Abstract: Loose sand is highly susceptible to liquefaction, and small changes in stress state can affect its liquefaction characteristics. Based on multi-directional cyclic simple shear tests, this study conducted cyclic simple shear tests on loose sand under different magnitudes and directions of static shear stress, and complex shear paths. The cyclic simple shear characteristics of loose sand under complex initial stress states are studied. The main conclusions are drawn as follows: (1) As the static shear stress ratio increases, the peak shear stress of the specimen increases, the increment of pore water pressure in the first cycle increases, and the specimen is more prone to liquefaction. The effect of the magnitude of initial static shear stress on excess pore water pressure is more significant at the early stage of shearing. (2) With the increase of the angle between the initial static shear stress and the main direction of dynamic shear stress, the peak shear stress of the specimen in the X direction decreases, and the pore water pressure of the specimen accelerates to increase. In addition, the increment in pore water pressure in the first cycle and the last cycle increases, and the difference between the cycles increases. The specimen is more prone to sudden liquefaction. (3) The specimen with 8-shaped shear path has the largest area of stress−strain hysteresis loops, which consumes the most energy per cycle, followed by the specimen with the circular shear path, and the specimen with the straight shear path has the smallest area. Complex shear paths can induce a sudden increase in pore water pressure at the beginning of shearing, increasing the increment in pore water pressure in each cycle and making it more prone to liquefaction. (4) The sequence of factors affecting the liquefaction of loose sand is the angle between the initial static shear stress and the dynamic shear stress, the shear path, and the magnitude of the initial static shear stress.

Key words: multi-directional cyclic simple shear test, loose sand, complex initial stress, liquefaction, pore water pressure, stress path, stress reversal