Rock Block loads in a tunnel in hard jointed rock: A comparison between estimations based on outcrop and underground structural survey

Underground excavations in hard jointed rock can undergo structurally induced damage or even collapse. An analysis of rock loads was made with reference to the relative orientation between the excavation direction and joint attitudes, taking into account for the variability of joint attitudes around their mean values. The analysis procedure was built into the framework of the Goodman and Shi (1985) block theory and allows for the evaluation of the maximum rock load frequency distributions, using a Monte Carlo simulation of joint set attitudes. An appreciation of the influence of joint frequency was accounted for according to Mauldon(1992). An analysis of the worst load condition was performed according to the probability distribution of the largest value. This procedure was applied to an alpine motorway tunnel in order to compare the rock load estimations that arise from two different exploration surveys: an outcrop survey and a direct underground survey.