Comparison between analytical and numerical stability analyses using FLAC for shallow tunnels in soft rock

Different analytical methods based on limit equilibrium conditions are available in order to design shallow tunnels in soft rock. The results of some methods show the relation of support resistance and ratio overburden to tunnel diameter. The analytical methods are based on different assumptions, and therefore the results obtained are different. In order to evaluate these different results continuum mechanic models using FLAC were investigated. FLAC calculations show that there is a very critical value of the ratio overburden to tunnel diameter for tunnel excavation collapse. Back analysis of reported tunnel collapses give the same critical ratio. Analytical methods do not reproduce this effect based on continuum mechanical investigations. In analytical methods it is assumed that the ratio of horizontal to vertical stress above the tunnel is equal to the coefficient of passive earth pressure. FLAC calculations show that this assumption is not sufficient in order to reproduce continuum mechanical effects and that the ratio of horizontal to vertical stress provides a great variety. Thus, analytical methods are first estimates, but numerical investigations, like FLAC, should be used to design tunnels.