Coupling elasto-plastic behaviour of unsaturated soils with piecewise linear large-strain consolidation

Abstract

Slurried soils or tailings are often deposited in layers that undergo complex stress paths in terms of desiccation and loading, as a given layer may undergo variable degrees of desiccation before burial by subsequent layers, which has implications for geotechnical stability and geo-environmental performance. Proper analysis therefore requires not only an effective coupling of unsaturated flow with large-strain consolidation, but also inclusion of hysteresis effects. This paper presents the development and testing of a coupled unsaturated flow–large-strain consolidation model, UNSATCON, initially formulated for only monotonic dewatering, to include such effects. UNSATCON operates by a novel numerical algorithm that ensures mass conservation and handles the saturated/unsaturated transition smoothly. Differently from conventional integration schemes commonly embedded in finite-element formulations, this algorithm, tracking the void ratio change explicitly for large deformation, is shown to be applicable for most constitutive models of unsaturated soils under one-dimensional or isotropic stress conditions. Three constitutive models are implemented: a modified state surface model and the original and a minor variant of the Glasgow coupled model (GCM). The numerical model is evaluated using a recently published experimental study on multilayer deposition of thickened gold tailings. The model adequately reproduces the essential features of the dewatering processes, including the irrecoverable deformation induced by desiccation, the interlayer water exchange governed by deformation and deformation-dependent hysteretic retention behaviour. All three implemented constitutive models can reasonably simulate these experimental observations, despite some trivial discrepancies in magnitude. Point-level inspection, however, reveals some fundamental differences that may have important practical implications. For example, the additional plastic strain that occurs in GCM type models over multiple wetting–drying cycles may explain observed trends in increased strength in the simulated experiment.