Three-dimensional Analysis of a Tunnel Excavation in a Jointed Rock Mass

A tunnel at a shallow depth, lying between several and 55 meters, was excavated in a jointed rock mass. Models of the rock mass and tunnel were elaborated basing on the data from an object in Carpathian flysch (Poland). The tunnel behaviour was analysed by using FLAC3D program and Coulomb-Mohr criterion and ubiquitous model of the rock mass with 10 combinations of the joint systems orientation (with respect to the tunnel axis). The tunnel shape is horse-shoe and its height equals 4.5 m. In each tunnel cross-section, 16 rock bolts and 20 cm shotcrete layer (with the strength increasing with time) were mounted. In cases of unfavourable orientation of joint system and unstable conditions, rock bolting of the tunnel heading face was installed and modelled. The numerical analysis was carried out for each excavation step, equal to 1.5 m. The entire deformation process and stress redistribution were registered starting with the cross-section in originally intact rock mass (i.e. before the tunnel heading face reached it), to the section located far from the face, in already supported and stabilized tunnel. The results obtained show the effect of discontinuities orientation on the stress distribution and displacement magnitude. The first signs of tunnel approaching heading face appear in a cross-section situated in a distance of 7 to 9 m from it. The processes of stress and displacement redistribution are long-range and occur in a distance of many meters from the already excavated tunnel face. The important result of the analysis was the determination of the ground response curves representing decompression and support loading as a function of the excavation advance. These results allow for better design of a proper support system of the tunnel.