Abstract: The mechanical effects of the buoyancy acting on deep foundation under different circumstances are studied by model tests.The study subject is the problem of reduction of groundwater buoyancy derived from sand and clay.Moreover,the variation of buoyancy with water level and the dynamic process of buoyancy with time are analyzed.The results indicate that the measured buoyancy of sand is identical with the theoretical buoyancy calculated by Archimedes’s law,while for clay,the measured value,varying with water level and time,is less than the theoretical one.And the difference can be expressed as the reduction coefficient.The reduction coefficient is small,ranging between 0.3 and 0.4,for the lower water level at initial stages.With the increase of time,the value of reduction coefficient gradually becomes large.And it reaches 0.70 ultimately.Therefore,a reasonable design of deep foundation should consider the effect of buoyancy.