1、Designation: D7702 11Standard Guide forConsiderations When Evaluating Direct Shear ResultsInvolving Geosynthetics1This standard is issued under the fixed designation D7702; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year o
2、f last revision. A number in parentheses indicates the year of last reapproval. Asuperscript 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 involvi
3、nggeosynthetic 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
4、which the adequacy of agiven professional service must be judged, nor should thisdocument be applied without consideration of a projects 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 do
5、cument 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 or D62431.4 This guide does not address selection of peak orlarge-displacement shear strength va
6、lues for design. Refer-ences on this topic include Thiel (33), Gilbert (12), Koernerand Bowman (16), and Stark and Choi (31).1.5 The values stated in SI units are to be regarded asstandard. No other units of measurement are included in thisstandard.1.6 This standard does not purport to address all o
7、f 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:2D653 Terminology Relati
8、ng to Soil, Rock, and ContainedFluidsD5321 Test Method for Determining the Coefficient of Soiland Geosynthetic or Geosynthetic and Geosynthetic Fric-tion by the Direct Shear MethodD6243 Test Method for Determining the Internal and Inter-face Shear Resistance of Geosynthetic Clay Liner by theDirect S
9、hear MethodD4439 Terminology for Geosynthetics3. Terminology3.1 DefinitionsFor definitions of terms relating to soil androck, refer to Terminology D653. For definitions of termrelating to geosynthetics and GCLs, refer to TerminologyD4439.3.2 Definitions of Terms Specific to This Standard:3.2.1 adhes
10、ion, nca,the component of shear strength indi-cated by the term ca, in Coulombs equation, t = ca+ s tan f3.2.2 Mohr-Coulomb friction angle f, nangle of frictionof a material or between two materials, degrees) the angledefined by the least-squares, “best-fit” straight line through adefined section of
11、 the shear strength-normal stress failureenvelope; the component of the shear strength indicated by theterm f, in Coulombs equation, t =c+s tan f3.2.3 coeffcient of friction, na constant proportionalityfactor relating shear to normal stress for a defined failurecondition over a specific range of nor
12、mal stresses.3.2.4 cohesion c, nthe component of shear strength indi-cated by the term c, in Coulombs equation, t =c+s tan f3.2.5 direct shear strength test, nfor geosynthetics, aprocedure in which the interface between a geosynthetic andany other surface, under a range of normal stresses specified
13、bythe user, is stressed to failure by the horizontal movement ofone surface against the other.3.2.6 failure envelope, nLine on the shear stress-normalstress plot representing the combination of shear and normalstresses that would result in a shear failure.3.2.7 secant friction angle, fsec, ndegrees)
14、 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.1This guide is under the jurisdiction ofASTM Committee D35 on Geosyntheticsand is the direct responsibi
15、lity of Subcommittee D35.04 on Geosynthetic ClayLiners.Current edition approved Feb. 1, 2011. Published April 2011. DOI:10.1520/D7702112For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume inf
16、ormation, refer to the standards Document Summary page onthe ASTM website.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.4. Significance and Use4.1 The shear strength of soil-geosynthetic interfaces andgeosynthetic-geosynthetic inte
17、rfaces is a critical design param-eter for waste containment systems, mining applications, damdesigns involving geosynthetics, reinforced slopes, and liquidimpoundments. Since geosynthetic interfaces often serve as aweak plane on which sliding may occur, shear strengths ofthese interfaces are needed
18、 to assess the stability of a wastemass or ore body over a lining system or the ability of a finalcover to remain on a slope. Accordingly, project-specific sheartesting using representative materials under conditions similarto those expected in the field is recommended for final design.Shear strengt
19、hs of geosynthetic interfaces are obtained byeither Test Methods D5321 (geosynthetics) or D6243 (geosyn-thetic clay liners). This guide touches upon some of the issuesthat should be considered when evaluating shear strength data.Because of the large number of potential conditions that couldexist, th
20、ere may be other conditions not identified in this guidethat could affect interpretation of the results. The seeminglyinfinite combinations of soils, geosynthetics, hydration, andwetting conditions, normal load distributions, strain rates,creep, pore pressures, etc., will always require individualen
21、gineering evaluations by qualified practitioners. Along thesame lines, the list of references provided in this standard is notexhaustive, nor are the findings and suggestions of anyparticular reference meant to be considered conclusive. Thereferences and their related findings are presented herein o
22、nlyas examples available in the literature of the types of consid-erations that others have found useful when evaluating directshear results.4.2 The figures included in this guide are only examplesintended to demonstrate selected concepts related to sheartesting of geosynthetics. The values shown in
23、 the figures maynot 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 (1776) first presented a theory for shear failure,showing that a material experiences failure at a criticalcom
24、bination of normal and shear stress, and not through somemaximum normal or shear stress alone. In other words, theshear stress on a given failure plane was shown to be a functionof the normal stress acting on that plane (5):t5fs! (1)If a series of shear tests at different values of normal stressis p
25、erformed, 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 (24). As the number of tests increases, a failureenvelope (line tangent to the failure circles) for the mat
26、erialbecomes apparent (Fig. 1).5.2 In general, the failure envelope described by Equation(1) is a curved line for many materials (5). For most geotech-nical engineering problems, the shear stress on the failure planeis approximated as a linear function of the total or effectivenormal stress within a
27、 selected normal stress range, as shownin Fig. 1. This linear approximation is known as the Mohr-Coulomb failure envelope. In the case of total stresses, theMohr-Coulomb failure envelope is expressed as:t5c 1stan f (2)where:t = shear stress,s = normal stress,f = friction angle (degrees), andc = cohe
28、sion (or adhesion between two materials)In the case of effective stresses, the linear failure envelope is:t5c 1 s2u!tanfor (3)t5 c1s tanf (3)where:u = Pore pressure,s = effective stress,f = drained friction angle (degrees), andc = effective stress cohesion or adhesion5.3 Since most laboratory direct
29、 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 and D6243 provide recommendations forshear displacement rates intended to all
30、ow excess pore waterpressures generated during shearing to dissipate. Recom-mended shear rates are 0.2 in/min for geosynthetic (non-GCL)interface tests, 0.04 in/min for geosynthetic/soil (includingGCLs) interface tests (37), and 0.004 in/min for GCL internalshear tests (7). However, as shown by Ober
31、meyer et al (23),even slower displacement rates may be needed for GCLs andhigh-plasticity clay soils to ensure that positive pore pressuresdo not develop during shearing. If tests involving GCLs orclays are loaded or sheared too quickly, excess pore waterpressures could develop, and results may not
32、be representativeof field conditions, which are often assumed to be drained. Theassumption of drained conditions is reasonable because drain-age layers are common in liner systems and because fieldloading rates are generally slow (11, 7). From Equation (3),positive pore pressures that are not allowe
33、d to dissipate willdecrease the measured shear stress. Tests that are shearedundrained may yield erroneous results similar to those dis-cussed in Section 9.5.4 Combinations of shear stress and normal stress that fallon the Mohr-Coulomb failure envelope indicate that a shearFIG. 1 Curved Mohr failure
34、 envelope and equivalent Mohr-Coulomb linear representation (from Wright, 2005).D7702 112failure will occur. Combinations below the failure enveloperepresent a non-failure state of stress (1).Astate of stress abovethe envelope cannot exist, since shear failure would havealready occurred.6. Measureme
35、nt and Reporting of Shear Strength by TestMethods D5321/D62436.1 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
36、 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 horizontal displacement of themoving section of the shear box.6.2 The test is
37、run until the horizontal displacement exceeds75 mm (3 in.) or other value specified by the user. 75 mm ofdisplacement is the practical upper limit of most direct sheardevices.6.3 The testing laboratory plots the test data as a graph ofapplied shear force versus horizontal displacement. The peakshear
38、 force and the shear force at the end of the test areidentified. The horizontal displacements associated with theseshear forces are also determined. An example set of shear-displacement plots for a typical textured geomembrane/reinforced GCL interface is shown in Fig. 2a. Typical shear-displacement
39、behavior of geosynthetic interfaces is discussedfurther 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 cor
40、rected 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
41、 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,Equation (2). The y-intercept
42、 of the straight line withx=0axisis the adhesion intercept, cafor interface strength or cohesionintercept c, for internal strength. The slope of the straight lineis the peak coefficient of friction. Taking the inverse tangent ofthe peak coefficient of friction yields the peak angle of friction,fpeak
43、. This exercise is then repeated to determine the largedisplacement cohesion and the large displacement angle offriction, fLD.7. Evaluation of the Mohr-Coulomb Failure Envelope7.1 Traditionally, the laboratory-reported Mohr-Coulombstrength parameters c and f have been used to assess thestability of
44、slopes containing geosynthetics using limit equi-librium methods. Although Test Methods D5321 and D6243call for the testing laboratory to draw a best-fit line through theshear stress-normal stress data and determine c and f,itisstrongly recommended that the design engineer also evaluatethe data to d
45、etermine the appropriate strength parameters to beused in a slope stability analysis.7.2 It is important to note that the reported Mohr-Coulombparameters only define the failure envelope for the range ofnormal stresses tested. Extrapolation of both friction angle andcohesion intercept outside the ra
46、nge of normal stresses testedmay not be representative. Extrapolating the failure envelopebelow the lowest normal stress tested can overestimate shearstrength, since the failure envelopes for many geosyntheticinterfaces can curve sharply to the origin. Similarly, extrapo-lating the failure envelope
47、above the highest normal stressFIG. 2 Typical Shear-Displacement Curves (a) and Peak and Large Displacement Failure Envelopes (b) for a Textured Geomembrane/Needlepunch Reinforced GCL InterfaceD7702 113tested can overestimate shear strength, since the failure enve-lope for many geosynthetic interfac
48、es flatten at high loads (27).If some extrapolation is required, a conservative and safemethod would be as follows (34):Extrapolation of the shear strength envelope to lowernormal loads would go from the result tested at the lowestnormal load back through the (0,0) origin.Extrapolation of the shear
49、strength envelope to high normalloads would go from the result tested at the highest normal loadwith a horizontal line of constant shear strength.Any extrapolation of shear strengths with resultingstrengths greater than these suggestions cannot be defended bythe test results.7.3 In the sample laboratory report shown in Fig. 2b, thepeak Mohr-Coulomb failure envelope, in kPa, is described by:tpeak= 24.9 + s tan 23 (tpeak= 520 + s tan 23, in psf). Thelarge-displacement Mohr-Coulomb failure envelope, in kPa, isdescribed by: tLD= 18.2 + s tan 12 (tLD= 380
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