Abstract: During the highway tunnel construction, soft rock tunnels under the condition of high geostresses are prone to having large deformation problems. Huangjiazhai Tunnel of Mazhu Expressway Project was selected as a typical tunnel in the highly geostressed soft rock mass. The construction deformation monitoring data indicate that the deformation rate of the excavated rock mass is fast with a large accumulated total deformation over a long observation period. An obvious creep deformation can be identified form the data. In this paper, the limits of the plastic zone and deformations in the soft rock mass from the numerical analysis results for the four different excavation sequences are presented. These four excavation sequences are: No support and excavation at twosteps; excavation at twosteps with support; excavation around the core support and threestep excavation with support. The results indicate that the initial support can effectively reduce the areas of the plastic zone and minimize the deformation. After the support, the maximum deformation in the surrounding soft rock mass due to the excavation transferred from the top of the arch to the bottom of the tunnel. The maximum deformation as well as the area of the plastic zone in the excavation around the core support are obviously smaller than those in other twostep excavation sequences. When use the threestep exaction with support, the deformation in the surrounding rock mass is the smallest among all other excavation sequences but the subsidence in the top of the arch is the largest. Since horizontal convergence in the excavated tunnel, it is suggested that the threestep excavation with support be used in the Huangjiazhai Tunnel excavation.

Key words: Soft Rock Mass with High Geo-stresses, Large Deformation, Optimization of Excavation Methods