Underground excavation in jointed rocks: a scheme to evaluate the rock load spectrum, taking joint orientation scatter into account

Tunneling in hard jointed rock can be prone to failures that are controlled by the rock mass structure. This kind of dangerous event can take place suddenly and greatly depends on the rock joint geometry. At the design stage, care must be taken to extrapolate the possible rock condition in depth, in order to define effective support measures. In fact, the stabilizing support action relies on the anticipated characteristics of the failure mechanisms. Rock mass classification schemes could be used to obtain first estimates of support requirements, but such evaluations often give a rough range of possible rock loads, mainly for poor rock mass rating. This paper shows a useful procedure to better constrain rock load estimation, which explicitly accounts lor rock jointing structure. The proposed approach is built on the Goodman and Shi (1985) block theory framework for tunnels and takes the natural scatter of joint set attitudes into account in an appropriate manner by means of a simulation scheme. A rock mass structure gives rise to different critical block types, whose worst configuration is that which envelops the tunnel shape. The interdependence of the relative tunnel-joint system orientation can be ascertained by scanning the full range of possible tunnel bearings using the simulation scheme. This analysis gives the sensitivity of the tunnel driving direction to the rock mass condition (tunnel rock load spectrum). This approach has been applied to the reanalysis of a sector of an Alpine tunnel excavated in a metamorphic environment. It gives a coherent appraisal of the strong link between the natural joint attitude scatter and the rock load. The statistics of the computed rock loads can be used to obtain a realistic bound for the extimation obtained from the rock mass classification schemes; these statistics are directly dependent on the geometry and simple mechanics.