A multivariate method for identifying structural domain boundaries in a rock mass

The mechanical and hydraulic behavior of rock masses vary from one domain to another because of the presence of discontinuities. Hence, structural domain boundaries should be determined according to the properties of these discontinuities. In this work, a new method for identifying structural domain boundaries based on discontinuity orientation and trace length is proposed. The new method is performed in three steps. First, Schmidt plots are divided into equal-area patches on the hemisphere. Second, the discontinuities occurring in each patch are ranked according to their trace lengths. Third, the Wald–Wolfowitz runs test is used to analyze the sequence of discontinuities. The results obtained by applying the new method to four simulated discontinuity data sets were more rigorous than Miller’s method in that Miller’s method only considers discontinuity orientation whereas the new method considers both orientation and trace length. Both the proposed method and Miller’s method gave the same, reliable, results when they were applied to discontinuity data collected from four adjacent adits at the Songta dam site. Only PD232 and PD252 corresponded to the same structural domain according to the two methods, because the discontinuity data collected from PD232 and PD252 had not only similar orientation distributions but also similar trace length distributions. The other advantage of the new method is that it can be used to evaluate the homogeneity of structural populations with small sample sizes.