Effect of soft lens on the behavior of ordinary and reinforced stone columns located in saturated sandy bed

One of the effective methods to improve the behavior of problematic bed containing soft lens is using stone column. The capability of this soil improvement method increases by simultaneous use of the stone column and geosynthetic encasement, especially in the presence of soft lens near to the ground surface or where the possibility of bulging failure is probable. In this research, the efficiency of using stone column embeded in a saturated sandy bed containing soft lens have been investigated in terms of bearing capacity and failure mode. Also, the efficiency of reinforcing the stone column has been investigated in the presence of soft lens with different thicknesses and placement level. The results of numerical analysis obtained by finite element software (Abaqus) and validated by laboratory results of small-scale physical model experiments in real stress conditions shows that in the presence of soft lens, the change in the length of a stone column has no effect on the occurrence of bulging failure and decreasing placement level of the soft lens, increases the bulging failure occurrence proportional. On the other hand, increasing the thickness of the soft lens reduces the bearing capacity of the ordinary and reinforced stone column. The phenomenon of bulging can occur at the level of the lens placement and up to a depth of about 4 times the diameter of the ordinary column because of existence of a soft lens in a relatively loose sandy bed, while mechanism of failure is not bulging anymore if using encasement.