Abstract: Initial shear stress has effect on soils in side slope,sloping ground and retaining wall, etc. Serious residual strain will occur in soft clay under initial shear stress. But, original Iwan’s model ignored this effect. In this paper, an empirical degradation model of soft clay has been incorporated into Iwan’s series-parallel model to account for cyclic degradation. Simultaneously, an ideal plastic element is paralleled to simulate the development of residual strain. Subsequently, the dynamic stress-strain relationship of each soft clay stratum is described with modified Masing’s rules. In this paper, a modified Iwan’s model is utilized in one-dimensional seismic response analysis. It’s observed that the constitutive model should consider this effect when initial shear stress existed and the earthquake is strong. Otherwise,velocities, accelerations and shear strains may be underestimated.

Key words: nonlinearity, soft clay, residual strain, modified Iwan’s model, seismic response analysis