Using pressuremeters - Cambridge Insitu

Using pressuremeters

This booklet is an introduction to pressuremeter testing usingour instruments. It is intended to be an aid to people trying todecide whether to use pressuremeter testing, and what type ofpressuremeter would be appropriate for their project. Peoplewanting to buy pressuremeter equipment will find some of theinformation useful.

It is primarily a technical guide. For information about costsplease contact us directly on [email protected]

It is a brief guide only. Further details on all aspects can befound on our website: http://www.cambridge-insitu.com

Usingpressuremeters A guide topressuremeter testing

Furggwanghorn, Switzerland

An introduction to pressuremeters 3Inserting the pressuremeterConstruction and calibrationAdvantages and limitations of the pressuremeter test

How to decide what pressuremeter to use 7Self Boring Pressuremeter (SBP)73mm High Pressure Dilatometer (HPD73)95mm High Pressure Dilatometer (HPD95)47mm Reduced Pressuremeter (RPM)

Additional considerations 10Self BoringPre-boringPre-boring with the 47mm RPM

Special tests 12Horizontal testsCreep testsConsolidation testsPermeability testing

Projects where our pressuremeters have been used 14The underground research facility at Mol, BelgiumTesting waste and investigating barrier wallsKolkata Metro East-West Project

Worked examples 16Case A. A self bored pressuremeter test in London ClayCase B. A pre-bored pressuremeter test in chalk Case C. A pre-bored pressuremeter test in competent rock

References 23

Contents

2 USING PRESSUREMETERS

Disassembled CDU Portable power pack

A kit of parts for a self boring pressuremeter

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Pressuremeters are devices for carryingout insitu testing of soils and rocks forstrength and stiffness parameters. Theyare generally cylindrical, long with respectto their diameter, part of this length beingcovered by a flexible membrane.

Pressuremeters enter the ground bypushing, by pre-boring a hole into whichthe probe is placed, or by self boring (fig.1)where the instrument makes its own hole.Once in the ground, increments ofpressure are applied to the inside of themembrane forcing it to press against thematerial and so loading a cylindrical cavity.A test consists of a series of readings ofpressure and the consequentdisplacement of the cavity wall (fig. 2), and the loading curve so obtained may be analysed using rigorous solutions forcylindrical cavity expansion andcontraction. It is the avoidance ofempiricism that makes the pressuremetertest potentially so attractive.

The test is usually carried out in a verticalhole so the derived parameters are thoseappropriate to the horizontal plane.

Inserting the pressuremeterThe interpretation of the pressuremeter testmust take account of the disturbance causedby the method used to place the probe in theground. The least disruptive of the methods isself boring where disturbance is often smallenough to lie within the elastic range of thematerial and is therefore recoverable. This is the only technique with the potential todetermine directly the insitu lateral stress, σho, the major source of uncertainty whencalculating the coefficient of earth pressure at rest, ko. However all methods allow theconfining stress to be inferred.

The disturbance caused by pre-boring andpushing is never recoverable. However for anypressuremeter test it is possible to erase thestress history of the loaded material by taking it to a significantly higher stress than it haspreviously seen, and then to reverse thedirection of loading. The point of reversal is anew origin and the stress:strain response willbe that due to the undisturbed properties of thematerial. In fig. 2 the three types of test are

shown. The tests were carried out at the samelocation (a heavily over-consolidated Gault claysite) at similar depths and give similar resultsfor strength and stiffness. Although the loadingpaths appear very different there aresimilarities in the unloading paths andwhenever a small rebound cycle is taken.These cycles are of particular importance. No matter how disturbed the material prior toinsertion all types of pressuremeter test havethe potential to make repeatable measument of shear stiffness and the reduction of stiffnesswith increasing strain.

Pre-boring

A pocket is formed in the ground byconventional drilling tools and the instrument is subsequently placed in the pre-formed hole.The major defect in this method is the completeunloading of the cavity that takes place in theinterval between removing the boring tool andpressurising the probe. The material must becapable 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. Howevercomparing the pre-bored curve to the self-bored shows how much further the cavity mayhave 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


insertion disturbance can erased. The methodcan be used in dense sand if drilling muds areused to support the open borehole but it isunlikely to be suitable for loose sands. TheMénard pressuremeter widely used in France is an example of a pre-bored device. In the UKthe High Pressure Dilatometer (the terms“dilatometer” and “pressuremeter” areinterchangeable in this context) is available and is used in rocks, hostile materials such asboulder clay, and dense sands. See fig. 3. A pre-bored operation will require theassistance of a drilling rig. Unlike the otherinsertion methods, if the hole is cored then itmay be possible to make laboratory tests onmaterial that is directly comparable to thatbeing tested by the pressuremeter. Pre-boredpressuremeter testing in a vertical hole has

been carried out to depths greater than 500metres and depths of 200 metres are routine.

Pushing

As the name suggests, pushed-inpressuremeters are forced into the ground soraising the state of stress in the surroundingsoil. A special case of this approach is theCone Pressuremeter (CPM) where a 15cm2

cone is connected to a pressuremeter unit ofthe same diameter. The disturbance caused tothe material is total and the only parameter thatcan be obtained from the loading path is thelimit pressure of the soil. The ‘pushed’ curve infig. 2 is an example of a CPM test and shows a clear plateau after the cavity has beenexpanded by about 15%. Strength parameters

are derived from the contraction curve andstiffness parameters from the response ofsmall rebound cycles. The method is fast andcan make a test in any material into which acone can be inserted. The coupling of theprofiling capability of the cone with the ability to make direct measurements of strength andstiffness is especially attractive. However as fig. 2 indicates the stresses required to make a satisfactory test are much higher than for theother methods, and at these levels of stress itis probable that crushing of the soil particles istaking place. This may be a significant factorespecially for tests in sand. Also obtainingreaction for pushing the probe may presentdifficulties – a jacking force of 10 tonnes ormore 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

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Self boring

Fig. 4 shows a schematic of the Cambridge self boring pressuremeter (SBP). Theinstrument is a miniature tunnelling machinethat makes a pocket in the ground into whichthe device very exactly fits. The foot of thedevice is fitted with a sharp edged internallytapered cutting shoe. When boring, theinstrument is jacked into the ground, and thematerial being cut by the shoe is sliced intosmall pieces by a rotating cutting device. Thedistance between the leading edge of the shoeand the start of the cutter is important and canbe optimised for a particular material. If tooclose to the cutting edge the ground suffersstress relief before being sheared. If the cutteris too far behind the shoe edge then theinstrument begins to resemble a close endedpile. In stiff materials the usual setting is flushwith the cutting shoe edge. The cutting devicetakes many forms. In soft clays it is generally a small drag bit, in more brittle material a rockroller is often used.

The instrument is connected to the jackingsystem by a drill string. This is in two parts, anouter fixed casing to transmit the jacking forceand an inner rotating rod to drive the cutterdevice. The drill string is extended in one metrelengths as necessary to allow continuousboring to take place. All the cut material isflushed back to the surface through theinstrument annulus, there is no erosion of thecavity wall. Normally water is used but air anddrilling muds have been applied with success.

Self boring is effective in materials from loosesands and soft clays to very stiff clays andweak rock. It will not operate in gravel andmaterials hard enough to damage the sharpcutting edge. In principle the probe can bemade to enter the ground with negligibledisturbance. In practice, self boring results in a small degree of disturbance that must beassessed before deciding a value for the insitulateral stress. Experience has shown that theself boring disturbance is low enough to remainwithin the elastic range of the material.

The SBP requires a modest amount ofreaction. On some soft clay sites it is possiblefor the self boring kit to operate without supportfrom other drilling tools. The minimum intervalbetween tests is one metre. Where tests aremore widely spaced or in materials withoccasional bands of hostile layers the SBP canbe used in conjunction with a cable percussionsystem, or be driven by a rotary rig using

special adaptors. Self boring in a vertical holeis routinely carried out to depths of 60 metresor more.

The self boring method is also used as a lowdisturbance insertion system for other devicessuch as load cells and permeameters.

Construction and calibrationThere are many designs of pressuremeter incurrent use, some of which are of complexconstruction. Fig. 5 is a view of the inside of a 6 arm Cambridge self boring pressuremeter.There are transducers for measuring the radialdisplacement of the membrane at 6 places andthe total and effective pressure being applied tothe cavity wall. The electronics for the signalconditioning including the conversion fromanalogue to digital is contained in the probeitself. Apart from supplying power, the output of the probe may be connected directly to theserial port of a small computer. This approachis necessary in order to obtain a high resolutionfree of noise. Pressuremeters with localinstrumentation are able to resolve withoutdifficulty displacements of 0.5 microns andpressure changes of 0.1kPa.

Pressuremeters can be expanded using air ora non-conducting fluid such as light transformeroil. There are automated systems forpressurising the equipment. Automation allowsthe expansion of the cavity to occur at aconstant rate of strain. It is conventional to logthe output of the pressuremeter on computerand to plot the loading curve in real time.

Meticulous calibration of the equipment is vital.The transducers must be calibrated regularlyboth for sensitivity and drift. Almost allpressuremeters suffer the defect that the output of the transducers is governed by themovements and pressure on the inside of themembrane, where what is required is thedisplacements and stresses acting on thecavity wall. The properties of the pressuremetermembrane can be a significant source ofuncertainty. It requires an amount of work tomake it move, and an additional component tokeep it moving. This is relevant to tests in softsoils. The membrane contribution may beestimated by carrying out membraneexpansion tests in free air.

The other major influence on themeasurements is system compliance, or thecontribution of the probe itself to the measuredstiffness. This can be a significant source of

Fig. 5 Inside a 6 arm SBP

5

error if the probe is used in very stiff soils orweak rock. This contribution may be estimatedby inflating the instrument to full working loadinside a metal sleeve of known elasticproperties.

The importance of the various calibrationsdepends on the type of pressuremeter andwhere it is being used. For example thecontribution of the hose supplying pressure to the probe is highly relevant if volumechanges are being measured at the surface,but is of no importance at all for a probe withinternal instrumentation, such as theCambridge family of devices.

Advantages

• A large number of fundamental soilproperties are obtained from a single test.

• To derive these properties, no empiricalcorrecting factors are needed.

• Measurements are made insitu at theappropriate confining stress.

• A large volume of material is tested –a typical test loads a column of material 0.5metres high and extending to more than 10times the expanded cavity radius. This is theequivalent of at least 1000 triaxial tests on38mm samples.

• Representative loads are applied – in theexample shown in fig. 2 about 12 tonnes isbeing applied to the cavity wall.

• Results can be obtained quickly as all thedata logging and most of the analysis iscarried out by automated systems.

• Commercial operation has shown that theinstruments, though more complex thanconventional site investigation equipment,are reliable.

• There are many materials whose propertiescan only be realistically determined by insitumeasurement.

• The pressuremeter test is particularlyappropriate for predicting the performance of laterally loaded piles.

• Pressuremeter tests are routinely used tocalibrate finite element models of complexgeotechnical problems.

Limitations

• The instrument will not penetrate gravels,claystones or the like, so generallypressuremeter testing requires support from conventional drilling techniques.

• Failure planes and deformation modes arenot always appropriate to those occurring in the final design. An estimate of theanisotropy of the material will be required in order to derive vertical parameters fromlateral values.

• Many familiar design rules and empiricalfactors are based on parameters obtainedfrom traditional techniques. It is not alwayspossible to use them with pressuremeterderived values, even if the insitu parametersmore accurately represent the true state ofthe ground.

• Only two stress paths can in practice befollowed, undrained and fully drained.

• The instruments and their associatedequipment are complex by conventional siteinvestigation standards and can only beoperated by trained personnel.

• Use of an inappropriate analysis to interpreta pressuremeter test can result in seriouslymisleading parameters.


Advantages and limitations of the pressuremeter test

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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 pressuremeterInsertion methods Self boring

Initial Diameter 83-89mm, depending on the configuration

Length of material sacrificed At least 1 metre of material 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 portable 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

73mm High Pressure Dilatometer (HPD73)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 material 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 highlevel 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.

95mm High Pressure Dilatometer (HPD95)Insertion methods Pre-bored hole or pocket



Length of material sacrificed At least 2 metres of material 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.


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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 Ménard 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 profile used to identify suitable locations for the pressuremeter test.

It is usually the case that our testing is one partonly of the operations being carried out in aborehole, and we are operating as specialistsub-contractors to the Main Contractor. Thispart of the booklet is concerned with makingclear the separation between what we supplyand what we need.

Self boringThis comes in three varieties:

A stand-alone drilling systemrequiring no additional equipment

There are not many circumstances where thisis possible but it does happen. Usually it will be a green field site. The system consists ofhydraulic rams to jack the probe, a small motorto rotate the inner drill string, and a water pumpto provide circulating fluid. A portable hydraulicpower pack and control panel distributes powerto the various units (fig. 3.1).

One difficulty is that kentledge for the hydraulicrams is limited, so in practice suitable materialwill be of low to medium strength only. The SBPmust drill every metre of the borehole soadditional testing is not an option. An amplewater source is required.

A stand-alone drilling systemoperating underneath a cable percussion rig

This is a common way of working, using all the special self boring drilling parts alreadymentioned but working in conjunction with acable percussion rig (fig. 3.2). The rig places a column of water well casing to a depth justabove where the first test is required,hammering it in the last 0.5 metre. The SBPsystem couples to the top of the casing columnand the skin friction on the casing is enough toallow self boring into most materials. An amplewater 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’. Therig open-holes to the next test depth, carryingout additional testing if required.

There are some locations that only a reducedheight cable percussion rig can access, so thecombined system is versatile.

If the hole is left open for a long time then thetested zones begin to collapse so a reasonablyquick operation is important.

A system for operating under a 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 asif it were a core barrel, but the adapter has athrust bearing to separate down-thrust fromrotation. Everything above the adapter spins,everything below is static and the probe entersthe ground without being rotated (fig. 3.3opposite).

Water needs to be supplied at appropriateflows. This means that the rig pump and waterswivel must be in good order, because the SBPwater path is a narrow annulus compared tonormal drill rod. Air mist can be used but ismore difficult and only suitable for relativelyshallow holes.

This system allows core to be taken in the test intervals. In material with a tendency tocollapse or in boreholes deeper than 40 metresit is the only appropriate option.

Pre-boringFor pre-boring the problems of getting theprobe into the ground are the responsibility ofthe drilling contractor. The additional issues tobe considered are these:


Additional considerations

Fig. 3.1 Self boring, stand-alone system

Fig. 3.2 Cable percussion system

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Size of borehole. It sometimes happens thatthe same size borehole is cored from surfaceto some considerable depth, and the HighPressure Dilatometer (HPD) must test layers inthis borehole. Because the probe is a close fitto the nominal core size this can be risky. Anymaterial falling down onto the probe can makeit difficult to recover the device.

Wireline coring. We are often asked toconsider adapting the equipment to work with a wireline coring system. The fit of the probe to the cavity has to be reasonably close for asuccessful test. It is not practical to test thecavity made by a wireline system with a probesmall enough to pass through the wireline corebit. There are wireline systems able to core attwo diameters but it is not advisable to use thewireline cable for lowering the pressuremeter. If the probe becomes trapped the wire cablewill not be able to exert more than a nominalforce to help pull it back. We thereforerecommend lowering the probe on rods. Theserods must has a diameter no greater than thediameter of the borehole less two times thediameter of the umbilical connecting the probe

to the pressure source on the surface. Thisumbilical must be taped at intervals to the rodto prevent loops occurring.

Inflation method. The HPD can be inflatedwith oil or air. The decision about what methodto use depends on circumstances. The besttest is obtained with oil because it allowspressure to change without large temperaturealteration. In good rock where certainty overtiny displacements is important this is an issue,especially where surface temperature isconsiderably different from the downhole state. However oil raises environmental issues.We use bio-degradeable transformer oil tominimise the risk.

Oil also gives a slower overall test, as time hasto be allowed for oil to return to the surface.There are ways of speeding up the process butit means adding an additional umbilical to thesystem, making the lowering and raisingprocedure more complex and time consuming.

For speed and convenience air inflation is usedin most circumstances. However, oil is always

used when calibrating the pressure capability ofthe probe on the surface because it is inherentlysafe in the event of a failure of any part.

Speeding up testing. The easiest way toaccelerate the test rate is to reduce the numberof lowering and raising events. We sometimestest a borehole that has been completely coredprior to our arrival. In such circumstances theprobe is lowered to the deepest location first,then tests are carried out in reverse order todepth. Normally the deepest part will be thetightest fit because the core barrel has madethe fewest passes.

Material with cavities. Limestone in particularcan be prone to solution cavities. Testing in thismaterial is frustrating because if the HPDmembrane is not completely supported at allplaces then it will burst at pressures too low togive useful data. Where such testing is requiredwe advise that the boreholes be cored inadvance of our arrival. They should then begrouted up. Once we are on site the groutedholes can be re-cored, with the grout coreavailable for inspection to prove the integrity of the cavity wall. The grout will be weakcompared to the limestone so no reinforcementtakes place.

Pre-boring with 47mm RPMThe difficulty with this device is that thediameter is smaller than the customary drillparts a drilling contractor can be expected tokeep. The holes for the RPM need to be formedwith drill bits and drill rods based on the AWsize. Typically the RPM is used to target certainlayers at significant depth, and in thesecircumstances there is no alternative but tomake a large diameter borehole first, then drilla 51mm or similar diameter pocket out of thebase of the larger hole. Provided the pocket forthe RPM is not too long (no more than sixmetres) then we supply the necessary rods totake the probe and umbilical from its small holeinto the larger hole. At this point we expect tocouple to whatever drill rods are available via a suitable adapter.

For very shallow tests, within 5 metres of thesurface, we can sometimes make the boreholeourselves using a powered hand auger.

Successful tests can also be made using theRPM to ream out an existing cone penetrometer(CPT) hole. This technique has been appliedwith some success in weak chalk, takingadvantage of a hole made by a 102cm CPT.

Fig. 3.3 Rotary rig system

The pressuremeter is normally used to carryout a cavity expansion test in a vertical hole.There are other more specialised tests that can be made and this section gives someexamples.

Horizontal testsFig. 4.1 shows an example of a self boringpressuremeter working horizontally. Thelocation is more than 200 metres below groundin a test tunnel researching the properties ofBoom Clay as a possible barrier medium forthe long term storage of nuclear waste. It wasnot permitted to use water as a drilling fluid, so the SBP was adapted to drill with air. Thecamera flash is reflecting off some of thereturning soil particles.

Horizontal testing has also been carried outwith pre-bored pressuremeters and inclined

holes are common-place. If one axis is arranged to be vertical when thepressuremeter is used horizontally then thiscan inform the analysis, because the verticalinsitu stress is normally known.

Creep testsFig. 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 hourduration. For each step the creep displace-ments, expressed as a percentage of the cavitydiameter, were plotted against log elapsedtime. The slope of this trend gives a stressdependent rate.

In this material the creep is substantial andmade it difficult to obtain an unload/reloadcycle, even after a long creep hold.

Consolidation testsThe SBP can carry out a holding test to obtainconsolidation parameters. It is a modification ofa normal undrained expansion test. Near thepoint where the cavity would be unloaded it isinstead held at that expansion and the excesspore water pressure (pwp)that has beengenerated is allowed to dissipate. As it does sothe effective stress at the cavity wall starts torise and the cavity wants to expand.

This triggers an automatic control system toreduce the total pressure at the cavity wall tocompensate. The net result is that the cavityremains at a constant diameter for as long asthe test is conducted. There is a closed formsolution for this situation [ref 6] that uses theparameters derived from the expansion phaseof the test and the time taken for 50% of thegenerated excess to dissipate.

Fig. 4.3 shows the dissipation data from twopwp cells, their mean and the total pressureresponse, plotted in a normalised form. Any of the profiles can give a value for thehorizontal 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

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Permeability testingFigs. 4.4 and 4.5 show the result of apermeability test carried out with a self boringpressuremeter. The procedure exploits theability of the pressuremeter to bore a pocket in the ground that it exactly fits. The stressconditions are, more or less, representative ofthe insitu state and are acting on the body ofthe probe, giving an excellent seal. As aconsequence the drill string now provides apipe from the surface down to the bottom of theprobe allowing access to the formation. For lowpermeability material the pipe work is filled withwater, is sealed off and is connected to theoutput of a small constant flow pump. This thenpressurises the water column. Fig. 4.4 showssteps of pressure, and the flow rates requiredto establish each step. Fig. 4.5 plots the flowrates 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 onegeometry – the tested pocket is zero lengthand the permeability is the mean of thehorizontal and vertical characteristics. If timeallows, then the probe can be pulled back togive a pocket of some length and the testrepeated. This gives a second permeabilityvalue where the horizontal characteristic ishaving a greater influence. Further pulling backallows additional values to be obtained. By abest fit process it is possible to identify theanisotropy factor for the horizontal and verticalconditions. In practice reconciling the data ismore complex than this implies because asmore and more of the material is exposed tothe test then a scale effect related to thevariability of the fabric becomes apparent [ref 26].

The permeability testing is an addition to theconventional expansion test, and is a way ofobtaining more data from one self boringepisode. If k is higher than 10-7m/sec then thesame concept can be used, but constant flow is not required and a falling head test can becarried out, measuring the height of the watercolumn 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

The front cover of this booklet gives anindication of the range of projects andenvironments where the pressuremeter testcan be used. It is widely used off-shore as wellas on land, in deserts, in mountains and intropical conditions.

Every project has its own set of problems anddifficulties that have to be overcome. This mightmean man-handling equipment in remotelocations, 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-containedoperation. Most of the time we have to workwith a local drilling contractor and operativeswho will be unacquainted with our equipmentand unused to what is required for a successfulpressuremeter test. This is not a major difficulty,and our engineers are accustomed to lookingafter the on-site training involved.

Our pressuremeters have been used on someof the world’s major civil engineering projects,such as Crossrail in London or the proposedcrossing of the Padma river in Bangladesh.What follows is a selection of some of the more unusual projects.

The underground researchfacility at Mol, BelgiumWe have at intervals over the last 15 yearsmade visits to the SCK-CEN facility at Mol,Belgium, to carry out pressuremeter testing inthe underground research facility HADES. Thisis a system of shafts and tunnels some 224metres below ground level in a zone of BoomClay in a highly plastic condition. The clay hasinteresting self healing properties whenfractured, displays extremely low levels ofpermeability and offers a possible solution to the problem of the disposal of high levelnuclear waste. Since 2000 the facility has beenrun by an expert group called EURIDICE andpressuremeter testing has been used duringthe construction of the facility and after toexamine the engineering properties of the clay.We ourselves began work there in 1999 with aself boring pressuremeter. We were not allowedto introduce water into the formation and sodrilled using air from a modified drill rig toimplement the self boring process. Specialcasing and drilling parts were designed by uswith some help from the drilling contractor togive the ideal flow path for delivering the airand returning the cuttings. The bulk of thetesting has been horizontal. Speed is important

in this material – it must be bored and testedas rapidly as possible because after one hourthe material will close onto the probe withsufficient force to make extracting theequipment almost impossible.

Successful pre-bored tests have also beencarried out with a 95mm HPD. This allowed a larger pressure to be applied and a greatercavity expansion achieved than is possible with a self bored probe.

Linkshttp://www.euridice.be/http://www.sckcen.be/en/Our-Research/Research-facilities/HADES-Underground-laboratory

Testing waste andinvestigating barrier wallsWe were approached by Dr Neil Dixon ofLoughborough University (now Professor ofEngineering) about the possibility of using apressuremeter to investigate the mechanicalproperties of municipal solid waste. Most of thework took place at a landfill site in Calvert,Buckinghamshire. The waste was a mixture ofresidential and commercial residue, not well-


Projects where our pressuremeters have been used

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sorted, in various stages of degradation anddepending on its age could be lightly to heavilycompacted.

The primary purpose of the testing was toobtain engineering parameters that wouldpermit the interaction between the body of thewaste and the components of the protectivebarriers to be modelled and quantified.

Self boring, pushing and pre-boring were all attempted. If metallic materials wereencountered then the damage caused toequipment could be spectacular. The mostsatisfactory results were obtained with a 95mmHPD, where the large expansion capabilityproved to be helpful. The pockets for this werecut dry, using a modified bit resembling a largehole cutter. The waste is heterogeneous, maybe partially saturated and of no particularparticle size so the results were variable andthe analytical processes were not necessarilyappropriate. However shear stiffness fromunload/reload cycles proved to be a plausibleand repeatable parameter, and it was possibleto relate the stiffness values to stress level.

Partly as a result of this work we becameinterested in the properties of the barriersthemselves, and have (in conjunction withCambridge University) carried out research

work on the mechanical properties of man-made and natural barriers, with specialattention being paid to permeability.

ReferenceDixon, N, Whittle, R, Jones, DRV, Ng’ambi, S(2006) Pressuremeter Tests in Municipal Solid Waste:Measurement of Shear Stiffness.Géotechnique, 56(3), pp 211-222.

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

Kolkata Metro East-WestprojectTwenty four self bored tests were carried out at four critical locations along the alignment ofthe proposed metro in Kolkata, India. Thepressuremeter testing component of the siteinvestigation had been specified by W S Atkins.For the most part the tests were conventionalin material that behaved either as a clay orsand. What was different about this project wasthe technical and practical difficulties that hadto be overcome to achieve success.

The work was conducted on a 24 hour basis at pavement locations in the heart of the city.

The only rig available turned out to be a small,light and rather old quill drive system wheremost of the controls had long broken down.Rate of rotation and advance was down to theskill of the driller, who knew his rig and how tocoax results from it. On more than oneoccasion the rig rotation system broke downwhilst driving the pressuremeter, and theboring was completed by rotating rods by hand.

In some ways a worse problem was aninadequate water pump, as no boring ispossible if the pump is not delivering asufficient flow. However as our report noted atthe time, these issues were a problem for therate of progress of the fieldwork rather than thetests themselves, which were of reasonablequality.

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 complextransport infrastructure in such locations andthis project is typical of the kind of testing weare asked to do.

Fig. 5.1 Self boring in Kolkata

The following pages are examples ofpressuremeter tests from a range of materialswith illustrations of how engineeringparameters can be derived.

CASE A. Analysis of a selfbored pressuremeter test in London ClayThe most straightforward test to analyse is anundrained cavity expansion and contraction inclay, where a self boring pressuremeter hasbeen used. The insertion disturbance is likely to be small and the undrained path means it is easy to calculate radial and circumferentialstresses and strains directly from thedisplacement and pressure measurementsmade by the instrument. There are a number ofanalyses that can be applied; what is describedhere is one approach. The test itself was overwater so depth is referred to bed level.

Fig. 1. Field curve

The test is logged as a set of readings ofpressure and displacement. At intervals theloading is interrupted to make a smallunload/reload cycle. These cycles can also be taken on the final contraction.

Fig. 2. Lift-off

The first action when analysing the data is toselect a plausible co-ordinate of stress anddisplacement that represents the origin for thecavity expansion. The stress value is the point where some movement is apparent. The displacement ordinate is close to zero, a feature of self boring.

Fig. 3. Shear strength (a)

Having selected an origin, displacement can beconverted to strain and the data analysed. Thisfigure shows the result of plotting the loadingdata on semi-log scales and identifying theultimate slope and intercept. These give shearstrength and limit pressure [ref 9].

16

Worked examples

Fig. 1 Field curve

Fig. 2 Lift-off

Fig. 3 Shear strength (a)USING PRESSUREMETERS

CAMBRIDGE INSITU 17

Fig. 4. Shear strength (b)

This is a similar procedure but applied to thefinal contraction data. It is of special interestbecause the origin at the start of unloading isan observable point – the origin used for theinitial loading is always uncertain due todisturbance [ref 19].

Fig. 5. Shear modulus (a)

This is a simple approach to derive an estimateof the shear modulus, by taking the slope of the chord bisecting a cycle of unloading andreloading. In a linear elastic material theunloading and reloading data would coincide.Here the cycle appears hysteretic, indicatingthat modulus varies with strain.

Fig. 6. Shear modulus (b)

This non-linear stiffness behaviour can berepresented by a power law. Here the reloadingdata from the previous plot are redrawn on log-log scales and the slope and interceptidentified. These two parameters allow thecurrent shear stress to be predicted at anystrain [ref 3].

The two parameters are referred to as α (theshear stress constant) and β, the non-linearexponent. β will take a value between 0.5 and1, where 1 is a linear elastic response. Thesemay be combined to give secant shearmodulus Gs for a particular value of shearstrain γ, as follows: Gs =αγβ -1.

This expression is good for values of shearstrain down to 10-4 , the resolution limit of ourprobes. This is not small enough to predict Gmax which is probably found at a shear strainnearer 10-5.

Fig. 4 Shear strength (b)

Fig. 5 Shear modulus (a)

Fig. 6 Shear modulus (b)

Fig. 7. Stiffness/strain

The trend of declining stiffness with strain isdrawn here for each cycle. Because the test isvirtually undrained the three cycles give almostexactly the same result. The lines come fromthe power law results, the data points fromapplying 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 forcomparison with the measured data. The non-linear stiffness parameters are assumedcorrect. A tiny alteration to the origin reconcilesloading 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 aset of parameters describing the strength andstiffness of the material, and the insitu stressstate. There are differing levels of uncertainty in these values. One method for resolving thisuncertainty is to see if the parameter set can

reproduce the measured field curve. Everymeasured data point could be calculated if theunderlying stress:strain curve was known. The soil model used here assumes a non-linear elastic/perfectly plastic stress:straincurve for which there is a closed-form solution.

The essence of such solutions is to define thestress and strain required to make the materialyield, then integrate this condition betweenknown boundaries. In the implementationshown here only the insitu horizontal stress is treated as a free variable.

CAMBRIDGE INSITU 19

Fig. 9. Test in chalk

The picture is slightly misleading because itshows the final output of the analysis. Theadditional features of this test compared withthe 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 betweenthe point of first contact and cavity strainzero. This is a consequence of unloading the cavity prior to the test.

• The initial part of the expansion containsshort duration stress holds, to monitor thecreep 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 thanthe 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 reference stress and displacement

It is not possible to discover the initial stressstate by inspection, so a method is usedwhereby estimates are back-calculated fromthe yield stress. The plot above consists ofthree views. The main display shows about2mm of the initial expansion. The slope of thestiffest part has been used to estimate initialshear modulus. The onset of plasticity is wherethe data points move away from the slope line.Initially the reference stress is guessed, thedisplacement ordinate of that stress giving anorigin for calculating strain. An analysis formobilised shear stress near failure is carriedout, and a calculated failure stress derived. Thisshould coincide with the observed value. If not,the guess of cavity reference stress is adjustedand the cycle repeated until a match is found.

The chart on the left shows the creepdisplacements from the holds included in theexpansion phase of the test. Creep seems to

fall to a minimum in the vicinity of the cavityreference stress estimate and increases againnear the yield stress estimate.

This analysis is used regardless of whether theloading conditions are drained or undrained. It is expected to give a higher bound estimatefor reference stress but a lower bound value for 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 nowdescribed. This is a test in weak chalk madewith a pre-bored pressuremeter. The pocket forthe probe was made by rotary coring. Theanalysis is harder because the disturbancecaused by pre-boring and the complete

unloading of the cavity prior to the probe beingplaced means that little can be gleaned fromthe initial response. It is also complicatedbecause the material is highly permeable andtherefore the test is a drained loading. Thismeans that it is not so easy to derive radial

strain and circumferential stress frommeasured pressuremeter co-ordinates ofpressure and displacement. Account has to be taken of dilatant properties, possiblecohesion and the ambient pore water state.

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

Fig. 11. Friction angle

Because the expansion is drained a differentanalysis for strength is required. The gradient of a log-log plot of effective stress and strain isused to produce a value for the internal angleof friction and dilation. Ambient water pressureand the residual friction angle have to beknown or estimated [ref 16].

Fig. 12. Stiffness/strain

Non-linear modulus parameters are obtained inthe same manner as a test in clay. Because thetest is drained each cycle plots a higher trend,related to the mean effective stress. The cycleon the final unloading shows the stiffestresponse because the mean stress is thatwhich 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 thesame non-linear elastic/perfectly plastic shearstress:shear strain curve as for the undrainedcase. It is less well constrained:

• Cohesion is also unknown as well as theinsitu lateral stress.

• Shear modulus parameters must be adjustedfor stress level.

• Poisson’s ratio is required, and this probablyhas to be guessed.

• Ambient water pressure and residual frictionangle are required.

• The solution takes no account of tensilestrength which begins to be an issue asmaterial approaches a rock like condition.

Despite these cautions the procedure iscapable of producing plausible matches to thefield data.

In this example the cavity reference pressurefrom the yield stress analysis gives the best fitcurve but it has been necessary to make aslight adjustment to the origin for strain. Theshear modulus at yield is nearly 3 times greaterthan the value provided by the initial slope, atypical result for a pre-bored test. The solutionis also able to provide a value for the limitpressure of the material [ref 4].


Fig.11 Friction angle

Fig. 12 Stiffness/strain

Fig. 13 Drained curve modelling

CAMBRIDGE INSITU 21

Fig. 14. Elastic deformation only

The example is from a test in intact limestone.Although not obvious, there are twounload/reload cycles in this test, virtuallyindistinguishable from the loading path. Theonly parameter that is sensible to take from thistest is an estimate of shear modulus from thelatter part of the loading, giving a value greaterthan 15GPa, or in terms of Young’s modulus40GPa. The total displacement once the probehas contacted the cavity wall is only about 80microns, so careful calibration of the probe forcompliance effects is essential. A shearmodulus of 15GPa is about the limit of what the probe can determine before the calibrationuncertainty exceeds the apparent value.

In general it is the poorer material that is ofmost interest, especially those where corerecovery is poor or does not produce intactsamples for laboratory testing. The finalexample is from a test carried out in weatheredlimestone.

Fig. 15. Elastic deformation with tensile failure

The test shows two cracks forming, one at4.8MPa and another at 8.1MPa. The event istoo fast for any data points to be recorded sothe plot shows a sudden jump at these stresslevels.

The slope of the loading curve changes as aresult of the tensile failure. Slope ‘A’ is stifferthan slope ‘B’ which is stiffer than slope ‘C’. Not so obvious is the fact that the reload cycles have a different slope and are notrepresentative of the properties of the intactrock – 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 rockIf failure in shear is an identifiable point in apressuremeter test then it is always possiblethat analyses for strength and initial stressstate can be carried out. In rock, the material

can be so good that the probe reaches itsmaximum working pressure with only elasticdeformation being seen. All that can be easilyderived from such tests is a value for shear

modulus. It it important to derive this from aslate in the test as possible so that the formingof the pressuremeter cover against the rock isnot confused with movement of the rock itself.

Fig. 16. Interpreting creep readings

In this figure, creep readings are plotted on theleft and the data in the main display are ‘end ofcreep readings’ only. No parameters are quotedexcept for the initial slope and its intercept onthe displacement axis. Although the materialappears to have failed in shear, the loadingcurve is actually three lines of differing slopeswith the shear failure stress not yet reached.After each crack has occurred, creepdisplacements reduce in magnitude.

Curiously, the one parameter that it is possibleto identify with only limited uncertainty is thehorizontal cavity reference pressure, Po. Thefirst crack appears at 4MPa total radial stress. At this point the circumferential stress must be zero or below. It follows that Po can be nogreater than 2MPa, and if the tensile strengthwere known, could be narrowed down evenfurther.


Fig. 16 Interpreting creep readings

CAMBRIDGE INSITU 23

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References

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23. MUIR WOOD, D. (1990)Strain dependent soil moduli and pressuremeter tests. Géotechnique, 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 interpretation of the pressuremeter test, Géotechnique 22 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. Géotechnique, 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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30. WINDLE, D. and WROTH, C.P.(1977)“The Use of a Self-boring Pressuremeter to determine the Undrained Properties of Clays.” Ground Engineering, September.

31. WITHERS, N.J., HOWIE, J., HUGHES, J.M.O. and ROBERTSON, P.K. (1989)“Performance and Analysis of Cone Pressuremeter Tests in Sands.” Géotechnique 39, No. 3, pp 433-454.

32. WROTH, C.P. (1984)“The Interpretation of In Situ Soil Tests.” Twenty Fourth Rankine Lecture, Géotechnique 34, No. 4, pp 449-489.


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