4.1 The shear strength of soil-geosynthetic interfaces and geosynthetic-geosynthetic interfaces is a critical design parameter for many civil engineering projects, including, but not limited to waste containment systems, mining applications, dam designs involving geosynthetics, reinforced slopes, and liquid impoundments. Since geosynthetic interfaces often serve as a weak plane on which sliding may occur, shear strengths of these interfaces are needed to assess the stability of earth materials resting on these interfaces, 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 recommended for final design. Shear strengths of geosynthetic interfaces are obtained by either Test Method D5321 (geosynthetics) or D6243 (geosynthetic clay liners). This guide touches upon some of the issues that should be considered when evaluating shear strength data. Because of the large number of potential conditions that could exist, there may be other conditions not identified in this guide that could affect interpretation of the results. The seemingly infinite combinations of soils, geosynthetics, hydration, and wetting conditions, normal load distributions, strain rates, creep, pore pressures, etc., will always require individual engineering evaluations by qualified practitioners. Along the same lines, the list of references provided in this guide is not exhaustive, nor are the findings and suggestions of any particular reference meant to be considered conclusive. The references and their related findings are presented herein only as examples available in the literature of the types of considerations that others have found useful when evaluating direct shear test results.

4.2 The figures included in this guide are only examples intended to demonstrate selected concepts related to direct shear testing of geosynthetics. The values shown in the figures may not be representative and should not be used for design purposes. Site specific and material-specific tests should always be performed.