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.

Constitutive modelling of unsaturated gold tailings subjected to drying and wetting cycles

“…The experimental dataset has been extracted from reference 23 with the aim of investigating the effect of hydraulic stress history on the mechanical behaviour of unsaturated gold mine tailings. The previous study by Qi et al 24 adopted the GCM for interpreting the behaviour of this material, with their analyses being limited to the behaviour during drying/wetting cycles. In comparison, the current study considers the response of the materials to simple shear loading on the samples that experienced drying/wetting cycles.…”

Unsaturated soil behaviour in thickened and filtered tailings

“…Fortunately, these somewhat complex interactions have been conceptualised by several models in the field of unsaturated soils, and many of these models (Basic Barcelona, or BBM, state surface or SSM, and Glasgow coupled or GCM) have been implemented in geotechnical codes, although for small strain conditions. Qi et al (2017aQi et al ( , 2020 reset these models into a large strain consolidation framework, the UNSATCON code, which has been applied to multilayer drying box tests (up to 1 m deep multilayer deposits, 1 by 1 m in plan), as well as field data from both hard rock and oil sands mines (Qi & Simms 2018a, Simms 2021). Here we do not focus on the details of the particular soil models (e.g.…”

“…Total stress or Bishop's stress the parameters of the relevant soil model (e.g. BBM, GCM) for the void ratio dependency of the SWCC and the shape of the plastic surface can be determined analytically (Qi et al 2020). The data requirements for determining the slopes of the elastic surface in Figure 5b, require a compression test on a previously desiccated sample, such as the test in Figure 4.…”

“…The resulting profile is seen in some of our drying box tests on clayey tailings, as illustrated in Figure7. Detailed comparisons of the model output with a gold tailings dry box test are presented inQi et al (2020).…”

Evaluation of candidate polymers to maximize geotechnical performance of oil sands tailings

“…The oil sands tailings literature is replete with studies comparing numerically-predicted long-term behaviour of tailings deposits based on laboratory-measured LSC parameters and field-scale pilots 6 with differing levels of complexity, in particular, studies of polymer-flocculated fluid fine tailings (Jeeravipoolvarn 2005;Jeeravipoolvarn 2010;Jeeravipoolvarn et al 2015;Qi et al 2017;Qi et al 2020, among many others). These verification exercises demonstrate that finite strain consolidation modelling can predict the behaviour of polymer-amended tailings deposits, with good agreement in the field for at least several years.…”