Some consequences of inelastic rock-mass deformation on the tunnel support loads predicted by the Einstein and Schwartz design approach

This paper examines the effects of inelastic deformation of a rock-mass on the tunnel support loads predicted by the Einstein and Schwartz approximate design procedure. This procedure calculates the loads imposed on a circular support in a rock-mass subjected to non-uniform in situ stresses, under the assumptions that these stresses are 'relaxed' (i.e., allowed to interact with the lining) after the support is installed, and that the rock-mass remains elastic. Results presented in this paper indicate that, for very deformable rock-masses i.e., rock-masses classified as 'Very Poor' and those falling into the category of soils according to the Geological Strength Index (GSI) or the Rock-Mass Rating (RMR), the Einstein and Schwartz procedure over-predicts the loads and deformations on the support. Thus, for these cases the procedure is conservative. For relatively stiff rock- masses i.e., rock-masses of 'Very Good' to 'Poor' quality, the Einstein and Schwartz procedure under-predicts the loads and deformations on the support. For these cases the procedure is not conservative. It is also found that the support loads computed on the assumption that the rock-mass is subject to a uniform in situ stress, where this stress is the mean of the (actual) maximum and minimum stresses, provide useful practical insights into the loads on the support under non-uniform conditions.