ABSTRACT

Abstract Under differential compressive stress, rocks exhibit complex deformation patterns, including near-linear stress-strain relationships, dilatational strain-hardening and strain-softening behavior which changes with confining stress. This complex behavior reflects the microstructure of the rock and changes in microstructure resulting from crack growth and interaction. Many of these patterns can be analyzed using linear elastic fracture mechanics to account for the strain energy of the microcracks and to model the growth and interaction of microcracks both under critical and subcritical crack growth conditions. While continuum models replicate many observations, micromechanical processes affecting the macroscopic deformation of porous rocks composed of assemblages of grains require explicit modeling of the grains. 1 Introduction

In response to differential compressive stresses, most rocks exhibit complex patterns of strain. Initially, the slope of the stress-strain curve increases, as low aspect ratio micro-cracks, often representing incomplete grain contacts in elastic rocks, close. This compaction is succeeded by a near-linear stress-strain relationship, often interpreted as elastic deformation, though frictional sliding occurs between internal microcrack surfaces and at grain contacts, before the peak of the stress-strain curve, strain-hardening occurs in varying degree, accompanied by dilatation. Deformations greater than those corresponding to the strength of the rock usually follow strain-softening and are accompanied by extreme dilatation at low levels of confining stress. Increasing confining stress suppresses dilatation, which may even be succeeded by compaction of porous rocks at high confining stresses. Dilatation is a result of opening mode or extensile microcrack growth within the rock, and compaction involves comminution of grains and displacements of granular particles. This complex behaviour reflects the microstructure of the rock and changes in it as a result of crack growth rather than intrinsic properties of its constituent minerals. The influence of microstructure and new microcracks on macroscopic deformation is the central theme of this paper.