Estimation of nonlinear time-dependent soil behavior in strong ground motions using vertical array data

Strong motion records made by vertical arrays allow estimation of nonlinear soil behavior in layers at different depths. As an example, records obtained during the 1995 Hyogo-ken Nanbu earthquake at Port-Island, SGK, and TKS sites are used to estimate the stress-strain dependencies of soils. For different layers, different types of nonlinear stress-strain curves are determined, showing the best-fit approximation to the observed data. Examining consecutive parts of the records, temporal changes of the curves are traced. At PI and SGK sites, the obtained dependencies are found to vary with time in surface layers showing a progressive reduction of shear modulus, and remain stable in deeper layers. At TKS site the behavior of soils is described by a nonlinear stress-strain curve not changing with time. Thus, the conclusion can be made that the stronger is the input motion, the more complex is nonlinear behavior of soils.