Stability of tunnels according to depth and variability of rock mass parameters

Even with the most recent advances in rock mechanics, the stability assessment of underground tunnels continues to involve many uncertainties in the parameters which can significantly affect the calculated factors of safety. In the present work, a straight characterization which can help to evaluate the risk of a tunnel collapse with respect to the depth of excavation is proposed. Following a consolidated Limit Analysis approach and by means of the Hoek-Brown failure criterion, the galleries are divided into shallow, intermediate and deep according to the variability of the mechanical parameters of the rock mass with depth. A solution to the problem of impending collapse in the case of variable rock properties is proposed and examples are presented and discussed both from an analytical and a numerical standpoint.