Abstract: The stability of the excavated tunnels in the high geostress soft rock mass is usually poor. Geohazards with large tunnel deformations, such as, collapse and roof squeeze, are frequently encountered in the tunnel excavation projects in these problematic rock mass. It is critically important to select an appropriate construction method. This paper presents the analytical results of a tunnel excavation support in the high geostress soft rock mass, and the analytical model was calibrated with the observed construction results. The following conclusions can be made: 1) Stepped excavation method is an appropriate construction method for the tunnel excavation in the soft rock mass with high geostresses. It has the advantage of small tunnel deflection and is reliable in the construction safety; 2) The plastic zones within the soft rock mass are initiated at the middle portion of the side walls and develop upward to the crown of the tunnel; 3) The deformations at the bottom and crown of the tunnel are toward the excavated area, whereas, larger stresses could be found a the middle portion of the tunnel walls and the deformation of these areas are toward the rock mass. Larger vertical stresses are also encountered at the middle portion of the side walls and horizontal stress concentrations are encountered at the bottom and the crown of the excavated tunnel. The supporting structures have large tensile stress at the middle portion of the side walls and larger compressive stresses could be found at the crown and bottom areas that supporting the tunnel.

Key words: high geo-stress areas, tunnel in soft rock mass, stepped excavation, finite element analysis