Effect of tunnel size on support pressure

Abstract

This technical note reviews existing empirical approaches for estimating support pressure and studying the effect of tunnel size on support pressure, and discusses a concept proposed by RK Goel for emphasising the effect of tunnel size on support pressure. Some of the empirical approaches have been developed for flat roofs, and some for arched roofs. For underground openings with flat roofs, the support pressure is generally found to vary with the width or size of the opening; in arched roof tunnels, it is found not to depend on it. Support pressure is directly proportional to the size of the tunnel opening for weak or poor rock masses, but not for good rock. Goel et al. have suggested different correlations for predicting support pressures in non-squeezing and squeezing ground conditions; the respective formulae for these correlations are cited. Analysis of these correlations, using measured support pressures from 25 tunnel sections, has provided a good opportunity to study the effect of tunnel size on support pressure in arched roof tunnels. In such tunnels, tunnelling through good rocks shows almost no influence of tunnel size on support pressure. Through weak rocks experiencing squeezing conditions, the support pressure increases with tunnel size.