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.
Frequently homogeneous models are used for the design of tunnels, applying empirical relations to account for the jointing influence on strength and deformability. Those methods may result in more or less realistic results in generally fractured rock masses, but will not be able to provide realistic results in strongly anisotropic rock masses.
The structure of the rock mass plays an important role for the behavior of tunnels. The generally reduced shear strength, as well as missing tensile strength perpendicular to the foliation can lead to shearing along the discontinuities, as well as their opening. Depending on the relative orientation between tunnel axis and foliation orientation the modes change between opening of discontinuities, shearing, shearing and opening, as well as buckling. The discontinuities also influence the development of the displacements in longitudinal direction, as well as the orientation and magnitude in the cross section. Results from numerical simulations and examples of measured displacements shall demonstrate this [1,2].