A nested-surface model for the stress-strain-time behavior of soils

A nested-surface model has been developed for the stress-strain-time behavior of soil. The model was designed for engineering analyses of deformations and excess pore pressures in geotechnical structures. The model consists of three major components: (1) A constitutive model for the stress-strain-time behavior of soil, (2) A model for the flow of pore water, (3) A finite element model to implement the constitutive and pore water flow models in engineering analyses.; The constitutive model is based on the Critical State theory of soil behavior. It includes capabilities for anisotropic elastoplasticity, deviatoric creep and delayed compression. The constitutive model uses multiple, nested loading surfaces to represent the elastoplastic stiffness and strength of a soil. The loading surfaces are three-dimensional generalizations of the consolidation and dilation surfaces of critical state soil mechanics. Each loading surface is a composite surface, consisting of an ellipsoidal consolidation surface and a bullet-shaped dilation surface. The shapes of the consolidation and dilation surfaces are independently adjustable in the p-{dollar}bar sigma{dollar} stress plane. The loading surfaces are also adjustable in the deviatoric stress plane to obtain circular or non-circular cross sections. Anisotropic strength and stiffness properties are modeled by means of the non-circular loading surfaces and by kinematic hardening. Strains due to plasticity, deviatoric creep and delayed compression contribute to hardening of the loading surfaces.; The pore water flow model is based on Darcy's flow equation. The model is formulated for use in problems with constrained, fully saturated seepage of a pore fluid such as water. The model includes capabilities for anisotropic permeability and for permeability that varies with the void ratio of the soil.; The finite element model couples the constitutive and seepage models for simultaneous solution of soil displacements and pore pressures. The finite element model is based on the theory of rate independent, incremental plasticity. The stress-strain equations are integrated iteratively by means of a cutting-plane algorithm. A cutting-plane algorithm is used rather than the more conventional radial-return algorithm because of the non-circular loading surfaces in the constitutive model. Time dependent equations for deviatoric creep, delayed compression and seepage are integrated by means of the generalized mid-point method. The solution for displacements and pore pressures is obtained from the system of nonlinear, finite element equations by the Newton-Raphson or Modified Newton techniques.; The model has been evaluated by comparing calculated results with known analytic solutions and with published experimental data. The evaluation included solutions of drained and undrained problems as well as of the fully coupled, displacement-pore pressure problem. Results show the model is capable of accurately predicting displacements, stresses and pore pressures. Results also show the model is useable with cohesionless and cohesive soils in the loose (wet) and dense (dry) states when loaded on the high or low pressure sides of the critical state.; The model is significant for several reasons. It extends prior models based on Critical State theory to cohesionless and cohesive soils. It is useable in both the dilative and compressive regimes of loading. It is capable of accurately predicting the effects of anisotropic elastoplasticity, deviatoric creep and delayed compression.