Abstract: To study the shear deformation mechanism and strength characteristics of vibroflotation crushed stone column composite foundation in sandy silty soil, two model tests of crushed stone pile composite foundation with different displacement rates of pile were carried out by using large stacking ring shear apparatus. The relationship between shear stress and shear displacement, deformation of composite foundation under different stress states are systematic analyzed. The test results show that: 1) The sandy silty composite foundation has a feature of elastoplastic object and the stress level S=0.5 can be used as the elastoplastic object’s dividing line roughly. 2) In the process of large deformation destruction of composite foundation, the stone columns and soil have a good deformation consistency, meanwhile, the shear deformation is inhibited with the increase of displacement rate of pile. 3) At present, a few specifications in China suggest that the number of stress ratio of pile with soil is 1 for the shear strength calculation of composite foundation, which means that only pile area replacement is considered. As a result, the calculation results of shear strength are close under different pile replacement rates. The permutation rate of 22.7% has a 1.32° gap compare with 40.3% while the pile replacement rate of 22.7% has a 3.65° gap compare with 40.3% in the model test. It indicates that the reinforcement mechanism of stone columns on composite foundation should not be limited to the factor of displacement rate, the change of drainage effectiveness of composite foundation factor should also be considered.

Key words: stone column, composite foundation, shear failure, shear strength