This paper addresses the issue of the tensile strength of incipient discontinuities in rock and presents preliminary results from a series of laboratory studies. In most rock masses rock discontinuities, as veins or incipient fractures, often retain some tensile strength that may approach that of the parent rock. This fact is of high importance to rock mass strength but is generally ignored, neglected or underestimated. Samples of incipient rock discontinuities including joints, bedding and mineral veins have been tested in direct tension. It has been confirmed that incipient, visible and discrete discontinuities, that might be recorded as ‘joints’ in a rock mass characterisation programme can indeed have high tensile strength, approaching that of parent rock. Others are of course far weaker. The factors contributing to tensile strength have been examined. It is concluded that the degree of incipiency of rock discontinuities needs to be differentiated in the process of rock mass classification and engineering design and this can best be done with reference to the tensile strength relative to that of the parent rock.

1 Introduction

Rock discontinuity is a collective term which encompasses joints, beddings, faults, layers and fissures, etc. Generally these fractures develop from an original incipient state with high tensile strength to fully mechanical discontinuities with zero tensile strength (Hencher & Knipe 2007). Incipient discontinuities have a controlling influence on many projects in terms of strength, deformability and permeability. For engineering simplification purpose, incipiency of rock discontinuities is usually neglected or underestimated. ISRM standard defines discontinuities have very low or zero tensile strength (ISRM 1978). Rock mass classification schemes such as Q system, RMR and RQD are defined on the basis of this engineering assumption which means that they all fail to resolve the issue of incipiency of discontinuities as well as varying degrees of tensile strength.

Degree of incipiency of rock discontinuities should not be neglected as it influences the overall strength of rock. Figure 1a presents an example of joint incipiency provided by Hencher when shearing a rock core. Light rock bridge segment is revealed after shearing test. Another incipient joint is shown in Figure 1b, intact rock bridges indicated by red arrows are shown after breaking along this fracture with geological hammer.

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