Viscohypoplastic bentonite (VIBE) model

The design of bentonite buffer in nuclear waste repositories relies on Thermo-Hydro-Mechanical (THM) constitutive models for bentonite. Although a number of advanced models have been developed in the past, one aspect has been overlooked so far: time. This omission is surprising given that repository design lifetime is at least a hundred thousand years. A new THM viscohypoplastic bentonite (VIBE) model is presented that incorporates time-dependent behaviour, notably stress relaxation and creep. Building on a previous double-structure hypoplastic model, the formulation was revised to address key shortcomings. A new expression for the microstructural void ratio was introduced, and an alternative macrostructural water retention curve was proposed - suitable also for general unsaturated soil mechanics and offering an alternative to the van Genuchten model. A link between cooling-induced microstructural shrinkage leading to predicted reduced water retention - contradicting experimental evidence - was eliminated. It was proposed that the macrostructure responds differently when loading is applied externally (nonlinear and irreversible) or results from internal microstructural swelling (largely reversible). Unlike the original formulation, the new model was expressed explicitly in terms of net stress rate as the primary unknown through algebraic manipulations facilitated by the new formulation. Time-dependent effects were incorporated while preserving the model's nonlinear mechanical response. A temperature-dependent relaxation rate function was introduced to reflect experimental evidence. The model was implemented in a finite element code, showing good predictive capability for stress relaxation. Predictions of various other experiments are also presented to demonstrate improvements over the original model.