Geomechanical Appraisal of the Deformation Potential of a Deep Tunnel in a Volcanic Rock Mass

Ophiolitic complexes can be defined as an association of ultra basic volcanic rocks (ultra-mafic) and basic (mafic) constituents of the oceanic crust as a result of a phenomenon of abduction clash between continental plates. An intricate structure in which volcanic rocks are intruded in the direction of the schistosity of the metamorphic rocks is further complicated by large scale over thrusts which create tectonic melanges at the base of such mega structures, affecting it by secondary tectonic contacts and inverse faults. The whole entity is found in considerable tectonic disorder where packages of peridotites or pillow lavas of various sizes "float" inside a sheared shale-like mass. It is understandable that tunnelling in such a formation requires a good appraisal of the rock mass deformation potential as it will have a direct impact in the support behaviour, particularly in the deeper sections with more than 250 m overburden. This paper deals with the geomechanical characterisation of the heterogeneous rocks mass and the different models employed to simulate its behaviour during the tunnel excavation and support.