Influence of Foliation Orientation on Tunnel Behavior

The relative orientation of foliation or closely spaced bedding has an important influence on the behavior of underground structures. Not only the potential failure mode is controlled by the relative orientation between underground opening and foliation strike, but also the longitudinal development of the displacements. Case histories and numerical simulations have shown that the magnitude of the displacements can differ almost by an order of magnitude for cases where the strike is parallel and perpendicular to the tunnel axis respectively. The reason is the usually low shear strength parallel to the foliation, which can lead to shearing or buckling failure modes. The knowledge of the influence of the relative foliation orientation on the mechanical behavior of the ground is extremely important for excavation and support design. The paper will show the results of numerical simulations, as well as monitoring data from tunnels in foliated rocks. Recommendations for excavation and support strategies are given.