Parameter Optimization and Sensitivity Analysis for Disturbed State Constitutive Model

Constitutive models for geologic materials and interfaces involve a number of parameters that need to be determined from appropriate laboratory tests. Because the test behavior is influenced by a number of factors such as material variability in test specimens, initial density, mean pressure, and stress paths, the parameters determined from such tests need to be averaged or optimized. The averaging procedure is often used. However, in view of the importance of the parameters in analysis and design, it is desirable and necessary to use advanced procedures such as optimization methods so as to find their improved and realistic values. This paper presents an optimization procedure for the determination of parameters in the unified disturbed state concept constitutive models. A series of multiaxial laboratory tests on a sand under different initial mean pressures, density, and stress paths are used to evaluate the optimized parameters. The stress-strain and volume change behavior is then back-predicted using the parameters from the conventional averaging procedure and the proposed optimization procedure. The results show that the optimized parameters provide improved predictions of the test data. The optimized parameters are used in a finite element procedure to predict cyclic behavior in a boundary value problem involving a shake table test. The proposed procedure can provide a useful methodology for the optimization of parameters for a wide range of available (plasticity, creep, damage, etc.) constitutive models. It can lead to improved analysis and design of geotechnical problems, particularly while using computer (finite element) procedures.