Effect of in- Situ stress on the ground reaction curve, the radius of the plastic zone and stress distribution around the tunnel in the convergence- confinement method

By excavation of a tunnel in a rock mass, the stresses in the medium redistributed and base on theses new stresses a plastic zone created in the tunnel periphery. Ground reaction curve and radius of plastic zone are two important parameters in the design of tunnels lining and are key components of convergence- confinement method. Analytical relationships assuming isotropic stress field around the tunnel to determine these two parameters have already been proposed. In this paper, two-dimensional numerical analysis assuming plane strain conditions are conducted on circular tunnels. In contrast to analytical models, the weight of the rock mass as well as non-isotropic stress conditions considered and their effect on the ground reaction curve, the radius of the plastic zone and stress distribution around the tunnel is investigated. In accordance with the results obtained for the tunnel wall for a given lining pressure, with increasing horizontal stress ratio, the amount of pressure required to create plastic zone (critical pressure) and the radius of the plastic zone decreases. In the crown of the tunnel, with increasing the horizontal stresses, critical pressure and the radius of the plastic zone increases, and resistance of the crown against the plastic deformations decreases.