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Part1 Test methods

By C Stoewahse, N Dixon, DRV Jones, W Blumel and

P Kamugisha*

IntroductIonGeosynthetics are increasingly being used for engineering applications.They are typically used in conjunction with soil and other types ofgeosynthetics and have a number of roles such as reinforcement,separation, drainage and as barriers. They are employed in a range ofdiverse applications such as landfill barriers and construction of steepsoil slopes.

The use of geosynthetics in a structure introduces potential planesof weakness, resulting in a requirement to assess the stability alonginterfaces between soil and geosynthetic and between geosynthetic andgeosynthetic. Figure 1 shows a schematic of a landfill containmentsystem. It demonstrates the range of materials that can be used andemphasises the importance of the soil/geosynthetic systems in con-trolling slope stability of both the capping system and the side slopesduring construction (ie prior to waste placement).

Shear behaviour of side slope*CStcewahse, Gesellschaft fiir interfaces also influences post-Grundbau and Umwelttechnik, waste placement barrier perfor-Braunschwelg

Dixon and p Kamugtsha mance. Settlement of the waste canDepartmentof Civil and Building, result in overstressing of theEhgineering,Loughborough geomembrane via transfer of load

through shear at the interface andDRV Jones, Golder Associates (UK),biottt~ngham 'his can lead to loss of barrierWBliimel, lnstitutfiirGrundbau, integrity.Bodenmechanik und Figure 2 shows a relativelyEnergiewasserbau,Universityof steep landflill side slope being linedHarm over

with a geomembrane and illus-

trates the importance of interface shear strength in ensuring the con-structability of such structures.

Assessment of stability requires detailed knowledge of thestress/strain behaviour of interfaces, as post peak shear strengths areoften mobilised resulting from the strain incompatibility of soils,geosynthetics and waste materials.

It is common practice to measure interface shear strength in a directshear apparatus (DSA) as used in soil mechanics but with a muchlarger shear plane. Design parameters are obtained by carrying outperformance tests (ie using site specific materials and relevant bound-ary conditions).

There are three standards in common use that provide guidance ontesting procedures; BS 6906:1991,ASTM D5321-92 and a German recom-mendation for landfill design GDA E 3-8 of 1998 (Gartung and Neff,1998).The final version of a preliminary European standard (prEN WI00189015) is imminent. In addition, a significant number of researchpapers have been published on this topic in the past 15 years.

It would appear therefore that there is adequate information andguidance to ensure high quality testing is carried out. However this isnot the case. There is growing evidence that tests specified to obtainparameters for design, and those reported in the literature, often lacksufficient control on the key factors affecting the measured values.This results in uncertainty regarding the likely variability of mea-sured shear strengths, and in some instances is leading to the use ofover estimated interface strengths in design.

This paper provides a summary of the key factors influencing mea-sured shear strength behaviour. It gives guidance, references key pub-lications on the main issues controlling the measurements andincludes test results that illustrate typical observed behaviour.

A companion paper (Dixon et al, 2002, to be published in GroundEngineering March) discusses the selection of characteristic strengthparameters from laboratory results for use in design.

While the issues included here are important for the assessment ofall geosynthetics, there are specific additional considerations for thetesting of geogrids, geonets and geosynthetic clay liners (GCL) that arenot covered. Some of the recommendations given are relevant fordirect shear tests entirely on soil, although care should be exercised inapplying the findings of this study as important issues specific to soiltests have not been included.

Variability of measured Interface strengthsIn recent years, research has been conducted to quantify the likelyvariability of test results and to identify key factors that controlmeasured strengths. As part of the development of the new Europeangeosynthetic test standard, inter-laboratory comparison tests wereconducted in an effort to quantify the likely scatter in measuredstrengths resulting from the use of different operators and testequipment (Gourc gr Lalarakotoson, 1997).

Tests were carried out in seven commercial and research laborato-ries —two each in France, Germany and UK and one in Italy —using

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geosynthetic materials supplied by the co-ordinator and obtained fromone source. The interface shear strengths between a range of geosyn-thetic materials and standard sand were measured.

Two similar, and complementary, inter-laboratory comparison testprogrammes were conducted by a working group of the GermanSociety for Geotechnical Engineering in 1995 and 1996, as part of theirresponse to development of the European standard (Bl(imel andStoewahse, 1998). The latter programme incorporated a more detailedspecification of the testing procedure. These programmes, each involv-ing approximately 20 laboratories, produced a range of measuredstrengths that is similar to the European study. Figures 3 and 4 showtest results from the German studies for a non-woven geotextile v sandinterface.

The significant variability of the shear stress v displacement curvesin Figure 3 is typical. The different laboratories produced a range ofpeak and large displacement shear strengths, and widely varyingstress vs displacement relationships. Figure 4 shows the distributionof peak failure envelopes obtained by the laboratories.

In addition to the large variation of results, of particular concern isthat some laboratories produced high, and hence unsafe, shearstrengths. Inspection of the data in Figure 3 shows clearly that some ofthe results are significantly in error (indicated by the shape of theshear stress v displacement curves). An experienced engineer wouldnot use these results and would require repeat tests to be carried out.By removing this spurious data the variability could be reduced sig-nificantly.

However, it is worth noting that all tests were conducted by labora-tories experienced in measuring geosynthetic interface shear strength,and that the laboratories knew their results would be compared withthose from a large number of other laboratories (ie their competitors).It can only be assumed that those who submitted the spurious testresults must have considered them to be correct. This indicates thatsome laboratories lack the experience to interpret the results theyobtain.

There are three categories of factors that lead to variability of mea-sured interface shear strength:

a) test apparatus designb) operator/test procedurec) variability of both geosynthetic and soil materials.Both the European and German test programmes used clearly

defined common test standards and samples from a common source,but involved different operators and a range of different DSA designs.

Repeatability can be improved by using one design of DSA and oneoperator, although the results may have a consistent error. Test pro-

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grammes have been carried out under these conditions at HannoverUniversity (Bliimel et al, 1996) and Loughborough University (Dixon etal, 2000).

Scatter of results from these tests "under conditions of repeatabili-ty" would be primarily due to variation in the geosynthetic and soiltest material. Some results of these studies are shown in Figure 5,together with the results of inter-laboratory tests, as coefficient ofvariation (standard deviation/mean) v normal stress for interfaces

36 GROUND ENGINEERING FEBRUARY 2002

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Direct shear apparatus (DSA)with different top box supports

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Tnble 1:Key elements of test stnndnrds

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Test apparatus

nseeeL1 setIndex tests + some guidanceon perfor mance testing

DSA 'about 300mmsquare'rsn

N081 88018Index tests only

DSA minimum shear area300mm square

ASTM 08321.88Performance tests

DSA minimum shear area300mm square

Specific requirements of DSA o „applied through rigid load plateMeasure vertical deformationsDesign of box not specified

Design should allow forsand dilation, o„+2%Fluid filled membrane systemsallowed for application of o„Measure vertical movement ofloading plate at end of test

o„applied by device that maintainsa constant uniform <r„fordurationof test+ 2%Design should allow for soildeformation during shearing

Number of tests conducted

Material conditioning

Method of fixing geosynthetics

Soil properties

9tests in total, o„=50, 100and 200,kPa (3 tests at each o,)Highlights need to conduct tests indifferent directions and on differentsides of geosyntheticSandandgeosynthetic20 + 5 C

Clamp or glued to rigid sub-stratum

Complying with fraction B(1.18mmto 600 Ijm) BS4550Compacted dry II~ = 1.65~

1.7Mg/m'erformancetests, compact soilatw,to92+ 2% p~

4 tests in total, v, = 50,2 x 100and 150kPa

Minimum of 3o„,user definedTest different directions and sides

User definedTake care not to damage geosyntheticduring placementMeasure p and w after test

Sand and geosynthetic 20'+ 2'C Soil and geosynthetic 21'C+ 2'CHumidity 65% + 2% if applicable Humidity 65% + 5% if applicable

Fix geosynthetic to rigid support to Clamping outside shear area orprevent any relative displacement gluing to rigid sub-stratumbetween specimen and support(eg glue, friction support in sheararea or clamped outside area)

Standard sand in accordance withEN 196-1(1.6mm to 0.08mm)Compacted wof 2% topd=1.75Mg/m3

Maximum particle size andgap size (top/bottom base)

Sand v geosynthetic (index) notspecifiedSoil vs. geosyn. (performance) gap isp~/2 or 1mm for fine grained soils.Maximum particle size < 1/8thbox depthGeosynthetic v geosynthetic gapnotspecified

Maximum particle not applicableGap size = 0.5mm

Maximum particle size<1/6th box depthSoilvgeosynthetic gap)d of soilGeosynthetic v geosynthetic gap notspecified

Locationof materialsinDSA

Shearing rate

Geosynthetic v geosyntheticrigid sub-stratum (ie not soil)Soil v geosynthetic either rigidsub-stratum, geosynthetic orsoil in top box.Depth of soil layer not specified

Geosynthetic v geosynthetic andsand v geosynthetic (index)2mm/minSoil v geosynthetic, variable ratedepending on drainage

Sand v geosynthetic, rigidsub-stratum in bottom box andsand in top boxDepth of sand layer = 50mm

Sand v geosynthetic 1mm/min

Geosynthetic v geosynthetic rigidsub-stratum (ie no soil)Soil v geosynthetic, geosyntheticsupported by rigid sub-stratumSoil either in top or bottom boxDepth of sand layer not specified

Geosynthetic v geosynthetic, Smm/minif no material specificationSoil v geosynthetic, slow enough todissipate excess pore pressuresIf no excess pore water pressuresexpected use 1mm/min

Derivation of shear strengthparameters

Obtain 8,5,from best fit straightline through all 9pointsDisregard any apparent adhesion(a) values

Best fit straight lines through allpoints (peak and residual) toobtain, 5R, 5„,nR and o „

Failure envelopes defined by bestfit straight lines to obtain strengthparameters 8,8,and Y intercepts

Specific reporting requirements All plots and calculationsDescribe failure modeReports 'f sand

'For comparison of index test results, All plots and calculationsall graphs and data have to besubmitted to judgement of an

engineer'escriptionof 'post peakbehaviour observed in each

test'8

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604E~Performance tests

DSA minimum shear area300mm square, for geosyntheticswithout surface structure andfine grained soil 100mm square

Design of DSA not specifiedMeasurement of normal andfriction stresses and of verticalmovementCalibration measurementsrecommended to determine thestress acting in the friction plane

3tests with 3different normalstresses and 2 repeating tests withthe mean value, which should matchthe expected normal stress insitu

Soil mechanical laboratoryconditions

Recommendations aboutsupport and fixation ofgeosynthetics depending on theindividual test case

Cohesive soils with not morethan95o%%d ~,'onthewetside'r as proposed by thelandfill designer. Not less than 24hpreconsdolidation time undernormal stress equal to the test.Noncohesive soils compacted tomedium density or as proposedby the landfill designer.

Maximum particie size d <1/15th of box lengthGap size is depending on testmaterials and has tobe chosen sothat there cannot developadditional normal forces by theff arne and secondary frictionplanes; chosen gap size has to bereported

Geosynthetic v geosynthetic rigid sub-stratum(ie normally no soil)Soil v geosynthetic, geosyntheticsupported by rigid sub-stratumSoil either in top or bottom box.Depth of soil layer not specified

Geosynthetic v geosynthetic and non-cohesive soil v geosynthetic,0.167 to1mm/minGeotextile v cohesive soil0.167mm/min.Geosynthetic liner v cohesive soil0.005mm/min

Tests should be performedindependently by a second institutionBest fit straight lines through allpoints(peak and residual) toobtaintestvaluesof 5,5„a anda,Derivation characteristic values.Disregard any apparent adhesion(u) values for non-cohesive soilsand for cohesive soils in specialconstruction cases

Detailed report about the testequipment, procedures andobservations during testing, aboutthe measured data and the furtherevaluation

between sand and a geotextile as well as betweena geotextile and a geomembrane. Each point rep-resents a number of tests on materials from thesame source conducted at the same normalstress.

The two important trends that can beobserved are the reduced scatter of dataobtained if tests are carried out in one labora-tory (not surprisingly), and an increase in thecoefficient of variation with decreasing nor-mal stress for all repeatability testing. The lat-ter is of practical importance to landfill coversystem design due to the increased uncertaintyin measured interface strengths at low normalstresses.

Unfortunately, rather than provide confidencein the ability to undertake reproducible tests,the results of these inter-comparison test pro-grammes cast doubt on the applicability ofaspects of current test procedures. The mainfactors that result in variability of test data arediscussed in subsequent sections.

Detailed evaluation of the results from theinter-laboratory comparison test programmesand discussions with participants indicates thatone of the main reasons for the scatter of mea-sured strength data is the different DSA devicesused for test performance. Bliimel andStoewahse (1998) investigated some effects ofdesign of DSA device on test results and thiswork has been extended by Stoewahse (2001).

Most of the DSA used for shear testing withgeosynthetics has a top box or frame with a testarea of about 300mm by 300mm. The lower parthas an equal test area or a box that is longerthan the upper one in the direction of shearmovement so that the shear area is kept con-stant during the test. Stoewahse (2001) used fourtypes of DSA with different top box supportsand with different loading and load controllingsystems (Figure 6).

The floating top box designed by Casagrandeis supported at one point only and is able torotate around this support (Figure 6a). Thedesign of the top box is well known from soiltesting and is in accordance with BS 1377:Part 7and ASTM D 3080. However, there are only a fewdevices of this type with sufficiently large testareas for interface friction testing.

The type of DSA mainly used for geosynthet-ic interface testing is constructed in a mannersuch that the top box is fixed (Figure 6c).

This device was designed specifically to mea-sure interface shear behaviour. The fixed topbox was introduced on the premise that a pre-formed shear plane (ie the interface) is locatedbetween the bottom and top boxes, and henceformation of a shear plane with associated volu-metric straining of material in the top box doesnot occur.

However, this assumption is wrong for testingof many combinations of materials and hencethere are concerns regarding the magnitude andtime dependent variation of the vertical stresson the interface during shearing.

Vertical stress is usually applied by air orwater pressure via a membrane, and hence isknown only on the top of the sample. Frictionbetween the test material and internal walls ofthe top box both during application of normalstress and shearing will alter the actual verti-cal stress acting in the shear plane by anunknown amount.

Five of these devices are in use in the UK andmore than forty in the US. Approximately 25 ofthese fixed top box devices are in use inGermany.

A device was modified in co-operation withmanufacturer Wille GeoTechnik in Goettingen,

Germany to overcome the problem of unknownvertical stress in the shear plane with the fixed-box-DSA (Figure 6d).

The modification allows the average verticalstress acting on the interface to be determinedby measuring the vertical support forces to thetop box. The pressure applied to the top of thesample is then regulated to keep the resultingvertical force on the interface at a constantvalue.

Another approach to improve the DSA wasmade by separating the loading system from theupper box and allowing it to move vertically, butnot to rotate during the test (Figure 6b). Thevertically moveable top box together with a con-trol system ensures that the vertical stressapplied to the interface remains constant duringthe testing process (Stoewahse, 2001).

This configuration was selected as the stan-dard DSA design incorporated in the GermanDIN 18 137-3.

Variation of resuttscausedby differentypes of test apparatusResults of friction tests on the sand-geotextileinterface conducted in different DSA-types, allwith a shear plane of 300mm square, arepresented below. The normal stresses appliedwere between 20kPa and 200kPa. At each normalstress at least two individual tests with threedifferent densities of dry sand were conducted.

Additionally, direct shear tests on the sandwere performed in all devices. Referencevalues for the internal shear strength of thesand were obtained in triaxial tests. The sandused for the test is the standard sand accordingto EN 196-1. This sand was also used in theEuropean and German inter-laboratory compar-ison test programmes.

In Figure 7 the peak shear strength vs nor-mal stress measured with different types of DSAis plotted for the interface between very densesand and a geotextile. It is obvious that the fixedbox device gives considerably higher peak shearstrength values than the other DSA. The resultsobtained with the other three devices seem to becomparable. As discussed above, using the fixedbox gave higher values caused by restraintforces (Stoewahse, 2001).

In Figure 8, the friction values 8, derived bylinear regression, for the sand and geotextileinterface are plotted v void ratio of the sand. Forcomparison, the results of direct shear testswith this sand are shown in a similar diagramtogether with some data from triaxial tests.These were performed to obtain calibration datafor the angle of internal friction g.

It can be seen that for all void ratios investi-gated the friction parameters measured withboth fixed box devices are somewhat higherthan those from the other DSA types. The valuesof p obtained with the vertically movable boxare in good accordance with the results of thetriaxial tests.

In Figure 9 shear force T and normal force Nv displacement s developed during the test areplotted for the DSA types with fixed and withvertically movable top box. In the upper graphthe shear force v displacement curves areshown. The applied load was P = 9kN which isequal to a normal stress of 100kPa. This loadvalue P is marked in the lower diagram by a fullline.

The observed variation of N in the fixed topbox is due to friction forces acting at its internalwall. During shear these forces are increasingand this affects the related shear force T. In thevertically movable box the force N acting nor-mal to the interface remains nearly constant

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during the whole test and closely matches the applied load P. It can beconcluded that the fixed top box design should not be used for testswholly on soil. This research has led to the DSA type with verticallymovable top box being recommended for shear testing on soil andgeosynthetic interfaces in Germany.

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Test procedureThe four standards listed in the introduction provide useful guidance ontesting procedures and evaluation of measured data for both specifierand operator. ASTM gives guidance for performance testing of soil vgeosynthetic and geosynthetic v geosynthetic interfaces. BS6906Part 8essentially covers only index tests on these two types of interface,although some limited guidance on performance testing is provided inAppendix A.

The proposed European standard is restricted to index tests on stan-dard sand v geosynthetic interfaces. The BS and ASTM are about 10years old and therefore do not include recent developments, and theproposed CEN document is of limited use for designers, as it only cov-ers index testing. GDA E3-8 is specifically devoted to landfill design andgives detailed recommendations for performance testing of all kinds ofinterfaces for liner systems and covers, although it is not available inEnglish.

Table 1 summarises the scope and guidance provided by the teststandards. This section of the paper provides a summary of the guid-ance given by each of these standards and comments on key elementsof the test procedure, including references to papers detailing recentresearch. Example results are given showing the influence of selectedaspects of the test procedure.

None of the guidelines specify the construction of the testing devicealthough detailed specification of the DSA exists in all the standardsfor direct shear tests on soils. As there are many DSA with a fixed topbox or similar specifications in use, it is important that a full descrip-tion of the testing equipment is provided with the test results. Theinvestigating laboratory should comment on the key question of howthe effective normal stress on the interface is calculated or measuredduring shearing (Bliimel and Stoewahse, 1998, and Bliimel et al, 2000).

In fixed top box DSA, the gap between the top and bottom boxes mustbe set prior to shearing. Advice from the test standards is both ambigu-ous and outdated. The gap size must be so small that no soil particlescan migrate out of the box but it must also be large enough so that noconstraints are induced. This is nearly impossible to achieve in a fixedbox DSA as shown above.

Bemben and Schulze (1998)demonstrated that the gap size has a sig-niflicant effect on the measured strength. Unfortunately, they did notdescribe the type of DSA they used in the study. The use of a gap sizevalue in accordance with ASTM D5321 and BS6906 can lead to signif-ican errors if additional considerations on the materials to be testedare not made.

Practical experience of the authors indicates that the accuracy withwhich the gap can be adjusted is not less than +0.5mm in a 300mmsquare DSA. This is in the typical range of grain size d~ for sands,which is recommended as the gap size in both ASTM D5321 andBS6906.

For friction tests with geosynthetics, their compressibility has to beconsidered. In tests with vertically movable and tilting top boxes theinitial gap size is not as important as there is an immediate relief ofconstraints if the gap chosen at the start of the test is too small.

The thickness of a soil layer placed in the top box and the surfaceroughness of the load plate can also affect the test results. For a sand-geotextile interface Stoewahse (2001) varied the thickness of the sandlayer in the top box and used two load plates, one with a smooth and theother with a rough surface. The tests were performed in a 300mmsquare DSA with a fixed top box. Results are shown in Figure 10.

It can be concluded that in a 300mm square box a sample thicknessof at least 50mm is sufficient for sandy materials. For fine-grainedcohesive soils it was found that the sample thickness could be reducedto about 30mm to shorten the time required for consolidation beforestarting the test. Generally it is recommended that a rough load platebe used for these tests.

Stretching of geosynthetics during friction testing may occur andhas to be prevented by modified fixing techniques. If the stress-dis-placement curves or the geosynthetic samples indicate that stretchinghas occurred, the test should be omitted and repeated with a sufficientfixing technique.

Tests should be carried out in a temperature controlled environ-ment (20'C +2'C) and using materials conditioned to this temperature.Pasqualini et al (1993) documented temperature effects on frictionbehaviour of geosynthetic interfaces and showed that they can besignificant.

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As water is likely to be present in most of the applications ofgeosynthetics in landfill design it is recommend that friction tests beperformed with submerged materials, unless special conditions have tobe taken into consideration by the design engineers.

For tests with clayey soils this is very important as such materialcan absorb water. Swelling leads to a reduction in interface shearstrength. Consideration should be given to whether the softened stateis relevant for stability analyses of liner systems. In performance test-ing of geosynthetic v geosynthetic interfaces it is also important to usesite-specific soils in the top box (ie overlying the upper geosynthetic).Jones and Dixon (1998) showed that grading, particle size and particleshape have a direct influence on the shear strength of a geomembranev non-woven geotextile interface. In a liner system the soil above a geo-textile affects the micro scale distribution and magnitude of normalstress at the friction interface.

The shearing rates specified in the standards for geosynthetic vgeosynthetic and sand v geosynthetic tests are appropriate. Stark et al(1996) and Stoewahse (2001) showed that peak shear strength valuesare independent of shearing rate for the range 0.03mm to 40mm perminute.

However, for performance testing, appropriate shearingrates must be specified according to the critical conditions expected onsite (ie drained or undrained). Drained tests can take many hours oreven days when involving cohesive soils and therefore are seldom car-ried out, although effective strength parameters are often required indesign!

Reporting of resultsThe evaluation of different landfill slope failures has shown that acontributing factor is a lack of communication between theparticipating parties. It is important to inform the testing engineerabout the project details and the site-specific conditions to ensurecorrect testing conditions are applied and thus appropriate resultsobtained.

In addition, the test method and results must be documented in amanner such that design engineers —who might not be knowledgeableabout the testing practice —are able to interpret the results and usethem in stability calculations to produce a rigorous design.

From the inter-laboratory comparison test programmes it has beenfound that even apparently minor changes in testing conditions canaffect the results significantly. Therefore, a detailed test report is nec-essary and must include information on the following:

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SummaryInterface shear strengths involving geosynthetics are often measuredincorrectly. At present, available test standards provide limitedguidance and this is leading to inconsistencies. Results of inter-laboratory comparison testing programmes in Europe and Germanyhave shown a large scattering of friction test results. A significantelement of the data scattering is believed to be due to the construction ofthe testing devices.

Additional research on the influence of several testing boundaryconditions has been undertaken at Loughborough University, UK, andHannover University, Germany. Experimental and numerical investi-gations have shown a significant effect of the support of the upper boxon the test results. Kinematical restrictions of the upper box causeconstraint forces in the system and this leads to high values of mea-sured interface shear strengths.

A direct shear apparatus with a vertically movable top box has beendeveloped at Hannover University. Friction tests on different geosyn-thetic interfaces gave reliable values of shear strength. Results ofdirect shear tests in this device on sand are in good agreement with tri-axial test results. The direct shear device with a vertically movable boxis recommended as a standard device for interface testing.

Moisture conditions, temperature, particle size and grading of thesoils involved are some of the many additional factors influencing testresults. This paper gives advice on the selection of testing conditionsand other factors that must be reported with the results and consideredin the application of the test data.

Information on the variation of test data and guidance on obtainingcharacteristic values of interface strength parameters for use indesign calculations is given in the companion paper.

0 20 40 80 80 100

8 (mm)

~ Description of test device: Including support of the top box, loadapplication, etc.

~ Test set up and boundary conditions: Shearing rate, samples testeddry or submerged, consolidation time, method of fixing thegeosynthetics to resist stretching, exact location of interface in relationto top and bottom boxes, gap between top and bottom boxes (if relevant),placement method of soils (eg compaction effort and layer thickness)and density and water content before and after the test.

~ Full material descriptions: Geosynthetics: manufacturer, mass perarea, thickness, polymer, description of the structure, etc; soils: origin,soil mechanical classification, other mechanical parameters,

~ Description of sampling methods employed: Geosynthetics:sample preparation, eg pre-soaking; soils: any form of pretreatment likecrushing of aggregates, drying, adding of water.

~ Test results: Shear stress v displacement curves, peak shear stress vnormal stress plots, large displacement shear stress v normal stressplots, volumetric changes v displacement if relevant, soil mechanicsparameters at beginning and end of test, shear strength parameters 5and tz and the method of derivation (eg linear regression).

~ State of the materials after the test: Stretching of thegeosynthetics, abrasion of geomembrane textures, orientation ofgeotextile fibres, post-shearing damage such as tearing of stitchbonding or welding points, development of additional shear zones ingeocomposites and also in soils, changes of water content etc.

The list above is necessarily incomplete. With the development ofnew geosynthetics other aspects might become important. For exam-ple, creep aspects are not usually considered in short-term frictiontests but can occur under compression during application of the nor-mal stress (eg when testing geocomposite drains made from polymerfoam pieces).

Further details are given in the standards summarisedin Table I, especially in GDA E3-8. Procedures for evaluation of testresults and derivation of characteristic values are presented in thecompanion paper "Geosynthetic interface shear behaviour: Part 2Characteristic values for use in design" (to be published in GE March2002).

AcknowlediementsProduction of this paper was part funded by The British Council and theGerman Academic Exchange Service as part of the British GermanAcademic Research Collaboration Programme. Patience Kamugisha isfunded by Golder Associates and Loughborough University and thework of Carl Stoewahse was conducted at Hannover University fundedby a number of German civil engineering companies.

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