Discontinuity spacings in rock

The possible distributions of discontinuity spacings along a straight line through a rock mass are considered. Unless there is a large predominance of evenly spaced discontinuities, any combination of evenly spaced, clustered and randomly positioned discontinuities leads to a negative exponenial form of frequency vs spacing value curve. The negative exponential form was confirmed by field discontinuity scanline surveys in three tunnels. Utilizing the negative exponential distribution, a relation between the Rock Quality Designation (RQD∗) and mean discontinuity frequency per metre (λ) is established: RQD∗ = 100 e−0.1λ(0.1λ + 1). The agreement with actual RQD was within 5% at 27 survey locations mainly in chalk and mudstone. Recommendations are made concerning the method of presenting discontinuity spacing data, the scanline length necessary for reasonable estimates of discontinuity frequency and the number of sample values required in a discontinuity survey.