Size Effect on the Stability of Tunnel Excavation

Size effect of tunnel excavation in fractured rock mass was analyzed with UDEC in this paper. For simplicity, two series of orthogonal equidistant joints were conceived in the calculation area. Change the excavation size and keep the constitutive model and its parameters of the rock mass around the tunnel to be constant during the solution. The result show, with the increasing of excavation size, vertical displacement of tunnel roof become larger, and the excavation stability turn weaker. So excavation size has great effect on the stability of fractured tunnel. In addition, another key factor, the joint set dip angle, whose effect to the tunnel stability was unearthed by comparing the results of horizontal and 45°joints. The larger the dip angle the weaker the stability at the same tunnel size.