A discrete anisotropic plasticity approach has been developed for the analysis of tunnels in horizontally laminated rock masses under high stress. By allowing lateral shear along the discrete laminations using joint elements, the mechanistic behaviour around a circular opening differs considerably from that predicted using isotropic plasticity methods and is also different from simulations using an anisotropic yield function coupled with an isotropic flow rule. Isotropic methods were found to adequately predict the the behaviour when the lamination thickness was on the order of the tunnel radius. Below this threshold the discrete anisotropy method predicts gravity-driven travelling, haunch instability, beam failure or chimney failure. The work present here has been tested against the sedimentary rocks from the Niagara Tunnel Project, in Niagara Falls, Ontario, Canada and agreement was achieved. This method could be an important consideration for nuclearwaste storage in sedimentary rocks, such as the Cobourg limestone or Queenston siltstone in Canada or the Opalinus claystone in France and Switzerland.
Anisotropic ground arises from parallel laminations within a rock mass, whether due to sedimentary bedding, joints or tectonic fabric. Anisotropy is both modulus and stress dependent (Chappell, 1989) and therefore not all rock masses with parallel structure or fabric will exhibit the same degree of anisotropic behaviour. Sedimentary and meta-sedimentary rocks are typically the first to be considered as laminated however, analogous flow behaviour can occur in igneous and metamorphic rocks creating laminations which behave similarly to sedimentary. To account for the anisotropic conditions it is possible to measure the properties of the rock mass parallel and perpendicular to the lamination direction. Traditionally isotropic material properties are used for engineering design, but by incorporating discrete joint elements to represent laminations, it is possible to capture the anisotropic behaviour of a rock mass.