We analyze the seismic responses of soil-pile-base isolated structure system for variety of ground types under constant input motion. And we discussed the effectiveness of an isolation system on soft grounds using three dimensional effective stress analysis. Analytical results such as base shear coefficient and shear strain of isolator show that the seismic responses are reduced even on the soft clay ground and liquefiable ground. There is a difference in the acceleration ratios of building to ground, for the different ground eases. However, the response acceleration values of building are nearly the same levels for the different ground cases. We also found out that the response motions of the ground which is lower than the base mat level, are close to the input motion to the building base.
