Quantifying the significance of isotropic joints in structural analysis

Structural analysis of a rock mass is a necessary component in the prediction of in situ block size distribution, the analysis of slope stability and hydromechanics. The success of such analyses is dependent upon the ability to characterise the populations of discontinuities in the rock mass based on measured samples usually obtained via drill core or exposure mapping. Clustering algorithms are used to characterise sets of discontinuities statistically. This technique can yield reasonable results when the majority of sampled discontinuities are assigned to sets. However many sites show evidence of isotropically distributed joints that remain unassigned during such a cluster analysis. This paper attempts to quantify the significance of relatively small numbers of isotropic discontinuities in structural analyses. Neglect of the isotropic component is shown to introduce potentially large errors in estimation of in situ block size distributions and stability analyses.