Abstract

Soft rocks are of great geological and geotechnical interest, due to their mechanical and structural characteristics falling between those of a soil and those of a rock. This presents several challenges for constitutive and numerical modelling. Here we propose a calibration strategy of an advanced kinematic hardening model with structure degradation against experimental data on soft rocks, accounting for their features at the micro- and macro-scales. The experimental results show a highly structured material, with strength and stiffness being controlled by inter-granular bonding as well as by mean effective stress. The clay content of soft rock samples seems to be a dominant factor: the specimens show lower stiffness and strength when the content is higher. Accordingly, the samples have been classified into two different types (A and B). The constitutive model reproduces well both the mechanically strong and stiff behaviour of Type A as well as the more ductile mechanical response of Type B specimens observed during drained triaxial compression tests. With increasing confining pressures, the transition from brittle and dilatant to more ductile and contractant behaviour observed for both types of rocks is well captured by the adopted constitutive model.

Introduction

The application of engineering science to the study of soils and rocks requires conceptual and mathematical models, able to describe the real behaviour of these materials [1]. Soft rocks are borderline cases between soft soils and hard rocks, with a stiffer and stronger structure compared to soft soils, and are not influenced by large-scale discontinuities, unlike hard rocks [2].

Experimental observations have shown similarities in the behaviour of soft rocks and natural clays, even though their structure may have originated from different processes [1, 3-7]. A key point in the experimental and modelling investigation of this class of materials is the characterisation of their pre- and post-gross yield behaviour, where the term "gross yield" indicates a state in the effective stress space outside the elastic domain at which a significant discontinuity in stress-strain response can be easily identified [8]. Behaviour before gross yield is generally stiff, but not necessarily elastic, as structure degradation may start with stress changes within the structure locus [1]. After gross yield, the structure is progressively lost with increasing plastic strains, causing the mechanical response to converge towards that of the reconstituted material [9].

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