Rock masses may present remarked stress anisotropy, and so the geostatic horizontal stresses may be highly anisotropic and larger than the overburden stress. Therefore, the alignment of a tunnel with one of the geostatic principal stress directions is improbable; thus, far-field axial shear stresses are likely to be present in the direction of the tunnel. In addition, rock masses may have important material anisotropy due to the presence of structure, e.g. bedding, foliation and stratification; so, a tunnel is also likely to be misaligned with the principal material directions of the rock mass. Those misalignments are often neglected in tunnel design and yet, their effects are not well explored in the literature. In this paper, the deformation patterns and the 3D face effects on tunnels misaligned with the principal directions of stress and material anisotropy are explored.

1. INTRODUCTION

Rock masses may present remarked stress and material anisotropy and, yet, tunnel design often neglects the tunnel orientation with the principal directions of stress and material anisotropy. However, the importance of the tunnel orientation with respect to the geostatic principal stress directions is well recognized. Experimental tunnels had been constructed in an Underground Research Laboratory (URL) in France, where the URL was placed at 490m depth in an anisotropic Claystone rock mass. Tunnels were excavated parallel and perpendicular to the major horizontal stress (σH). A supported circular tunnel of radius 2.6m aligned with sH showed a horizontal convergence of 37 to 58mm and a vertical convergence of 24 to 30mm. A similar tunnel aligned with the minor horizontal stress (σh) showed horizontal convergence of 19mm to 34mm and vertical convergence of 112mm to 158mm (Armand et al., 2013). These field data illustrate the importance of the tunnel orientation with respect to the geostatic principal stresses. Indeed, it has been long known that a tunnel should be aligned with the geostatic major principal stress to minimize stress concentrations around the opening (Goodman, 1989).

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