Abstract：In order to investigate the influence of vertical water pressure oscillation on the liquefaction of saturated sand，one dimensional side-constrained model of saturated sand is presented based on biphase continuous media theory. The development of liquefaction and the liquefaction thickness are analyzed. It is shown that the developing rate of liquefaction increases with the increment of load and the decrement of sand strength or the permeability，but the deformation of sand increases with the decrement of sand strength and increment of permeability and load. If the permeability of sand is homogeneous，the pore pressure and the strain rise sharply near the place with the smallest permeability. This solution may explain why the fracture occurs in the sand column in some conditions. With the development of liquefaction，the strength of the soil foundation becomes smaller and smaller. The foundation will slide or flow when the limited state arrives. That means，landslides or debris flow will occur.
