1. INTRODUCTION
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.
Tunnel design in a jointed rock mass must account for the interaction of the joint pattern and the opening. Destressing due to the excavation causes displacements, mainly along the joints and the tunnel supports undergo a non uniform rock pressure. Though a complete mechanical analysis of a blocky system intersected by a tunnel can be performed by means of numerical methods such as DEM (Cundall, 1971) or DDA (Shi, 1989), it is also evident that only a few blocks (Key Blocks), located around the tunnel perimeter, must be properly supported in order to maintain the opening stability. A quite simple basic failure scheme of a falling or sliding monolith can therefore be used to draw design considerations. The KB concept has been exploited for tunnels (Goodman & Shi, 1985), in a rather essential but convenient approach, by defining the different block types and the stabilising forces according to the limit equilibrium condition. Further progress in tunnel KB analysis has been made by finding analytical relations between the joint frequency and the relative probabilities of block occurrence for continuous joints (Mauldon, 1992) and more recently (Mauldon, 1994, 1995) also considering impersistent joints. It refers to a reanalysis of a newly built alpine motorway tunnel and the purpose was to show how the information on rock mass structure, obtained at different levels, can be used to build simple analysis schemes of structurally controlled loads. The block theory framework was the core of the evaluation scheme where the KB variability was considered through a stochastic simulation of joint attitudes. A comparison is made between the estimations of possible loads due to two independent rock mass descriptions of the tunnel site: the first is based on an outer surface geo-structural survey; the second uses systematic underground exploration along a pilot tunnel, that was bored in advance of the full tunnel excavation. The computed maximum loads are also analysed according to the extreme value statistic. The results show that the data from the surface survey can provide a sound model of jointed rock mass to forecast tunnel rock loads.