# ASTM D7702D7702M-14

Designation: D7702/D7702M − 14Standard Guide forConsiderations When Evaluating Direct Shear ResultsInvolving Geosynthetics1This standard is issued under the fixed designation D7702/D7702M; the number immediately following the designation indicates theyear of original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of lastreapproval. A superscript epsilon (´) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This guide presents a summary of available informationrelated to the evaluation of direct shear test results involvinggeosynthetic materials.1.2 This guide is intended to assist designers and users ofgeosynthetics. This guide is not intended to replace educationor experience and should only be used in conjunction withprofessional judgment. This guide is not intended to representor replace the standard of care by which the adequacy of agiven professional service must be judged, nor should thisdocument be applied without consideration of a project’s manyunique aspects. Not all aspects of this practice may beapplicable in all circumstances. The word “Standard” in thetitle of this document means only that the document has beenapproved through the ASTM consensus process.1.3 This guide is applicable to soil-geosynthetic andgeosynthetic-geosynthetic direct shear test results, obtainedusing either Test Method D5321/D5321M or D6243/D6243M.1.4 This guide does not address selection of peak orlarge-displacement shear strength values for design. Refer-ences on this topic include Thiel (1)2, Gilbert (2), Koerner andBowman (3), and Stark and Choi (4).1.5 The values stated in either SI units or inch-pound unitsare to be regarded separately as standard. The values stated ineach system may not be exact equivalents; therefore, eachsystem shall be used independently of the other. Combiningvalues from the two systems may result in non-conformancewith the standard.1.6 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:3D653 Terminology Relating to Soil, Rock, and ContainedFluidsD5321/D5321M Test Method for Determining the ShearStrength of Soil-Geosynthetic and Geosynthetic-Geosynthetic Interfaces by Direct ShearD6243/D6243M Test Method for Determining the Internaland Interface Shear Strength of Geosynthetic Clay Linerby the Direct Shear MethodD4439 Terminology for Geosynthetics3. Terminology3.1 Definitions—For definitions of terms relating to soil androck, refer to Terminology D653. For definitions of termsrelating to geosynthetics and GCLs, refer to TerminologyD4439.3.2 Definitions of Terms Specific to This Standard:3.2.1 adhesion, caor c, n—the y-intercept of the Mohr-Coulomb shear strength envelope; the component of shearstrength indicated by the term ca, in Coulomb’s equation, τ =ca+ σ tan δ.3.2.2 failure envelope, n—curvi-linear line on the shearstress-normal stress plot representing the combination of shearand normal stresses that define a selected shear failure criterion(for example, peak and post-peak). Also referred to as shearstrength envelope.3.2.3 Mohr-Coulomb friction angle δ,n—angle of frictionof a material or between two materials (degrees), the angledefined by the least-squares, “best-fit” straight line through adefined section of the shear strength-normal stress failureenvelope; the component of the shear strength indicated by theterm δ, in Coulomb’s equation, τ =c+σ tan δ.3.2.4 Mohr-Coulomb shear strength envelope, n—the least-squares, “best-fit” straight line through a defined section of theshear strength-normal stress failure envelope described the1This guide is under the jurisdiction ofASTM Committee D35 on Geosyntheticsand is the direct responsibility of Subcommittee D35.04 on Geosynthetic ClayLiners.Current edition approved May 1, 2014. Published June 2014. Originallyapproved in 2011. Last previous edition approved in 2013 as D7702_D7702–13a.DOI:10.1520/D7702_D7702M–14.2The boldface numbers in parentheses refer to a list of references at the end ofthis standard.3For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at service@astm.org. For Annual Book of ASTMStandards volume information, refer to the standard’s Document Summary page onthe ASTM website.Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States1equation τ =ca+ σ tan δ. The envelope can be described forany chosen shear failure criteria (for example, peak, post-peak,or residual).3.2.5 secant friction angle, δsec,n—(degrees) the angledefined by a line drawn from the origin to a data point on theshear strength-normal stress failure envelope. Intended to beused only at the shearing normal stress for which it is defined.3.2.6 shear strength, τ,n—the shear force on a given failureplane. In the direct shear test it is always stated in relation tothe normal stress acting on the failure plane. Two differenttypes of shear strengths are often estimated and used instandard practice:3.2.6.1 peak shear strength, n—the largest value of shearresistance experienced during the test under a given normalstress.3.2.6.2 post-peak shear strength, n—the minimum, orsteady-state value of shear resistance that occurs after the peakshear strength is experienced.3.2.6.3 Discussion—Due to horizontal displacement limita-tions of many commercially available shear boxes used todetermine interface shear strength, the post-peak shear strengthis often specified and reported as the value of shear resistancethat occurs at 75 mm [3 in.] of displacement. The end user iscautioned that the reported value of post-peak shear strength(regardless how defined) is not necessarily the residual shearstrength. In some instances, a post-peak shear strength may notbe defined before the limit of horizontal displacement isreached.4. Significance and Use4.1 The shear strength of soil-geosynthetic interfaces andgeosynthetic-geosynthetic interfaces is a critical design param-eter for many civil engineering projects, including, but notlimited to waste containment systems, mining applications,dam designs involving geosynthetics, mechanically stabilizedearth structures, and reinforced soil slopes, and liquid im-poundments. Since geosynthetic interfaces often serve as aweak plane on which sliding may occur, shear strengths ofthese interfaces are needed to assess the stability of earthmaterials resting on these interfaces, such as a waste mass orore body over a lining system or the ability of a final cover toremain on a slope. Accordingly, project-specific shear testingusing representative materials under conditions similar to thoseexpected in the field is recommended for final design. Shearstrengths of geosynthetic interfaces are obtained by either TestMethod D5321/D5321M (geosynthetics) or D6243/D6243M(geosynthetic clay liners). This guide touches upon some of theissues that should be considered when evaluating shearstrength data. Because of the large number of potentialconditions that could exist, there may be other conditions notidentified in this guide that could affect interpretation of theresults. The seemingly infinite combinations of soils,geosynthetics, hydration, and wetting conditions, normal loaddistributions, strain rates, creep, pore pressures, etc., willalways require individual engineering evaluations by qualifiedpractitioners. Along the same lines, the list of referencesprovided in this guide is not exhaustive, nor are the findingsand suggestions of any particular reference meant to beconsidered conclusive. The references and their related find-ings are presented herein only as examples available in theliterature of the types of considerations that others have founduseful when evaluating direct shear test results.4.2 The figures included in this guide are only examplesintended to demonstrate selected concepts related to directshear testing of geosynthetics. The values shown in the figuresmay not be representative and should not be used for designpurposes. Site specific and material-specific tests should al-ways be performed.5. Shear Strength Fundamentals5.1 Mohr first presented a theory for shear failure, showingthat a material experiences failure at a critical combination ofnormal and shear stress, and not through some maximumnormal or shear stress alone. In other words, the shear stress ona given failure plane was shown to be a function of the normalstress acting on that plane (5):τ 5 f~σ! (1)If a series of shear tests at different values of normal stressis performed, and the stress circle corresponding to failure isplotted for each test, at least one point on each circle mustrepresent the normal and shear stress combination associatedwith failure (6). As the number of tests increases, a failureenvelope (line tangent to the failure circles) for the materialbecomes apparent (Fig. 1).FIG. 1 Curved Mohr Failure Envelope and Equivalent Mohr-Coulomb Linear Representation (from Wright (7)).D7702/D7702M − 1425.2 In general, the failure envelope described by Eq 1 is acurved line for many materials (5). For most geotechnicalengineering problems, the shear stress on the failure plane isapproximated as a linear function of the total or effectivenormal stress within a selected normal stress range, as shownin Fig. 1. This linear approximation is known as the Mohr-Coulomb shear strength envelope. In the case of total stresses,the Mohr-Coulomb shear strength envelope is expressed as:τ 5 ca1σ tan δ (2)where:τ = shear stress,σ = normal stress,δ = friction angle (degrees), andca= adhesionIn the case of effective stresses, the linear failure envelope is:τ 5 ca 1~σ 2 u! tan δ (3)orτ 5 ca 1σ’ tan δ’where:u’ = pore pressure,σ’ = effective normal stress,δ’ = drained friction angle (degrees), andca’ = effective stress adhesionNOTE 1—Adhesion, ca, is commonly associated with interface shearstrength results. Cohesion, c, is often associated with internal shearstrength results involving soils or GCLs. Mathematically, these terms arethe identical; simply the y-intercept of the Mohr-Coulomb shear strengthenvelope, or in other words, the component of shear strength indicated bythe term ca, in Coulomb’s equation, τ = ca+ σ tan δ.NOTE 2—The end user is cautioned that some organizations (forexample, FHWA (8), AASHTO (9) along with state agencies who areusing these documents) are currently using the Greek letter, Delta (δ), todesignate wall-backfill interface friction angle and the Greek letter, Rho(ρ), to designate the interface friction angle between geosynthetics andsoil.5.3 Since most laboratory direct shear tests do not includepore pressure measurements, shear strength results reported bylaboratories are normally expressed in terms of total normalstress. For direct shear tests involving geosynthetics, TestMethods D5321/D5321M and D6243/D6243M provide recom-mendations for shear displacement rates intended to allowdissipation of pore water pressures generated during shearing.Recommended shear rates are 0.2 in./min for geosynthetic(non-GCL) interface tests, 0.04 in./min for geosynthetic/soil(including hydrated GCLs) interface tests (10), and 0.004in./min for hydrated GCL internal shear tests (11). However, asshown by Obermeyer et al. (12), even slower displacementrates may be needed for GCLs and high-plasticity clay soils toensure that positive pore pressures do not develop duringshearing. If tests involving GCLs or clays are loaded or shearedtoo quickly, excess pore water pressures could develop, andresults may not be representative of field conditions, which areoften assumed to be drained. The assumption of drainedconditions is reasonable because drainage layers are commonin liner systems and because field loading rates are generallyslow (13, 11). From Eq 3, positive pore pressures that are notallowed to dissipate will decrease the measured shear stress.Tests that are sheared undrained may yield erroneous resultssimilar to those discussed in Section 9. Drained and undrainedstrengths are not interchangeable from a design perspective.5.4 Combinations of shear stress and normal stress that fallon the Mohr-Coulomb shear strength envelope indicate that ashear failure will occur. Combinations below the shear strengthenvelope represent a non-failure state of stress (14). A state ofstress above the envelope cannot exist, since shear failurewould have already occurred.6. Measurement and Reporting of Shear Strength by TestMethods D5321/D5321M / D6243/D6243M6.1 The shear resistance between geosynthetics or betweena geosynthetic and a soil is determined by placing the geosyn-thetic and one or more contact surfaces, such as soil, within adirect shear box. A constant normal stress representative offield stresses is applied to the specimen, and a tangential(shear) force is applied to the apparatus so that one section ofthe box moves in relation to the other section. The shear forceis recorded as a function of the shear displacement of themoving section of the shear box.6.2 The test is run until the shear displacement exceeds 75mm [3 in.] or other value specified by the user. Note that 75mm of displacement is the practical upper limit of most directshear devices.6.3 The testing laboratory plots the test data as a graph ofapplied shear force versus shear displacement. The peak shearforce and the shear force at the end of the test are identified.The shear displacements associated with these shear forces arealso determined. An example set of shear-displacement plotsfor a typical textured geomembrane/reinforced GCL interfaceis shown in Fig. 2a. Typical shear-displacement behavior ofgeosynthetic interfaces is discussed further in Section 9.6.4 The shear stresses applied to the specimen for eachrecorded shear force are calculated by dividing the shear forceby the specimen area. For tests in which the area of specimencontact decreases with increased displacement, a corrected areashould be calculated, unless other technical interpretationarrangements are made ahead of time between the engineer andthe testing laboratory.6.5 The testing laboratory plots the peak shear stress andpost-peak (also known as large displacement) shear stressversus applied normal stress for each test conducted. Anexample set of shear stress-normal stress plots for a typicaltextured geomembrane/reinforced GCL interface is shown inFig. 2b.6.6 The testing laboratory then draws a least-squares “best-fit” straight line through the peak shear stress data points, Eq 2.The intercept of the straight line with the y-axis (x = 0) is theadhesion, cafor interface strength or cohesion intercept c, forinternal strength. Taking the inverse tangent of the slope of thestraight line yields the peak angle of friction, δpeak. Th