Modeling of Passive Piles –An Overview

Kok Sien Ti

PhD student, Department of Civil Engineering, University Putra Malaysia, Serdang, Selangor, Malaysia

Email: koksienti@yahoo.com

Bujang B.K. Huat, Jamaloddin Noorzaei, Mohd Saleh Jaafar

Department of Civil Engineering, University Putra Malaysia, Serdang, Selangor, Malaysia

Email: bujang@eng.edu.upm.my

Gue See Sew

CEO, G&P Geotechnics Sdn Bhd Bandar Tasik Selatan, KL, Malaysia

ABSTRACT Piles subjected to lateral loading from the moving soils is known as passive piles. These passive load induces forces and bending moment in piles that can lead to serviceability problems or even failure of the piles itself. This area of research is not recent and there were countless works that has been documented for the last three decades. In this paper, a review was conducted based on existing literatures and then classified to address four topics, namely; pile subjected to horizontal soil movement, pile-supported embankment, piles for slope stabilization, piles adjacent to deep excavation and failures of piles in open excavation. In addition, this paper intends to present a general overview on the state-of-the-art research for passive piles. Based on the summary, passive piles in open excavation may indeed proved to be another recent and significant area of research that should be emphasized, as this will have invaluable contribution to the deep foundation industry.

KEYWORDS: passive pile, horizontal moving soil, embankment, slope, excavation.

INTRODUCTION When piles are subjected to soil movement, these piles are known as passive piles. Soil

movement is encountered in practice when piles are placed in an unstable slope, landslides, adjacent to deep excavation, tunnel operation, marginally stable riverbank with high fluctuating water level and also in piles supporting bridge abutment adjacent to approach embankments. The design of such piles may be based on the assumptions that forces from moving soil will act against the piles and ‘squeeze’ past the piles. On the other hand, active piles referred to a pile subjected to a external horizontal force. Refer Figure 1 for the differences between active and passive piles.

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Figure 1: (a) Active pile with length, L and diameter, B subjected to a point horizontal load, Q, acting with a distance e, from ground surface (Broms, 1965). (b) Passive pile in an embankment

subjected to horizontal loading from moving soil past the piles (Ellis and Springman, 2001).

The difference between load-transfer relationship for laterally loaded pile groups was defined in detail by Bransby (1996). In brief, only the passive lateral-definitions was described here. Pile displacement relative to the initial pile position is defined as δp, and the soil displacement midway between piles is defined as δs (Figure 2). An equivalent uniform displacement is defined as δeq so that, swept areas A1=A2. Therefore, δ=δeq-δp.

This paper attempts to present an overview mainly for single pile in four topics, namely; pile subjected to horizontal soil movement, pile-supported embankment, pile for slope stabilisation, piles adjacent to deep excavation and failures of piles in open excavation. The author is aware that reviews had been done by some researchers extensively on a particular topic and complete efforts done may not be reported in this paper. Nevertheless, this paper presents an overview and brief description on collaborations from various researchers and present the discussion mainly to emphasize the significance of research on piles in open excavation.

Figure 2: Passive lateral loading (Displacement definitions), adapted from Bransby (1996).

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PILES SUBJECTED TO LATERAL SOIL MOVEMENT Early works in analyzing piles by using finite element code has been carried out by Desai &

Appel (1976), whom developed a three-dimensional finite-element code for the analysis of piles in a group in moving soil. Linear analysis was used for the piles and nonlinear analysis for the soil. Subsequently, Shie & Brown (1991) and Brown & Shie (1992) did further work with finite elements and produced some useful results where else Wang (1986) did a comprehensive study of the various analytical methods that can be employed in analyzing a closely-spaced group of piles in moving soil.

Poulos et al (1995) presented model test on single piles subjected to lateral soil movement embedded in calcareous sand. Influence of pile head fixity condition, the ratio of the depth of moving soil to the pile embedded length, pile diameter and stiffness on the pile’s maximum bending moment are investigated and relationship are presented in forms of normalized expressions. Subsequently, with adapting the same method, Chen et al (1997) proceed to present model tests on pile groups subjected to lateral soil movement in the same soil material, where the group effect on the lateral response of a pile in a group was found to be dependant on a number of factors, namely position of the pile in the group, the pile spacing, the number of piles, and the head fixity condition. In both papers, an existing boundary element program was used to produce theoretical predictions, which agrees well with the experimental results. In this case, noth Poulos et al (1995) and Chen et al (1997) have established similar testing and procedure to induce a triangular profile of horizontal soil movement with depth on the piles as shown in figure 3 below.

Chen and Poulos (1997) then proceeded to presents design charts for analyzing the lateral response of vertical piles subjected to lateral soil movements, by using a simplified boundary-element analysis, using a specified free-soil soil movement profiles. However, only if accurate assessment of horizontal soil movement can be made, it is generally possible to obtain a satisfactory prediction of the pile response by the present method.

Figure 3: Model pile groups subjected to lateral soil movement (Chen et al, 1997)

As for recent analytical method adopted, researchers like Xu and Poulos (2001) employed a 3D coupled boundary element approach to analyze the response of vertical piles subjected to passive loadings such as soil shrink/swelling, soil surface surcharge, tunneling, soil movements arising from driving piles and cavity formation in soil. A number of theoretical expressions for soil movements are developed and presented. These expressions have been incorporated into the pile-soil governing equation previously developed by the authors.

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Besides that, Chaudhuri (2005) describes another analytical method for evaluating pile foundation response to lateral ground movement. A model using beam on linear and nonlinear elastic foundations has been analyzed using the finite difference method. Dimensionless plots are developed to evaluate pile behaviour for different soil and geometric conditions. These plots can be used for estimating pile displacements and moments caused by lateral soil movement. Failure mechanisms are examined and are related to relative soil-pile stiffness and soil yielding.

Pan et al. (2000, 2002) conducted a series of laboratory model tests in soft clay was conducted to investigate the behaviour of single and coupled piles subjected to lateral soil movements and to determine the ultimate soil pressure acting on the pile shaft. The test results also indicated that different distributions of limiting soil pressures along the pile shaft were developed for the single and coupled passive piles. Following this, Pan et al (July 2002) carried out a three-dimensional finite element analyses to investigate the behaviour of single piles subjected to lateral soil movements and to determine the ultimate soil pressures acting along the pile shaft. The finite element analyses program ABAQUS was used for the analyses. The von- Mises constitutive model was employed to model the non-linear stress-strain soil behaviour. The pile was assumed to have linear elastic behaviour. The three-dimensional finite element mesh used in the analysis was optimized taking into account the computing capacity limitations.

In addition to recent development in finite element modeling, Miao et. al (2006) carried out three-dimensional finite element analyses to investigate the response of a single pile when subjected to lateral soil movements. The pile and soil were modelled using 20-node quadrilateral brick element with reduced integration. For compatibility between the soil-pile interface elements, 27-node quadrilateral brick elements with reduced integration were used to model the soil around the pile adjacent to the soil-pile interface. A Mohr-Coulomb elastic-plastic constitutive model with large-strain mode was assumed for the soil. The analyses indicated that the behaviour of the pile was significantly influenced by the pile flexibility, the magnitude of soil movement, the pile head boundary conditions, the shape of the soil movement profile and the thickness of the moving soil mass.

PILES FOR SLOPE STABILIZATION The slip surface of landslides in stiff clays of highly weathered rocks is substantially weaker

(softer) than the materials above or below it. This zone has been referred to as the discrete shear zone. One way to improve the resistance at the weak shear zone is by using isolated shear piles, also known as dowel piles. Shear piles are reinforced cylindrical piles that pass through the landslide and are anchored at their lower end in stable soil or bedrock. The pile anchorage provides lateral bearing resistance near the base of the moving ground. Design of the reinforcement steel is controlled by the maximum bending moment developed in the pile.

Poulos (1995) reported that the prediction of soil lateral movements requires a knowledge of the distribution of lateral soil modulus and limiting lateral pile-soil pressure with depth, and the free-field horizontal soil movements. For problems involving slope instability, a distribution of free-field soil movements such as that shown in figure 4 appears to be appropriate.

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Figure 4: Basic problem of a pile in unstable slope: free-field soil movement, Poulos (1995)

Some of the successful applications of such techniques have been reported by Esu and D’Elia (1974), Ito and Matsui (1975), Sommer (1977), Wang et al. (1979), Ito et. al. (1982), Nethero (1982), Morgenstern (1982), Gudehus and Schwarz (1985), Reese et al (1992) and Rollins and Rollins (1992). Driven timber piles have been used to reinforced the slope stability of very soft clays in Sweden, while cast-in-place reinforced concrete piles as large as 1.5m diameter have been used in Europe and the United States to stabilize active landslides in stiff clays (Bulley 1965 and Offenberger 1981). In Japan, steel tubes piles of 300mm diameter have been used to stabilize active landslides areas (Taniguchi 1967) and Fukouka (1977) presented three real-life cases in Kanogawa Dam, Hokuriku Expressway in Fukue Prefecture and Higashitono where steel piles were used to improve the factor of safety of landslides.

Viaggiani (1981) suggested that a sliding slope may be stabilized by increasing its safety factor by a few percent besides presenting modes of failure for rigid and flexible pile as shown in figure 5 below. Rigid pile refers to pile with the moment of resistance that is more than the maximum bending moment induced. Flexible pile refers to pile with the moment of resistance equal to the induced bending moment at certain location. In the latter case, plastic hinge(s) may be formed in the pile. On the first column of each mode shows the pile displacement, followed by the soil reaction, shear force and bending moment.

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Figure 5: (a) Modes of failure for rigid pile (after Viaggiani, 1981). (b) Modes of failure for

flexible pile (after Viaggiani, 1981).

Reese et al. (1992) have presented a p-y approach for assessing the improvement in slope stability which arises from using piles. Rowe and Poulos (1979) developed a two-dimensional finite element approach that allowed for the three dimensional effect of soil flowing through rows of piles. A three dimensional finite element approach has been developed by Hassiotis & Chameau (1984) for the analysis of stabilization of surcharged slope with drilled piles.

Chow (1996) presented an approach to analysis piles for slope stabilization, where piles are modeled using the modulus of subgrade reaction and the pile-soil-pile interaction considered using the theory of elasticity. Two case histories, one for single pile and the other for pile group, are analyzed which show that the numerical model can predict the general characteristics of the piles reasonably well. However, this study suggest that the design of the piles based on the computed response from single pile analysis, may be conservative.

Lee et al. (1995) presents a simplified approach to the study of a row of piles used for slope stabilization based on an uncoupled formulation in which the pile response and slope stability are considered separately. The pile response when subjected to external lateral soil movement from slope instability is analyzed by a modified boundary element method. A conventional simplified Bishop slip circle approach is employed to analyze the slope stability. Figure 6 below showed the portion of the piles embedded in the sliding slope subjected to large lateral soil movements where vertical soil movements are ignored here.

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Figure 6: Simplified piled-soil stability analysis (Poulos, 1995)

Where s=spacing between piles, H =slope height, D=depth from toe of slope to a hard base, α=slope inclination angle, L=pile length and d=pile diameter.

Jeong et. al.(2003) and Won et al (2005) presented a numerical comparison of predictions by limit equilibrium analysis and 3D numerical analysis for a slope-pile system to investigated the response of the pile groups by using both coupled and uncoupled analysis. Special attention is given to the coupled analysis based on the explicit-finite-difference code, FLAC 3D. Coupled analyses were performed for stabilizing piles in a slope, in which the pile response and the slope stability are considered simultaneously and subsequently the factors of safety are compared to a solution for a homogeneous slope using an uncoupled analysis (limit equilibrium analysis). It is found that the factor of safety in slope is much more conservative for an uncoupled analysis than for a coupled analysis based on three-dimensional finite element analysis.

Thompson and White (2006) conducted a full-scale pile load test to investigate the soil-structure interactions for small-diameter piles subject to lateral soil movement, in which piles installed through a shear box were indirectly loaded by uniform lateral translation of soil. Instrumentation of the shear boxes and pile reinforcement indicated the load distributions that developed along the piles. The load test analyses which succeeded the pile load tests support the claim that the distributed loads which are achieved during the pile loading vary linearly with depth. The product of the analysis, which answers a central question of the research, is directly incorporated into the proposed design methodology for the soil displacement grouted micropiles. It is apparent from the pile load tests that small-diameter pile elements provide effective passive resistance to lateral soil movement.

Laudeman and Chang (2004) discussed and tabulated a summary of available design methods based on a simple slope configuration analyzed by using finite element method. Based on this analysis, it is concluded that the finite element method appears to be effective to analyze this difficult problem.

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Guo (2006) presents a new closed-form solutions for a free-head pile embedded in an elastic-plastic, non-homogeneous soil by simulating pile-soil interaction using a series of springs distributed along the pile shaft where the stiffness of each spring is theoretically related to soil modulus, pile-soil relative stiffness and loading properties; and the limiting force is catered for by a new generic expression. Ranges of input parameters are provided, in light of predictions carried out to date against 62 tested piles in clay and sand.

PILE-SUPPORTED EMBANKMENTS The consolidation of embankments on clay can cause significant vertical and horizontal

movements of the soil beneath and adjacent to the embankment. Piles supporting the bridge abutments may therefore be subjected to axial and lateral loads which are induced by these soil movements. The design of the piles therefore requires an assessment of the consequent axial force, axial movement, bending moment and lateral deflection developed in the piles. Figure 7 shows a schematic section through such a structure, illustrating the forms of interaction which tend to increase lateral structural loading and displacement, and hence may result in unserviceable behaviour of the abutment or bridge deck.

Figure 7: Schematic section through a full-height piled bridge abutment constructed on soft clay illustrating the forms of lateral loading (Ellis and Springman, 2001).

De Beer & Wallays (1972) proposed a semi-empirical method to estimate the maximum bending moment for piles subjected to asymmetrical surcharges. They assumed that a constant lateral pressure distribution acted on the pile in the soft layer. The magnitude of this lateral pressure was a function of the total vertical overburden pressure, apparent angle of friction and the slope of a fictitious embankment of material of unit weight 18kN/m3. They suggested that the lateral loading was caused by horizontal consolidation and creep, implying that their method was primarily intended to design piles in the long term. The method cannot be used to calculate the variation of bending moment with depth along the pile.

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Therefore, they conservatively recommended that the piles should be reinforced over their whole length to carry the maximum calculated bending moment. After calibrating the method against a few case studies, they demonstrated that the method is only suitable if a large margin of safety is provided against overall instability of the soil mass, i.e, the factor of safety against overall instability should be greater than 1.6. The approach is very simple and proposes that a condition on ultimate foundation capacity may be used to assess the likelihood of significant soil-structure interaction. In reality, the mechanism used assess the foundation capacity is likely to be a poor representation of the mechanism provoking soil-structure interaction. Also factors such as the variation of the strength of the soft clay with depth, relative soil-pile stiffness and any resulting displacement are ignored.

Begemann & De Leeuw (1972) used Airy’s stress function to derive closed form solutions for calculation of horizontal deformation and earth pressure distributions with depth in a layer of soft clay which is subjected to surcharge loading. They assumed that the soft soil is an elastic, homogeneous and isotropic material resting on a rigid base. In addition, they assumed that the soil has an infinite dimension in the intermediate principal stress (horizontal) direction and that it deforms in an undrained condition. Two types of boundary conditions were studied: vertical loading without surface shear stress, and zero horizontal deformation at the loading surface. Numerical examples were given to calculate lateral deformation of soil and earth pressure on both flexible and rigid piles. However, no comparison was given with any field observations.

Based on research work by De Beer & Wallays (1972) and Begemann & De Leeuw (1972), summarizing nineteen field observations, and an assumption of soil elasticity, Oteo (1977) derived design charts for calculating the maximum lateral displacement and bending moment in relatively flexible piles in soft soil subjected to adjacent surcharging. The effect of soil-pile interaction was accounted for in a basic way. For stiff piles, he followed the maximum pressure method proposed by Begemann & De Leeuw (1972), which is discussed.

Tschebotarioff (1973) summarized research work on piled abutments and suggested that even with a factor of safety of 1.5 against rotational failure of the entire structure, the design of a bridge abutment on soft clay should take account of additional lateral load on the piles. Based on the results of model tests at Princeton University in the 1940s and field measurements in New Jersey, he recommended an empirical approach of triangular lateral pressure distribution in the soft clay layer, with maximum pressure acting on the piles at mid-depth. The magnitude of vertical stress included the combined weights of backfill and half of the height of the soft clay layer. Once this pressure distribution was known, the bending moment was calculated using equations from a structural handbook, by assuming full fixity at the pile cap and pin support at the interface between the soft clay and the underlying soil. Although this method allowed simple assessment of ultimate bending moment capacity required in piles, it was not possible to estimate deformations, and the shear stress transfer mechanism underneath the embankment due to the lateral soil movement was not recognized.

Poulos & Davis (1980) and Poulos (1994) derived a theoretical method to analyse the distributions of pressure and bending moment along a single pile subjected to a known lateral soil movement. The soil in the analysis was assumed to be an ideal, isotropic elastic material, having a Young’s modulus and Poisson’s ratio which are unaffected by the presence of pile (which was modelled as a thin vertical strip). Parametric studies using the finite difference method were carried out to study some of the factors influencing the development of pile moments and displacements, such as relative pile flexibility, boundary conditions, shape and magnitude of soil movement profile and pile diameter. In addition, some comments were given regarding values of soil parameters required for practical problems.

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Some comparisons were made between observed pile bahaviour and predictions given by the theory, and reasonable agreement was obtained. This method would be difficult to apply in practice because the distribution of horizontal soil movement with depth is required as one of the input parameters. Horizontal ground movement cannot be known in advance, although ground movement distribution can be estimated from inclinometers installed on other similar sites or from finite element analysis.

Franke (1977) reported the design method adopted in Germany. The first step was to calculate overall stability of the retaining structure against circular slip failure using the method of slices. The additional resistance to slip failure provided by piles which pass through the slip plane was ignored. In cases where the factor of safety was not considered sufficiently high to guard against significant lateral soil movement, it was recommended that the piles should be designed to withstand a uniformly distributed lateral pressure of 10.5cu acting on them in the soft soil layer. This approach could be particularly over-conservative under certain circumstances, since such pressures only result from fully developed plastic flow of soil past the piles.

Randolph & Houlsby (1984) used classical plasticity theory to derive exact solutions for limiting lateral resistance of a circular pile in cohesive soil. Their analyses were based on a perfectly plastic soil response. They reported that the limiting pressures that can develop were 9.14cu and 11.94cu for perfectly smooth and perfectly rough piles respectively.

Baguelin et. al. (1977) examined the mechanism of ‘load-transfer’ relationship, expressed by lateral load on the pile based on the relative pile-soil lateral displacement and stiffness properties of the soil. An analytical solution for the force required to displace a pile laterally relative to a distant rigid circular boundary was derived. For the case of a rigid adherent circular disc (representing the cross-section of the pile, which is displaced through a homogeneous linear elastic material (plane strain), a load-transfer relationship was derived. The influence of pile section (square or circular), disturbance of a soil zone around the pile and plastic yielding of soil in an undrained manner were also studied by means of the finite element method. In addition, some simplified three dimensional analyses were carried out to investigate the effects of pile length, boundary and loading conditions. The findings from the simplified analyses were compared with a case record and good agreement was obtained.

Baguelin et. al. (1977)’s load- transfer relationship was reinterpreted by Springman (1989) and Springman and Bolton (1990) for relative pile-soil displacement and was subsequently also used by Stewart et. al (1993) to describe pile-soil interaction in plane strain analyses of undrained passive pile loading. The work described above has been extended by Bransby (1995), using results from FEM analyses. A power law stress-strain relationship for the soil was introduced and group effects were considered using rectangular boundaries. However, the work was restricted to the undrained case. For an isolated pile, Baguelin’s method also corresponds approximately to that used by Bransby, although the boundary conditions are slightly different. However, the rectangular boundaries used by Bransby allow meaningful investigation of group effects. Bransby’s relationships were considered the most relevant, since the work corresponds to that used previously by Springman and Stewart et al. in the case of isolated piles but also has the advantages that non-linear stress-strain behaviour and group effects can be incorporated.

Using the data from centrifuge model test by Bransby (1995), Bransby and Springman (1996) then proceeded to model the test by a three dimensional finite element analyses to study the short-term behaviour of pile groups subjected to lateral pressures by deformation of a clay layer under an adjacent surcharge load.

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Subsequently also from Bransby (1995), Bransby (1996) present a technical note which investigates the definition of the load transfer relationships for undrained passive and active lateral-pile loading suitable for pile design in clay. Results from a series of two-dimensional finite element analyses were presented with interaction bahaviour being explained. Bransby and Springman (1999) then used similar method to examine the effects of pile-soil-pile interaction on the load-transfer response for passive lateral loading in undrained soil. Interaction factors suitable for design use to account for increasing lateral pressure on piles have been produced for a range of pile spacing and power law soil exponents.

On the basis of the work described by Baguelin et. al. (1977), Stewart (1992) and Stewart et. al (1993) proposed a modified method to relate the lateral pressure acting on the pile to an approximate relative soil-pile displacement. This method attempts to eliminate the possibility of invalid solutions from Springman & Bolton’s formulation when the relative pile-soil stiffness is low and to provide a better representation of pile group behaviour. The method approximates the behaviour of a pile group as a single beam with fixed support at the base and a moment at the top which prevents rotation whilst allowing lateral deflection. However, no shear force is considered to act at the pile cap level, leading to the assumption that the same horizontal load acts on all rows of piles.

Analytical solutions were obtained to compute the maximum moment in the piles and the pile cap deflection. In order to match the observed behaviour from model centrifuge tests carried out by Stewart (1992), a non-linear stress-strain curve for kaolin was incorporated into the method to account for the observed non-linear behaviour. Moreover, corrections were applied to the embankment geometry to account for distinct variations from the infinite strip load assumed in the proposed analysis. The analytical results compare well with the two centrifuge tests. Recommendations were made that the applied embankment loading should be limited to less than three times the undrained shear strength of the soft stratum to avoid significant plastic deformation in the soft layer.

Carter (1982) also used the finite element method to investigate the bending moments and axial forces induced in a single pile embedded in the centre of an axissymmetric mesh and isotropic, perfectly elastic soil mass. Loading on a circular arc and surface strip loading extending to one side of the pile were studied with various pile geometry, end fixities and relative soil- pile stiffness. A series of normalized charts were produced, and these may be used for design. The technique allows good representation of the three dimensional nature of the problem, although pile groups cannot be analyzed directly unless a rigid pile cap can be approximated by imposing zero rotation of the pile head.

The effects of vertical drains in the clay layer for embankment was investigated by Ellis (1997) through a series of geotechnical centrifuge tests examining the effect of clay layer depth and the rate of embankment construction. The results confirmed the existence of established interaction effects due to lateral displacement of clay past piles, which was initially investigated by Springman and Bolton (1990).

Following this, Ellis and Springman (2001) presented an accompanying series of plane strain finite element analyses for the undertaken series of geotechnical centrifuge tests of full-height piled bridge abutments constructed on soft clay to study the soil-structure interaction effects. Although some aspects of the structure do not conform to a plane strain analysis (most notably the piles), incorporation of the soil-structure by adapting Randolph (1981)’s method. Success of the method is illustrated by good comparison with the centrifuge test results.

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For piles to support a retaining wall, the recommendations of Terzaghi et al. (1996) embodied in charts, are based on the use of the equivalent-fluid method and a perusal of the charts shows the importance of the selection of an appropriate material for the backfill. A comparison of values from theory with values from the charts shows that the charts are close to those from Rankine’s theory for active earth pressure. Therefore, the assumption is implicit in the Terzaghi charts that the wall is capable of some movement without distress if the pressures from the backfill were greater than the chart values.

Poulos (1996) reviewed some available methods for design of piles through embankment and presented comparison between these methods for maximum bending moment in the piles, lateral pile head deflection, maximum axial force in pile and axial pile head settlement. Some of the methods being reviewed are the method of Tschebotarioff (1973), De Beer and Wallays (1972), Stewart (1992), an approach used by a road authority in Australia, a simplified analysis of pile downdrag and design charts developed from boundary element analyses of pile-soil interaction. It concludes that none of the previously available methods appears able to provide a consistent means of estimating the lateral response of piled embankment (Refer to figure 8 and table 1 below).

Figure 8: Idealized hypothetical case (Poulos, 1996)

Table 1: Summary of results for idealized hypothetical case.

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PILES ADJACENT TO DEEP EXCAVATION In dense urban environment where land is scare and buildings are closely spaced, deep

excavation for basement construction and other underground facilities such as mass rapid transit stations and cut-and-cover tunnels is unavoidable. These deep excavations would cause lateral soil movement behind the excavation. Such soil movement would in turn induce lateral loading on adjacent pile foundations, inducing additional bending moment and deflection on the piles.

For example, Finno et al. (1991) reported the analyses and performance of groups of step-tapered piles located adjacent to a 50-ft-deep tieback excavation. Evaluation of effects of movements on the adjacent piles were then carried out using a plane strain finite element code. However, accuracy of the approach could not be clearly verified due to less certainty with the selection of soil parameters, especially the soil’s modulus because of the lack of detailed laboratory or in situ testing. The uncertainty of modeling the equivalent bending stiffness of the pile group in plane strain resulted in lower and upper bound solution.

Poulos & Chen (1996) did a two stage analysis by use of the finite-element method and the boundary-element method to analyze the response of piles due to unsupported excavation-induced lateral soil movement in clay. Initially, a plane strain finite element method was used to simulate the excavation procedure and to generate free-field soil movements, that is the soil movement that would occur. These generated soil movements are then used as input into a boundary element method to analyze the pile’s response as shown in figure 9 below:

Figure 9: “Standard” problem analyzed (Poulos and Chen, 1996)

On the contrary, Goh et al. (2003) used the soil movements measured by an in-soil inclinometer as the free-field lateral soil movement. Goh et al. (2003) presented the results of an actual full-scale instrumented study carried out to examine the behaviour of an existing pile due to nearby excavation of a 16m deep excavation with the in-soil inclinometer was installed about 6m away . The instrumented existing pile was analyzed as a single where it is discretized into a finite number of discrete (linear elestic) beam elements. The interaction between the soil and the pile is modeled by a series of non-linear horizontal springs represented by a hyperbolic equation.

Recent efforts in centrifuge modelling of passive piles adjacent to unbraced excavation was done by Leung et al. (2000), Leung et al. (2003), Ong et al. (2006) and Leung et al (2006). Initially, Leung et al. (2000) presented the results of centrifuge tests of an adjacent single pile behind a unstrutted stable and failed wall of a deep excavation in dense sand. Refer Figure 10 below. The research also investigates the influence of head fixity and shows that behind the stable

Vol. 14 [2009], Bund. P 14 wall, the pile head deflection and maximum bending moment for the free-headed pile decreases exponentially with increasing distance from the wall. Subsequently, Leung et al. (2003) extended the centrifuge test to pile groups, incorporating the effects of interaction factors between the piles with different head fixities.

Figure 10: Centrifuge model setup (Prototype dimension given in parentheses) by Leung et. al. (2000)

Following Leung et al. (2000), further investigation was done for single pile behind stable wall (Ong, 2006) and instable wall (Leung, 2006) in clay. It is concluded that calculated pile response is in good agreement with the measured data if correct shear strength obtained from post-excavation was used in numerical analysis.

The numerical analysis was based on a simplified model (Chow and Yong, 1996) and was used to back-analyze the responses of single pile subjected to lateral soil movements in clay. In this model, the pile is modeled as a series of linear elastic beam elements and the soil is idealized using the modulus of subgrade reaction. This numerical method has been also adopted successfully to back-analyze the centrifuge model tests on a single pile in sand reported by Leung et al (2000) earlier. The approach is similar to that of two much earlier studies by Goh et al. (1997) and Poulos and Chen (1996), where free-field soil movement profiles were used as the input soil movements, except for the case of Poulos and Chen (1996), the soil is modeled as an elastic continuum.

PILES IN OPEN EXCAVATION Recent years have seen rapid development with countless high-rise structures and massive

infrastructures. As land’s availability was becoming more scarce, it is inevitable that deep foundations had become the primary choice for these structures. Recently, there are increasing number of cases where piles failed in open excavation. These conditions are likely to happen when deep excavation had to be carried out after the installation of piles. Although some countries practice excavation work before piles installation to protect the integrity of the piles, it is inevitable in some conditions where limited construction space does not permit open excavation, especially where construction of multi-level basements were involved. Excavation in soft soil will further complicate the work as excessive horizontal soil movement from the soft soil will induce additional load on the piles. There are limited resources reporting on the behaviour of these piles.

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Thasnanipan et al. (1998) presented four case histories of pile damage associated with excavation works in Bangkok soft clay. The failures were confirmed by high strain dynamic load test and also modelled by a two-dimensional finite element to predict the response of the piles.

In addition, Kok et al. (2009) presents a case study in West Malaysia for passive piles failure in very soft marine clay in open excavation. In this particular case, the thickness of the very soft marine clay ranges from 5m to 7m from the ground surface. Preliminary site investigation reported SPT ‘N’ value of zero for the very soft marine clay. Deep foundation using piles were used in design to support the superstructure. During pile cap construction, the piles were subjected to uncontrolled open excavation, inducing bending moment in the piles and resulted in some cracked and broken piles. Some of the pictures of the broken pile groups are shown in Figures 11 and 12.

Figure 11: Picture showing a 3-pile group of broken piles (Kok et al., 2009)

Figure 12: Picture showing a group of 6 pile group of broken piles (Kok et al., 2009)

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The analyses were carried out using an existing three-dimensional finite element code, PLAXIS 3D FOUNDATION with a non-linear soil model, Hardening-Soil to predict the response of these piles during excavation. The configuration of the final excavation phase generated in the analysis is shown in figure 13 below.

Figure 13: Excavation profile for final phase of staged construction (Kok et al, 2009)

Regardless of all the efforts done on passive pile behaviour adjacent to supported or unsupported excavation, there are still limited resources on pile response in an open excavation. To date, there is limited method available, especially in terms of design charts to predict the response of piles in soft clay excavation, resulting in excessive lateral soil movements which may induce bending moment which exceeds the pile’s cracking moment, resulting in broken piles.

CONCLUSION A comprehensive literature review was conducted to examine the current state of knowledge

regarding passive piles. Of all the publications compiled and reviewed for passive loading on piles, numerous efforts were found starting from over the last three decades, where major emphasis was placed on the first three groups. Various approaches from theoretical and analytical to finite element method has been adapted to predict the response of the piles, also incorporating the influence of group interaction factors.

On the contrary, significant of piles adjacent to excavation gained interest of researchers from the beginning of the last decade as field test and centrifuge test data has been presented for single and pile groups adjacent to unbraced excavation; behind stable and unstable wall. Together with this, method of analysis by using plane strain finite element method incorporated with subgrade reaction and elastic continuum formulation has been widely used. The similarities of the applicability of these methods is that a specified free-field soil movement have to be inputted in the numerical method to analyze the response of the piles. The author believes that all of these state-of-the-art method are widely accepted as it is supported by comparison with field and lab data. Figure 14 shows the summary of literature review done on passive piles.

4.10m

3.10m

9.5mBroken pile

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Although research on piles adjacent to an excavation seemed to be recent, there seemed to be a missing gap in research for piles in an open excavation. Only few cases of piles failures have been reported. It is also important to note that previous researchers have emphasied that effective and reliable prediction of the pile’s response could only be carried out only if the accurate magnitude of lateral soil movement could be known, which for most cases is a source of great uncertainty. Therefore, the author would like to highlight that a three-dimensional modelling would offer another excellent alternative to study the response of this piles in an open excavation. Conclusively, this review indirectly highlights another area of research that is significant and would contribute to the existing literature and the deep foundation industry.

Figure 14: Summary of literature review-Lateral passive loading on piles.

ACKNOWLEDGEMENT Authors wishes to record their appreciation to the Ministry of Higher Education Malaysia for

funding this research under the Fundamental Research Grant Scheme.

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