| A new method is developed for the evaluation of the seismic response of horizontally layered saturated soil deposits composed of layers of sand and clay, i.e., composite soil deposits. The method is implemented in a computer program named D-MOD, which combines a newly developed generalized cyclic degradation-pore water pressure generation model for clay, a modified cyclic stress degradation model for sand, and a modified general stress-strain model for soils, with the previous work of Lee and Finn and of Vucetic and Dobry, on the seismic response and liquefaction of horizontally layered sand deposits.; The D-MOD analysis utilizes a multi-degree-of-freedom one dimensional lumped parameter soil system model. It incorporates all main aspects known to govern the seismic response and associated pore water pressure generation and dissipation in composite soil deposits, such as: (i) nonlinear stress-strain behavior; (ii) pore water pressure generation controlled by the amplitude and number of strain cycles; (iii) irregular seismic input accelerations and corresponding irregular cyclic stress-strain response of soils; (iv) degradation of soil stiffness and strength caused by the pore water pressure generation and deterioration of soil microstructure; and (v) the pore water pressure dissipation and redistribution. The development of the method included laboratory testing, a parametric study using D-MOD, and a verification of D-MOD on three well documented case histories.; The results of cyclic testing provided experimental evidence for several previously used concepts, but never thoroughly experimentally verified. Parametric studies and case history analyses reveal that, besides the characteristics of the incoming seismic motion at the base and the stratification (geometry), the seismic response of a saturated composite soil profile is governed in a complex manner by the development of seismic strains, cyclic degradation of soil stiffness and strength, and the simultaneous generation, dissipation and redistribution of positive as well as negative cyclic pore water pressures. Calculated seismic responses indicated that when seismically induced shear strains are above certain threshold values (ranging roughly between 0.01% for sands and 0.1% for clays) the truly nonlinear modelling of the degrading stress-strain behavior is needed. The D-MOD analysis incorporates such modelling of the nonlinear and degrading soil properties, and therefore represents a useful tool for the description and understanding of the behavior of fully saturated composite soil deposits during strong earthquakes. |
