1、Designation: D7702 11D7702 13Standard 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, th
2、e year of 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 information related to the evaluation of direct shear test result
3、s 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 replacethe standard
4、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 this document means on
5、ly 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 D62431.4 This guide does not address selection of peak or large-displacement s
6、hear strength values for design. References on thistopic include Thiel (33), Gilbert (12), Koerner and Bowman (16), and Stark and Choi (31).1.5 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.1.6 This standard does not purport
7、 to address all of 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:2D653 Ter
8、minology Relating 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
9、 ShearMethodD4439 Terminology for Geosynthetics3. Terminology3.1 DefinitionsFor definitions of terms relating to soil and rock, refer to Terminology D653. For definitions of term relatingto geosynthetics and GCLs, refer to Terminology D4439.3.2 Definitions of Terms Specific to This Standard:3.2.1 ad
10、hesion, nca,the ,The y-intercept of the Mohr-Coulomb shear strength envelope; the component of shear strengthindicated by the term ca, in Coulombs equation, = ca + tan 3.2.2 Mohr-Coulomb friction angle , nangle of friction of a material or between two materials, degrees) the angle definedby the leas
11、t-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 1 This guide is under the jurisdiction of ASTM Committee D35 on Geosynthetics and is the dire
12、ct responsibility of Subcommittee D35.04 on Geosynthetic Clay Liners.Current edition approved Feb. 1, 2011Jan. 15, 2013. Published April 2011. DOI:10.1520/D770211Last previous edition published 2011 as D770211. DOI:10.1520/D7702132 For referencedASTM standards, visit theASTM website, www.astm.org, o
13、r 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 intended only to provide the user of an ASTM standard an indication of what changes ha
14、ve 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 current versionof the standard as published by ASTM is to be considered the official do
15、cument.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States13.2.3 coeffcient of friction, na constant proportionality factor relating shear to normal stress for a defined failure conditionover a specific range of normal stresses.3.2.3 cohe
16、sion c, Mohr-Coulomb shear strength envelope, nthe component of shear strength indicated by the term c, inCoulombs equation, least-squares, “best-fit” straight line through a defined section of the shear strength-normal stress failureenvelope described the equation = ca + tan . The envelope can be d
17、escribed for any chosen shear failure mode (for example,peak, post-peak, or residual).3.2.5 direct shear strength test, nfor geosynthetics, a procedure in which the interface between a geosynthetic and any othersurface, under a range of normal stresses specified by the user, is stressed to failure b
18、y the horizontal movement of one surfaceagainst the other.3.2.4 failure envelope, 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) the angle defined by a line drawn f
19、rom 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 thenormal st
20、ress acting on the failure plane. Two different types of shear strengths 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
21、 of shear resistance that occurs after the peak shear strength isexperienced.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 resis
22、tance that 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 shear strength. In some instances,a post-peak shear strength may not be defined before the limit of horizontal dis
23、placement is reached.3.2.7 shear strength envelope, ncurvi-linear line on the shear stress-normal stress plot representing the combination of shearand 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-geo
24、synthetic interfaces and geosynthetic-geosynthetic interfaces is a critical design parameter formany civil engineering projects, including, but not limited to waste containment systems, mining applications, dam designsinvolving geosynthetics, reinforced slopes, and liquid impoundments. Since geosynt
25、hetic interfaces often serve as a weak planeon which sliding may occur, shear strengths of these interfaces are needed to assess the stability of earth 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. Ac
26、cordingly,project-specific shear testing using representative materials under conditions similar to those expected in the field is recommendedfor final design. Shear strengths of geosynthetic interfaces are obtained by either Test Methods D5321 (geosynthetics) or D6243(geosynthetic clay liners). Thi
27、s guide touches upon some of the issues that should be considered when evaluating shear strengthdata. Because of the large number of potential conditions that could exist, there may be other conditions not identified in this guidethat could affect interpretation of the results. The seemingly infinit
28、e combinations of soils, geosynthetics, hydration, and wettingconditions, normal load distributions, strain rates, creep, pore pressures, etc., will always require individual engineering evaluationsby qualified practitioners. Along the same lines, the list of references provided in this standard is
29、not exhaustive, nor are thefindings and suggestions of any particular reference meant to be considered conclusive. The references and their related findingsare presented herein only as examples available in the literature of the types of considerations that others have found useful whenevaluating di
30、rect shear test results.4.2 The figures included in this guide are only examples intended to demonstrate selected 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-
31、specific tests should always be performed.5. Shear Strength Fundamentals5.1 Mohr (1776) first presented a theory for shear 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 wor
32、ds, the shear stress on a givenfailure plane was shown to be a function of the normal stress acting on that plane (5):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 eac
33、h circle must represent the normal and shear stress combination associated with failure (24).As the number of tests increases, 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 Equation (1) is a curve
34、d line for many materials (5). For most geotechnicalengineering problems, the shear stress on the failure plane is approximated as a linear function of the total or effective normal stressD7702 132within a selected normal stress range, as shown in Fig. 1. This linear approximation is known as the Mo
35、hr-Coulomb failure shearstrength envelope. In the case of total stresses, the Mohr-Coulomb failure shear strength envelope is expressed as:5c1tan (2)where: 5 shear stress, 5 normal stress, 5 friction angle degrees!,andc 5 cohesion or adhesion between two materials! 5 ca1 tan (2)where: 5 shear stress
36、, 5 normal stress, 5 friction angle degrees!,andc a 5 adhesionIn the case of effective stresses, the linear failure envelope is:5c 12u!tan (3)or5c1 tan 5ca 1 tan (3)or5ca 1(2u) tanwhere:u = Pore pressure, = effective normal stress, = drained friction angle (degrees), and = drained friction angle (de
37、grees), andc = effective stress cohesion or adhesionc = effective stress adhesionNOTE 2Adhesion, 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 identic
38、al; simply the y-intercept of the Mohr-Coulomb shear strength envelope, or in otherwords, the component of shear strength indicated by the term ca, in Coulombs equation, = ca + tan .NOTE 3The end user is cautioned that some organizations (e.g. FHWA,AASHTO along with state agencies who using these do
39、cuments) are currentlyusing the Greek letter, Delta (), to designate wall-backfill interface friction angle and the Greek letter, Rho (), to designate the interface friction anglebetween geosynthetics and soil (Reference: AASHTO, 2010. LRFD Bridge Design Specifications, 5th Edition, American Associa
40、tion of State Highwayand Transportation Officials, Washington, D.C. and FHWA, 2009. Mechanically Stabilized Earth Walls and Reinforced Soil Slopes, Design andConstruction Guidelines, U.S. Department of Transportation, Federal Highway Administration, Washington DC, FHWA NHI-10-024 Vol I, NHI-10-025Vo
41、l II, and as FHWA GEC011).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
42、 recommendations for shear displacement rates intended to allow excess dissipation of pore waterFIG. 1 Curved Mohr failure envelope and equivalent Mohr-Coulomb linear representation (from Wright, 2005).D7702 133pressures generated during shearing to dissipate. shearing. Recommended shear rates are 0
43、.2 in/min for geosynthetic (non-GCL)interface tests, 0.04 in/min for geosynthetic/soil (including hydrated GCLs) interface tests (37), and 0.004 in/min for hydrated GCLinternal shear tests (7). However, as shown by Obermeyer et al (23), even slower displacement rates may be needed for GCLs andhigh-p
44、lasticity clay soils to ensure that positive pore pressures do not develop during shearing. If tests involving GCLs or claysare loaded or sheared too quickly, excess pore water pressures could develop, and results may not be representative of fieldconditions, which are often assumed to be drained. T
45、he assumption of drained conditions is reasonable because drainage layersare common in liner systems and because field loading rates are generally slow (11, 7). From Equation (3), positive pore pressuresthat are not allowed to dissipate will decrease the measured shear stress. Tests that are sheared
46、 undrained may yield erroneousresults similar to those discussed in Section 9. Drained and undrained strengths are not interchangeable from a design perspective.5.4 Combinations of shear stress and normal stress that fall on the Mohr-Coulomb failure shear strength envelope indicate thata shear failu
47、re will occur. Combinations below the failure shear strength envelope represent a non-failure state of stress (1).Astateof stress above the envelope cannot exist, since shear failure would have already occurred.6. Measurement and Reporting of Shear Strength by Test Methods D5321/D62436.1 The shear r
48、esistance between geosynthetics or between a geosynthetic and a soil is determined by placing the geosyntheticand 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
49、 is applied to the apparatus so that one section of the box moves in relationto the other section.The shear force is recorded as a function of the horizontalshear displacement of the moving section of the shearbox.6.2 The test is run until the horizontalshear displacement exceeds 75 mm (3 in.) or other value specified by the user. Note that75 mm of displacement is the practical upper limit of most direct shear devices.6.3 The testing laboratory plots the test data as a graph
