Stability of Undrained Deformation of Fluid-saturated Geo-materials

The presence of pore fluids in geomaterials can alter deformation processes and facilitate or delay material failure. Dilation/contraction of geomaterials in the course of inelastic undrained deformation causes a reduction/increase in pore pressure altering the effective compressive stress. This results in an increase/decrease in shear stress that can be sustained by the material over the corresponding case in drained deformation. We use a perturbation approach to investigate whether undrained conditions inhibit or promote localization or diffused instability as compared to the drained case. The effect of the material strain-rate sensitivity on the undrained stability is evaluated. INTRODUCTION: Inelastic volume changes in fluid-saturated geomaterials tend to cause a change in pore fluid pressure. Under drained conditions, pore pressure remains constant as its alterations are equilibrated by the pore fluid flow. Under undrained conditions, changes in pore pressure persist. Pore pressure drop in undrained shearing of dilatant geomaterials causes reduction of effective compressive stress and, consequently, the increase of the shear stress sustained by the material (dilatant strengthening) over the corresponding drained shear strength [Rice, 1975]. Undrained shear stress “weakening” over the corresponding drained strength is observed in contracting geomaterials (Fig. 1). Rice [1975] has used perturbation analysis to show that undrained dilatant strengthening is limited by instability at the state (C), Fig. 1, corresponding to the peak stress in the underlying drained response, state (T). For contractive materials, Vardoulakis [1985, 1996] has predicted strong instability of undrained deformation at small strains corresponding to the shear stress peak (T−), Fig. 1. However, neither of these predictions agree with the results from displacement-controlled biaxial compression experiments on saturated sands under undrained conditions (e.g., Han and Vardoulakis [1991]). These experiments indicate that loss of stability (manifested by subsequent localization of deformation into shear bands) occurs at plastic strains significantly higher than the ones corresponding to the state (C) for dense (dilatant) sands, and to the state (T−) for loose (contractive) sands. In this paper, we reconsider the earlier theoretical analyses (and assumptions) to resolve the above disagreement between the theory and experiment. Ö In A.S. Khan and O. Lopez-Pamies (Eds.), Plasticity, Damage and Fracture at Macro, Micro and Nano Scales (Proceedings of PLASTICITY ‘02), NEAT Press, 2002.