Characterizing discontinuities in naturally fractured outcrop analogues and rock core: the need to consider fracture development over geological time

This paper reviews aspects of the procedures for characterizing rock masses from outcrop mapping and core logging. It is argued that current definitions of discontinuities and joints are too simple and too coarse to deal adequately with the range of geological features that are found in the field and that range from open fractures through to incipient joint traces. A generic approach is proposed that differentiates between discontinuities on the basis of relative tensile strength compared to the intact parent rock. Examples are provided of how fracture frequency and extent vary with degree of weathering and erosion, and it is suggested that the concept of dynamic development of geological discontinuities needs to be appreciated by geotechnical engineers and structural geologists when analysing fracture networks. This concept has major implications for the use of rock mass classifications to zone the rock mass into engineering units.