Plastic radii and longitudinal deformation profiles of tunnels excavated in strain-softening rock masses

We describe techniques to estimate plastic radii and longitudinal deformation profiles of tunnels excavated in rock masses. The longitudinal deformation profile, a graph that relates a fictitious internal pressure to the distance to the tunnel face, is necessary to assess adequate distance to the face for the purpose of installing support. Traditional application of this method usually relies on an elastic representation of the longitudinal deformation profile. A more realistic approach has been proposed recently that accounts for the elastoplastic nature of rock masses. It is based, however, on assuming elastic–perfectly plastic rock mass behaviour, an assumption which is more or less realistic, but only for low quality rock masses with a geological strength index (GSI) below 35. We extend this approach to the case of strain-softening rock masses representing a wider range of rock masses (25 < GSI < 75). Based on studying various numerical techniques to estimate these curves, we propose a simplified approximate equation of the plastic radius of a tunnel excavated in a strain-softening rock mass, which can be combined with existing longitudinal deformation profile estimation techniques to analytically obtain a more realistic approach to calculating longitudinal deformation profiles for strain-softening rock masses.

► The confinement-convergence method in strain-softening rock-masses is reviewed.
► Based on rigorous techniques, a formulation is derived to obtain the plastic radius.
► LDPs are numerically obtained in tunnels excavated in SS rock masses.
► An existing formulation is extended to estimate LDPs of tunnels in SS rock masses.