Keynote lecture: The influence of geological structure on the engineering of underground openings in discontinuous rock masses

Field investigations of underground openings in discontinuous rock masses have revealed an important observation. Block failures that were documented following the excavation could be generally correlated with a small group of joint half-space combinations ( joint pyramids , or "JP&s;s ). Although in a highly discontinuous rock mass with several representative joint sets the number of possibly removable JP&s;s is very large, only a limited number was found to create the most common and widespread failures; these JP&s;s consistently determine the most critical blocks, i.e. the key blocks of the excavation. Two factors determine the likelihood that a removable JP will create a critical key block for a given free face of the excavation. One is the frequency or mean spacing value of each joint set that creates the boundaries of the given JP. A JP that is made of members of the most frequent sets is more likely to be represented. However this likelihood must be modified by the JP shape. A narrow JP yields blocks that have their apex remote from the free face and therefore is less likely to form critical blocks than a JP that has an apex adjacent to the free face in question. The results of field investigations are given to demonstrate the applicability of the approach.