The effect of gravity on the stability of a deep tunnel

Abstract

In this note, the possible effect of gravity on the stability of a deep circular tunnel is evaluated through the application of a simple approximation of the gravity force, in the elastoplastic model. The conditions of stability under gravity loading of a deep unsupported tunnel have been derived by analyzing the conditions for which the support pressure is a monotonic function of the tunnel closure (or equivalently of the radius of the plastic interface). On the basis of these results, a stability diagram was formulated that provides a quick and powerful means of assessing whether a deep underground cavity will or will not collapse without support system. This analysis does not include, however, the problem of excessive closure, i.e. a deep tunnel may be stable, but yet require a support system to restrict closure to a tolerable amount. A stability analysis should therefore always be supplemented by a ground reaction curve calculation. (TRRL)