ABSTRACT

Dedicated to Charles Fairhurst, who was among the first to write about extensile splitting and spalling in rocks subjected to compression.

ABSTRACT: We are beginning a study of deformation and fracture around underground openings through experiments on thick-walled hollow cylinders of rock that incorporate several features: plane strain loading, the ability to impose different stress paths, and 'freezing' of the fracture geometry under load. The elastic moduli calculated from the measured deformations are different than the moduli reported from uniaxial compression tests. The 'unconfined' strength of the rock surrounding the hole is two to three times typical uniaxial strengths, and a confining stress in the hole strengthens the rock. Although the load is axisymmetric, there is a preferred direction of failure. Failure is by brittle spalling, resulting in triangular failed regions with pointed tips. The extent of failure is influenced by stress path and perhaps by strain rate.

1 INTRODUCTION

For decades, laboratory tests on rocks have sought to achieve a homogeneous state of stress on a sample of rock, and to increase these stresses until the entire sample fails. The aim of this procedure is to define the strength of the rock as a function of the stress invariants and also to obtain a constitutive law describing the pre-failure and post-failure stress-strain behavior. The results of these measurements have often been applied to the design of underground openings to determine, with varying degrees of success, the zones of rock subject to failure and the resulting deformations. In many cases the predictions do not match the observations, and the actual mechanism of failure is often different than expected.

This is not surprising when one compares the geometry and loading conditions around an underground opening to those in a laboratory core test. The rock next to an excavation is generally under polyaxial stress in a plane strain condition, but with only one surface that can dilate. It is subject to stress and strain gradients and is loaded by adjacent rock rather than a testing machine. The stress path imposed on the rock during excavation may be quite different than in a typical laboratory test. It is important to determine what effects these differences have on the strength, deformations, and failure mechanisms of the rock, and whether failure in this situation is controlled solely by the stress state. It is important also to determine whether stress-strain behavior and macroscopic failure mechanisms observed in core tests are inherent material properties or if they are influenced by features such as core geometry, loading geometry, and stress path.

It is therefore instructive to perform laboratory experiments on model openings, such as hollow cylinders. Hollow cylinders have often been used to test the strength of rock under true polyaxial states of stress(Hoskins1 969). More recently, thick-walled hollow cylinders of rock subjected to axisymmetric loading on the external diameter, and sometimes axial loading as well, have been used to model boreholes or tunnels(e.g. Daemen & Fairhurst 1971, Santarelli & Brown 1987).

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