1、Designation: D7702 13D7702/D7702M 13aStandard Guide forConsiderations When Evaluating Direct Shear ResultsInvolving Geosynthetics1This standard is issued under the fixed designation D7702;D7702/D7702M; the number immediately following the designation indicatesthe year of original adoption or, in the
2、 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 information related to the evaluation of dir
3、ect shear test results involvinggeosynthetic materials.1.2 This guide is intended to assist designers and users of geosynthetics. This guide is not intended to replace education orexperience and should only be used in conjunction with professional judgment. This guide is not intended to represent or
4、 replacethe standard of care by which the adequacy of a given professional service must be judged, nor should this document be appliedwithout consideration of a projects many unique aspects. Not all aspects of this practice may be applicable in all circumstances.The word “Standard” in the title of t
5、his document means only that the document has been approved through the ASTM consensusprocess.1.3 This guide is applicable to soil-geosynthetic and geosynthetic-geosynthetic direct shear test results, obtained using eitherTest Method D5321 or D6243.1.4 This guide does not address selection of peak o
6、r large-displacement shear strength values for design. References on thistopic include Thiel (331),)2, Gilbert (122), Koerner and Bowman (163), and Stark and Choi (314).1.5 The values stated in either SI units or inch-pound units are to be regarded as standard. No other units of measurement areinclu
7、ded in this separately as standard. The values stated in each system may not be exact equivalents; therefore, each system shallbe used independently of the other. Combining values from the two systems may result in non-conformance with the standard.1.6 This standard does not purport to address all o
8、f the safety concerns, if any, associated with its use. It is the responsibilityof the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatorylimitations prior to use.2. Referenced Documents2.1 ASTM Standards:3D653 Terminology Relating
9、 to Soil, Rock, and Contained FluidsD5321 Test Method for Determining the Shear Strength of Soil-Geosynthetic and Geosynthetic-Geosynthetic Interfaces byDirect ShearD6243 Test Method for Determining the Internal and Interface Shear Resistance of Geosynthetic Clay Liner by the Direct ShearMethodD4439
10、 Terminology for Geosynthetics3. Terminology3.1 DefinitionsFor definitions of terms relating to soil and rock, refer to Terminology D653. For definitions of termtermsrelating to geosynthetics and GCLs, refer to Terminology D4439.3.2 Definitions of Terms Specific to This Standard:3.2.1 adhesion, ca,
11、ncthea,The y-intercept of the Mohr-Coulomb shear strength envelope; the component of shear strengthindicated by the term ca, in Coulombs equation, = ca + tan .1 This guide is under the jurisdiction of ASTM Committee D35 on Geosynthetics and is the direct responsibility of Subcommittee D35.04 on Geos
12、ynthetic Clay Liners.Current edition approved Jan. 15, 2013May 1, 2013. Published April 2011June 2013. Originally approved in 2011. Last previous edition published 2011 approved in 2013as D770211. DOI:10.1520/D77021313. DOI:10.1520/D7702_D7702M13A.2 The boldface numbers in parentheses refer to a lis
13、t of references at the end of this standard.3 For referencedASTM standards, visit theASTM website, www.astm.org, or contactASTM Customer Service at serviceastm.org. For Annual Book of ASTM Standardsvolume information, refer to the standards Document Summary page on the ASTM website.This document is
14、not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Becauseit may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropri
15、ate. In all cases only the current versionof the standard as published by ASTM is to be considered the official document.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States13.2.2 failure envelope, ncurvi-linear line on the shear stress-no
16、rmal stress plot representing the combination of shear andnormal stresses that define a selected shear failure criterion (for example, peak and post-peak). Also referred to as shear strengthenvelope.3.2.3 Mohr-Coulomb friction angle , nangle of friction of a material or between two materials, degree
17、s)materials (degrees),the angle defined by the least-squares, “best-fit” straight line through a defined section of the shear strength-normal stress failureenvelope; the component of the shear strength indicated by the term , in Coulombs equation, = c + tan .3.2.4 Mohr-Coulomb shear strength envelop
18、e, nthe least-squares, “best-fit” straight line through a defined section of the shearstrength-normal stress failure envelope described the equation = ca + tan . The envelope can be described for any chosen shearfailure modecriteria (for example, peak, post-peak, or residual).3.2.4 failure envelope,
19、 nLine on the shear stress-normal stress plot representing the combination of shear and normal stressesthat would result in a shear failure.3.2.5 secant friction angle, sec, ndegrees)(degrees) the angle defined by a line drawn from the origin to a data point on theshear strength-normal stress failur
20、e envelope. Intended to be used only at the shearing normal stress for which it is defined.3.2.6 shear strength, , nthe shear force on a given failure plane. In the direct shear test it is always stated in relation to thenormal stress acting on the failure plane. Two different types of shear strengt
21、hs are often estimated and used in standard practice:peak shear strength, nthe largest value of shear resistance experienced during the test under a given normal stress.post-peak shear strength, nthe minimum, or steady-state value of shear resistance that occurs after the peak shear strength isexper
22、ienced.NOTE 1Due to horizontal displacement limitations of many commercially available shear boxes used to determine interface shear strength, thepost-peak shear strength is often specified and reported as the value of shear resistance that occurs at 75 mm (3 in.) of displacement. The end user iscau
23、tioned that the reported value of post-peak shear strength (regardless how defined) is not necessarily the residual shear strength. In some instances,a post-peak shear strength may not be defined before the limit of horizontal displacement is reached.3.2.6.1 peak shear strength, nthe largest value o
24、f shear resistance experienced during the test under a given normal stress.3.2.6.2 post-peak shear strength, nthe minimum, or steady-state value of shear resistance that occurs after the peak shearstrength is experienced.3.2.6.3 DiscussionDue to horizontal displacement limitations of many commercial
25、ly available shear boxes used to determine interface shear strength,the post-peak shear strength is often specified and reported as the value of shear resistance that occurs at 75 mm 3 in. ofdisplacement. The end user is cautioned that the reported value of post-peak shear strength (regardless how d
26、efined) is notnecessarily the residual shear strength. In some instances, a post-peak shear strength may not be defined before the limit ofhorizontal displacement is reached.3.2.7 shear strength envelope, ncurvi-linear line on the shear stress-normal stress plot representing the combination of shear
27、and normal stresses that define a selected shear failure mode (for example, peak and post-peak).4. Significance and Use4.1 The shear strength of soil-geosynthetic interfaces and geosynthetic-geosynthetic interfaces is a critical design parameter formany civil engineering projects, including, but not
28、 limited to waste containment systems, mining applications, dam designsinvolving geosynthetics, reinforced slopes, and liquid impoundments. Since geosynthetic interfaces often serve as a weak planeon which sliding may occur, shear strengths of these interfaces are needed to assess the stability of e
29、arth materials resting on theseinterfaces, such as a waste mass or ore body over a lining system or the ability of a final cover to remain on a slope. Accordingly,project-specific shear testing using representative materials under conditions similar to those expected in the field is recommendedfor f
30、inal design. Shear strengths of geosynthetic interfaces are obtained by either Test MethodsMethod D5321 (geosynthetics) orD6243 (geosynthetic clay liners). This guide touches upon some of the issues that should be considered when evaluating shearstrength data. Because of the large number of potentia
31、l conditions that could exist, there may be other conditions not identified inthis guide that could affect interpretation of the results. The seemingly infinite combinations of soils, geosynthetics, hydration, andwetting conditions, normal load distributions, strain rates, creep, pore pressures, etc
32、., will always require individual engineeringevaluations by qualified practitioners. Along the same lines, the list of references provided in this standardguide is not exhaustive,nor are the findings and suggestions of any particular reference meant to be considered conclusive. The references and th
33、eir relatedfindings are presented herein only as examples available in the literature of the types of considerations that others have found usefulwhen evaluating direct shear test results.4.2 The figures included in this guide are only examples intended to demonstrate selected concepts related to di
34、rect shear testingof geosynthetics. The values shown in the figures may not be representative and should not be used for design purposes. Sitespecific and material-specific tests should always be performed.D7702/D7702M 13a25. Shear Strength Fundamentals5.1 Mohr (1776) first presented a theory for sh
35、ear failure, showing that a material experiences failure at a critical combinationof normal and shear stress, and not through some maximum normal or shear stress alone. In other words, the shear stress on a givenfailure plane was shown to be a function of the normal stress acting on that plane (5):5
36、f! (1)If a series of shear tests at different values of normal stress is performed, and the stress circle corresponding to failure is plottedfor each test, at least one point on each circle must represent the normal and shear stress combination associated with failure (246).As the number of tests in
37、creases, a failure envelope (line tangent to the failure circles) for the material becomes apparent (Fig. 1).5.2 In general, the failure envelope described by EquationEq 1 (1) is a curved line for many materials (5). For most geotechnicalengineering problems, the shear stress on the failure plane is
38、 approximated as a linear function of the total or effective normal stresswithin a selected normal stress range, as shown in Fig. 1. This linear approximation is known as the Mohr-Coulomb shear strengthenvelope. In the case of total stresses, the Mohr-Coulomb shear strength envelope is expressed as:
39、 5 ca1 tan (2)where: 5 shear stress, 5 normal stress, 5 friction angle degrees!,andc a 5 adhesion5ca1tan (2)where: = shear stress, = normal stress, = friction angle (degrees), andca = adhesionIn the case of effective stresses, the linear failure envelope is:5ca 1 tan (3)or5ca 1(2u) tan5ca 1(2u) tan
40、(3)or5ca 1 tan where:u = Pore pressure,u = pore pressure, = effective normal stress,FIG. 1 Curved Mohr failure envelope and equivalent Mohr-Coulomb linear representation (from Wright, 2005).Wright (7).D7702/D7702M 13a3 = drained friction angle (degrees), andc = effective stress adhesionca = effectiv
41、e stress adhesionNOTE 1Adhesion, ca, is commonly associated with interface shear strength results. Cohesion, c, is often associated with internal shear strength resultsinvolving soils or GCLs. Mathematically, these terms are the identical; simply the y-intercept of the Mohr-Coulomb shear strength en
42、velope, or in otherwords, the component of shear strength indicated by the term ca, in Coulombs equation, = ca + tan .NOTE 2The end user is cautioned that some organizations (e.g. FHWA, (for example, FHWA (8), AASHTO (9) along with state agencies who areusing these documents) are currently using the
43、 Greek letter, Delta (), to designate wall-backfill interface friction angle and the Greek letter, Rho (), todesignate the interface friction angle between geosynthetics and soil (Reference: AASHTO, 2010. LRFD Bridge Design Specifications, 5th Edition,American Association of State Highway and Transp
44、ortation Officials, Washington, D.C. and FHWA, 2009. Mechanically Stabilized Earth Walls andReinforced Soil Slopes, Design and Construction Guidelines, U.S. Department of Transportation, Federal Highway Administration, Washington DC,FHWA NHI-10-024 Vol I, NHI-10-025 Vol II, and as FHWA GEC011).soil.
45、5.3 Since most laboratory direct shear tests do not include pore pressure measurements, shear strength results reported bylaboratories are normally expressed in terms of total normal stress. For direct shear tests involving geosynthetics, Test MethodsD5321 and D6243 provide recommendations for shear
46、 displacement rates intended to allow dissipation of pore water pressuresgenerated during shearing. Recommended shear rates are 0.2 in/minin./min for geosynthetic (non-GCL) interface tests, 0.04in/minin./min for geosynthetic/soil (including hydrated GCLs) interface tests (3710), and 0.004 in/minin./
47、min for hydrated GCLinternal shear tests (711). However, as shown by Obermeyer et alal. (2312), even slower displacement rates may be needed forGCLs and high-plasticity clay soils to ensure that positive pore pressures do not develop during shearing. If tests involving GCLsor clays are loaded or she
48、ared too quickly, excess pore water pressures could develop, and results may not be representative of fieldconditions, which are often assumed to be drained. The assumption of drained conditions is reasonable because drainage layersare common in liner systems and because field loading rates are gene
49、rally slow (1113, 711). From Eq 3Equation (3), , positive porepressures that are not allowed to dissipate will decrease the measured shear stress. Tests that are sheared undrained may yielderroneous results similar to those discussed in Section 9. Drained and undrained strengths are not interchangeable from a designperspective.5.4 Combinations of shear stress and normal stress that fall on the Mohr-Coulomb shear strength envelope indicate that a shearfailure will occur. Combinat
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