The effect of rock anisotropy caused by one and consecutively by the combination of two planes of weakness on the loading of a circular tunnel is investigated using numerical analysis. Finite element method is employed. The rock mass discontinuities are simulated separately with joint elements. The effect of multiple jointed rock mass is examined by adding a second joint set at specific orientations. Parametric analyses are carried out for the joint friction angle, for the stress field, in combination with varying joint orientation, joint density and joint stiffness. The analysis results are used to identify the extent of the slip zones around the opening and to compare them to existing theoretical solutions. The variation of displacements along the periphery of the tunnel is also compared to existing solutions and the additional loading of the tunnel caused by the slip of the joint planes is presented and commented.

1 Introduction

Stratified rock masses are often encountered in underground works such as tunnels and mining. Their stratification is associated with bedding planes developed during sedimentation or at a later stage, during metamorphosis. When the stratification is at the scale of the underground construction, the behavior of the system is governed by the mechanical properties of the bedding planes, rather than the properties of the rock mass itself.

Wittke (1990) showed that the overstressed zones around a tunnel depend on the anisotropy of the rock mass, Amadei & Pan (1992) that the gravity-induced horizontal stresses depend on several parameters such as the type, degree and orientation of the rock anisotropy with respect to the ground surface. According to Gerrard (1977), intact rock anisotropy is stress dependent with a decrease in anisotropy associated with an increase in confinement. The stress dependency of rock anisotropy implies that linear elasticity may be of limited value when describing the deformability of anisotropic rocks and that it should be replaced by non-linear elasticity or more complex constitutive behavior if permanent deformation occurs. Acceptable predictions of rock behavior can still be achieved assuming linear anisotropic elasticity as long as the selected rock properties are determined in a stress range comparable to what is expected in situ. Being able to account for the directional character of anisotropic rocks instead of assuming them isotropic is certainly a step in the right direction, Amadei (1996).

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