Static and Dynamic Instability of Liquefiable Soils

Stanford University, Stanford CA

Investigators

Abstract

Abstract 0201317, Ronaldo Borja, Stanford University "Static and Dynamic Instability of Liquefiable Soils" The objective of this project is to develop a finite element model for analyzing the dynamic instability and flow liquefaction of soils, including the accompanying lateral flow and large ground movement during and following an earthquake. The model is based on a two-phase mixture theory with the following essential components: (a) a constitutive model that replicates the buildup of excess pore pressure prior to liquefaction; (b) a criterion for the onset of flow liquefaction instability; and (c) a constitutive response at residual state following flow liquefaction. A bounding surface plasticity model is used to model the anisotropic cyclic stress-strain behavior of soils before the onset of flow liquefaction, as well as to predict the accompanying pore pressure buildup. Large deformations are employed through the use of a finite deformation theory based on a multiplicative decomposition of the deformation gradient. A major component of the project is devoted to analyzing the results of monotonic undrained triaxial tests and cyclic undrained simple shear tests as a boundary-value problem. A universal assumption employed in interpreting the results of these tests is that the specimen is deforming homogeneously, suggesting that any measured specimen response may be interpreted as a material response. However, there are strong indications that this is far from being correct. Among the issues addressed by performing boundary-value problem simulations is whether or not the quasi-steady state condition commonly observed in saturated sand specimens of intermediate density is non-local. Also being investigated are the effects of imperfections and other perturbations on the response of soil specimens taken as a structural system. Finally, a systematic methodology is being devised for re-calibrating the constitutive model parameters in the finite deformation regime, and the finite element model is used to reanalyze the collapse of the Lower San Fernando Dam following the San Fernando earthquake of 1971.