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Standard Test Method for Torsional Ring Shear Test to Measure Drained Fully Softened Shear Strength and Stress Dependent Strength Envelope of Fine-Grained Soils
— 6 стр.

5.1 The ring shear apparatus maintains the cross-sectional area of the shear surface constant during shear and shears the specimen continuously in one rotational direction for any magnitude of shear displacement and along the entire specimen cross-sectional area.

5.2 The ring shear apparatus allows a reconstituted specimen to be consolidated at the desired normal stress prior to drained shearing. This simulates the field conditions under which complete softening develops in overconsolidated clays, claystones, mudstones, and shales that do not have a pre-existing shear surface, sheared bedding planes, joints, or faults as described by Skempton (19702 and 19773) and unfailed compacted fill slopes (Gamez and Stark 20144) because the fully softened strength corresponds to the peak shear strength of a normally consolidated fine-grained soil. The fully softened strength is only applicable to the soil zones that are subject to the environmental deterioration and applied shear stresses that lead to soil softening, deterioration of soil fabric, and strength loss, which may not be relevant to all slopes and all depths. The fully softened strength should be used in an effective stress/drained stability analysis using a stress dependent strength envelope for slopes with no prior shearing.

5.3 The ring shear test is suited to the determination of the drained fully softened shear strength because of the short drainage path through the thin specimen, small post-peak strength loss in a normally consolidated specimen, and the constant cross-sectional area.

5.4 The ring shear test specimen is annular so the angular displacement differs from the inner radius to the outer radius. This is not significant because a normally consolidated specimen does not exhibit a large post-peak strength loss so the difference in peak shear resistance at the inner radius and outer radius at different displacements is not significant and the ratio of the inner to outer radii of the ring is greater than 0.5 in accordance with Hvorslev (1936)6.

Note 1: Notwithstanding the statements on precision and bias contained in this test method: The precision of this test method is dependent on the competence of the personnel performing it and the suitability of the equipment and facilities used. Agencies that meet the criteria of Practice D3740 are generally considered capable of competent testing. Users of this test method are cautioned that compliance with Practice D3740 does not ensure reliable testing. Reliable testing depends on several factors; Practice D3740 provides a means of evaluating some of those factors.

1.1 This test method provides a procedure for performing a torsional ring shear test under a drained condition to measure the fully softened shear strength and stress dependent strength envelope of fine-grained soils (using a reconstituted normally consolidated specimen). The fully softened strength and the corresponding stress dependent effective stress strength envelope are used to evaluate the stability of slopes that do not have a pre-existing shear surface but have been subjected to environmental conditions and shear stresses that lead to soil softening, deterioration of the soil fabric, and strength loss. It has been shown (Skempton 19702 and 19773) that under these conditions and within the depth zones that have undergone softening, first-time slope failures can occur at effective stress levels that correspond to a fully softened strength envelope. It has also been shown empirically (Skempton 19702 and 19773) that fully softened strength of fine grained soils can be approximated by the peak strength of a reconstituted and normally consolidated specimen. In this test method, reconstituted and normally consolidated specimens are sheared at a controlled and constant displacement rate until the peak shear resistance has been obtained. Generally, the drained fully softened failure envelope is determined at three or more effective normal stresses. A separate test specimen must be used for each normal stress to measure the fully softened strength otherwise a post-peak or even drained residual strength will be measured if the same specimen is used at the same or at another effective normal stress because of the existence of a prior shear surface.

1.2 The ring shear apparatus allows a reconstituted specimen to be normally consolidated at the desired normal stress prior to drained shearing. The test results closely simulate the fully softened strength of stiff natural fine-grained soils (Skempton 19702 and 19773) and compacted fills of fine-grained soils (Gamez and Stark 20144). This simulates the mobilized shear strength in overconsolidated clays, claystones, mudstones, and shales in natural slopes and compacted fill in manmade slopes, such as, dams, levees, and highway embankments, after the soil has fully softened and attained the fully softened strength condition.

1.3 A shear stress-displacement relationship may be obtained from this test method. However, a shear stress-strain relationship or any associated quantity, such as modulus, cannot be determined from this test method because defining the height of the shear zone is difficult and needed in the shear strain calculations. As a result, the height of this shear zone is unknown, so an accurate or representative shear strain can therefore not be determined.

1.4 The selection of normal stresses and final determination of the shear strength envelope for design analyses and the criteria to interpret and evaluate the test results are the responsibility of the engineer or entity requesting the test.

1.5 Units—The values stated in SI units are to be regarded as the standard. The values given in parentheses are mathematical conversions to inch-pound units that are provided for information only and are not considered standard.

1.6 *This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.*

1.7 *This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.*