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Using pressuremeters


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This booklet is an introduction to pressuremeter testing using

our instruments. It is intended to be an aid to people trying to

decide whether to use pressuremeter testing, and what type of

pressuremeter would be appropriate for their project. Peoplewanting to buy pressuremeter equipment will find some of the

information useful.

It is primarily a technical guide. For information about costs

please contact us directly on [email protected]

It is a brief guide only. Further details on all aspects can be

found on our website: http://www.cambridge-insitu.com

Usingpressuremeters

A guide to

pressuremeter testing

Furggwanghorn, Switzerland


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An introduction to pressuremeters 3Inserting the pressuremeter

Construction and calibration

Advantages and limitations of the pressuremeter test

How to decide what pressuremeter to use 7Self Boring Pressuremeter (SBP)

73mm High Pressure Dilatometer (HPD73)


47mm Reduced Pressuremeter (RPM)

Additional considerations 10Self Boring

Pre-boringPre-boring with the 47mm RPM

Special tests 12Horizontal tests

Creep tests

Consolidation tests

Permeability testing

Projects where our pressuremeters have been used 14The underground research facility at Mol, Belgium

Testing waste and investigating barrier walls

Kolkata Metro East-West Project

Worked examples 16Case A. A self bored pressuremeter test in London Clay

Case B. A pre-bored pressuremeter test in chalk

Case C. A pre-bored pressuremeter test in competent rock

References 23

Contents


4/282 USING PRESSUREMETERS

Disassembled CDU Portable power pack

A kit of parts for a self boring pressuremeter


5/28CAMBRIDGE INSITU 3

Pressuremeters are devices for carrying

out insitu testing of soils and rocks for

strength and stiffness parameters. They

are generally cylindrical, long with respect

to their diameter, part of this length being

covered by a flexible membrane.

Pressuremeters enter the ground by

pushing, by pre-boring a hole into which

the probe is placed, or by self boring (fig.1)

where the instrument makes its own hole.

Once in the ground, increments of

pressure are applied to the inside of the

membrane forcing it to press against the

material and so loading a cylindrical cavity.

A test consists of a series of readings of

pressure and the consequentdisplacement of the cavity wall (fig. 2),

and the loading curve so obtained may

be analysed using rigorous solutions for

cylindrical cavity expansion and

contraction. It is the avoidance of

empiricism that makes the pressuremeter

test potentially so attractive.

The test is usually carried out in a vertical

hole so the derived parameters are those

appropriate to the horizontal plane.

Inserting the pressuremeter

The interpretation of the pressuremeter test

must take account of the disturbance caused

by the method used to place the probe in the

ground. The least disruptive of the methods is

self boring where disturbance is often small

enough to lie within the elastic range of the

material and is therefore recoverable. This

is the only technique with the potential to

determine directly the insitu lateral stress,ho, the major source of uncertainty when

calculating the coefficient of earth pressure

at rest, ko. However all methods allow the

confining stress to be inferred.

The disturbance caused by pre-boring and

pushing is never recoverable. However for any

pressuremeter test it is possible to erase the

stress history of the loaded material by taking

it to a significantly higher stress than it has

previously seen, and then to reverse the

direction of loading. The point of reversal is a

new origin and the stress:strain response will

be that due to the undisturbed properties of the

material. In fig. 2 the three types of test are

shown. The tests were carried out at the same

location (a heavily over-consolidated Gault clay

site) at similar depths and give similar results

for strength and stiffness. Although the loading

paths appear very different there are

similarities in the unloading paths and

whenever a small rebound cycle is taken.

These cycles are of particular importance.

No matter how disturbed the material prior to

insertion all types of pressuremeter test have

the potential to make repeatable measumentof shear stiffness and the reduction of stiffness

with increasing strain.

Pre-boring

A pocket is formed in the ground by

conventional drilling tools and the instrument

is subsequently placed in the pre-formed hole.

The major defect in this method is the complete

unloading of the cavity that takes place in the

interval between removing the boring tool and

pressurising the probe. The material must be

capable of standing open and so the method

is best suited to rock. As fig. 2 indicates it ispossible to make a test in stiff clay. However

comparing the pre-bored curve to the self-

bored shows how much further the cavity may

have to be expanded before the influence of

An introduction to pressuremeters

Fig. 1 A self boring pressuremeter approximately 1.25m x 0.08m

Fig. 2 Test curves for 3 types of probe in Gault clay at about 5mBGL


6/284 USING PRESSUREMETERS

insertion disturbance can erased. The method

can be used in dense sand if drilling muds are

used to support the open borehole but it is

unlikely to be suitable for loose sands. The

Mnard pressuremeter widely used in France

is an example of a pre-bored device. In the UK

the High Pressure Dilatometer (the terms

dilatometer and pressuremeter are

interchangeable in this context) is available

and is used in rocks, hostile materials such as

boulder clay, and dense sands. See fig. 3.

A pre-bored operation will require the

assistance of a drilling rig. Unlike the other

insertion methods, if the hole is cored then itmay be possible to make laboratory tests on

material that is directly comparable to that

being tested by the pressuremeter. Pre-bored

pressuremeter testing in a vertical hole has

been carried out to depths greater than 500

metres and depths of 200 metres are routine.

Pushing

As the name suggests, pushed-in

pressuremeters are forced into the ground so

raising the state of stress in the surrounding

soil. A special case of this approach is the

Cone Pressuremeter (CPM) where a 15cm2

cone is connected to a pressuremeter unit of

the same diameter. The disturbance caused to

the material is total and the only parameter that

can be obtained from the loading path is thelimit pressure of the soil. The pushed curve in

fig. 2 is an example of a CPM test and shows

a clear plateau after the cavity has been

expanded by about 15%. Strength parameters

are derived from the contraction curve and

stiffness parameters from the response of

small rebound cycles. The method is fast and

can make a test in any material into which a

cone can be inserted. The coupling of the

profiling capability of the cone with the ability

to make direct measurements of strength and

stiffness is especially attractive. However as

fig. 2 indicates the stresses required to make

a satisfactory test are much higher than for the

other methods, and at these levels of stress it

is probable that crushing of the soil particles is

taking place. This may be a significant factor

especially for tests in sand. Also obtainingreaction for pushing the probe may present

difficulties a jacking force of 10 tonnes or

more is not unusual.

Fig. 3 73mm and 95mm High Pressure Dilatometer Fig. 4 Self boring

Water

CableflowReturn

Flexiblemembrane

Porewaterpressuresensor

Flow ofslurriedsoil andwater

Rotatingcutter

Straingaugedspring

Expansionfollower

clamp

Cuttingshoe edge

Membrane


7/28CAMBRIDGE INSITU

Self boring

Fig. 4 shows a schematic of the Cambridge

self boring pressuremeter (SBP). The

instrument is a miniature tunnelling machine

that makes a pocket in the ground into which

the device very exactly fits. The foot of the

device is fitted with a sharp edged internally

tapered cutting shoe. When boring, theinstrument is jacked into the ground, and the

material being cut by the shoe is sliced into

small pieces by a rotating cutting device. The

distance between the leading edge of the shoe

and the start of the cutter is important and can

be optimised for a particular material. If too

close to the cutting edge the ground suffers

stress relief before being sheared. If the cutter

is too far behind the shoe edge then the

instrument begins to resemble a close ended

pile. In stiff materials the usual setting is flush

with the cutting shoe edge. The cutting device

takes many forms. In soft clays it is generally

a small drag bit, in more britt le material a rock

roller is often used.

The instrument is connected to the jacking

system by a drill str ing. This is in two parts, an

outer fixed casing to transmit the jacking force

and an inner rotating rod to drive the cutter

device. The drill string is extended in one metre

lengths as necessary to allow continuous

boring to take place. All the cut material isflushed back to the surface through the

instrument annulus, there is no erosion of the

cavity wall. Normally water is used but air and

drilling muds have been applied with success.

Self boring is effective in materials from loose

sands and soft clays to very st iff clays and

weak rock. It will not operate in gravel and

materials hard enough to damage the sharp

cutting edge. In principle the probe can be

made to enter the ground with negligible

disturbance. In practice, self boring resultsin a small degree of disturbance that must be

assessed before deciding a value for the insitu

lateral stress. Experience has shown that the

self boring disturbance is low enough to remain

within the elastic range of the material.

The SBP requires a modest amount of

reaction. On some soft clay sites it is possible

for the self boring kit to operate without support

from other drilling tools. The minimum interval

between tests is one metre. Where tests are

more widely spaced or in materials withoccasional bands of hostile layers the SBP can

be used in conjunction with a cable percussion

system, or be driven by a rotary rig using

special adaptors. Self boring in a vertical hole

is routinely carried out to depths of 60 metres

or more.

The self boring method is also used as a low

disturbance insertion system for other devices

such as load cells and permeameters.

Construction and calibrationThere are many designs of pressuremeter in

current use, some of which are of complex

construction. Fig. 5 is a view of the inside of

a 6 arm Cambridge self boring pressuremeter.

There are transducers for measuring the radial

displacement of the membrane at 6 places and

the total and effective pressure being applied to

the cavity wall. The electronics for the signal

conditioning including the conversion from

analogue to digital is contained in the probe

itself. Apart from supplying power, the output

of the probe may be connected directly to the

serial port of a small computer. This approach

is necessary in order to obtain a high resolution

free of noise. Pressuremeters with local

instrumentation are able to resolve without

difficulty displacements of 0.5 microns and

pressure changes of 0.1kPa.

Pressuremeters can be expanded using air or

a non-conducting fluid such as light transformer

oil. There are automated systems for

pressurising the equipment. Automation allows

the expansion of the cavity to occur at a

constant rate of strain. It is conventional to log

the output of the pressuremeter on computer

and to plot the loading curve in real time.

Meticulous calibration of the equipment is vital.

The transducers must be calibrated regularly

both for sensitivity and drift. Almost all

pressuremeters suffer the defect that the

output of the transducers is governed by the

movements and pressure on the inside of themembrane, where what is required is the

displacements and stresses acting on the

cavity wall. The properties of the pressuremeter

membrane can be a significant source of

uncertainty. It requires an amount of work to

make it move, and an additional component to

keep it moving. This is relevant to tests in soft

soils. The membrane contribution may be

estimated by carrying out membrane

expansion tests in free air.

The other major influence on themeasurements is system compliance, or the

contribution of the probe itself to the measured

stiffness. This can be a significant source of

Fig. 5 Inside a 6 arm SBP

5


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error if the probe is used in very stiff soils or

weak rock. This contribution may be estimated

by inflating the instrument to full working load

inside a metal sleeve of known elastic

properties.

The importance of the various calibrations

depends on the type of pressuremeter and

where it is being used. For example thecontribution of the hose supplying pressure

to the probe is highly relevant if volume

changes are being measured at the surface,

but is of no importance at all for a probe with

internal instrumentation, such as the

Cambridge family of devices.

Advantages

A large number of fundamental soil

properties are obtained from a single test.

To derive these properties, no empirical

correcting factors are needed.

Measurements are made insitu at the

appropriate confining stress.

A large volume of material is tested

a typical test loads a column of material 0.5

metres high and extending to more than 10

times the expanded cavity radius. This is the

equivalent of at least 1000 triaxial tests on

38mm samples.

Representative loads are applied in the

example shown in fig. 2 about 12 tonnes isbeing applied to the cavity wall.

Results can be obtained quickly as all the

data logging and most of the analysis is

carried out by automated systems.

Commercial operation has shown that the

instruments, though more complex than

conventional site investigation equipment,

are reliable.

There are many materials whose propertiescan only be realistically determined by insitu

measurement.

The pressuremeter test is particularly

appropriate for predicting the performance

of laterally loaded piles.

Pressuremeter tests are routinely used to

calibrate finite element models of complex

geotechnical problems.

Limitations

The instrument will not penetrate gravels,

claystones or the like, so generally

pressuremeter testing requires support

from conventional drilling techniques.

Failure planes and deformation modes are

not always appropriate to those occurring

in the final design. An estimate of the

anisotropy of the material will be required

in order to derive vertical parameters from

lateral values.

Many familiar design rules and empirical

factors are based on parameters obtained

from traditional techniques. It is not always

possible to use them with pressuremeter

derived values, even if the insitu parametersmore accurately represent the true state of

the ground.

Only two stress paths can in practice be

followed, undrained and fully drained.

The instruments and their associated

equipment are complex by conventional site

investigation standards and can only be

operated by trained personnel.

Use of an inappropriate analysis to interpret

a pressuremeter test can result in seriously

misleading parameters.

6 USING PRESSUREMETERS

Advantages and limitations of the pressuremeter test



The decision about what pressuremeter to use for a particular project is not clear cut and there will be budgetary constraints in addition to

technical considerations. This section of the booklet focuses on the technical issues. It is divided up by instruments, as there is considerable

overlap between the probes and the materials they can test.

Self boring pressuremeter

Insertion methods Self boring

Initial Diameter 83-89mm, depending on the configuration

Length of material sacrificed At least 1 metre of mater ial must be self bored before testing

Displacement system Direct strain sensing at 3 points equally spaced around the centre of the expanding region

Displacement resolution Better than 1 micron

Pressure resolution 0.1kPa

Maximum expansion capability 15% greater than the at rest diameter

Maximum working pressure 10MPa

Suitable for: Homogeneous clays (soft to very stiff), silts and sands, soft rocks such as flint-free chalk

Strengths The SBP gives the highest quality pressuremeter test with minimal insertion disturbance. It is the only device

able to measure the external pore water pressure and so can provide effective stress parameters. As an

addition to the expansion test it can incorporate a consolidation phase. With a slight modification it can also

be used to obtain good quality measurements of the permeability of the formation [ref 26].

Weakness If the cutting shoe edge is damaged (by gravel or a hard layer) then the insertion disturbance is not minimal

and the expansion capability may not be enough to erase the consequences.

There is no core recovery as such but all the cut material is returned to the surface as a completely disturbed

sample.

Additional notes In general self boring is a faster system than other methods for making a test pocket. It can also be less

demanding on supporting equipment. In some circumstances it can operate as a por table stand alone system

and It is often used in conjunction with a cable percussion rig.

There are versions of this instrument that have 6 displacement sensors and incorporate a three axis

inclinometer.

How to decide what pressuremeter to use


10/28


Insertion methods Pre-bored hole or pocket

Initial Diameter 73mm

Allowable pocket diameter 75mm to 83mm

Length of material sacrificed At least 2 metres of mater ial must be cored to give a pocket long enough to test




Maximum expansion capability 33% greater than the nominal pocket diameter (76mm)

Maximum working pressure 20MPa in normal use, 30MPa with some modifications

Suitable for: Stiff clays, sands and rock of all kinds

Strengths Pre-boring a hole means that core can be recovered, giving the possibility of carrying out laboratory tests

on the same material as the pressuremeter tests.

Weakness It can be difficult to core at this diameter in highly fractured or friable materials. If the material is prone to

collapse, and a pocket it lost, this can give rise to substantial gaps in the information obtained from a borehole.

Additional notes If the pocket size is 83mm then the expansion capability falls to 22%. Because a large pocket size implies a high

level of disturbance it is likely to be difficult to achieve a test that gives representative properties for the material.

The instrument also has a magnetic compass so that the orientation of the displacement axes can be known.


Insertion methods Pre-bored hole or pocket



Length of material sacrificed At least 2 metres of mater ial must be cored to give a pocket long enough to test




Maximum expansion capability 49% greater than the nominal pocket diameter (101mm)

Maximum working pressure 20MPa in normal use, 30MPa with some modifications

Suitable for: Stiff clays, dense sands and rock of all kinds

Strengths Pre-boring a hole means that core can be recovered, giving the possibility of carrying out laboratory tests

on the same material as the pressuremeter tests. Provided the pocket stands open then a test is almost certain.

Because it has a large expansion capability it is often used in transition materials where core recovery is likely

to be poor.

Weakness If the material is prone to collapse, and a pocket it lost, this can give rise to substantial gaps in the information

obtained from a borehole.

Additional notes This HPD has sometimes been fitted with a point and used as a push-in probe in very soft materials, typically

alluvial clay.

The instrument also has a magnetic compass so that the orientation of the displacement axes can be known.




47mm Reduced Pressuremeter (RPM)

Insertion methods Pre-bored hole and pushed



Length of material sacrificed Only 0.6 metres of material is required to make a test




Maximum expansion capability 52% greater than the at rest diameter

Maximum working pressure 12MPa

Suitable for: Medium to stiff clays, loose to dense sands and weathered or soft rock

Strengths Extremely compact, portable and versatile

Weakness Due to the small diameter the displacement sensing system is slightly more affected by instrument compliance

than the larger probes.

It can be difficult to make a hole for the probe at the required tolerance, as this is not a common size.

Although it can be pushed, in practice it will be difficult to do this in stiff material because of the high loads that

will be required.

Additional notes Because the probe is dimensionally similar to a Mnard pressuremeter it is often used to carry out this style

of testing, with the advantage that the high resolution of displacement allows good quality unload/reload cycles

to be incorporated.

The probe has also been used down a borehole formed by a 102cm cone penetrometer, with the cone profileused to identify suitable locations for the pressuremeter test.


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It is usually the case that our testing is one part

only of the operations being carried out in a

borehole, and we are operating as specialist

sub-contractors to the Main Contractor. This

part of the booklet is concerned with making

clear the separation between what we supply

and what we need.

Self boring

This comes in three varieties:

A stand-alone drilling systemrequiring no additional equipment

There are not many circumstances where this

is possible but it does happen. Usually it will

be a green field site. The system consists ofhydraulic rams to jack the probe, a small motor

to rotate the inner drill string, and a water pump

to provide circulating fluid. A portable hydraulic

power pack and control panel distributes power

to the various units (fig. 3.1).

One difficulty is that kentledge for the hydraulic

rams is limited, so in practice suitable material

will be of low to medium strength only. The SBP

must drill every metre of the borehole so

additional testing is not an option. An ample

water source is required.

A stand-alone drilling systemoperating underneatha cable percussion rig

This is a common way of working, using all

the special self boring drilling parts already

mentioned but working in conjunction with a

cable percussion rig (fig. 3.2). The rig places

a column of water well casing to a depth just

above where the first test is required,

hammering it in the last 0.5 metre. The SBP

system couples to the top of the casing column

and the skin friction on the casing is enough to

allow self boring into most materials. An ample

water source is required, not normally part of

a cable percussion operation.

If the test spacing is more than 2 metres thenthe operation is usually one test and out. The

rig open-holes to the next test depth, carrying

out additional testing if required.

There are some locations that only a reduced

height cable percussion rig can access, so the

combined system is versatile.

If the hole is left open for a long time then the

tested zones begin to collapse so a reasonably

quick operation is important.

A system for operating undera rotary rig

In this method we supply the pressuremeter,

a special drill string and a purpose-built adapterfor the rotary drill head. The probe is drilled as

if it were a core barrel, but the adapter has a

thrust bearing to separate down-thrust from

rotation. Everything above the adapter spins,

everything below is static and the probe enters

the ground without being rotated (fig. 3.3

opposite).

Water needs to be supplied at appropriate

flows. This means that the rig pump and water

swivel must be in good order, because the SBP

water path is a narrow annulus compared tonormal drill rod. Air mist can be used but is

more difficult and only suitable for relatively

shallow holes.

This system allows core to be taken in the

test intervals. In material with a tendency to

collapse or in boreholes deeper than 40 metres

it is the only appropriate option.

Pre-boring

For pre-boring the problems of getting the

probe into the ground are the responsibility of

the drilling contractor. The additional issues to

be considered are these:


Additional considerations

Fig. 3.1 Self boring, stand-alone system

Fig. 3.2 Cable percussion system



Size of borehole. It sometimes happens that

the same size borehole is cored from surface

to some considerable depth, and the High

Pressure Dilatometer (HPD) must test layers in

this borehole. Because the probe is a close f it

to the nominal core size this can be risky. Any

material falling down onto the probe can make

it difficult to recover the device.

Wireline coring. We are often asked to

consider adapting the equipment to work with

a wireline coring system. The fit of the probe

to the cavity has to be reasonably close for a

successful test. It is not practical to test the

cavity made by a wireline system with a probe

small enough to pass through the wireline core

bit. There are wireline systems able to core at

two diameters but it is not advisable to use the

wireline cable for lowering the pressuremeter.

If the probe becomes trapped the wire cable

will not be able to exert more than a nominal

force to help pull it back. We therefore

recommend lowering the probe on rods. These

rods must has a diameter no greater than the

diameter of the borehole less two times the

diameter of the umbilical connecting the probe

to the pressure source on the surface. This

umbilical must be taped at intervals to the rod

to prevent loops occurring.

Inflation method. The HPD can be inflated

with oil or air. The decision about what method

to use depends on circumstances. The best

test is obtained with oil because it allows

pressure to change without large temperaturealteration. In good rock where certainty over

tiny displacements is important this is an issue,

especially where surface temperature is

considerably different from the downhole

state. However oil raises environmental issues.

We use bio-degradeable transformer oil to

minimise the risk.

Oil also gives a slower overall test, as time has

to be allowed for oil to return to the surface.

There are ways of speeding up the process but

it means adding an additional umbilical to thesystem, making the lowering and raising

procedure more complex and time consuming.

For speed and convenience air inflation is used

in most circumstances. However, oil is always

used when calibrating the pressure capability of

the probe on the surface because it is inherently

safe in the event of a failure of any part.

Speeding up testing. The easiest way to

accelerate the test rate is to reduce the number

of lowering and raising events. We sometimes

test a borehole that has been completely cored

prior to our arrival. In such circumstances theprobe is lowered to the deepest location first,

then tests are carried out in reverse order to

depth. Normally the deepest part will be the

tightest fit because the core barrel has made

the fewest passes.

Material with cavities. Limestone in particular

can be prone to solution cavities. Testing in this

material is frustrating because if the HPD

membrane is not completely supported at all

places then it will burst at pressures too low to

give useful data. Where such testing is requiredwe advise that the boreholes be cored in

advance of our arrival. They should then be

grouted up. Once we are on site the grouted

holes can be re-cored, with the grout core

available for inspection to prove the integrity

of the cavity wall. The grout will be weak

compared to the limestone so no reinforcement

takes place.

Pre-boring with 47mm RPM

The difficulty with this device is that the

diameter is smaller than the customary drill

parts a dr illing contractor can be expected to

keep. The holes for the RPM need to be formed

with drill bits and drill rods based on the AW

size. Typically the RPM is used to target certain

layers at significant depth, and in these

circumstances there is no alternative but to

make a large diameter borehole first, then drill

a 51mm or similar diameter pocket out of the

base of the larger hole. Provided the pocket for

the RPM is not too long (no more than sixmetres) then we supply the necessary rods to

take the probe and umbilical from its small hole

into the larger hole. At this point we expect to

couple to whatever drill rods are available via

a suitable adapter.

For very shallow tests, within 5 metres of the

surface, we can sometimes make the borehole

ourselves using a powered hand auger.

Successful tests can also be made using the

RPM to ream out an existing cone penetrometer(CPT) hole. This technique has been applied

with some success in weak chalk, taking

advantage of a hole made by a 102cm CPT.

Fig. 3.3 Rotary rig system


14/28

The pressuremeter is normally used to carry

out a cavity expansion test in a vertical hole.

There are other more specialised tests that

can be made and this section gives some

examples.

Horizontal tests

Fig. 4.1 shows an example of a self boring

pressuremeter working horizontally. The

location is more than 200 metres below ground

in a test tunnel researching the properties of

Boom Clay as a possible barrier medium for

the long term storage of nuclear waste. It was

not permitted to use water as a drilling fluid,

so the SBP was adapted to drill with air. The

camera flash is reflecting off some of the

returning soil particles.

Horizontal testing has also been carried out

with pre-bored pressuremeters and inclined

holes are common-place. If one axis

is arranged to be vertical when the

pressuremeter is used horizontally then this

can inform the analysis, because the vertical

insitu stress is normally known.

Creep tests

Fig. 4.2 shows a test carried out with an HPD

in a rock glacier. At intervals during the test

the pressure was held constant for one hour

duration. For each step the creep displace-

ments, expressed as a percentage of the cavity

diameter, were plotted against log elapsed

time. The slope of this trend gives a stress

dependent rate.

In this material the creep is substantial and

made it difficult to obtain an unload/reload

cycle, even after a long creep hold.

Consolidation tests

The SBP can carry out a holding test to obtain

consolidation parameters. It is a modification of

a normal undrained expansion test. Near the

point where the cavity would be unloaded it is

instead held at that expansion and the excess

pore water pressure (pwp)that has been

generated is allowed to dissipate. As it does so

the effective stress at the cavity wall starts to

rise and the cavity wants to expand.

This triggers an automatic control system to

reduce the total pressure at the cavity wall to

compensate. The net result is that the cavity

remains at a constant diameter for as long as

the test is conducted. There is a closed form

solution for this situation [ref 6] that uses the

parameters derived from the expansion phase

of the test and the time taken for 50% of the

generated excess to dissipate.

Fig. 4.3 shows the dissipation data from two

pwp cells, their mean and the total pressure

response, plotted in a normalised form.

Any of the profiles can give a value for the

horizontal consolidation, but it is normal to

use the mean of the two pwp sensors.


Special tests

Fig. 4.1 Horizontal self boring in Boom Clay

Fig. 4.2 Creep testing in Switzerland



Permeability testing

Figs. 4.4 and 4.5 show the result of a

permeability test carried out with a self boring

pressuremeter. The procedure exploits the

ability of the pressuremeter to bore a pocket

in the ground that it exactly fits. The stress

conditions are, more or less, representative of

the insitu state and are acting on the body ofthe probe, giving an excellent seal. As a

consequence the drill string now provides a

pipe from the surface down to the bottom of the

probe allowing access to the formation. For low

permeability material the pipe work is filled with

water, is sealed off and is connected to the

output of a small constant flow pump. This then

pressurises the water column. Fig. 4.4 shows

steps of pressure, and the flow rates required

to establish each step. Fig. 4.5 plots the flow

rates against pressure, and gives a lineartrend. The slope of this trend is a function of

the permeability and a shape factor.

This is one result at this location, for one

geometry the tested pocket is zero length

and the permeability is the mean of the

horizontal and vertical characteristics. If time

allows, then the probe can be pulled back to

give a pocket of some length and the test

repeated. This gives a second permeability

value where the horizontal characteristic is

having a greater influence. Further pulling backallows additional values to be obtained. By a

best fit process it is possible to identify the

anisotropy factor for the horizontal and vertical

conditions. In practice reconciling the data is

more complex than this implies because as

more and more of the material is exposed to

the test then a scale effect related to the

variability of the fabric becomes apparent

[ref 26].

The permeability testing is an addition to the

conventional expansion test, and is a way ofobtaining more data from one self boring

episode. If kis higher than 10-7m/sec then the

same concept can be used, but constant flow

is not required and a falling head test can be

carried out, measuring the height of the water

column in the SBP drill rods.

Fig. 4.3 Consolidation testing in London Clay

Fig. 4.4 Permeability testing, raw data

Fig. 4.5 Permeability testing, result


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The front cover of this booklet gives an

indication of the range of projects and

environments where the pressuremeter testcan be used. It is widely used off-shore as well

as on land, in deserts, in mountains and in

tropical conditions.

Every project has its own set of problems and

difficulties that have to be overcome. This might

mean man-handling equipment in remote

locations, such as Tanzania or The Gambia,

or using helicopters to deposit equipment on

a rock glacier in Switzerland. It is not usual

for us to run a completely self-contained

operation. Most of the time we have to work

with a local drilling contractor and operatives

who will be unacquainted with our equipment

and unused to what is required for a successful

pressuremeter test. This is not a major difficulty,

and our engineers are accustomed to looking

after the on-site training involved.

Our pressuremeters have been used on some

of the worlds major civil engineering projects,

such as Crossrail in London or the proposed

crossing of the Padma river in Bangladesh.

What follows is a selection of some of the

more unusual projects.

The underground research

facility at Mol, BelgiumWe have at intervals over the last 15 years

made visits to the SCK-CEN facility at Mol,

Belgium, to carry out pressuremeter testing in

the underground research facility HADES. This

is a system of shafts and tunnels some 224

metres below ground level in a zone of Boom

Clay in a highly plastic condition. The clay has

interesting self healing properties when

fractured, displays extremely low levels of

permeability and offers a possible solution

to the problem of the disposal of high levelnuclear waste. Since 2000 the facility has been

run by an expert group called EURIDICE and

pressuremeter testing has been used during

the construction of the facility and after to

examine the engineering propert ies of the clay.

We ourselves began work there in 1999 with a

self boring pressuremeter. We were not allowed

to introduce water into the formation and so

drilled using air from a modified drill r ig to

implement the self boring process. Special

casing and drilling parts were designed by uswith some help from the drilling contractor to

give the ideal flow path for delivering the air

and returning the cuttings. The bulk of the

testing has been horizontal. Speed is important

in this material it must be bored and tested

as rapidly as possible because after one hour

the material will close onto the probe withsufficient force to make extracting the

equipment almost impossible.

Successful pre-bored tests have also been

carried out with a 95mm HPD. This allowed

a larger pressure to be applied and a greater

cavity expansion achieved than is possible

with a self bored probe.

Links

http://www.euridice.be/

http://www.sckcen.be/en/Our-Research/Research-facilities/HADES-

Underground-laboratory

Testing waste andinvestigating barrier walls

We were approached by Dr Neil Dixon of

Loughborough University (now Professor of

Engineering) about the possibility of using a

pressuremeter to investigate the mechanical

properties of municipal solid waste. Most of thework took place at a landfill site in Calvert,

Buckinghamshire. The waste was a mixture of

residential and commercial residue, not well-


Projects where our pressuremeters have been used



sorted, in various stages of degradation and

depending on its age could be lightly to heavily

compacted.

The primary purpose of the testing was to

obtain engineering parameters that would

permit the interaction between the body of the

waste and the components of the protective

barriers to be modelled and quantified.

Self boring, pushing and pre-boring were

all attempted. If metallic materials were

encountered then the damage caused to

equipment could be spectacular. The most

satisfactory results were obtained with a 95mm

HPD, where the large expansion capability

proved to be helpful. The pockets for this were

cut dry, using a modified bit resembling a large

hole cutter. The waste is heterogeneous, may

be partially saturated and of no particular

particle size so the results were variable andthe analytical processes were not necessarily

appropriate. However shear stiffness from

unload/reload cycles proved to be a plausible

and repeatable parameter, and it was possible

to relate the stiffness values to stress level.

Partly as a result of this work we became

interested in the properties of the barriers

themselves, and have (in conjunction with

Cambridge University) carried out research

work on the mechanical properties of man-

made and natural barriers, with special

attention being paid to permeability.

Reference

Dixon, N, Whittle, R, Jones, DRV, Ngambi, S

(2006)

Pressuremeter Tests in Municipal Solid Waste:

Measurement of Shear Stiffness.Gotechnique, 56(3), pp 211-222.

Link

http://hdl.handle.net/2134/4618

Kolkata Metro East-Westproject

Twenty four self bored tests were carried out

at four critical locations along the alignment of

the proposed metro in Kolkata, India. Thepressuremeter testing component of the site

investigation had been specified by W S Atkins.

For the most part the tests were conventional

in material that behaved either as a clay or

sand. What was different about this project was

the technical and practical difficulties that had

to be overcome to achieve success.

The work was conducted on a 24 hour basis

at pavement locations in the hear t of the city.

The only rig available turned out to be a small,

light and rather old quill drive system where

most of the controls had long broken down.

Rate of rotation and advance was down to the

skill of the driller, who knew his rig and how to

coax results from it. On more than one

occasion the rig rotation system broke down

whilst driving the pressuremeter, and the

boring was completed by rotating rods byhand.

In some ways a worse problem was an

inadequate water pump, as no boring is

possible if the pump is not delivering a

sufficient flow. However as our report noted at

the time, these issues were a problem for the

rate of progress of the fieldwork rather than the

tests themselves, which were of reasonable

quality.

Some of the expansions in the more clay-likematerial were turned into consolidation tests.

The tests typically took two hours to complete,

and were popular with the drilling crew.

There is an increasing need for complex

transport infrastructure in such locations and

this project is typical of the kind of testing we

are asked to do.

Fig. 5.1 Self boring in Kolkata


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Fig. 4. Shear strength (b)

This is a similar procedure but applied to the

final contraction data. It is of special interest

because the origin at the start of unloading is

an observable point the origin used for the

initial loading is always uncertain due to

disturbance [ref 19].

Fig. 5. Shear modulus (a)

This is a simple approach to derive an estimate

of the shear modulus, by taking the slope of

the chord bisecting a cycle of unloading and

reloading. In a linear elastic material the

unloading and reloading data would coincide.

Here the cycle appears hysteretic, indicating

that modulus varies with strain.

Fig. 6. Shear modulus (b)

This non-linear stiffness behaviour can be

represented by a power law. Here the reloading

data from the previous plot are redrawn on

log-log scales and the slope and intercept

identified. These two parameters allow the

current shear stress to be predicted at any

strain [ref 3].

The two parameters are referred to as (the

shear stress constant) and , the non-linear

exponent. will take a value between 0.5 and1, where 1 is a linear elastic response. Thesemay be combined to give secant shear

modulus Gs for a particular value of shear

strain , as follows: Gs = -1.

This expression is good for values of shear

strain down to 10 -4 , the resolution limit of our

probes. This is not small enough to predict

Gmax which is probably found at a shear strain

nearer 10-5.

Fig. 4 Shear strength (b)

Fig. 5 Shear modulus (a)

Fig. 6 Shear modulus (b)


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Fig. 7. Stiffness/strain

The trend of declining stiffness with strain is

drawn here for each cycle. Because the test is

virtually undrained the three cycles give almost

exactly the same result. The lines come from

the power law results, the data points from

applying Palmer (1972) directly to the data

[ref 25].

Fig. 8. Curve comparison

The parameters produced so far are used

to calculate a pressure/strain curve for

comparison with the measured data. The non-

linear stiffness parameters are assumed

correct. A tiny alteration to the origin reconciles

loading and unloading shear strength. Finally,

the initial reference stress is chosen for best fit

[ref 29].


Fig. 7 Stiffness/strain

Fig. 8 Curve comparison

At this stage of the process the analyst has a

set of parameters describing the strength and

stiffness of the material, and the insitu stress

state. There are differing levels of uncertainty

in these values. One method for resolving this

uncertainty is to see if the parameter set can

reproduce the measured field curve. Every

measured data point could be calculated if the

underlying stress:strain curve was known.

The soil model used here assumes a non-

linear elastic/perfectly plastic stress:strain

curve for which there is a closed-form solution.

The essence of such solutions is to define the

stress and strain required to make the material

yield, then integrate this condition between

known boundaries. In the implementation

shown here only the insitu horizontal stress

is treated as a free variable.



Fig. 9. Test in chalk

The picture is slightly misleading because it

shows the final output of the analysis. The

additional features of this test compared with

the self boring example in clay are:

The cavity wall is not pressing against

the instrument at the start of the test.

There is an appreciable difference between

the point of first contact and cavity strain

zero. This is a consequence of unloadingthe cavity prior to the test.

The initial part of the expansion contains

short duration stress holds, to monitor the

creep characteristics of the material.

There is a longer stress hold before the

start of each unload/reload cycle

Less evidence of hysteretic behaviour in

the cycles, so they appear more linear than

the clay.

The membrane collapses at the head

of water pressure at the end of the test,a feature of a drained expansion.

Fig. 10. Estimates of cavity referencestress and displacement

It is not possible to discover the initial stress

state by inspection, so a method is used

whereby estimates are back-calculated from

the yield stress. The plot above consists of

three views. The main display shows about

2mm of the initial expansion. The slope of the

stiffest part has been used to estimate initialshear modulus. The onset of plasticity is where

the data points move away from the slope line.

Initially the reference stress is guessed, the

displacement ordinate of that stress giving an

origin for calculating strain. An analysis for

mobilised shear stress near failure is carried

out, and a calculated failure stress derived. This

should coincide with the observed value. If not,

the guess of cavity reference stress is adjusted

and the cycle repeated until a match is found.

The chart on the left shows the creepdisplacements from the holds included in the

expansion phase of the test. Creep seems to

fall to a minimum in the vicinity of the cavity

reference stress estimate and increases again

near the yield stress estimate.

This analysis is used regardless of whether the

loading conditions are drained or undrained.

It is expected to give a higher bound estimate

for reference stress but a lower bound valuefor the strain origin [ref 20, 11].

Fig. 9 Test in chalk

Fig. 10 Estimates of cavity reference stress and displacement

A more difficult test to analyse is now

described. This is a test in weak chalk made

with a pre-bored pressuremeter. The pocket for

the probe was made by rotary coring. The

analysis is harder because the disturbance

caused by pre-boring and the complete

unloading of the cavity prior to the probe being

placed means that little can be gleaned from

the initial response. It is also complicated

because the material is highly permeable and

therefore the test is a drained loading. This

means that it is not so easy to derive radial

strain and circumferential stress from

measured pressuremeter co-ordinates of

pressure and displacement. Account has to

be taken of dilatant properties, possible

cohesion and the ambient pore water state.

CASE B. Analysis of a pre-bored pressuremeter test in chalk


22/28

Fig. 11. Friction angle

Because the expansion is drained a different

analysis for strength is required. The gradient

of a log-log plot of effective stress and strain is

used to produce a value for the internal angle

of friction and dilation. Ambient water pressure

and the residual friction angle have to be

known or estimated [ref 16].

Fig. 12. Stiffness/strain

Non-linear modulus parameters are obtained in

the same manner as a test in clay. Because the

test is drained each cycle plots a higher trend,

related to the mean effective stress. The cycle

on the final unloading shows the stiffest

response because the mean stress is that

which applied at the end of loading.

Fig. 13. Drained curve modelling

We have developed a closed-form solution for

a drained test in a c-phi material based on the

same non-linear elastic/perfectly plastic shear

stress:shear strain curve as for the undrained

case. It is less well constrained:

Cohesion is also unknown as well as the

insitu lateral stress.

Shear modulus parameters must be adjusted

for stress level.

Poissons ratio is required, and this probablyhas to be guessed.

Ambient water pressure and residual friction

angle are required.

The solution takes no account of tensile

strength which begins to be an issue as

material approaches a rock like condition.

Despite these cautions the procedure is

capable of producing plausible matches to the

field data.

In this example the cavity reference pressurefrom the yield stress analysis gives the best fit

curve but it has been necessary to make a

slight adjustment to the origin for strain. The

shear modulus at yield is nearly 3 times greater

than the value provided by the initial slope, a

typical result for a pre-bored test. The solution

is also able to provide a value for the limit

pressure of the material [ref 4].


Fig.11 Friction angle

Fig. 12 Stiffness/strain

Fig. 13 Drained curve modelling



Fig. 14. Elastic deformation only

The example is from a test in intact limestone.

Although not obvious, there are two

unload/reload cycles in this test, virtually

indistinguishable from the loading path. The

only parameter that is sensible to take from this

test is an estimate of shear modulus from the

latter part of the loading, giving a value greater

than 15GPa, or in terms of Youngs modulus

40GPa. The total displacement once the probe

has contacted the cavity wall is only about 80

microns, so careful calibration of the probe for

compliance effects is essential. A shear

modulus of 15GPa is about the limit of what

the probe can determine before the calibration

uncertainty exceeds the apparent value.

In general it is the poorer material that is of

most interest, especially those where core

recovery is poor or does not produce intact

samples for laboratory testing. The final

example is from a test carried out in weathered

limestone.

Fig. 15. Elastic deformation withtensile failure

The test shows two cracks forming, one at

4.8MPa and another at 8.1MPa. The event is

too fast for any data points to be recorded so

the plot shows a sudden jump at these stress

levels.

The slope of the loading curve changes as a

result of the tensile failure. Slope A is stiffer

than slope B which is stiffer than slope C.Not so obvious is the fact that the reload

cycles have a different slope and are not

representative of the properties of the intact

rock they will be under-estimates.

Fig. 14 Elastic deformation only

Fig. 15 Elastic deformation with tensile failure

CASE C. Analysis of a pre-bored pressuremeter tests in rock

If failure in shear is an identifiable point in a

pressuremeter test then it is always possible

that analyses for strength and initial stress

state can be carried out. In rock, the material

can be so good that the probe reaches its

maximum working pressure with only elastic

deformation being seen. All that can be easily

derived from such tests is a value for shear

modulus. It it important to derive this from as

late in the test as possible so that the forming

of the pressuremeter cover against the rock is

not confused with movement of the rock itself.


24/28

Fig. 16. Interpreting creep readings

In this figure, creep readings are plotted on the

left and the data in the main display are end of

creep readings only. No parameters are quoted

except for the initial slope and its intercept on

the displacement axis. Although the material

appears to have failed in shear, the loading

curve is actually three lines of differing slopeswith the shear failure stress not yet reached.

After each crack has occurred, creep

displacements reduce in magnitude.

Curiously, the one parameter that it is possible

to identify with only limited uncertainty is the

horizontal cavity reference pressure, Po. The

first crack appears at 4MPa total radial stress.

At this point the circumferential stress must

be zero or below. It follows that Po can be no

greater than 2MPa, and if the tensile strength

were known, could be narrowed down evenfurther.


Fig. 16 Interpreting creep readings



1. BAGUELIN, F., JEZEQUEL, J.F. and SHIELDS, D.H. (1978)

The Pressuremeter and Foundation Engineering. Transtech Publications, Clausthal, Germany ISBN 0-87849-019-1.

2. BELLOTTI, R., GHIONNA, V., JAMIOLKOWSKI, M., ROBERTSON, P. and PETERSON, R. (1989).

Interpretation of moduli from self-boring pressuremeter tests in sand. GotechniqueVol. XXXIX, no. 2, pp.269-292.

3. BOLTON M.D. and WHITTLE R.W. (1999)A non-linear elastic/perfectly plastic analysis for plane strain undrained expansion tests. GotechniqueVol. 49, No.1, pp 133-141.

4. CARTER, .I. P., BOOKER, J. R. & YEUNG, S. K. (1986).

Cavity expansion in cohesive frictional soils. Gotechnique36, No. 3,.pp 349-358.

5. CHANDLER, R.J., LEROUEIL, S. and TRENTER, N.A. (1990)

Measurements of the permeability of London Clay using a self boring permeameter. Gotechnique40, No. 1, pp 113-124.

6. CLARKE, B.G., CARTER, J.P. and WROTH, C.P. (1979).

In Situ Determination of Consolidation Characteristics of Saturated Clays. Design Parameters in Geotechnical Engineering, VII ECSMFE, Brighton,

Vol. 2, pp 207- 211.

7. ERVIN, M.C., BURMAN, B.C. and HUGHES, J.M.O.(1980).The use of a high capacity pressuremeter for design of foundations in medium strength rock. International Conference on Structural Foundations

on Rock, Sydney.

8. GHIONNA, V., JAMIOLKOWSKI, M., LANCELLOTTA, R. & MANASSERO, M (1989).

Limit Pressure of Pressuremeter Tests. Proc. of 12th ICSMFE, Rio De Janeiro.

9. GIBSON, R.E. and ANDERSON, W.F. (1961)

In situ measurement of soil properties with the pressuremeter, Civil Engineering and Public Works Review, Vol. 56, No. 658 May pp 615-618.

10. HABERFIELD, C.M and JOHNSTON, L.W (1990)

The interpretation of pressuremeter tests in weak rock theoretical analysis. Proc. 3rd Int.Symp.Pressuremeter, Oxford, pp. 169-178.

11. HAWKINS, P.G., MAIR, R.J., MATHIESON, W.G. and MUIR WOOD, D. (1990)Pressuremeter measurement of total horizontal stress in stiff clay, Proc. ISP. 3 Oxford.

12. HOULSBY, G.T and SCHNAID, F. (1994)

Interpretation of shear moduli from cone pressuremeter tests in sand. Gotechnique44, no.1, pp 147-164.

13. HOULSBY, G. and WITHERS, N.J. (1988)

Analysis of the Cone Pressuremeter Test in Clay. Gotechnique, Vol 38, No. 4, pages 573-587.

14. HUGHES, J.M.O. (1973).

An instrument for in situ measurement in soft clays. PhD Thesis, University of Cambridge.

15. HUGHES, J.M.O., ERVIN, M.C. (1980)

Development of a High Pressure Pressuremeter for determining the engineering proper ties of soft to medium strength rocks. Proc. 3rd Aus.-NZ Conf.Geomechanics, Brisbane, pp.292-296.

16. HUGHES, J.M.O., WROTH, C.P. and WINDLE, D. (1977)

Pressuremeter tests in sands, Gotechnique4, pp 455-477.

17. JARDINE, R.J. (1991)

Discussing Strain-dependent moduli and pressuremeter tests. Gotechnique41, No. 4., pp 621-624.

18. JARDINE, R.J. (1992)

Nonlinear stiffness parameters from undrained pressuremeter tests. Can. Gotechnique. 29, pp 436-447.

19. JEFFERIES, M.G. (1988)

Determination of horizontal geostatic stress in clay with self-bored pressuremeter.Can. Gotechnique. 25 (3), pp 559-573.

20. MARSLAND, A. and RANDOLPH, M.F. (1977).

Comparison of the Results from Pressuremeter Tests and Large Insitu Plate Tests in London Clay.Gotechnique27 No. 2 pp 217-243.

References


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21. MAIR, R.J. and WOOD, D.M. (1987)

Pressuremeter Testing. Methods and Interpretation. Construction Industry Research and Information Association Project 335. Publ. Butterworths,

London. ISBN 0-408-02434-8.

22. MANASSERO, M. (1989)

Stress-Strain Relationships from Drained Self Bor ing Pressuremeter Tests in Sand. Gotechnique39, No.2, pp 293-307.

23. MUIR WOOD, D. (1990)

Strain dependent soil moduli and pressuremeter tests. Gotechnique, 40, pp 509-512.

24. NEWMAN, R.L., CHAPMAN, T.J.P. and SIMPSON, B. (1991)

Evaluation of pile behaviour from pressuremeter tests. Proc. Xth European Conference on Soil Mechanics and Foundation Engineering, Florence,

May 1991.

25. PALMER, A.C. (1972)

Undrained plane-strain expansion of a cylindrical cavity in clay: a simple interpretat ion of the pressuremeter test, Gotechnique22 No. 3 pp 451-457.

26. RATNAM, S., SOGA, K. and WHITTLE, R.W. (2005)

A field permeability measurement technique using a conventional self boring pressuremeter. Gotechnique, 55. pp. 527-537. ISSN 0016-8505.

27. ROWE, P.W. (1962)

The Stress Dilatancy Relation for Static Equilibrium of an Assembly of Particles in Contact. Proceedings of the Royal Society. Vol. 269, Series A,

pp 500-527.

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Discussing Experience with the self boring rock pressuremeter. Ground Engineering, Jan/Feb, pp 30-32.

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Using non-linear elasticity to obtain the engineering proper ties clay a new solution for the self boring pressuremeter. Ground Engineering, Vol.32,

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The Use of a Self-boring Pressuremeter to determine the Undrained Properties of Clays. Ground Engineering, September.

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27/28CAMBRIDGE INSITULoad cell pressuremeter


28/28

Cambridge Insitu Ltd

38-39 High Street

Little Eversden

Cambridge CB23 1HE

England

Tel: +44 (0)1223 262361

Fax: +44 (0)1223 263947

[email protected]

http://www.cambridge-insitu.com

Primary contact: Clive Dalton

pressuremeters web

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