5.1 The procedure described in this test method for determination of the shear resistance of the soil and geosynthetic or geosynthetic and geosynthetic interface is intended as a performance test to provide the user with a set of design values for the test conditions examined. The test specimens and conditions, including normal stresses, are generally selected by the user.

5.2 This test method may be used for acceptance testing of commercial shipments of geosynthetics, but caution is advised as outlined in 5.2.1.

5.2.1 The shear resistance can be expressed only in terms of actual test conditions (see Note 4 and Note 5). The determined value may be a function of the applied normal stress, material characteristics (for example, of the geosynthetic), soil properties, size of sample, drainage conditions, displacement rate, magnitude of displacement, and other parameters.

Note 4-In the case of acceptance testing requiring the use of soil, the user must furnish the soil sample, soil parameters, and direct shear test parameters. The method of test data interpretation for purposes of acceptance should be mutually agreed to by the users of this standard.Note 5-Testing under this test method should be performed by laboratories qualified in the direct shear testing of soils and meeting the requirements of Practice D3740, especially since the test results may depend on site-specific and test conditions.

5.2.2 This test method measures the total resistance to shear between a geosynthetic and a supporting material (substratum) or a geosynthetic and an overlying material (superstratum). The total shear resistance may be a combination of sliding, rolling and interlocking of material components.

5.2.3 This test method does not distinguish between individual mechanisms, which may be a function of the soil and geosynthetic used, method of material placement and hydration, normal and shear stresses applied, means used to hold the geosynthetic in place, rate of shear displacement, and other factors. Every effort should be made to identify, as closely as practicable, the sheared area and failure mode of the specimen. Care should be taken, including close visual inspection of the specimen after testing, to ensure that the testing conditions are representative of those being investigated.

5.2.4 Information on precision among laboratories is incomplete. In cases of dispute, comparative tests to determine whether a statistical bias exists among laboratories may be advisable.

5.3 The test results can be used in the design of geosynthetic applications, including but not limited to: the design of liners and caps for landfills, mining heap leach pads, tailings impoundments, cutoffs for dams and other hydraulic barriers, geosynthetic-reinforced retaining walls, embankments, and base courses; in applications in which the geosynthetic is placed on a slope; for determination of geosynthetic overlap requirements; or in other applications in which sliding may occur between soil and a geosynthetic or between two geosynthetic materials.

5.4 The displacement at which peak strength and post-peak strength occurs and the shape of the shear stress vs. shear displacement curve may differ considerably from one test device to another due to differences in specimen mounting, gripping surfaces and material preparation. The user of results from this standard is cautioned that results at a specified displacement may not be reproducible across laboratories and that the relative shear displacement measured in this test at peak strength may not match relative shear displacement at peak strength in a field condition.