The effect of tunnel size on stability problems in rock masses

摘要

The effect of tunnel size on stability problems is usually analyzed through the estimation of rock support pressures. Rock mass classification systems serve as the only practical basis for estimating rock support pressures in complex underground structures. Several classification systems, including the Q-system, have been developed for estimating tunnel support pressures. The empirical equation relating the support pressure and the tunneling quality Q suggests that the rock support pressure is independent of the tunnel size. In this paper numerical experiments are conducted to investigate the effect of tunnel size on support pressures. It is shown that the support pressure increases significantly with tunnel size for an elastic-perfectly plastic rock mass. The maximum axial force on the shotcrete lining doubles as the diameter of the tunnel increases from 5 to 20 m. For an elastic rock mass the increase in support pressure with increasing tunnel size is not so significant. These numerical results confirm the observations from instrumented tunnel sections in the Himalayas.