Pressuremeter Test

CHAPTER 2

15

PRESSUREMETERS IN GEOTECHNICAL ENGINEERING

2.1 INTRODUCTION

Geotechnical engineering relies, in most of the cases, on empirical observations and

experience. One of the main reasons for relying upon observed results is that there are

practical restrictions involved in extracting quality soil samples and performing

laboratory testing. Another reason for the continued reliance on the observed results is the

inability to obtain exact and repeatable strength properties of soil and rock at a reasonable

cost. In order to solve these issues, geotechnical engineers have been trying to develop the

in-situ testing procedures. The main limitation to the in-situ testing is the selection of an

appropriate device that can provide reliable engineering parameters with least disturbance

to soil. Over the last half century, a lot of research has been carried out towards

developing new in-situ soil testing devices and data analysis procedures around the world.

These devices include cone penetrometers, pressuremeters, dilatometers etc. have been

developed with the aim to obtain better quality soil parameters.

This chapter presents state-of-the-art literature available on the various types of

pressuremeters, data analyses, interpretation and applications in geotechnical engineering.

2.2 DEFINITION OF A PRESSUREMETER A pressuremeter is basically a cylindrical probe with an expandable flexible membrane. A

uniform hydraulic or gas pressure can be applied to the walls of the test pocket through

the membrane to get the ground response curve, viz., applied pressure versus expansion

of the membrane. The ground response curve can be interpreted to obtain fundamental

soil properties and design parameters, such as strength, stiffness and in-situ horizontal

stress. Figure 2.1 describes the definition of a pressuremeter. The probe is lowered to the

test depth with drill or cone rods. The test section of the probe is connected with a cable

to the control unit at the surface. This control unit includes a pressure supply regulator

and a data logger. The pressuremeter test is the only test that can give directly values of

in-situ horizontal stress, stiffness and strength (Clarke, 1996).

CHAPTER 2 PRESSUREMETERS IN GEOTECHNICAL ENGINEERING

16

Data logger

Test

mod

ule

Prob

e

Drill rod

Borehole

Control cable/hose

Test control

Test pocket

Test pocket samediameter as probe

Self-boring head

Probe drilled into test pocket

Figure 2.1 The definition of a pressuremeter

Figure 2.3 Self-boring pressuremeter

(a)

(b)

Guard cell

Test section

Guard cell

Test section

Test pocketlarger diameterthan probe

Figure 2.2 Types of prebored pressuremeters(a) a tricell probe (b) a monocell

Probe pushed into test pocket

Probe lowered into test pocket

Friction reducer

External cone for full displacement

Test pocketlarger diameterthan probe dueto friction reducer

Figure 2.4 Full-displacement pressuremeter


17

2.3 HISTORY OF THE PRESSUREMETER

The roots of the pressuremeter extend back to Kogler, who developed the first preboring

pressuremeter in Germany in 1933. Owing to the limitations of the available materials,

Kogler could achieve only partial success. However, Louis Ménard, a French engineer,

made significant advances in the pressuremeter device, its analysis and its acceptance.

Therefore, Ménard is referred to as the father of the pressuremeters. The pressuremeter,

he developed was patented in 1955. Since then there have been many advances; not only

in the development of the equipment but also in the analysis of results, as described in the

following articles.

2.4 TYPES OF PRESSUREMETERS Pressuremeter probes are grouped on the bases of their method of installation (pre bored,

self-bored or pushed in) and method of measuring displacement (volume or radial). Based

on their method of installation, pressuremeters can be divided into following different

types:

(i) Pre-bored pressuremeter (PBPM)

This pressuremeter is lowered into the pre-bored test pocket and test is performed. This

PMT requires test pocket diameter slightly greater than the diameter of the probe. If a

stable test pocket can be drilled, any ground condition can be tested using this

pressuremeter. PBPMs are the most common group of pressuremeters and their versions

have been in commercial use since 1958.

The expansion of the membrane can be monitored at the surface using either

displacement transducer(s) (radial displacement type) or volume change transducers

(volume displacement type).

The original Ménard pressuremeter is a volume displacement type tricell probe (Figure

2.2a). The central cell is allowed to expand while the outer cells, also called guard cells,

are provided to ensure the true cylindrical expansion of the central cell. In the 1950’s, the

OYO Corporation of Japan developed a single-cell or monocell in which expansion was

measured through displacement transducers (Figure 2.2b).


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(ii) Self-boring pressuremeter (SBPM)

A self-boring pressuremeter (SPBM) creates its own test pocket in the ground. The probe

of this pressuremeter is headed by a sharp rotating cutter shoe, which removes enough

soil to accommodate the volume of the pressuremeter. During boring the instrument is

pushed slowly into the ground. The pushing is halted when the centre of the membrane

reaches the test level. These probes can be used in any soil provided the gravel content is

small, and in some weak rocks (Clarke, 1996).

The SBPMs were developed both in France (Jézéquel et al., 1968) and in the UK (Wroth

& Hughes, 1973). Both probes have a single-cell but use different expansion

measurement systems. The French SBPM has a volume measuring system, while the UK

probe uses displacement transducers. Figure 2.3 describes the principle of this SBPM.

(iii) Pushed-in pressuremeter (PIPM)

This pressuremeter (PIPM) is pushed into a soil, thus creating a disturbed zone around the

pocket during installation and can only be used in soils in which it is possible to push a

static-cone penetrometer. Reid et al. (1982) developed the first PIPM developed primarily

for offshore use. In 1988, Huang et al. developed another PIPM for on land use. In both

cases, a thick walled tube is pushed into the soil. The displaced soil enters the tube and is

withdrawn with it after the test. The membrane is inflated by oil and the measurement of

pressure and the volumetric strains are made.

(iv) Full-displacement pressuremeter (FDPM)

The idea of mounting a pressuremeter probe behind a cone penetrometer to enable tests to

be performed as a part of the cone penetration test operations was initiated in the early

1980’s in both France and Canada. However, these devices being large in diameter

required special equipment for their installation and were superceded by devices of

smaller diameter. The membrane is protected by a Chinese lantern. A friction reducer is

used to reduce the shear between the soil and the probe. This implies that at the start of a

test the membrane may not be in contact with the ground.

The first pressuremeter of this type was developed by Withers et al. (1986). It is headed

by a 15 cm2 solid cone, which is pushed into place by displacing the ground, as shown in

Figure 2.4. It measures the inflation pressure and the circumferential strain at three


19

locations 120° apart at the centre of the membrane. The instrument is inflated by Nitrogen

gas or oil when used for offshore operations (Withers et al. 1986). The latest Fugro device

(Zuidberg & Post, 1995) has been further simplified for easier assembly and better control

of the tests but the basic design has remained unchanged.

Based on the method of installation, the FDPM can be grouped with pushed-in

pressuremeters.

Apart from the Ménard pressuremeter, modern pressuremeters are single-cell and have

either a volume or radial displacement measuring system. Table 2.1 presents details of

commercially available pressuremeters in the UK indicating the group to which they

belong, their displacement measuring system and the ground in which they can operate.

2.5 SELECTION OF A PRESSUREMETER The selection of a pressuremeter mainly depends on the condition of ground under

investigation and the required design parameters. Figure 2.5 provides guiding principles

for selecting a suitable type of pressuremeter based on these factors.

2.6 MERITS AND LIMITATIONS OF PRESSUREMETERS Every equipment has certain merits and limitations when used for the determination of

the desired parameters. Likewise pressuremeters also have some merits and limitations.

Some of the merits and limitations are given below (Akbar, 2001):

Merits

• Any ground condition can be tested provided an appropriate type of pressuremeter is

used.

• The similarity between the expansion of the pressuremeter membrane and that of a

cylindrical cavity allows the use of available close form solutions for the determination

of pressuremeter parameters.

• The basic soil parameters, such as shear modulus, non-linear stiffness profile, total

horizontal stress and undrained shear strength for clays or angle of internal friction for

sands, can be determined by pressuremeters.

• The pressuremeter can also provide design parameters directly (Ménard test).


20

Table 2.1 Some details of commercially available pressuremeters in the UK (From Clarke, 1996)

Group Name Pressure capacity

MPa

Strain capacity

%

Diameter mm

Total length

m

Expanding length mm

L/D Displacement measurement

system

Ground conditions

Prebored Ménard pressuremeter GC

4 53 74 6.5 volume All soils

Ménard pressuremeter GB

20 53 74 6.5 volume Weak to moderately strong rocks

Oyo Elastometer 100 10 12 66 520 7.4 one diameter Stiff clays, dense sands and weak rocks

Oyo Elastometer 200 20 66 520 7.4 one diameter Weak to moderately strong rocks

High pressure dilatometer

20 25 73 1.5 455 6.1 three diameters Weak to moderately strong rocks

Self-bored Cambridge self-boring pressuremeter

4.5 15 84 1 500 6 three radii All soil containing little or no gravel

Weak rock self-boring pressuremeter

20 10 73 1 400 5.5 three radii Hard clays, very dense sands and weak rocks

Pushed-in Cone pressuremeter 4.5 50 44 1 450 10.3 three radii All soil amenable to static cone penetrometer testing


21

Probe

Pressure capacity

Ground conditions

Test type

Parameters

Figure 2.5 Guidelines for the selection of a pressuremeter (From Clarke, 1996)

Pressuremeter

Pre-bored Self-bored Pushed-in

low to medium high highlow to medium low to medium

all soils rock weak rocksoils containing little or no

soils in which a cone can be pushed

Ménard stress/strain stress/strain stress/strain

design parameters

Em, pL

σh by iteration

from loading

strength from limit pressure and unloading

curve

stress-strain curve

strength from loading curve

σh directly

Gur from unload/reload

cycle

strength from unloading curve and

correlations

σh from correlations

and unloading curv

CHAPTER 2 RESSUREMETERS IN GEOTECHNICAL ENGINEERING

22

• These devices can be used as ground profiling tools, as a control test for ground

improvement and to calibrate other devices.

Limitations

• The pre-boring technique influences the soil properties adjacent to the test pocket. This

technique may not be used in soils where the chances of borehole collapse are present.

Repeatable results are difficult to produce using the PBPM. Careful control of the

procedure can produce repeatable results; however, application of the data is usually

empirically based (Withers et al., 1986).

• The SBPM was developed with the aim to minimise ground disturbance. This instrument

provides soil properties of high quality; however, a lot of skill and experience is needed

for the operation of the SBPM. Moreover SBPMs cannot be used in difficult ground

conditions such as soils containing gravels and hard rocks.

• The cross-sectional area of the PIPMs requires a large reaction force for pushing in

cemented soils, hard clays and dense sands. The design of the tube of the PIPMs does not

allow their use in all types of soils and the boundary conditions at the start of the test can

vary (Withers et al., 1986).

• The FDPM is a relatively small pressuremeter, produces repeatable disturbance and is

operator independent. Owing to same diameters (43.7 mm) of the pressuremeter and the

cone, the rubber membrane experiences ground friction during pushing of instrument into

the ground. This friction force between the membrane and the ground can damage the

membrane or can pull the membrane out of the contracting ring. The membrane therefore

requires some sort of protection such as Chinese lantern.

2.7 THE SUITABILITY OF PRESSUREMETERS TO GROUND CONDITIONS Table 2.2 gives a summary of the suitability of pressuremeters in various ground conditions.

PBPMs can be used in any type of soil or rock in which the borehole remains stable with or

without mud. SBPMs are applicable in grounds having little or no gravels and in weak rocks.

PIPMs and FDPMs can be used in grounds where it is possible to push a cone. It means

dense sands, hard clays, gravely soils and rocks are not suitable for cone pressuremeters.

Since the probe supports the test pocket wall during installation, the test pocket wall stability


23

is not critical in case of SBPMs and FDPMs. However, care is required for ensuring the

stability of a borehole.

Table 2.2 The applicability of pressuremeters to ground conditions (From Clarke, 1995)

Ground type PBPM SBPM PIPM

Soft clays A A A

Stiff clays A A A

Loose sands B with support A A

Dense sands B with support B C

Gravels C by driving N N

Weak rock A B N

Strong rock A N N

A: very good; B: good; C: moderate; N: not possible

2.8 CALIBRATIONS In order to convert pressuremeter test data to applied pressure and strain and to correct for

system or membrane compliance, different types of calibrations are carried out. Table 2.3

presents the recommended frequency and relevance of the calibrations for the three groups of

pressuremeters (Clarke, 1996).

The calibrations of displacement and pressure transducers are carried out in laboratory by

comparing their voltage output with measured values of displacement and pressure

respectively. All radial displacement type probes contain transducers. Any hysteresis and

non-linearity for the transducers is checked during calibration to establish a confidence level

on the accuracy of the test results.

The membrane stiffness is obtained by inflating the probe in air. The pressure required to

inflate a membrane in air is referred to as the membrane stiffness. The membrane stiffness is

deducted from the applied pressure during a test in order to obtain the correct pressure at the

membrane/soil interface. This calibration applies to all pressuremeter tests and is carried out

whenever a membrane is replaced.


24

Table 2.3 Frequency and relevance of calibrations (From Clarke, 1996)

Calibration Pre-bored Self-bored and

Frequency Additional calibrations Relevance

Volume Radial pushed-in Soft ground Max. pres. 1 MPa

Most ground Max. pres. 4 MPa

Strong ground Max. pres. 20 MPa

Transducers if fitted Yes yes 1. start and finish of project 2. regular intervals during

project.

1. any change in the transducers 2. change to the lead connecting the

probe to the surface 3. following damage to probe which

involves thorough cleaning and drying.

critical critical critical

System compliance

yes No no 1. start and finish of project 2. regular intervals during

project.

1. change to the lead connecting the probe to the surface.

2. change to the control unit.

important important critical

Membrane stiffness

yes Yes yes 1. start and finish of project 2. regular intervals during

project 3. every time a membrane is

replaced.

critical important not important

Membrane compression

yes Yes yes 1. start and finish of project 2. regular intervals during

project. 3. every time a membrane is

replaced.

unnecessary not important critical

Membrane thining

no Yes yes not important not important critical

Zero yes Yes yes 1. prior to lowering into borehole

critical critical critical


25

Membrane compression is a change in the thickness of a membrane due to pressure and as it

expands. The changes in both cases are small and are only significant for tests in rocks using

displacement type probes. Calibration for the membrane thinning is necessary (in rocks) as

the transducers measure the movement of the inner surface of the membrane, which is

different from that of the outer surface.

Calibrations for the system compliance, membrane compression and thinning for volume

displacement type probes are carried out by pressurizing the probe in a metal cylinder. It

represents the volume change that occurs in the supply line (hose), testing equipment and

probe as the pressure is increased.

2.9 INSTALLATION Installation is the process by which a test pocket is created in order to install a pressuremeter

probe and perform the test. Installation technique can considerably affect the shape of a test

curve and the quality of the interpreted design parameters. Undoubtedly, the installation

technique should be consistent and should be designed in such a manner that either it creates

minimum disturbance or produces repeatable disturbance to the surrounding ground. The

installation techniques for different types of pressuremeters are discussed below:

a) Installation technique for PBPM

A pocket for a PBPM can be created using different techniques but the best technique is that

which removes all material and minimizes disturbance to the pocket wall (Clarke, 1996). The

borehole should be designed according to the size of the pressuremeter to be employed. The

design criteria include pocket size, minimum disturbance during drilling and removal of drill

rods and drill bit. The probe diameter has to be smaller than the pocket diameter to enable it

to be lowered into place (Clarke, 1995). As a guide, the ratio of the diameter of the pocket to

that of the probe must not be greater than 1.10 (Mair and Wood, 1987). American Society for

Testing and Materials (ASTM) recommends the ratio between 1.03 and 1.20 (Briaud and

Gambin, 1984). The borehole can be drilled using either shell and auger techniques or rotary

techniques (Clarke, 1996). It is recommended that the probe should be lowered down the hole

within 15 minutes of completing the drilling (Mair and Wood, 1987 so that conditions remain

undrained.


26

b) Installation technique for SBPM

An SBPM pocket is drilled in soft to stiff clay using an internal bit driven by rotating inner

rods. Mud is pumped down the inner rods to flush the cuttings back through the annulus

between the inner and outer rods to the surface where they are collected in a settling tank. The

mud pressure, speed of advance and cutter position are adjusted by the operator to ensure that

the SBPM replaces the ground as the probe advances. Although as the stiffness of the clay

increases it becomes more economical to use a separate rig to advance the borehole between

test locations. A separate rig is always recommended when using an SBPM in sands due to

potential borehole collapse (Clarke, 1996).

c) Installation technique for FDPM

A FDPM is pushed into the ground using a cone truck or a pushing rig or a jack. The ground

reaction is achieved by placing ground anchors into the ground for a pushing rig or a purpose

built reaction frame for the jack. The reaction force can also be obtained by placing the dead

weight on the purpose built reaction frame. The FDPM probe is fixed with the pushing rig or

the jack to push it into the soil up to the desired test level. A FDPM can be used in any soil in

which it is possible to push a static cone penetrometer without damaging the cone.

Some of the points, which should be kept in mind alongwith the previous discussion on

installation, are briefly described below:

• The minimum test spacing in a borehole should be one meter to ensure that no

disturbance is produced to the ground by the previous test above.

• It is common to specify tests at particular horizons to obtain a profile of ground

parameters. The centre of the expanding section should be at least half a metre plus half

the length of the expanding section below the base of the borehole to minimize the

effects of the borehole on a test curve.

• The standard pushing rate for a FDPM is 2 cm/s using a standard cone truck.

2.10 TEST PROCEDURE A test is defined as the application of pressure to a pocket wall. A pressuremeter test can be

performed as either


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• Stress-controlled test or

• Strain-controlled test

A true stress-controlled test is one in which a pressure increment is maintained for 60 to 120

seconds (Clarke, 1996). All pressuremeter tests are stress-controlled, but it is possible to

expand a membrane at an almost constant strain rate with small increments of pressure. This

is the procedure mostly employed when testing with an SBPM or a FDPM. The rate of strain

is 1%/min. and 5%/min. for the SBPM and the FDPM respectively (Clarke, 1996). The tests,

however, remain stress-controlled prior to the on-set of yield, unloading to yield in extension

and during an unload-reload cycle. This permits a better control at lift-off and prevents creep

effects prior to unload-reload and final unloading (Howie et al., 1990; Clarke, 1996). Tests in

rocks and tests using volume displacement type pressuremeter probes tend to be stress-

controlled.

The importance of the rate of strain can not be denied during a strain-controlled test as it

influences the test results. The undrained shear strength, su increases about 15% for every ten

times increase of strain rate for pressuremeter strain path (Penumadu et al. 1988). A high

strain rate test can affect the clay strength due to its viscous behaviour, whereas a too slow

test can result in partial drainage. The test procedure to be adopted is necessarily a

compromise (Houlsby & Withers, 1988).

Ménard test is a special stress controlled test used to obtain design properties directly from

volume displacement tricell pre-bored probes. The probe is lowered into the prebored pocket

and expanded in about ten to fourteen equal stress increments until the volume of the pocket

is approximately doubled in size. Each increment is maintained for one minute with readings

of volume being recorded at 15 s, 30 s and 1 min after applying the increment.

Shear modulus is determined from the unload-reload cycle by restricting it within a stress

range so that failure in extension does not occur. For clay, this stress range will typically be

equal to its undrained shear strength; while for sand it will be around 40% of the effective

pressure at the start of unloading. An unload-reload cycle can be excluded from a test if

strength is the only parameter required, as it is determined from the latter part of the loading

curve or from the unloading curve.


28

A pressuremeter test is continued to the maximum average cavity strain required which

should be less than the strain capacity of the probe. However, a test can be stopped earlier for

one of the following reasons (Clarke, 1996):

(a) The maximum pressure capacity of the probe is insufficient to yield the ground. This is

frequent when rocks are tested.

(b) The membrane bursts because of damage caused either during installation, during a test

by discontinuities in the ground, or by expansion up into the annulus between the

pocket wall and drill rods.

(c) The oil capacity runs out because of non-uniform expansion.

(d) The test pocket created is too large.

2.11 INTERPRETATION OF PRESSUREMETER DATA The analyses of the pressuremeter test data can be carried out to determine three main soil

parameters: in-situ total horizontal stress, stiffness or shear modulus and strength. Table 2.4

presents different methods for the determination of these parameters. These methods are

discussed below.

2.11.1 Horizontal Stress, σho Horizontal stress is defined as the in-situ geostatic stress. Its determination is subjective since

it depends on selecting a single point on a test curve. Several methods are available to

identify that point but all these techniques are based on assumptions. Figure 2.6 gives the

position of the horizontal stress for all the three types of pressuremeters. The available

methods include:

(a) The lift-off method.

(b) Methods based on shear strength.

(c) Methods based on test procedure.

(d) Fitting functions to the test curve.

(e) Empirical correlations with other data.


29

The detailed description of each method can be seen in Clarke (1995). Since ground

disturbance is least only in case of SBPM, the horizontal stress can be determined directly

from this test. For PBPM and FDPM probes, σho can be determined using the above-

mentioned methods.

Figure 2.6 Typical curves and positions of the reference datum, ao for the three types of pressuremeters (After Clarke, 1995)

Houlsby et al. (1988) have proposed a correlation presented in Table 2.4 to estimate σho for

clays using FDPM probe. However, they are not sure about the reliability of values

estimated from the method.

Yu et al. (1996) proposed a technique to estimate σ′ho in normally consolidated sands using

Jaky’s expression for the coefficient of earth pressure at rest: Ko = 1-sinφps. Table 2.4 gives

an equation to evaluate angle of internal friction, which may be used to determine Ko. Using

this Ko, the effective horizontal stress (σh) can be estimated using the basic correlation σh = Ko

σv , where σv is the vertical effective stress.

Displacement or volumetric strain

PBPM test

SBPM test

FDPM test

σhpo

σh

App

lied

pres

sure

PB

PM

ao

SB

PM

ao

FDP

M a

o


30

Table 2.4 Commonly used methods to interpret pressuremeter tests (From Clarke, 1996)

Soil type Parameter Probe Method Reference Equation All soils and

rocks Gur or Gu or Gr All probes Unload-reload cycle 0.5 (dp/dεc)

Clay σh PBPM Curve fitting Marsland and Randolph (1977) σh SBPM Directly from curve

Logarithmic model Gupta (2000)

pL = σh+su+suln[4I2

r/(4Ir-1)] σh FDPM Unloading curve Houlsby and Withers (1988) pL − su[1+ln(G/su)] Gu or Gr v εc All probes Unload-reload cycle Muir Wood (1990) su PBPM Modified limit pressure Amar et al. (1975) (plm−σh)/(5.5 to 15) su SBPM Latter part of loading curve Windle and Wroth (1977) (p−σh) = su[1+ln(G/su)+ln(∆V/V)] su FDPM Unloading curve Houlsby and Withers (1988) p = pL−2su[1+ln{sinh(εmax−ε)/sinh(su/G)}] Sand σh PBPM,

FDPM Not recommended

Ghionna et al. (1995) Nutt (1993)

Ko = [a (pL-uo)/σ 'vo] [(1+2Ko)/3]b/[(qc-uo)/ σ '

vo]b

(pL−σh)/σ'h = 1.98 + 19.1Dr σh SBPM Directly from curve Gu or Gr v εc All probes Unload-reload cycle Bellotti et al. (1989) φ' PBPM Not recommended Ménard plm = b 2(φ'−24)/4 b = 1.8 for wet and 3.5 for dry

sand and 2.5 on average φ' SBPM Latter part of loading curve Hughes et al. (1977) sin'φ = s/[1+(s-1)sin'φcv] φ' FDPM Unloading curve

Schnaid & Houlsby (1992)

Houlsby & Nutt (1993) Ghionna et al. (1995)

Yu et al. (1996)

φ' = 1.45[(qc−σh)/( σ'h)]+26.5 Dr = 1/3 [(qc−σh)/(σ'h)]+10 (pL−σh)/σ'h = 2.21 + 19.35Dr (qc−σh)/(pL−σh) = 3.80 + 9.84Dr φ'tc = 33.4 – 50.78ξo

φps = [14.7/ln(G/σ'vo)]*(q'c/pL) +22.7 ψ SBPM Latter part of loading curve Hughes et al. (1977) sinψ = s+(s−1)sinφ'cv For definition of symbols, refer to the list of symbols.


31

2.11.2 Shear Modulus, G The pressuremeter is an ideal test for determining the ground stiffness (Clarke, 1996). The

shear modulus is best obtained from any pressuremeter test curve where the soil behaves

elastically. Although the initial part of the pressuremeter test curve shows elastic behaviour

but it depicts the stiffness of the disturbed ground because of disturbance during the probe

installation. The best elastic behaviour in a pressuremeter test curve is that of the unload-

reload cycle if the stress range is insufficient to cause yield in extension (Clarke, 1995). Two

techniques are available to determine the average shear modulus from the unload-reload

cycle. Firstly, the average secant stiffness is estimated by fitting a line to all the data

comprising the unload-reload cycle, as shown in Figure 2.7a. The shear modulus is half of the

slope of the best-fit line. This modulus represents the average stiffness of elements of ground

radiating from the probe at the effective stress at the point of unloading, (Clarke, 1997). This

method can be used for any type of pressuremeter in any ground condition (Clarke, 1995).

The shear modulus may also be determined from the slope of single line drawn between the

two apexes of the loop (Houlsby & Schnaid, 1994). However, as per their observation this

method overestimates the stiffness values in the tune of 20-30% in comparison to the method

when all data points between the two apexes are considered.

A more pertinent shear modulus value can be obtained by selecting the data over a specified

strain range. A cavity strain range between 0.1% and 0.2% should be selected as this range

represents typical average strains in the ground beneath or adjacent to a structure (Clarke,

1996). The slope is determined from either the unloading part of the cycle, selecting the

maximum cavity strain as the origin or from the reloading portion with minimum cavity

strain as the origin (Figure 2.7a). The reloading portion estimates more consistent values

(Clarke, 1993).

Gupta (2000) has described a procedure to determine moduli at various percentages of yield

strength for the SBPM. This practice is often used in the interpretation of laboratory tests as

well but Houlsby & Nutt (1993) do not consider this practice appropriate for the

interpretation of pressuremeter tests.


32

Figure 2.7 The selection of shear moduli from an unload-reload cycle showing (a) unload and reload moduli and (b) the non-linear stiffness profile (After Akbar, 2001)

In clays, the effective stress does not change once yield occurs but, during an unload/ reload

cycle modulus varies with the strain. In the case of sands, with expansion of membrane the

effective stress increases resulting in an increase in shear modulus. In both cases, a non-linear

stiffness profile can be obtained from an unload-reload cycle simply by taking a secant using

the maximum or minimum cavity strain as an origin (Figure 2.7b).

2.11.3 Undrained Shear Strength, su Different methods are available for the determination of undrained shear strength of clays

using different pressuremeters. These methods do not provide same strength (Wroth, 1984).

For SBPM, the post peak strength is obtained from the latter part of the loading curve

considering soil as a perfectly plastic model as shown in Figure 2.8. For PBPM, the shear

strength is taken as a factor of the limit pressure, pL, the pressure required to double the cavity

Unload-reloadcycle

Non-linear profile

∆εc

GuorGr

Cavity strain

App

lied

pres

sure

Gu

Gr Maximum cavitystrain

Start of unloading

Minimum cavitystrain

Gur

Definition of shear modulus from an unload-reload cycle

Gur - secant modulus from whole cycle

Gu - secant unload modulus measured from maximum cavity strain, εcmax, in the cycle

Gr - secant reload modulus measured from minimum cavity strain, εcmin, in the cycle

Gu0.2% - secant unload modulus measured over εcmax and (εcmax-0.2%)

Gr0.2% - secant reload modulus measured over εcmin and (εcmin+0.2%) (a) (b)


33

size (Table 2.4). A summary of correlations developed by various researchers between pL and

su, for PBPM are given in Table 2.5.

Figure 2.8 Derivation of undrained shear strength from a SBPM test in clay assuming linear elastic perfectly plastic clay (From Clarke, 1997)

Houlsby et al. (1988) outlined a technique for the determination of the undrained shear

strength from the unloading part of the FDPM curve as shown in Figure 2.9. This method

simultaneously provides shear modulus, G and in-situ horizontal stress, σho; though

determining σho by this method is not reliable (Houlsby et al., 1988). As per the procedure,

the cavity expansion pressures are plotted against {-ln[{(m+1)/2}(εm-ε)]} values for the final

unloading part of the curve. Value of m is taken equal to 1 for cylindrical expansion and 2

for spherical expansion, ε = the cavity strain at any pressure and εm = the maximum strain

reached in that test. σho is determined by identifying the mid point between the limit pressure

(pL) and the intercept of the trend line over the plastic unloading part. The slope of the line is

2su (2+m)/3, which reduces to 2su for cylindrical expansion. The abscissa of the intersection

of the limit pressure and the plastic unloading line is 1+ln (Ir), where Ir is the rigidity index (=

G/su). So with known values of 1+ln (Ir) and su, G can be determined. The modulus of

elasticity, E and the Poisson’s ratio, υ are related to shear modulus by:

( )ν+=

12EG (2.1)

If υ is known, E can be determined.

0500

1000

1500

2000

2500

3000

3500

0 2 4 6 8 1 0 1 2 1 4 -2.8 -2.6

Pressure curve

Cavity strain, %

App

lied

pres

sure

, kP

a

-2.4 -2.2 -2 -1.8 -1.6 -1.42700

2750

2800

2850

2900

2950

3000

ln (volumetric strain)A

pplie

d pr

essu

re, k

Pa

Best-fit straight line to data replotted as pressure vs. ∆V/V = su


34

Table 2.5 Empirical relations between undrained shear strength and net limit pressure (After Clarke, 1995)

su Clay type Reference

(plm – σh)/k k= 2 to 5 Ménard (1975)

(plm – σh)/5.5 Soft to firm clays

(plm – σh)/8 Firm to stiff clays

(plm – σh)/15 Stiff to very stiff clays

Cassan (1972), Amar and Jézéquel (1972)

(plm – σh)/6.8 Stiff clays Marsland and Randolph (1977)

(plm – σh)/5.1 All clays Lukas and LeClerc de Bussy (1972)

(plm – σh)/10+25 Amar and Jézéquel (1972)

(plm – σh)/10 Stiff clays Martin and Drahos (1986)

plm /10+25 Soft to stiff clays Johnson (1986)

Figure 2.9 Derivation of shear strength, shear modulus and horizontal stress from FDPM curve by Houlsby and Withers (1988) method

2.11.4 Angle of Shearing Resistance, φ'

Hughes et al. (1977) provides a method for the determination of the peak angle of shearing

resistance for dense sands using the latter part of an SBPM test curve. The procedure is

presented in Figure 2.10. Initially the slope of the latter part, s, of the curve is determined and

value of the angle of shearing resistance at constant volume, φ'cv, is selected from the Table

2.6 for the soil type. Then the angle of shearing resistance is evaluated using the relationship

given in Table 2.4. As φ'cv is not critical in determining φ', Clarke (1996) recommends a

value of 35° to be used. Angle of dilation can also be determined using the relation given in

Table 2.4 using the values of s and φ'cv.

1+ln(Ir)

Limit pressure

Plastic unloading

2su

-ln[1.5( εmax - ε)]

App

lied

pres

sure

FDPM curve σho

Cavity strain

App

lied

pres

sure

-ln[{(m+1)/2}(εm-ε)]


35

It is not possible to determine the angle of shearing resistance directly from PBPM and

FDPM tests due to large installation ground disturbance. Mair & Wood (1987) recommend

that PBPM test data should not be used to evaluate φ', however, a correlation proposed by

Ménard for the purpose is presented in Table 2.4.

Figure 2.10 The determination of angle of shearing resistance from SBPM tests in sand (After Clarke, 1997)

Table 2.6 Typical values of φ'cv (after Robertson and Hughes, 1986)

Soil type φ'cv (Degrees)

Well-graded gravel-sand-silt 40

Uniform coarse sand 37

Well-graded medium sand 37

Uniform medium sand 34

Well-graded fine sand 34

Uniform fine sand 30

App

lied

pres

sure

, kP

a

Best-fit straight line to data replotted as ln(pressure) vs. ln(εc)

Slope is functionof angle of shearingresistance

ln (current strain)Cavity strain

0

1000

2000

3000

4000

5000 8.5

8.4

8.3

8.2

8.1

8.0

7.9

-3.6 -3.4 -3.2 -3.0 -2.8 -2.6 -2.40 2 4 6 8 10

ln (

effe

ctiv

e ap

plie

d pr

essu

re)


36

For FDPM, Houlsby and Nutt (1993) proposed two equations presented in Table 2.4, which

relate relative density, limit pressure and cone resistance. These equations require that there is

no excess pore pressure and ambient pore pressure is known. The relative density can be

determined using these equations if limit pressure and cone resistance are known from in-situ

testing. The angle of shearing resistance can be determined using correlation proposed by

Bowles (1996) between φ' and Dr, given as:

rD1528 +=′φ (in degrees) (2.2)

Yu et al. (1996) proposed a technique, presented in Table 2.4, for the determination of the

angle of internal friction of sands from the FDPM data. This technique requires the values of

the cone tip resistance, the pressuremeter limit pressure and the shear modulus. The shear

modulus to be used in the equation can be estimated from an unload-reload cycle as described

earlier.

2.11.5 Overconsolidation Ratio, OCR The excess pore pressure generated during a pressuremeter test in clay depends on the OCR

(Clarke, 1995). During the installation of a piezocone and a DMT, Mayne & Bachus (1988,

1989) developed a relation between the effective preconsolidation pressure, p'c and the excess

pore pressure, ∆u given below:

( )uc sGM

up/ln

4∆=′ (2.3)

where M is the critical state parameter ⎥⎦

⎤⎢⎣

⎡′−

′=

φφ

sin3sin6 .

The excess pore pressures developed during a pressuremeter test in clay is

( )⎥⎦⎤

⎢⎣⎡ ∆⎟

⎠⎞⎜

⎝⎛

VV

sGs

uu ln and at the limit pressure this term reduces to ⎟

⎠⎞⎜

⎝⎛

uu s

Gs ln , therefore, the

equation 2.3 takes the form:

Ms

p uc

4=′ (2.4)

2.12 FACTORS AFFECTING INTERPRETATION OF PRESSUREMETER TESTS The following factors can influence pressuremeter test results (Clarke, 1995):


37

1. The installation of the pressuremeter affects the initial size of cavity and the properties

of the surrounding ground.

2. The probe may not be vertical.

3. The vertical stress may not be the intermediate stress once yield has occurred.

4. The horizontal stress may not be uniform.

5. The ground may not behave as a continuum, especially if discontinuities are present.

6. The ground may not be homogeneous (either vertically or radially).

7. Drainage can occur during a test.

8. Ground properties are test rate dependent.

9. The expansion of the cavity may not be cylindrical.

10. The probe dimensions do not conform to those of a theoretical pocket.

The detailed description of the above items can be seen in Clarke (1995).

2.13 DESIGN APPLICATIONS OF PRESSUREMETERS RESULTS For the last half a century or so pressuremeter interpreted parameters have been used

increasingly in geotechnical design of various civil engineering structures (Clarke, 1995).

There are two different methods of design, viz., direct and indirect. The direct method, being

the mostly used, is based on the philosophy that performance of full-scale foundations can be

associated with parameters determined from empirical correlations based on theory. The

indirect methods are based on the mechanical behaviour of the ground; employ mechanical

properties in theoretically derived formulae modified by observation. Both methods are

included in Eurocode No.7: Geotechnical Design.

The Ménard method belongs to the direct method of design, the main applications of which

are:


38

Table 2.7 Parameters obtained from pressuremeter tests and their potential quality (From Clarke, 1995) Parameter Clay Sand Gravel Rock

Soft Stiff Loose Dense Weak Strong

Probe PBPM SBPM FDPM PBPM SBPM FDPM PBPM SBPM FDPM PBPM SBPM FDPM PBPM PBPM SBPM PBPM

σh A CE C A CE B C C

su BE A BE BE A BE CE B CE

c' B

φ' B B CE A CE CE A CE CE B

Gi A A A A B

Gur A A A A A A A A A A A A C A A A

pL BE A BE BE A BE CE A CE CE A CE CE CE B CE

Ch B A A B A A

A - excellent; B - good; C - possible; E - empirical

σh total horizontal stress su undrained shear strength c' cohesion φ' angle of shearing resistance Gi initial shear modulus Gur secant shear modulus from an unload-reload cycle pL limit pressure Ch coefficient of consolidation


39

Table 2.8 Correlations of PMT data with SPT, CPT and Laboratory data Data type Parameter Soil type Correlation Reference

PMT~SPT pL, N Sandy silty clay 7.21945.29 += CorL Np

where pL is in units of kPa.

Yagiz et al. (2008)

pL, qc Clay Lc pq 3=

where qc and pL are in units of kPa.

Wieringen (1982)

pL, qc, φ' Sand ( ) Lc pq 75.1tan15 φ′=

where qc and pL are in units of kPa and φ' in degrees.

Wieringen (1982)

Dense sand 10=Lc pq

Loose sand 5=Lc pq

Silt 6=Lc pq

Insensitive clay 3=Lc pq

PMT~CPT pL, qc

Very sensitive clay 5.1=Lc pq

Schmertmann (1977)

PMT Gur, su Clay uur sG 40=

where Gur and su are in units of MPa. Wong and Hwang (1977)

N, Dr Sand ( ) 42.012.0 )(25 ND vor−′= σ

where Dr is in % and σvo in kPa.

Yoshida et al. (1988)

N, φ' Sand ( ) 205.3 5.0 +=′ Nφ where φ' is in units of degrees.

Muramachi (1974)

SPT N, su Very soft to very stiff clays

Very soft to stiff clays

Nsu 64.6=

Nsu 86.7=

where su is in units of kPa.

Terzaghi and Peck (1967), Parcher and Means (1968) Tschebotarioff (1973)

qc, Dr Sand ( )[ ]5.0log6674 vocr qD σ ′+−=

where qc and σ'vo are in units of ksf.

Jamiolkowski et al. (1988)

CPT qc, φ' Sand ( ) 5.05.229 cq+=′φ

where qc is in units of MPa.

Meyerhof (1976)

For definition of symbols, refer to the list of symbols.


40

• Determination of bearing capacity and settlement of shallow foundations.

• Design of axially loaded piles.

• Design of horizontally loaded piles.

• Design of ground anchors.

The indirect method of design uses the in-situ stress, stiffness and strength in either design

rules or numerical methods. The stresses and mechanical properties that can be obtained from

pressuremeter tests and their potential quality for use in design are listed in Table 2.7.

2.14 CORRELATIONS OF PMT DATA WITH SPT AND CPT DATA

Some of the soil parameters required for design purposes can be directly obtained by

interpreting the pressuremeter ground response curve as discussed earlier in section 2.11.

However, other design parameters of interest can be determined by using available

correlations between PMT data and other in-situ testing equipments data. A problem using

these correlations could be that they are site specific and may not be applicable locally;

therefore a user should use them cautiously (Bowles, 1996). A care is therefore highly

required for use of different correlations. Table 2.8 presents the correlations of PMT, SPT,

CPT data along with their sources and soil type. 2.15 FULL-DISPLACEMENT CONE PRESSUREMETER The general information pertaining to different pressuremeters including their operation, test

procedures, interpretation of data and correlations of PMT data with other in-situ testing

devices and laboratory data is presented in articles 2.1 to 2.14. However, the main focus in

this research work was the full-displacement (cone) pressuremeter (for the reasons described

in Chapter 1) originally designed by Withers et al. (1986). Akbar (2001) developed

Newcastle Full-Displacement Pressuremeter (NFDPM) with some modification to Withers et

al. design. The NFDPM has been further modified during this research. The following

sections describe Withers et al. (1986) and Akbar (2001) cone pressuremeters in detail. The

modified version of NFDPM is presented in chapter-3.

2.15.1 The Withers et al. (1986) Full-Displacement Cone Pressuremeter This pressuremeter was developed at Cambridge In-situ to specifications prepared by Fugro

for offshore testing, as the other versions of the pressuremeters in the market were not cost-

effective for this purpose. Another aim was to enable tests to be performed as part of the cone


41

penetration test operation. The cone penetration test equipment (CPT) alone has proven

outstanding device for soil profiling and soil strength estimation but it does not give reliable

estimate of stiffness. The pressuremeter device, on the other hand, is well suited for

measuring both soil stiffness and the strength parameters (Houlsby & Withers, 1988; Houlsby

& Nutt, 1993, Yu et al. 1996, Powell & Shields, 1995, 1997). The cone pressuremeter, due to

the soil disturbance during its insertion into the ground, was not aimed to provide information

on in-situ stress and if it can be obtained, it may be regarded as a bonus (Houlsby & Nutt,

1993).

Figure 2.11 shows a cut away view of the full-displacement cone pressuremeter probe. The

overall length of the membrane is 448 mm. The outside diameter of the probe (with

membrane and Chinese lantern on) is the same as that of the 15 cm2 piezocone (43.7 mm)

mounted in front of it. The Chinese lantern is used to secure the membrane. The L/D ratio of

the probe is about 10. The minimum distance between the centre of the membrane and the

conical tip is 930 mm, which can be increased using spacers.

The circumferential strain is measured at three locations 120° apart at the middle of the test

section through strain-gauged springs. The springs move with the membrane and the output

signals from the gauges are recorded (after being amplified) using a data logger. Using the

calibration data, the change of the gauges output can be converted into the radial expansion of

the membrane. The inflation pressure, pore pressure (when fitted to the probe) and the

piezocone signals (passed independently through the probe) are also recorded at the surface.

The instrument was designed to withstand 10 MPa inflation pressure, with 20 tonnes pushing

and extraction forces. The radial expansion capacity that can be measured with the strain-

gauged springs is 50%, which is more than the limit of other Pressuremeters (Table 2.1).

Figure 2.12 shows the set-up of the FDPM prototype testing. The installation, test procedure

and the data interpretation have already been described. However, the theory behind the

FDPM data interpretation is explained below:


42

Figure 2.11 Details of full-displacement (cone) pressuremeter (After Withers et al., 1986)

Connection toamplifier sub

43.7 mm

Contraction ring

Chinese lantern

Membrane clampingring

Arm cover sleeve

Membrane

Strain gauged spring

3 strain sensing armsat 120o spacing

Instrument body

Membrane

Membrane clampingring

Chinese lantern

Contraction ring

Connection to conespacer and cone

224

mm

224

mm

705

mm

930

mm

to

cone

tip


43

Since the insertion of the cone pressuremeter subjects the soil to a more complex stress

history, the analysis procedure is not straightforward. However, Teh (1987) proved that the

stress distribution far behind the cone tip is similar to the distribution created by the

expansion of a cylindrical cavity from zero initial radius. Houlsby & Withers (1988),

therefore, consider the use of the simpler cylindrical cavity expansion theory justified,

provided that the bottom of the pressuremeter section is located more than about 10 diameters

behind the cone tip. The high L/D ratio compared to the other pressuremeters also reduces the

errors in the modelling of the expansion and contraction phases of the test by cylindrical

theory (Houlsby & Withers, 1988).

Houlsby & Withers (1988) have illustrated the changes occurring in the soil at various radii

during the FDPM installation, expansion and contraction, as shown in Figure 2.13. Before the

pressuremeter installation, three points A, C and E are located as shown in Figure 2.13(a),

where point A is initially on the centre-line of the pressuremeter. After installation of the

probe, point A moves to a radius Ri; the pressuremeter radius [Figure 2.13(b)]. Figures

2.13(c) and (d) show the location of point A after expansion and contraction phases at radii Re

and Rc respectively.

Point C lies within the plastic zone after the probe installation [Figure 2.13(b)] and remains in

the same zone after maximum pressuremeter expansion [Figure 2.13(c)]. Its radius at

maximum expansion is rce, which becomes rcc after the pressuremeter has been contracted to

dimension Rc [Figure 2.13(d)]. Point C with radius rcc lies on the elastic-plastic boundary.

Point E is initially at radius reo and remains in the elastic zone after the probe installation. At

maximum expansion, it lies on the elastic-plastic boundary with radius ree [Figure 2.13(c)].

The installation of the cone pressuremeter expands the cavity from zero initial diameter to a

finite size, which is further increased by 50% during the pressuremeter expansion. This

necessitates the use of large strain theory to analyse the pressuremeter data. Since the rigidity

index (Ir = G/su) of clay will typically be large (i.e. greater than 30), it implies that the strain

to the onset of plasticity is small, which justifies the use of small strain solution for the elastic

region (Houlsby & Withers, 1988). Gibson & Anderson (1961) have also used such a

combination of large and small strain analysis in the plastically and elastically deforming

regions.


44

Figure 2.12 Set-up of FDPM for prototype testing

In summary, pressuremeter expansion [Figure 2.13(c)] deforms the material between A and E

plastically during expansion, while the material outside E remains elastic. After the

contraction phase [Figure 2.13(d)], the material outside E remains elastic, that between E and

C has been loaded plastically and unloaded elastically, and the material between C and A has

been loaded plastically, and then unloaded adequately so that reverse plasticity (yielding in

extension) has occurred.

400

mm

705

mm

Cone rodsconducting hose

720

mm

Standard cone rod

Cone rod adaptor

Amplifier housing

Contracting ring

Pressuremetermodule

Contracting ringCone spacer

Cone

Push head Electric/hydraulic hose


45

(a)

(b)

(c)

(d)

A C E

E

E

E

C

C

C

Ri

Re

Rc

A

A

A

rce

rcc

ree

Figure 2.36 The stages of cone pressuremeter test: (a) in-situ condition, (b) after pressuremeterinstallation, (c) at maximum pressuremeter expansion, (d) during pressuremeter contraction.(After Houlsby and Withers, 1988).

reos u

E D C B A

s

t

σho

E

D

CBA

s us u

s

t

Figure 2.37 Stress states around the cone pressuremeter: (a) at maximum expansion,(b) during contraction (After Houlsby and Withers, 1988)

(a) (b)

Figure 2.13 The stages of cone pressuremeter test: (a) in-situ condition, (b) after pressuremeterinstallation, (c) at maximum expansion, (d) during pressuremeter contraction (After Houlsby andWithers, 1988)

Figure 2.14 Stress states around the cone presuremeter: (a) at maximum expansion, (b)during contraction (After Houlsby and Withers, 1988)


46

The positions of the stress points in Figure 2.13 are further elaborated by Houlsby & Withers

(1988) on a plot of shear stress t = (σr-σθ)/2 against normal stress s = (σr-σθ)/2. This is shown

in Figure 2.14 for the cylindrical expansion case. During elastic behaviour of a soil, s remains

constant while t remains constant during plastic behaviour. The stress states at maximum

expansion are shown in Figure 2.14(a), where the material inside E is deforming plastically,

while the material outside E remains elastic.

After contraction, the stress points move to the locations as shown in Figure 2.14(b). It is

evident from the figure that all the points first unload elastically and then those points inside

C unload plastically.

This theoretical study made by Houlsby & Withers (1988) forms the basis for their analysis

of the unloading part (contraction phase) of the cone pressuremeter curve provided that the

unloading should have commenced after very large expansion strains (∆R/Ri > 20 to 30%)

had been imposed on the soil. This is to load that soil mass far from the probe, which has not

been loaded during its installation. The unloading behaviour of that material should not have

any effect of probe installation. This is the reason for a large strain capacity into the FDPM

design.

Houlsby & Withers (1988) analysis for clays (cylindrical, plane strain) can be summarized as

below:

• The expansion occurs at constant pressure

( )ruhoL Isp ln1++= σ (2.5)

• Initial unloading is at slope 2G

• Plastic unloading occurs along the curve

[ ]( )⎪⎭

⎪⎬⎫

⎪⎩

⎪⎨⎧

⎟⎟⎠

⎞⎜⎜⎝

⎛⎥⎦

⎤⎢⎣

⎡−−+−=

ruL I

spp 1sinhlnsinhln12 max εε (2.6)

where,

pL = expansion limit pressure


47

p = pressuremeter pressure

σho = in-situ horizontal stress

su = undrained shear strength

Ir = rigidity index = G/su

G = shear modulus

ε = pressuremeter strain

εc = pressuremeter strain at maximum expansion.

Since sinh(ε) ≅ ε for small values of ε, equation 2.6 can be used to construct Figure 2.9 to

determine the shear modulus, undrained shear strength and in-situ horizontal stress as

described earlier.

The FDPM has the following advantages and disadvantages (Akbar, 2001):

Advantages • The soil disturbance due to insertion of the probe is repeatable and operator

independent. An analysis based on cavity expansion theory is possible to apply to

determine the strength and stiffness.

• The use of cone makes the installation of the probe easy, thus saving time and money.

• The CPT and pressuremeter experience together can help improving correlation of soil

strength and stiffness. The combined results can provide a better understanding of the

soil stratigraphy, strength and stiffness.

• The lateral load deformation response of the soil is measured directly. This information

is useful for the design of laterally loaded piles.

• It can measure the pore pressure with piezocone.


48

Disadvantages

• The insertion of the probe causes large soil disturbance, thus changing the soil

properties and in-situ stress state. The determination of in-situ stress by any method

remains empirically based and subjective.

• The probe is sophisticated with three displacement transducers, a pore pressure

transducer and transducer for cone signals.

• The membrane has to be secured using a Chinese lantern. While the use of a Chinese

lantern makes the probe assembling difficult, it can also get filled with dirt at any time,

thus affecting the performance of the probe.

2.15.2 Newcastle Full Displacement Pressuremeter (NFDPM)

Akbar (2001) developed the NFDPM during his Ph.D. at the University of Newcastle,

Newcastle Upon Tyne, UK. The main body of the probe of the NFDPM is made of high

strength stainless steel. Its diameter is 44.4 mm with a 420 mm long test section (L). With the

membrane in place, the outer diameter of the probe (D) is 48.2 mm. The L/D ratio is therefore

8.7. The diameter of the cone used is 50.8 mm (surface area = 28.5 cm2). Thus, the cone

creates a cavity with a diameter about 5% greater than that of the probe. The oversize cavity

helps in two ways during the probe installation in the ground. The dragging force on the

membrane due to friction with the soil is reduced; thus preventing the ends of the membrane

from being pulled out of the clamping ring. It can also be pushed safely through a soil

containing gravel. This eliminates the use of the Chinese lantern as well, thus making its

assembly simpler and cost-effective. A match of the probe diameter and that of the cone,

therefore, makes it possible to test any soil including gravely soils and glacial tills.

Figure 2.15 presents a view and a cross-section of the main body (part 1) along with some

accessories of this probe. Figure 2.16 shows a picture of the prototype NFDPM. The main

body has a 115 mm long and 10 mm wide slot in the middle (part 5 in Figures 2.15 a) for the

expansion arms assembly (Figure 2.17). A longitudinal hole of 8 mm diameter (7 in Figure

2.15 b) is drilled from one end (the end to which the hydraulic coupling is connected) up to

the central slot. This hole houses the transducer wires and transmits the pressurised gas,

which inflates the membrane. The radial grooves (8 in Figure 2.15 b) and longitudinal

grooves have been machined to allow the dry nitrogen gas (N2) pressure to reach everywhere


49

240

S lot for arm s assem blyto f it (5)

420

Conical t ip f it s here M ain body (1)

Hole f or guide screw (9)

240

M em branenot shown

(b) S ection at A-A (perpendicular to the paper)

(a) A cut-away v iew

Spot f ace ofc lam ping nut

115

Hole for pin

Radialgrooves (8)

M em brane (2)

A

Cone rod adapterf its here

Longitudinalhole, dia. 8 m m (7)

S lot for guidescrew (10)

Clam ping ring (3)

Clam ping nut (4)

A

Figure 2.15 The Newcastle full-displacement pressuremeter probe (After Akbar, 2001)

(a) A cut-away view

(b) Section at A-A (perpendicular to the paper)


50

underneath the membrane simultaneously. This allows the whole length of the membrane to

expand uniformly under uniform soil conditions around the probe.

Limited in-situ testing was carried out with the Newcastle full-displacement pressuremeter

(NFDPM). The subsoils at the test sites were soft to firm clays, glacial till and sand. A brief

description of different sites is presented below (Akbar, 2001):

1) Newburn Riverside

This site is located on the north bank of the river Tyne between Lemington and Newburn,

about 7 km west of Newcastle University. The site consists of soft clay, being the floodplain

of the river Tyne and is overlain by about 2 m of made ground. PMT testing up to a depth of

9.5 m, keeping test interval of 1 m, was carried out at this site.

2) Birtley Brick Pit site

This site consists of firm to stiff glacial laminated clay and is located next to the Redland

Brick factory in Birtley, Gateshead, about 10 km from Newcastle University. A total depth of

8 m was explored at this site using the NFDPM.

3) Stella Gill expansion site

This site is located in Stella Gill Industrial Estate; Pelton Fell in Chester-Le-Street about 15

km from Newcastle University. The site consists of boulder clay, which is defined as an

unstratified and irregular mixture of boulders, cobbles, gravels, sand, silt and clay of glacial

origin, (also known as ‘glacial till’). Only one PMT test (1.6 m) could be conducted due to

limited reaction force.

4) Blyth beach sand dunes

Blyth town is located along the North-sea coast, about 20 km north of Newcastle University.

The testing was carried out on the sand dunes located along the beach. NFDPM in-situ testing

up to depth of 3 m was carried out at this site.

The new pressuremeter worked well at all the sites. The data obtained were processed using

current pressuremeter theories. Akbar (2001) concluded the following on the basis of the

testing carried out with the NFDPM:


51

Figure 2.16 A photograph of the assembled NFDPM (From Akbar, 2001)

Figure 2.17 The expansion arms (After Akbar, 2001)

Clampingnuts (4)

Rubber membrane (2)

Clampingrings (3)

Hydraulicfitting

Cone

66 m

m

Magnet 1

Magnet 2

Arm 1

Arm 2

Hair springHall effecttransducer (HET)

Hole for pin(pivot)

(a) At zero expansion of membrane

Seat forHET

(b) Maximum limit of arms expansion

44.4

mm


52

1. NFDPM proved robust enough to test dense/hard soils and sensitive enough to test soft

soils.

2. The expansion measurement system is simple, robust and shows repeatable and non-

hysteretic calibrations. The test operation and control are also simple as the pressure

and cavity expansion can be monitored in engineering units on the computer screen

during a test.

3. The applied pressure-cavity expansion curve data can be analysed using current cavity

expansion theories to determine the strength and stiffness of soils. The results obtained

compare well with those from other instruments.

4. Due to the limited amount of test data generated, any relationship could not be

established between the pressuremeter data and the drained shear strength of sands.

The amount of testing was inadequate to ascertain full confidence in the robustness of the

test probe and Akbar (2001) recommended to do more testing for this purpose.

2.15.3 Comparison of the FDPM and the NFDPM The analysis of the FDPM presented in section 2.15.1 shows that although the soil is

disturbed during probe installation, yet the unload-reload cycle and unloading portion of the

pressuremeter ground response curve can be analysed to find out strength and stiffness of the

soils. The results compare well with those from other sources (Houlsby & Withers, 1988).

The design parameters of sand can also be determined from this pressuremeter data using

Van Wieringen’s (1982) correlation discussed earlier.

With these developments in mind and to benefit from the existing experience, the NFDPM

was designed with a much simpler and robust system (Akbar, 2001) to yield a similar stress-

strain curve, as does the FDPM. The NFDPM uses a dummy cone instead of a piezocone.

The expansion is measured at the centre of the probe using a single transducer rather than

three. There is no pore pressure measurement provision. The cone diameter is larger than the

probe diameter to allow its use in all soils without damaging the membrane.

The expansion measurement system in the NFDPM can record the cavity strain to about 44%

of its original size, which is close to that for the FDPM (50%) and more than the minimum

specified (20%-30%) by Withers et al. (1986). The NFDPM stress-strain curve can, therefore,

be analysed in the same way as that from the FDPM.


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2.16 SUMMARY Three types of pressuremeters are available in the industry; namely pre-boring (PBPM), self-

boring (SBPM), and full-displacement pressuremeter (FDPM). The type of ground to be

tested and the design parameters required make the bases for the choice of the pressuremeter

type.

Some ground disturbance can not be avoided while testing with any type of pressuremeter.

The ground disturbance is at maximum level with the FDPM, at moderate level with the

PBPM and least with the SBPM. However, in the case of the FDPM, since the deflated

diameter (due to the installation method) remains constant every time, the amount of

disturbance created during its installation is always the same in similar soils, which reduces

the level of uncertainty.

Pressuremeter data can be interpreted to determine shear modulus, undrained shear strength,

angle of shearing resistance and horizontal in-situ stress values of subsurface soils.

In-situ total horizontal stress can be directly determined from a SBPM test curve provided the

disturbance to the test pocket during its installation has been minimum. With other

pressuremeters, there are a number of techniques to estimate its value; however, the selection

of any method is subjective.

Since the Newcastle full-displacement pressuremeter was designed to work in the same way

as the FDPM developed by Withers et al. (1986), the analysis procedures developed for it can

be applicable to the NFDPM as well.

Pressuremeter Test

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