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. The analysis of these effects in the past, have primarily been restricted to two-dimensions (e.g. plane strain, plane stress). In other words, stress changes around the advancing tunnel face have been largely ignored due to the complexity of three-dimensional analysis.

Results from this study, however, show that complex load paths experienced near the tunnel face, due to the gradual excavation and advance of the tunnel, can significantly influence the response of the rock mass. In situ observations at the Atomic Energy of Canada Limited's Underground Research Laboratory (AECL's URL) have similarly shown that stress rotation in areas of high tangential stresses near the tunnel face significantly contributes to the strength degradation and failure of the rock.

Three-dimensional finite-element results demonstrate that as the tunnel face approaches and passes through a unit volume of rock, the spatial and temporal evolution of the 3-D stress ?eld encompasses a series of deviatoric stress increases and/or decreases as well as several rotations of the principal stress axes. 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 signi?cance of these effects relates to the stress dependency of microfracture initiation and propagation, brittle fracture damage and rock strength degradation. Implications with respect to the new Gotthard base tunnel, currently under construction in Switzerland, are presented using examples from the nearby Furka tunnel.