A new versatile constitutive law for modelling the monotonic response of soft rocks and structured fine‐grained soils

Abstract

Summary This paper deals with a new critical state–based constitutive model for soft rocks and fine‐grained soils. The model, formulated in the single‐surface plasticity framework, is characterised by the following main features: (i) a generalised three‐invariant yield surface capable of reproducing a wide set of well‐known criteria, (ii) the dependency of the elastic stiffness on the current stress state by means of a hyperelastic formulation, (iii) the ability of simulating the plastic strain–driven structure degradation processes by a set of appropriate isotropic hardening laws, and (iv) a nonassociate flow rule in the meridian plane. The adopted formulation is hierarchical, such that the various features of the model can be activated or excluded depending on the specific kind of geomaterial to be modelled and on the quality and quantity of the related available experimental results. The constitutive model was implemented in a commercial finite element code by means of an explicit modified Euler scheme with automatic substepping and error control. The procedure does not require any form of stress correction to prevent drift from the yield surface. The performance of the model is first analysed by means of a wide set of parametric analyses, in order to highlight the main features and to evaluate the sensitivity of the formulation with reference to the input parameters. The model is then adopted to simulate the experimental response observed on three different geomaterials, ranging from soft clays to soft rocks.

Unsaturated structured soils: constitutive modelling and stability analyses

“…The mechanical behaviour of the solid skeleton is here described by employing a modified version of the WR2 model proposed by Boldini et al [7], named WR2-Unsat. The original WR2 is a critical state-based hardeningplasticity model, aimed at reproducing the main aspects of the mechanical response of structured soils under monotonic loading paths.…”

“…In the following, the basic constitutive equations are briefly outlined, referring to the triaxial plane where all the stress and strain paths investigated in this study are confined. Details on the original multiaxial formulation for fully saturated conditions and its predictive capabilities in reproducing experimental results can be found in Boldini et al [7].…”

“…In this model, thus, the flow rule can be associated or not, depending on the values assigned to the shape parameters. The formulation of both yield surface and plastic potential function is very flexible and can be adapted to reproduce well-known criteria, such as non-associated Mohr-Coulomb or Modified Cam Clay (MCC) models (see [7]).…”

“…Many of them are based on critical state soil mechanics and are formulated within the single-surface isotropic hardening-plasticity framework (e.g. [7,34,38]).…”

“…For this, in the following, a relatively simple model accounting for both structure degradation and partial saturation is proposed. The model is formulated extending to unsaturated conditions a recently proposed versatile constitutive formulation for saturated structured soils [7]. The model aims at reproducing the main features of the hydro-mechanical response of unsaturated cemented soils under monotonic loading conditions, adopting a single-surface isotropic hardening elasto-plastic formulation based on a single stress variable to account for partial saturation effects.…”