1、Designation: D7702/D7702M 13aD7702/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 th
2、e 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 di
3、rect 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 o
4、r 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
5、this 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 D5321D5321/D5321M or D6243D6243/D6243M.1.4 This guide does not ad
6、dress selection of peak or large-displacement shear strength values for design. References on thistopic include Thiel (1)2, Gilbert (2), Koerner and Bowman (3), and Stark and Choi (4).1.5 The values stated in either SI units or inch-pound units are to be regarded separately as standard. The values s
7、tated in eachsystem may not be exact equivalents; therefore, each system shall be used independently of the other. Combining values from thetwo systems may result in non-conformance with the standard.1.6 This standard does not purport to address all of the safety concerns, if any, associated with it
8、s 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 to Soil, Rock, and Contained FluidsD5321D5321/D5
9、321M Test Method for Determining the Shear Strength of Soil-Geosynthetic and Geosynthetic-GeosyntheticInterfaces by Direct ShearD6243D6243/D6243M Test Method for Determining the Internal and Interface Shear Strength of Geosynthetic Clay Liner bythe Direct Shear MethodD4439 Terminology for Geosynthet
10、ics3. Terminology3.1 DefinitionsFor definitions of terms relating to soil and rock, refer to Terminology D653. For definitions of terms relatingto geosynthetics and GCLs, refer to Terminology D4439.3.2 Definitions of Terms Specific to This Standard:3.2.1 adhesion, ca, or c, nthe y-intercept of the M
11、ohr-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 Geosynthetic Clay Liners.Current ed
12、ition approved May 1, 2013May 1, 2014. Published June 2013June 2014. Originally approved in 2011. Last previous edition approved in 2013 asD7702D7702/D770213. DOI:10.1520/D7702_D7702M13A.13a. DOI:10.1520/D7702_D7702M14.2 The boldface numbers in parentheses refer to a list of references at the end of
13、 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 not an ASTM standard and is i
14、ntended 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 appropriate. In all cases only the cu
15、rrent 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-normal stress plot representing
16、 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 (degrees), the angle definedby the l
17、east-squares, “best-fit” straight line through a defined section of the shear strength-normal stress failure envelope; thecomponent of the shear strength indicated by the term , in Coulombs equation, = c + tan .3.2.4 Mohr-Coulomb shear strength envelope, nthe least-squares, “best-fit” straight line
18、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 criteria (for example, peak, post-peak, or residual).3.2.5 secant friction angle, sec, n(degrees) the angle defined by a line dr
19、awn from the origin to a data point on the shearstrength-normal stress failure 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 theno
20、rmal stress acting on the failure plane. Two different types of shear strengths are often estimated and used in standard practice:3.2.6.1 peak shear strength, nthe largest value of shear resistance experienced during the test under a given normal stress.3.2.6.2 post-peak shear strength, nthe minimum
21、, 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 commercially available shear boxes used to determine interface shear strength,the post-peak shear strength is often specified and r
22、eported 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 defined) is notnecessarily the residual shear strength. In some instances, a post-peak shear strength may not be defined b
23、efore the limit ofhorizontal displacement is reached.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 limited to waste containment systems, mini
24、ng applications, dam designsinvolving geosynthetics, reinforced mechanically stabilized earth structures, and reinforced soil slopes, and liquid impoundments.Since geosynthetic interfaces often serve as a weak plane on which sliding may occur, shear strengths of these interfaces are neededto assess
25、the stability of earth materials resting on these interfaces, such as a waste mass or ore body over a lining system or theability of a final cover to remain on a slope. Accordingly, project-specific shear testing using representative materials underconditions similar to those expected in the field i
26、s recommended for final design. Shear strengths of geosynthetic interfaces areobtained by either Test Method D5321D5321/D5321M (geosynthetics) or D6243D6243/D6243M (geosynthetic clay liners). Thisguide touches upon some of the issues that should be considered when evaluating shear strength data. Bec
27、ause of the large numberof potential conditions that could exist, there may be other conditions not identified in this guide that could affect interpretationof the results. The seemingly infinite combinations of soils, geosynthetics, hydration, and wetting conditions, normal loaddistributions, strai
28、n rates, creep, pore pressures, etc., will always require individual engineering evaluations by qualifiedpractitioners. Along the same lines, the list of references provided in this guide is not exhaustive, nor are the findings andsuggestions of any particular reference meant to be considered conclu
29、sive. The references and their related findings are presentedherein only as examples available in the literature of the types of considerations that others have found useful when evaluatingdirect shear test results.4.2 The figures included in this guide are only examples intended to demonstrate sele
30、cted concepts related to direct 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.5. Shear Strength Fundamentals5.1 Mohr first presented a theory for
31、 shear failure, showing that a material experiences failure at a critical combination of normaland shear stress, and not through some maximum normal or shear stress alone. In other words, the shear stress on a given failureplane was shown to be a function of the normal stress acting on that plane (5
32、):5f! (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 (6).As the number of tests i
33、ncreases, 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 Eq 1 is a curved line for many materials (5). For most geotechnical engineeringproblems, the shear stress on the failure plane is approximat
34、ed as a linear function of the total or effective normal stress withinD7702/D7702M 142a 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 expresse
35、d as:5ca1tan (2)where: = shear stress, = normal stress, = friction angle (degrees), andca = adhesionIn the case of effective stresses, the linear failure envelope is:5ca 1 2 u! tan (3)or5ca 1 tan where:u = pore pressure, = effective normal stress, = drained friction angle (degrees), andca = effectiv
36、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
37、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 (for example, FHWA (8), AASHTO (9) along with state agencies who are using thesedocuments) are currently using the Greek lette
38、r, Delta (), to designate wall-backfill interface friction angle and the Greek letter, Rho (), to designatethe interface friction angle between geosynthetics and soil.5.3 Since most laboratory direct shear tests do not include pore pressure measurements, shear strength results reported bylaboratorie
39、s are normally expressed in terms of total normal stress. For direct shear tests involving geosynthetics, Test MethodsD5321D5321/D5321M and D6243D6243/D6243M provide recommendations for shear displacement rates intended to allowdissipation of pore water pressures generated during shearing. Recommend
40、ed 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 0.004 in./min forhydrated GCL internal shear tests (11). However, as shown by Obermeyer et al. (12), even slower displacement rates may ben
41、eeded for GCLs and high-plasticity clay soils to ensure that positive pore pressures do not develop during shearing. If testsinvolving GCLs or clays are loaded or sheared too quickly, excess pore water pressures could develop, and results may not berepresentative of field conditions, which are often
42、 assumed to be drained. The assumption of drained conditions is reasonablebecause drainage layers are common in liner systems and because field loading rates are generally slow (13, 11). From Eq 3,positive pore pressures that are not allowed to dissipate will decrease the measured shear stress. Test
43、s that are sheared undrainedmay yield erroneous results similar to those discussed in Section 9. Drained and undrained strengths are not interchangeable froma design perspective.5.4 Combinations of shear stress and normal stress that fall on the Mohr-Coulomb shear strength envelope indicate that a s
44、hearfailure will occur. Combinations below the shear strength envelope represent a non-failure state of stress (14). A state of stressabove the envelope cannot exist, since shear failure would have already occurred.FIG. 1 Curved Mohr failure envelopeFailure Envelope and equivalentEquivalent Mohr-Cou
45、lomb linear representationLinear Representa-tion (from Wright (7).D7702/D7702M 1436. Measurement and Reporting of Shear Strength by Test Methods D5321/D5321MD5321/D6243 / D6243/D6243M6.1 The shear resistance between geosynthetics or between a geosynthetic and a soil is determined by placing the geos
46、yntheticand one or more contact surfaces, such as soil, within a direct shear box. A constant normal stress representative of field stressesis applied to the specimen, and a tangential (shear) force is applied to the apparatus so that one section of the box moves in relationto the other section. The
47、 shear force is recorded as a function of the shear displacement of the moving section of the shear box.6.2 The test is run until the shear displacement exceeds 75 mm 3 in. or other value specified by the user. Note that 75 mmof displacement is the practical upper limit of most direct shear devices.
48、6.3 The testing laboratory plots the test data as a graph of applied shear force versus shear displacement. The peak shear forceand the shear force at the end of the test are identified. The shear displacements associated with these shear forces are alsodetermined. An example set of shear-displaceme
49、nt plots for a typical textured geomembrane/reinforced GCL interface is shownin Fig. 2a. Typical shear-displacement behavior of geosynthetic interfaces is discussed further in Section 9.6.4 The shear stresses applied to the specimen for each recorded shear force are calculated by dividing the shear force by thespecimen area. For tests in which the area of specimen contact decreases with increased displacement, a corrected area should becalculated, unless other technical interpretation arrangements are m
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