersed in

the natural pore fluid, the ion concentrations in this latter can be evaluated from the

concentrations measured in the supernatant solution, being the soil’s natural water content

known.

The concentrations estimated in the pore fluid, taking into account the water content

profiles shown in Figure 4.14, are plotted against the depth from the ground surface in

Figure 4.15 for I9b, I9c, I12, and Ki boreholes.

Besides Na+, other ions such as Ca2+, Mg2+ and K+ were clearly detected, while other

cations were found only in traces.

In borehole I9b (Figure 4.15), after the first 8 m characterised by very low values, the

sodium concentration increases with depth quite regularly. The materials above and below

the slip surface, identified in this location at about 13 m depth (Figure 4.14), do not seem

characterised by significantly different concentrations. On the contrary, a noticeable

increase of Na+ concentration within the shallow 5 m is seen in I9c, followed by a drop to

very low values at 7-8 m depth, close to the slip surface, and then to an increase again. The

sodium concentrations in I12 are very low in the first 13 m, that is in the landslide soil, and

much higher in the stable soil underneath.

4. Pore fluid composition in clays of marine origin

106

0

5

10

15

20

25

30

0 0.25 0.5

dept

h (m

)

0 0.25 0.5 0 0.25 0.5Ion concentration in the pore fluid (mol/l)

I9b I12I9c Na+

K+

Ca2+

Mg2+

0 0.25 0.5

K6

0 0.25 0.5

K5bis

0 0.25 0.5

K50

5

10

15

0 0.25 0.5

dept

h (m

)

K4

0 0.25 0.5

K3

0 0.25 0.5

K2

0 0.25 0.5

K1bis0

5

10

15

0 0.25 0.5

dept

h (m

)

K1

Measuring technique:- I9b, I9c, I12: ICP spectrometer- Ki: ion-selective electrodes

slipsurface

slipsurface

slipsurf.

slipsurface

slipsurface

slipsurface

slipsurface

slipsurface

slipsurface

slipsurface

slipsurface

Figure 4.15 Concentration of Na+, K

+, Ca

2+, Mg

2+ estimated in the pore fluid against

depth from the ground surface in boreholes I9b, I9c and I12 (Di Maio et al., 2015a) and in

boreholes Ki. The depth of the slip surface at corresponding locations is also indicated.

4. Pore fluid composition in clays of marine origin

107

The concentrations of K+, Ca2+ and Mg2+ remain generally negligible with respect to that

of sodium in I9b, I9c and I12. A trend of increasing concentration with depth of K+ and

Mg2+, however, can be seen at high depths, especially in I9b. On the contrary, the calcium

concentration is sometimes significant in the shallow 5 metres.

It is worth noting that the molar concentrations of the main cations in I9b at the maximum

investigated depth, about 28 m, are the following: c(Na+) = 0.44 mol/l, c(K+) = 0.007

mol/l, c(Ca2+) = 0.02 mol/l, c(Mg2+) = 0.017 mol/l. They are quite close, in values and

proportions, to the those of seawater: c(Na+) = 0.47 mol/l, c(K+) = 0.01 mol/l, c(Ca2+) =

0.01 mol/l, c(Mg2+) = 0.053 mol/l, considering a salinity of 35 g/l.

The concentrations of Na+, K+ and Ca2+ in the pore fluid of the soil extracted from

boreholes Ki (Figure 4.15) are very similar. The values of Na+ are very low in the first 8 m,

in the landslide body, and then increase with depth in the stable soil underneath (Figure

4.14). Potassium is practically absent. On the contrary, calcium concentrations are

sometimes significant in the shallow material and decrease noticeably to 8 m depth,

becoming practically negligible in the stable soil.

Chemical analyses were carried out also on soil samples extracted from boreholes S3, S8,

S9 and S11, whose locations are shown in Figure 2.17. The natural water content profile in

these boreholes was not determined. Therefore, the test results are shown in Figure 4.16 in

terms of the sodium concentration in the supernatant solution. The values thus refer to a

constant mass of solids in a certain amount of water, independently of the natural water

content. The concentration profile in the supernatant solution of samples from I9b is also

reported for comparison

4. Pore fluid composition in clays of marine origin

108

0 5 10 15 20 25 30 35 40

00.

005

0.01

depth (m)

S3

00.

005

0.01

S1

1

00.

005

0.01

S9

I9b

Na+

conc

entr

atio

n in

the

supe

rnat

ants

olut

ion

(mol

/l)

00.

005

0.01

S8

00.

005

0.01

K1

K1b

isK

2K

3K

4K

5K

5bis

K6K

i

slip

su

rfac

e

slip

su

rfac

e

slip

su

rfac

e

slip

su

rfac

e

Figure 4.16 Na+ concentration in the supernatant solution and of soil samples from

boreholes S8, S9, S11, Ki (Costa della Gaveta landslide) and S3 (Varco d’Izzo landslide).

The depth of the shear surface, as found by inclinometer measurements, is also indicated.

109

The figure shows that the concentrations in I9b and S9, located along the same cross

section of the landslide, are similar. The concentration increase with depth is quite regular

and at the transition from the landslide material to the stable soil (at 25 m depth in S9, at 13

m in I9b) the concentration does show sudden variations. In S8, in which the slip surface

was found at 37 m depth, the concentrations seem lower than in S9, while they are much

higher in S11, located outside the landslide body, and in S3 (Varco d’Izzo landslide), in

which the slip surface was found at about 11 m depth.

In Figure 4.16 Na+ concentration measured in the supernatant solution of the material

extracted from boreholes Ki is also shown. Being these results independent of the natural

water content, also the measurements carried out on the shallow material are reported. The

values of concentration in Ki boreholes seem consistent to each other and to those of

boreholes I9b, S9 and S8. However, they must be considered carefully, since the presence

of lapideous fragment could make the samples not representative of the in situ condition.

The concentrations of Na+, K+ and Ca2+ evaluated in the pore fluid of the soil extracted

from boreholes I12, I9b and I9c, and on specimens from boreholes Ki whose water content

was considered reliable, are plotted against the depth from the ground surface in Figure

4.17. The figure confirms that the concentration profiles in Ki boreholes are consistent to

one another and that, notwithstanding the different measurement technique, they appear

consistent to those relative to I12, I9b and I9c. The values estimated in boreholes S8 and

S9, assuming two values of water content, w = 15% and w = 20%, are plotted in the figure

as well. The shaded area highlights the possible field of variation of the sodium

concentration in these latter boreholes with the water content ranging between 15% and

20%. From the depth of 15 m on, the concentrations in S8 and S9 seem anyway lower than

those evaluated in the other boreholes.

4. Pore fluid composition in clays of marine origin

110

0

5

10

15

20

25

30

0 0.1 0.2 0.3 0.4 0.5

dept

h (m

)Na+ (mol/l)

0 0.05 0.1

Ca2+ (mol/l)

0 0.05 0.1

K+ (mol/l)

0 100Ca2+ (mmol/l)

K1 K1bis K2 K3 K4K5 K5bis K6 I12 I9bI9c S8 (w=15%) S8 (w=20%) S9 (w=15%) S9 (w=20%)

Figure 4.17 Concentration of Na+, K

+ and Ca

2+ in the pore fluid against depth from the

ground surface, evaluated on the material extracted from boreholes Ki, I12, I9b and

I9c.The values relative to S8 and S9 considering w=0.15 and w=0.2 are also reported. The

shadowed area indicates the range of concentration for S8 and S9 for 0.15<w<0.2.

The supernatant solution was also analysed by electrical conductivity measurements

carried out by means of a 4-electrode conductivity probe, equipped with a temperature

sensor to account for the dependence of the conductivity on temperature.

As an example, Figure 4.18 shows the values of electrical conductivity of the supernatant

fluid of the soil extracted in I9b as a function of depth. The values are compared to the

sodium concentration and to the ionic strength I, evaluated as:

4. Pore fluid composition in clays of marine origin

111

∑=i

ii zcI 2

2

1

where c is the concentration of the i-th ion and z is its valence.

The ionic strength was estimated considering the cation concentrations evaluated by the

spectrometer and assuming, to preserve electroneutrality, that the negative charge is

counterbalanced by monovalent anions, such as Cl-. As a matter of fact, the figure shows a

good agreement between the trends of ionic strength and of electrical conductivity. It also

shows a qualitative agreement between these latter and the concentration of Na+, the

largely prevailing cation. The measurement of the electrical conductivity of the supernatant

solution can thus be a tool for a quick estimation of the ion concentration trend.

0

5

10

15

20

25

30

0 250 500 750 1000

dept

h (m

)

electrical conductivity (µS/cm)

Electric conductivity

Na+

I9b - Ionic Force

Na+ concentration and ionic strength (mmol/l)

Electrical conductivity

Na+

Ionic strength

I9b

Figure 4.18 Sodium concentration, electrical conductivity (normalised at 25°C) and ionic

strength of the supernatant solutions of samples from borehole I9b (Di Maio et al., 2015a).

4. Pore fluid composition in clays of marine origin

112

Summing up, the pore fluid composition of the Costa della Gaveta soil varies in space

greatly, both from one borehole to another and in a single borehole with depth from the

ground surface. Sodium is the main dissolved cation and its concentration increases

noticeably with depth, reaching values similar to those of seawater also in terms of relative

proportions among the most abundant cations. In some boreholes, such as I12, located

upslope, close to the landslide’s source area, pore ion concentrations in the landslide

material are significantly lower than those in the stable underlying soil. Conversely, in

boreholes such as S9 and I9b, located in the landslide channel, the pore ion concentration

increase is gradual and the concentration profile does not show discontinuities at the

transition from the landslide material to the stable soil.

The evaluated differences in pore fluid composition can determine significant differences

in the mechanical behaviour of the material and, in particular, in the residual shear

strength, as shown in section 2. Furthermore, the current ion concentration profiles,

characterised by significant gradients, show a non-equilibrium condition, in which the ion

concentrations are still changing in time, reasonably decreasing, thus possibly leading to

further variations in the material properties and strength.

As a consequence of the observed spatial variations of ion concentration, the assumption of

a unique value of residual shear strength along the entire slip surface of the Costa della

Gaveta landslide, at any depth, can be misleading even under the hypothesis of

homogeneous subsoil. Additionally, the use of distilled water to reconstitute the material to

submit to laboratory mechanical tests, and/or the use of distilled water as cell fluid, can

lead to an underestimation of the residual friction angle with respect to that actually

available in situ.

4. Pore fluid composition in clays of marine origin

113

4.3 ELECTRICAL RESISTIVITY OF THE SYSTEM

SOLID SKELETON – PORE FLUID

The composition of the pore fluid was shown to vary noticeably in the Costa della Gaveta

slope and significant variations in the residual shear strength due to variations in pore fluid

composition have been evaluated. However, the pore fluid composition can be determined

directly only on soil samples of small size. The possibility of extending the results to a

large volume is of practical interest. To this aim, first studies have been carried out in order

to investigate possible correlations between the pore ion concentration and the electrical

resistivity of the soil determined on specimens and slurries in laboratory. The possibility of

extending the results to large volumes by means of the electrical resistivity tomography

(ERT) technique is currently under study.

In soils which do not contain significant amounts of clay minerals, the electric current

flows mostly through the fluid in the pores. Thus, higher values of resistivity are expected

for soils with low porosity, low degree of saturation and/or low ion concentration in the

pore fluid. A simplified formula for the resistivity is the following (Archie, 1942):

ρ = a·ρw·n-m

in which n is the porosity of the soil, a and m depend on the type of soil and ρw is the

electrical resistivity of the fluid in the pores.

By means of ERT carried out in situ and electrical laboratory tests on soil specimens,

Cosentini et al. (2013) showed that, for a pyroclastic soil, the relation can provide reliable

information on the degree of saturation of the material. In clay soils, however, the

electrical conduction along the surfaces of the electrically charged clay minerals is

significant, and the Archie’s law was proven not to be applicable (e.g. Frohlich and Parke,

1989). Alternative models to study the electrical conduction in clays have been described

by Santamarina et al. (2001).

Typical ranges of variation of the electrical resistivity for different materials are shown in

Figure 4.19. The resistivity of clays and shales is in the order of a few Ωm to 100 Ωm,

4. Pore fluid composition in clays of marine origin

114

while sand and gravels exhibit values which are 2 to 3 orders of magnitude higher. Values

in the range 2-100 Ωm are evaluated for fresh water, while salt water can exhibit values

lower than 1 Ωm. Depending on pore fluid composition, is therefore reasonable to expect

that the electrical resistivity of clay soils can increase or decrease with the fluid content.

Figure 4.19 Typical ranges of electrical resistivity of some soils, rocks and fluids (Palacky,

1987).

The electrical resistivity of a clay soil depends on the compositions of both the solid

skeleton and the pore fluid. The type of dependence has been investigated by tests carried

out by means of a 4-electrode conductivity probe on clay slurries. The results (Figure 4.20)

show that, if the used fluid is distilled water, the electrical resistivity increases with the

fluid content and the clay mineralogy influences the resistivity significantly. In fact, at the

same fluid content, kaolin slurries exhibit an electrical resistivity one order of magnitude

higher than that of bentonite slurries, while intermediate values are determined on slurries

of the Costa della Gaveta material (mainly illitic and kaolinitic). It is worth noting that the

electrical resistivity of the slurries is about 100 times lower than that of the used fluid,

notwithstanding the very high water content. This suggests that most of the electrical

conduction occurs through the system solid skeleton – adsorbed water. Conversely, if the

pore fluid is a concentrated salt solution, the electrical resistivity of the slurry is controlled

by that of the fluid, it does not depend on clay mineralogy significantly and exhibits only

small variations with the fluid content.

4. Pore fluid composition in clays of marine origin

115

0.1

1

10

100

1000

10000

0 200 400 600 800

ρ (Ω

m)

w (%)sol. 1M NaCl

kaolin - water

bentonite - water

distilled water

CdG - water

all the materials - 1 M NaCl solution

1 M NaCl solution

Figure 4.20 Electrical resistivity of clay slurries prepared with water or 1 M NaCl solution

against the fluid content, w (Di Maio et al., 2015c).

Some preliminary tests were carried out on specimens of the Costa della Gaveta soil

trimmed from the material cored in boreholes Ki. The specimens were not properly

undisturbed, as an effect of the presence of hard fragments. However, they reasonably

maintained their natural pore fluid.

The specimens were put into Plexiglas cylindrical tubes (3 cm diameter, 6 cm length)

which have two small openings for the insertion of two electrodes, at a mutual distance of

2 cm. The bases of the specimens were regularised and two metallic plates were applied on

them. The contact between the soil surface and the plates was improved by moistening

these latter with a tiny amount of a water-based gel prior to the application.

The specimens were tested by means of a ad hoc sample system, made as 4-electrode

measurements in order to avoid electrode impedance effects. The geometrical scheme is

shown in Figure 4.21. In this system, a georesistivimeter (Syscal Pro, Iris company) injects

a direct current through the metal plates at the bases and measures the potential drop by

means of the electrodes driven into the specimen.

The device provides the values of potential drop V, current intensity I and electrical

resistivity ρ calculated according to a semi-space geometry as in the field. In the laboratory

scheme, the resistivity is corrected by considering the actual specimen’s geometry,

4. Pore fluid composition in clays of marine origin

116

assuming one-dimensional conditions for the current flow, i.e. ρ = V/I·A2/l, where A is the

specimen’s section and l is its length.

ADC

V

Figure 4.21 Geometrical scheme for the measurement of the electrical resistivity of soil

specimens. The picture on the right shows the Syscal Pro device.

Figure 4.22 shows, against the depth from the ground surface, the electrical resistivity

measured on several specimens extracted from different boreholes. Some decrease of

electrical resistivity with depth can be seen. However, data scattering is high and the

observed variations, in the order of a few Ωm, are in the same magnitude of the

experimental error.

Figure 4.23 compares the results obtained on the “undisturbed” specimens to those

obtained by means of the same testing technique on specimens reconstituted with distilled

water or with 1 M NaCl solution. The results obtained on slurries of the same soil prepared

with water or NaCl solutions at various concentrations, and tested by means of the 4-

electrode conductivity probe, are also shown.

The figure highlights that the values obtained with different devices are consistent as a

whole, but they are not comparable directly, mostly because of the different electrical

fields. The results relative to the “undisturbed” specimens lie between those of the

specimens reconstituted with distilled water and with 1 M NaCl solution. However, as seen

also in figure Figure 4.22, the variations of resistivity are practically negligible, between 3

and 6 Ωm, and without a clear dependence on pore ion concentration or on depth. The

possible trends are probably masked by the high error magnitude.

4. Pore fluid composition in clays of marine origin

117

0

3

6

9

12

15

0 3 6 9

dept

h (m

)

electrical resistivity - ρ (Ωm)

K1

K1bis

K2

K3

K4

K5

K5bis

K6

Figure 4.22 Electrical resistivity of specimens of Costa della Gaveta soil against the depth

from the ground surface.

0.1

1

10

100

0 100 200 300 400w (%)

ρ (

Ωm

)

distilled water

1 M NaCl

0.1 M NaCl

0.3 M NaCl

reconst. with water water

reconst. 1 M NaCl sol.

undist.

slurriessoils

Figure 4.23 Electrical resistivity of Costa della Gaveta soil (I12 and K5bis boreholes):

slurries prepared with distilled water and with 1M NaCl solutions at various

concentrations, reconstituted specimens and partially undisturbed specimens (Di Maio et

al., 2015c).

4. Pore fluid composition in clays of marine origin

118

The electrical resistivity has been also evaluated in situ by means of electrical resistivity

tomography (ERT) with 1.5 m resolution. Two measurements were carried out: one along

a cross section through boreholes I12 and P12 (ERT1) and one very close to boreholes Ki

(ERT2).

The electrical resistivity distribution of the ERT2 (Figure 4.24) shows the presence of two

electrical layers: a shallow one, about 8 m thick, characterised by values of resistivity

higher than 40 Ωm, and a deeper one with values lower than 20 Ωm. Differently, the ERT1

shows generally lower and much more uniform values, in the range 10-20 Ωm.

SW NE

K1 K2 K3

K4 (15 m)

I12b (20 m)

K5 K6

Unit Electrode spacing 3 m

P12I12Canale

SW bore

hole

sK

i

I12

I12b

I12

bore

hole

sK

i

I12

I12b

I12

unit electrode spacing = 3 m

slip surface

slip surface

ERT2

ERT1

ERT2

ERT1

Figure 4.24 Electrical resistivity distribution obtained by ERT (Di Maio et al., 2015c).

Figure 4.25 compares the values of resistivity evaluated on “undisturbed” specimens to

those evaluated by the ERT in situ at the same location (ERT2, borehole K6). The field

values are higher than those obtained in laboratory. However, the values of ERT1 are

closer to those of laboratory specimens than those of ERT2.

4. Pore fluid composition in clays of marine origin

119

0

3

6

9

12

15

0 20 40 60

dep

th (m

)

electrical resistivity (Ωm)

Specimens

ERT2

K6

range of ERT1

Figure 4.25 Electrical resistivity evaluated on “undisturbed” specimens and in situ by

ERT2 at a corresponding location. The resistivity range found in ERT1 is also indicated.

The values of electrical resistivity evaluated in situ are generally higher than those of

laboratory specimens (Jones, 1995; Straface and Rizzo, 2013). The difference is generally

attributed to scale effects, including the much higher heterogeneity in composition and

structure of the material in situ.

In order to understand whether the ERT investigation can be used to evaluate variations in

pore fluid composition, an improvement of the test procedure, for instance by the use of

the cross hole method with a higher resolution, is currently under examination. The

possibility to perform measurements on larger specimens, in order to increase the

representativeness of the soil volume in laboratory tests, is also being considered.

120

5 CONCLUSION

In this work the results of several laboratory tests have been presented. The aim of the

work was twofold: 1. to study the influence of pore fluid composition on the residual shear

strength and on creep behaviour and 2. to analyse the natural pore fluid composition of a

clayey slope affected by landslides, trying to understand how the processes observed in

laboratory can influence the landslide behaviour.

The residual shear strength of the Costa della Gaveta soil is influenced by the composition

of the pore fluid significantly. The residual friction angle of specimens reconstituted with

distilled water falls in the range 8°-10°, while the material reconstituted with concentrated

NaCl solutions exhibits values which can reach 20°. Even higher values are found for the

material prepared with KCl solutions. The relation between pore solution concentration

and residual friction angle is not linear, with higher gradients in the range 0-1 mol/l, that is

in the range in which the evaluated values of natural pore fluid concentration fall.

Therefore, significant variations of the residual friction angle in situ due to different pore

fluid compositions are expected, even under the hypothesis of homogeneous soil. As a

corollary, both the assumption of a unique friction angle along the slip surface of the Costa

della Gaveta landslide (up to 40 m deep) and the evaluation of the residual friction angle in

laboratory on specimens reconstituted with distilled water can be misleading.

In order to evaluate the creep behaviour of the material along a slip surface in residual

condition, several shear tests under controlled shear stresses were carried out on pre-

sheared specimens of the Costa della Gaveta soil and of a sodium bentonite. The

specimens were prepared with a concentrated NaCl solution and sheared under constant

displacement rate in a bath of the same solution until the residual strength was achieved.

Subsequently, the apparatuses were modified so that the specimens could be subjected to

5. Conclusion

121

an average applied stress, along the slip surface, lower than the residual strength obtained

in the solution but higher than that obtained on the same material reconstituted with – and

submerged in – distilled water. The applied stress caused only negligible displacements

with a primary creep pattern. The subsequent exposure to distilled water, which was

frequently replaced in order to keep the concentration gradient between the cell fluid and

the pore fluid as high as possible, caused displacements with increasing rate with a tertiary

creep pattern, until the specimens re-experienced “failure”. This was attributed to the loss

of strength caused by the ion concentration decrease in the pore fluid due to ion diffusion.

In fact, the available strength evaluated soon after “failure” was found to be very close to

the driving shear stress.

On the basis of the experimental results, it is reasonable to hypothesise that the pore

solution concentration decrease in situ can be a “hidden” cause of the viscous movements

of the landslide.

The pore fluid composition of the soil at Costa della Gaveta was found to vary

significantly with depth from the ground surface. In particular, sodium was found to be the

most abundant cation, with concentrations ranging from negligible values at the surface to

values close to those representative of seawater at different depths, depending on the

location. Furthermore, the relative proportions of the main cations were found in

agreement with those in seawater as well, consistently with the marine origin of the

investigated clay formation.

In some boreholes located close to the head of the landslide, pore ion concentrations in the

landslide material are significantly lower than those in the stable underlying soil. This is

the zone in which the colluvial material of the source area accumulates. Conversely, in

boreholes located in the landslide channel, the pore ion concentration increase is gradual

and the concentration profiles do not show discontinuities at the transition from the

landslide material to the stable soil.

The quantitative evaluation of how the processes observed in laboratory – and in particular

the decrease in strength and the increase in shear displacements associated to the reduction

in pore solution concentration – influence the behaviour of the Costa della Gaveta

landslides is currently under examination and represents the aim of the future study.

122

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Anson, R. W. W., Hawkins, A. B. (1998). The effect of calcium ions in pore water on the residual shear strength of kaolinite and sodium montmorillonite. Géotechnique, Vol. 48, No. 6, pp. 787-800.

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