Physical Basis and Validation of a Constitutive Model for Soil Shear Derived from Microstructural Changes

Abstract

Previous work indicated that rates of change of shear stress, effective normal stress, and void ratio of a sheared soil are proportional to applied values of shear and effective normal stress; initial proportionality values decay exponentially with strain to become zero at the steady-state condition. This paper proposes that the physical basis for this behavior is an underlying stochastic process in which particles move at random shear strains into the steady-state flow structure under the action of shear stress, countered by frictional resistance generated by the effective normal stress. The resulting dynamical systems model with physical properties closely fits 130 undrained and drained triaxial and true-triaxial shear tests, exhibiting strain softening or strain hardening, using various stress paths, conducted on uncemented, resedimented clays at various overconsolidation ratios (OCRs) and uncemented sands and silts at various relative densities. Parameters varied orderly with OCRs (clays) and confining pressure (silts and sands). The model’s value is that based on a simple hypothesis of particles moving into the steady state at random shear strains, it closely matches data from a variety of tests. Present limitations of the model are that it only applies to static loading and not yet to generalized stress paths found in field situations.