ABSTRACT

Anisotropic, jointed rock masses can result in relatively complex conditions that are not easily approximated with numerical models using general rock mass strength parameters. In this case study, numerical models using either rock mass continua or fracture networks are examined in the context of tunnels within horizontally bedded shales with high horizontal stress. This case study demonstrates the scale and orientation effects of rock mass discontinuities on the understanding of rock mass reaction and potential failure mechanisms. A case history from the Hanlan tunnel project in Mississauga, Canada is reviewed. Continuum models for the tunnels were created using rock mass strength parameters from the Generalized Hoek-Brown Failure Envelope using laboratory testing data and Geological Strength Indices. Due to high horizontal stress and the relatively low Geological Strength Indices in the horizontally bedded rock mass, continuum models of the tunnels exhibit extensive conjugate shear failure planes through the tunnel haunches, resulting in very large zones of plasticity that are not typically observed based on local tunneling experience. Adjusting the rock mass strength parameters for a pseudo-intact rock condition and explicitly modeling fracture networks changes the model ground reaction and more accurately reflects observed behaviour.