EFFECTS OF LARGE PRESHEARING ON CYCLIC BEHAVIOR OF SAND

ABSTRACT In order to study pore water pressure response and liquefaction characteristics of sand which has previously experienced liquefaction, two series of cyclic triaxial shear tests were run on loose saturated sand specimens in a static manner. In the first test series, sepcimens were subjected, at the end of the cyclic loading, to a large axial strain oriented to the triaxial compression under very low effective confining pressures in order to produce a state of liquefaction. After reconsolidation, the specimens were again subjected to the same sequence of cyclic stresses as before. The test results showed that the pore water pressure response and, hence, resistance to liquefaction of the presheared specimen was essentially the same in the second cyclic loading as the response of pore water pressures exhibited by the virgin specimen during the first cyclic loading. It was speculated that a newly formed cross anisotropic structure of the specimen during the application of the large preshear towards the triaxial compression had the same vertical axis of symmetry as that of the virgin specimen which was formed under gravity by the method of pulviation in water. In the second series of tests, a large axial strain oriented towards the triaxial extension was applied, at the end of the first cyclic loading, to a specimen under very small effective confining pressures, leading eventually to a state of liquefaction. After reconsolidation, the specimen was again subjected to the same sequence of cyclic stresses as before. It was found that the pore water pressure response in the second cyclic loading of the presheared specimen was different from the pore water pressure response exhibited by the virgin specimen during the first cyclic loading. This difference was ascribed to the fact that a newly formed cross anisotropic structure of the specimen formed during the application of the large preshear towards the triaxial extension had a horizontal axis of symmetry, as opposed to the vertical axis of symmetry preexistant in the virgin specimen formed by the method of pluviation in water. As a result of these observations, it was argued that a subsequent liquefaction of a sand which has previously been liquefied depends not only on the magnitude of preshear but more significantly on the direction of the preshear which is executed under very small confining pressures at the end of the previous cyclic loading.