Shear Testing of Soft Rock Masses

Previous studies aimed at improving our understanding of rock mass behavior have incorporated laboratory techniques, case studies, or numerical methods. This study extends the laboratory based data by examining the behavior of relatively large scale jointed, soft rock mass samples in direct shear. The rock mass samples tested in this study were fabricated by cutting joint sets into a soft, synthetic siltstone block. Therefore, the development of new procedures and equipment for fabricating and shear testing the rock mass samples was required. The visual and measured data recovered from this testing was used to identify and model the prepeak behavior and failure mechanisms occurring within the samples. The observed prepeak behavior of the rock masses was found to comprise either sliding along one of the joint sets or rotation of a portion of the rock mass defined by the jointing pattern. All samples ultimately failed by shearing through intact rock coincident with the shear plane defined by the testing apparatus.