Abstract: When digging large-span long-diversion tunnel in soft rock, because of the poor stability of surrounding rock, one of the pivotal questions is to ensure the stability of the surrounding rock and safe construction. In this paper, a 3D model of the diversion tunnel is established based on engineering exploration and analysis of environmental geological conditions. Then, the surrounding rock stability after excavating is analyzed based on unloading rock mass mechanical theory and method. After taking the effects of dynamic unloading into consideration, the calculation results show that large deformation occurs in surrounding rock, which mainly locates in the partial regions near the confluence of faults and tunnel, while the arch subsidence of other cavity axis section is about 10mm, the relative displacements around the tunnel are within the specification tolerance, and the abutment tunnel is stable during and after excavation processes. The influences of unloading effect, such as obvious plastic zones, local deformation and stress concentration, are especially remarkable in faults and unconformity contact planes, which should be caused enough attention, that is, it is necessary to take supporting measures in time or take advanced support during excavation processes. Finally, some feasible suggestions about the construction method are put forward according to the calculation results. The research results can offer important support for the design and construction of diversion tunnel.

Key words: sort rock, tunnel, unloading, geology forecast