BOUNDS FOR TUNNEL ROOF SUPPORT.

A modified Mohr-Coulomb yield criterion and its associated flow rule are used in a plane strain approach to predict lower and/or upper bounds for the supporting forces necessary to prevent a homogeneous isotropic rock mass, either intact or with granular particulate characteristics, from breaking into a horizontal circular or rectangular tunnel. Although the use of the normality condition often leads to the prediction of excessive rates of volume dilatation, this assumption greatly facilitates the analysis and leads to a first order approximation of the true solution. The results suggest that, even for relatively weak rocks, the overall stability is of secondary importance and local stability problems associated with adverse geological conditions are probably of greater significance. Refs.