Role of Topography and Anisotropy When Selecting Unlined Pressure-Tunnel Alignment

Abstract

This paper shows how analytical solutions, proposed by the writers to predict in-situ stresses in rock masses with smooth and irregular topographies, can help in the selection of the alignment of unlined pressure tunnels near slopes and valley walls. The proposed methodology can be applied to ridges and valleys in isotropic or anisotropic rock masses subject to gravity or to combined gravitational and tectonic loading. The analytical solutions are two-dimensional and assume plane or generalized plane strain. They can be used to substitute existing design charts for pressure tunnels based on the finite-element method. It is found that the safe alignment of unlined pressure tunnels depend greatly on the extent of tensile regions in valley walls, which itself depends on parameters such as valley geometry, the degree of rock anisotropy, the orientation of the planes of rock anisotropy, and the in-situ loading conditions (gravity, gravity and tectonic).