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
