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9 <br />conditions. The soil specimen is confined by a flexible, <br />cylindrical rubber membrane reinforced with wire or <br />by stacked plates confining the specimen in a standard <br />membrane. The specimen is placed between a base <br />pedestal and top plate, as shown in Figure 11. A <br />vertical load (Pv) is applied to the specimen from the <br />top plate, and the soil is sheared by moving the top <br />plate horizontally at a constant rate (Ph). The stiff <br />frame of the wire-reinforced membrane or stacked <br />plates provides lateral confinement. The wire- <br />reinforced membrane or stacked plates allow for <br />horizontal displacements along the shear plane during <br />shearing, but maintain a constant specimen height by <br />adjusting the vertical load. <br />Drainage is prevented though the base pedestal and <br />the top plate, and thus volume change does not occur <br />and pore water pressures are generated in the DSS <br />test. There are some systems that use backpressure <br />and measure pore water pressures, but the most <br />common method is to equate the change in vertical <br />stress required to maintain a constant height with the <br />equivalent pore water pressure that would have <br />developed under undrained conditions. <br />Figure 11: Direct simple shear test apparatus. <br />The DSS test is most suitable for evaluating the <br />undrained shear strength (Su) of soft cohesive soils <br />(clays and elastic silts). The undrained shear strength is <br />defined as the peak horizontal shear stress achieved <br />during testing. <br />The DSS test should not be confused with the DS test, <br />which was discussed earlier. The DS test utilizes a <br />shear box with two rigid sections and forces a shear <br />plane to develop between the two. An important <br />difference is that the DS test measures drained shear <br />strength parameters and is not suitable for predicting <br />undrained shear strengths, as discussed earlier. In <br />contrast, the DSS test measures only undrained <br />strengths. The DSS test is not as commonly used as the <br />DS test. <br />Torsional Ring Shear Test <br />The torsional ring shear test is used when investigating <br />the shearing resistance of soils at very large strains or <br />displacements. The test is performed as described in <br />ASTM D6467 and D7608. <br />In the torsional ring shear test, a remolded soil <br />specimen in the shape of a ring with a rectangular <br />cross-section is confined by an external ring. Porous <br />ceramic plates are placed at the top and bottom of the <br />soil specimen. An effective normal stress (V’n) is <br />applied to the specimen through the top plate, as <br />illustrated in Figure 12. The specimen is sheared by <br />continuously rotating the lower half of the specimen in <br />one direction while the upper half reacts against a <br />torque arm that is held in place by a proving ring or <br />load cell at each end. The torque arm measures the <br />load as the soil specimen is sheared along a horizontal <br />plane that passes through the specimen. The torque <br />applied to the upper porous plate is used to calculate <br />the average shear stress on the failure surface. <br />Figure 12: Torsional ring shear test apparatus. <br />DSS Reference: Bowles (1988) <br />Top Plate <br />Base Pedestal