AbstractMultidirectional cyclic shearing induced by earthquakes involves variations not only in amplitude but also in direction. To investigate the undrained cyclic shear responses of sand under this complex stress state, several series of simple shear tests were conducted on reconstituted Toyoura sand specimens prepared by water sedimentation and dry deposition methods, at the relative density of about 34% and initial vertical stress of 100 kPa, with the variable-direction dynamic cyclic simple shear system. By applying cyclic stresses in two mutually perpendicular horizontal directions with a certain phase difference, the multidirectional complex cyclic stress path was achieved. The cyclic unstable behavior, pore pressure response, and liquefaction resistance were analyzed with varying phase differences. Results indicate that the increase of phase difference delays the occurrence of unstable cyclic response. The development trend of normalized pore pressure is not affected by the phase difference or sample preparation method. A modified pore pressure generation model is developed to quantify the normalized pore pressure development of sand. Moreover, the effect of phase difference on liquefaction resistance is not monotonic. The liquefaction resistance of sand under multidirectional cyclic loading is approximately 70% of that under unidirectional cyclic loading. Also, the liquefaction resistance of specimens formed by dry deposition is less than that of specimens formed by water sedimentation.

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