Gyratory Shear Apparatus; Design, Testing Procedures, and Test Results on Undrained Sand

When a saturated mass of undrained sand in situ is subjected to cyclic loading, the stresses within any small element remain uniform, although changing cyclically. However, in cyclic laboratory tests the stresses on rigid boundaries are nonuniform and their cumulative effect during cyclic loading causes progressive redistribution of water content and density within specimen. The purpose of this investigation was to measure such redistribution in sand subjected to 1 cyclic simple shear X tests and 2 gyratory shear Y tests, using a gyratory apparatus designed to perform both types of tests. Negator springs mounted on a gyratory arm apply a constant horizontal force which for X tests is transformed into a reciprocating shear force, and for Y tests produces a constant, rotating shear force. The cylindrical test specimen is enclosed in a rubber membrane, which is supported on the outside by a flat coil spring whose leaves are slightly separated at start of test. At end of test the specimen is frozen permitting expanding water to discharge into a burette and cut into 64 elements for determination of distribution of water content and relative density. The report includes 1 description of apparatus and appurtenant test equipment, with shop drawings, 2 description of test procedures, 3 typical test results, and 4 discussion. Measured redistribution of density shows clearly that loose and medium dense specimens are compacted in zones adjacent to the rigid top and bottom boundaries and are loosened in zones along midplane and adjacent to membrane. With increasing number of cycles, redistribution and softening in loose and medium-dense specimens continue until steady-state deflections develop.