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Pressuremeter test (PMT)

General principles Types of equipment Introduction to expansion of cylindrical cavity Estimation of soil properties form pressuremeter tests

in clays Estimation of soil properties form pressuremeter tests

in sands A Hong Kong case Appendix: Analysis of pressuremeter test

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References

Mair, R.J., and Wood, D.M. 1987. Pressuremeter testing methods and interpretation. CIRIA Ground Engineering Report: In-situ Testing. Butterworths, London.

Briaud, J-L. 1992. The pressuremeter. A.A. Balkema, Rotterdam.

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Introduction to PMT

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Introduction to PMT The principal attraction of the pressuremeter test is that, in theory, the

boundary conditions are controlled and well defined, as are the stress and strain conditions in the surrounding soil mass. The basis of the test is the expansion of a long cylindrical membrane installed in the ground.Characteristics of the ground can be deduced from measurements of the pressure and the change in volume or radius of the expanding membrane.

Various pressuremeter devices are currently available. Some are installed in pre-formed holes, others are self boring, and others are pushed into the ground from the base of boreholes.

A further attraction of the self-boring type of pressure-meter is that it potentially offers the closest approach to undisturbed soil testing of any in-situ test by its ability to tunnel its way into the ground with minimal soil disturbance prior to a test being carried out.

The following parameters can be deduced from a test: Deformation modulus (i.e. compressibility) Strength

(a) undrained strength for clays or weak rocks, Cu(b) angle of shearing resistance for sands, '(c) angle of dilation for sands, in-situ total horizontal stress, ho

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1933, Kogler developed the first pre-boring pressuremeter in Germany, but did not pursue his idea.

1955, Menard developed a pre-boring pressuremeter in France and started his company.

1957, Fukuoka in Japan developed a K-value tester to obtain lateral soil moduli.

1963, Menard published the first equations and charts relating pressuremeter results to foundation settlement and bearing capacity.

1965, Jezequel in France developed the first self-boring pressuremeter at LCPC.

1982-1986, joint effort on cone pressuremeter. 1988, ASTM D4719-87 Pressuremeter testing in soils.

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General principles

The membrane is expanded against the surrounding soil by means of water, gas or oil under pressure. Outward radial deformation of the soil occurs as the membrane expands. The objectives of the test is to obtain the relationship between the applied pressure and deformation of the soil. Deformation of the soil can be obtained by monitoring the volume of fluid injected into the central part of the pressuremeter. The radial deformation of the soil is directly measured by caliper or feeler arm.

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Three distinct phases are usually evident in tests in soil. The initial curved portion (Phase I) is attributed to expansion of the membrane until it comes into full contact with the sides of the borehole (point A), also to deformation of any softened zone. Phase 2 is approximately linear until point B is reached, marking the onset of plastic behavior of the soil closest to the pressuremeter. Phase 3 continues with the zone of soil in a plastic condition increasing in radius until a limit pressure pL is approached. The meaning of the three phases is important in the interpretation of parameters from pressuremeter tests.

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Pressuremeter test (PMT)

General principles Types of equipment Introduction to expansion of cylindrical cavity Estimation of soil properties form pressuremeter tests in clays Estimation of soil properties form pressuremeter tests in sands An Hong Kong case Appendix: Analysis of pressuremeter test

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Types of equipment Maximum pressures of 2.5-10.0 MPa for use in soils; 10.0-20.0 MPa in

hard soils and weak rocks The length of the flexible part not less than 6 diameters to simulate

cylindrical cavity expansion Three major types Menard-type pressuremeters (MPM devices), for use in a pre-

formed hole. Types 1 and 2 based pressure application and measuring of responses. Self-boring pressuremeters (SBP): they are in essence miniature

tunneling machines which are steadily jacked into the ground. The soil displaced by the instrument enters the cutting head where it is broken into small pieces by a rotating cutter, then flushed to the surface. The push-in pressuremeter (PIP): normally inserted by pushing

either into an undersize pre-cored hole or into the bottom of a borehole without any pre-coring.

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Type 1 Menard pressuremeter test (After Weltman and Head 1983)

A central measurement cell filled with water to which pressure is applied by gas pressure controlled at the surface. The change in radius of the borehole during expansion of the membrane is obtained by measuring the change in volume of the water-filled central cell.

Nitrogen

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Type 2 Menard pressuremeter test (After Weltman and Head 1983)

The membrane is expanded under gas or oil pressure, and the displacement of the borehole wall is directly measured by feeler-arms or displacement transducers inside the membrane.

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Self-boring pressuremeters (SBP)-Camkometer

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The push-in pressuremeter (PIP) for offshore industry

-Expansion by oil-Limited usage

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Comparison of products

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Comparison of products

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Test procedures

ASTM D4719-00 Standard Test Method for Prebored PressuremeterTesting in Soils

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Summary1. There are two principal categories of pressuremeter test: the MPM test and the SBP test. MPM devices

are installed in pre-formed holes and SBP devices bore their own way into the ground. (A third specialist category of pressuremeter test, the PIP test, has been developed principally for offshore use.)

2. MPM tests can be performed in most soils and weak rocks, provided suitable installation methods are available. SBP tests can be performed in most soils and some very weak rocks, provided there are no hard obstructions of medium gravel size or larger. If such obstructions are present, SBP tests can sometimes still be undertaken in conjunction with a conventional boring rig. Gravels are unsuitable for any type of pressuremeter test.

3. There are two major types of pressuremeter, distinguished by their pressure application and deformation measuring systems: either pressurised liquid expands the membrane and cavity volume changes are measured (type 1), or pressurised gas expands the membrane and cavity radius changes are measured by means of displacement transducers (type 2). In some cases, pressurised oil is used to expand the membrane in type 2 devices designed for use in weak rocks.

4. Calibration of any pressuremeter is a vital part of the test procedure. The system stiffness, affecting volume change or radius measurements, is particularly import-ant for tests in stiffer soils or weak rocks. The membrane resistance is more important for tests in soft soils. Calibrations have to be undertaken regularly throughout all test series. Without proper calibration, pressuremeter tests are usually meaningless.

5. As with any in-situ test, control of installation procedure critically affects the success of any pressuremeter test: This is particularly important for pressuremeter tests, for which the installation procedure has to be carefully selected for each set of ground conditions. The methods of forming the hole (if an MPM test) and installing the pressuremeter should be chosen to minimise disturbance to the surrounding ground.

6. At the outset of any programme of pressuremeter tests on a new site, several exploratory tests should be undertaken. Different methods of hole formation should be explored in the case of MPM tests, and different cutter settings for SBP tests. The quality of any pressuremeter test can usually be judged by the corrected pressure-volume (or pressure-radius) curve, which should be plotted on site immediately following the test.

7. Any pressuremeter test programme requires the highest degree of operator skill and site supervision.

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Pressuremeter test (PMT)

General principles Types of equipment Introduction to expansion of cylindrical cavity Estimation of soil properties form pressuremeter tests in clays Estimation of soil properties form pressuremeter tests in sands A Hong Kong case Appendix: Analysis of pressuremeter test

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Pressuremeter cavity

Initial state: p0, h0, V0, 0During test: p, h, V, V = 2h

h = constant V = V - V0Cavity strain: c = ( 0)/0= circumferential strain in the wall of

the cavity, typically up to 10%.

Test result plotted: p:V or p:V

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Equilibrium equation

Circumferential strain: = y/rRadial strain: r = dy/drCavity strain: c = yc/0=

(-0)/0

0=+ rdr

d rr

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Elastic solutions

If a pressuremeter is expanded in elastic soil, it does not matter whether or not drainage is able to occur. No pore water pressure development.

The expansion of the cavity that appears to be a compressive process turns out to be entirely a shearing process. Properties deduced with reference to this analysis concern the shearing and not the compression of the surrounding soil.

0;2 === Vry

rc

rryy cc == 0

20

0 2 rG chrr

==

20

0 2 rG ch

== 0=++ zr

r

(Timoshenko and Goodier 1934)

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Shear moduli from elastic solutions

At the edge of the cavity, r = , r = p

At the start of the test, = 0, the initial shear modulus Gi is

= 00 2 ch Gp

=

cddpG

021

dVdpV

ddpG

ci 02

1 =

= Note: dV= L 20d, V0= L 02, c=d/0

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Solution in elastic-perfectly plastic soil Yielding occurs at the wall of

the cavity when

Indefinite deformation when

The undrained shear strength can be determined based on the equation

uh cp += 0

)]/(log1[0 ueuhL cGcp ++=)/(log VVcpp euL +=

Luh ppcAt + )( 0

Gibson and Anderson (1961)

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Pressuremeter test (PMT)

General principles Types of equipment Introduction to expansion of cylindrical cavity Estimation of soil properties form pressuremeter tests

in clays Estimation of soil properties form pressuremeter tests in

sands An Hong Kong case Appendix: Analysis of pressuremeter test

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Estimation of soil properties form pressuremeter tests in clays

Shear modulus In-situ total horizontal stress Undrained shear strength Coefficient of horizontal

consolidation

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Shear modulus Determination of shear moduli from

initial slope of pressuremeterexpansion curve and from slope of unload-reload cycle

where ec and V are, respectively, the cavity strain and current volume of the cavity

The initial slope of the p:c or p:Vcurves can be used to estimate an initial modulus Gi; but it is more satisfactory to determine Gi from the slope of an unloading-reloading cycle after expansion has clearly reached a plastic phase.

dVdpVGor

ddpG

c

== 21

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Initial and secant moduli

c is approximately half the shear strain in the wall of the cavity; The secant modulus of a particular cavity strain value is approximately the same as half the slope of the pressuremeter curve at that c.

G is shear-strain dependent.

cc

si ddpG

ddG = =

== )(

21;)(

0

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Example

Results of a typical SBP test in a very stiff clay at a depth of 43.3 m at Zeebrugge, Belgium (Wroth 1982). The unload-reload cycle has an amplitude of 300 kPa; the slope is twice the shear modulus, Gur = 47 MPa.

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Comparison of shear moduli for London clay

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In-situ total horizontal stress Estimation by lift-off method Applicable to SBP tests only When the pressure in the

cavity increases from zero and exceeds ho, the cavity is able to expand and lift off occurs. Lift-off is detected at the

point of departure of the p:c data from the initial linear relationship, which results from the compliance of the strain arm system.

6 m in Gault clay, Dalton and Hawkins 1982)

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In-situ total horizontal stress Estimation by graphic iteration

relationship between the reference cavity pressure and the in-situ total horizontal stress

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Undrained shear strength from the slope of the p:logeV/Vcurve

For a pressuremeter being expanded in an impermeable soil which does not change in volumeduring the test, the local slope of the p:logeV/V curve gives an indication of the shear stress at the wall of the cavity:

= dp / d[logeV/V]

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Undrained shear strength from limit pressure

The limit pressure of a cylindrical cavity in elastic-perfectly plastic soil is

when the cavity expands indefinitely, V/V =1. cu then is

Np can be correlated with G/cu; plcan be determined by extrapolation to volume change = volume of pressuremeter.

)]/(log1[0 ueuhL cGcp ++=

)]/(log1/[)( 0 uehLu cGpc +=)/(log1;/)( 0 uepphLu cGNNpcOr +==

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Comparison of Cu

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Summary for interpretation in clays1. Most of the experience to date with pressuremeter testing in soils has been

with clays. In a number of well-documented cases, it has been possible to compare the parameters deduced from pressuremeter tests with those obtained by other in-situ tests and laboratory tests. The greatest potential for pressuremeter testing in clays lies in the measurement of in-situ horizontal stress and modulus.

2. In-situ horizontal stress can be more reliably measured with the SBP device.3. Modulus is best measured in an unload-reload cycle, which may be

performed in any type of pressuremeter test. It is particularly important to perform unload-reload cycles in MPM or PIP tests, because the initial modulus is seriously affected by disturbance on installation.

4. Undrained shear strength obtained from any type of pressuremeter test tends to be significantly larger than values obtained by other means, and should therefore be viewed with caution when applied to existing design methods.

5. Other design parameters, such as coefficient of consolidation and effective stress parameters, can, in principle, be obtained from certain pressuremetertests in clay, but there is to date very little experience of these.

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Pressuremeter test (PMT)

General principles Types of equipment Introduction to expansion of cylindrical cavity Estimation of soil properties form pressuremeter tests in clays Estimation of soil properties form pressuremeter tests

in sands A Hong Kong case Appendix: Analysis of pressuremeter test

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Estimation of soil properties from presuremeter tests in sands

PMTs can be used to estimate values of shear modulus, in-situ total horizontal stress, and peak angles of friction and dilation for sand deposits. To date, analysis of PMT tests in sands is not so well developed as analysis of tests in clays.

Shear modulus and friction angle are studied in this section.

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Shear modulus

dVdpVGor

ddpG

c

== 21

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Angle of shearing resistance and dilation

Hughes et al. 1977 (pages 143-147 in Appendix) Failure initiated at the cavity wall

- dilaion anglecv critical state friction angle peak friction anglec intercept, not cohesion

'sin)1(1'sin

cvss

+='sin)1(sin cvss +=

Acsup c ++= )2/ln()ln( 0)'sin1()( ' 00 += hup

h02h0-p p

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Angle of shearing resistance and dilation

There are two possible approaches to determine from MPTs Estimate the in-situ vertical stress and the maximum likely

value of the ratio of horizontal to vertical effective stresses,K0. Take as reference state the point at which this estimated in-situ stress condition is regained. Choose the reference state so that the plot of log (p-u0):

log c is straight over as large a portion of the test as possible. Once an acceptable curve has been achieved in a plot of log

(p-u0): log c, the slope of its latter linear portion, s, can be measured and values of and the angle of dilation, , determined from a chart.

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Approach 1

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Approach 2

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Relations among , , cv and sExample

s = 0.425Assume cv = 35Find = 39and = 5.5

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SummaryObtaining representative samples of sands is extremely difficult. Values of ' determined from triaxial or shear box tests on reconstituted disturbed material are generally conservative for very dense deposits, because it is very difficult to reproduce the same density in the laboratory. Laboratory tests are not suitable for estimating either in-situ horizontal stress or in-situ elastic modulus. In-situ tests are therefore essential to obtain these parameters. Elastic modulus can be estimated from seismic tests, and from empirical relationships from CPT or SPT data, but no in-situ test other than a SBP test has the potential to provide reasonable estimates of in-situ horizontal stress. Values of ' can be empirically correlated with CPT or SPT data, but pressuremeter tests (particularly SBP tests) are a promising alternative in-situ test for sand deposits, because elastic modulus, in-situhorizontal stress, and ' can all be estimated. However, at present the experience is very limited in comparison with pressuremeter testing of clays and weak rocks.

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An Hong Kong caseby Fugro Geotechnical Services (HK) Ltd

2004

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