Numerical modelling of three-dimension stress rotation ahead of an advancing tunnel face

Abstract

As underground excavations and construction works progress into deeper and more complex geological environments, understanding the three-dimensional redistribution of excavation-induced stresses becomes essential given the adverse consequences such stresses will have on the host rock strength and the subsequent excavation stability. This paper presents the results from a detailed three-dimensional finite-element study, which explores near-field stress paths during the progressive advancement of a tunnel face. These results demonstrate that as the tunnel face approaches and passes through a unit volume of rock, the spatial and temporal evolution of the three-dimensional stress field encompasses a series of deviatoric stress increases and/or decreases as well as several rotations of the principal stress axes. Particular emphasis is placed on the rotation of the principal stress axes as being a controlling factor in the direction of fracture propagation. If this orientation changes in time, i.e. during the progressive advancement of the tunnel face, the type of damage induced in the rock mass and the resulting failure mechanisms may also vary depending on the type and degree of stress rotation. The significance of these effects is discussed in terms of microfracture initiation and propagation, brittle fracture damage and rock strength degradation. Further analysis is also presented for varying tunnelling conditions including the effects of tunnel alignment with respect to the initial in situ stress field, excavation sequencing and elasto-plastic material yielding. Implications with respect to the new Gotthard base tunnel, currently under construction in Switzerland, are presented using examples from the nearby Furka tunnel.