Abstract
The effect of tunnel size on stability problems in jointed rock masses is usually analyzed through the estimation of rock support pressures. The Q-system of rock mass classification has been widely used to estimate the rock support pressure from the rock mass quality Q. The empirical equation relating the support pressure and the tunnelling quality Q suggests that the unit support pressure is independent of the tunnel size. In this paper numerical experiments are conducted to investigate the effect of tunnel size on rock 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 in the shotcrete lining doubles as the diameter of the tunnel increases from 5 to 20 m. For linearly elastic rock mass the increase in support pressure with increasing tunnel size was computed to be small. These numerical results confirm the observations from instrumented tunnel sections in the Himalayas. (A) This paper was presented at Safety in the underground space - Proceedings of the ITA-AITES 2006 World Tunnel Congress and the 32nd ITA General Assembly, Seoul, Korea, 22-27 April 2006. For the covering abstract see ITRD E129148. Reprinted with permission from Elsevier.
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