Impacts of geological conditions on instability causes and mechanical behavior of large-scale tunnels: a case study from the Sichuan-Tibet highway, China

Tunneling in complicated geological conditions frequently encounters problems with squeezing ground, large ground deformation, rockbursts, asymmetric deformation, non-linear rheology of surrounding rock, water inrush, and failure of supporting structures. In this paper, the Sichuan-Tibet highway, which is located at the north-west edge of the Sichuan basin and the eastern margin of the Qinghai-Tibet plateau, serves as a case study to investigate potential hazard sources for tunnels constructed in complex and difficult mountainous areas. Consideration first is given to the following 5 sets of conditions: crossing a fault zone, shallow tunnels in soft rock, crossing a landslide deposit, and deep tunnels in hard and soft rock formations. A program of field tests was performed to study the changes in rock pressure and the performance of support structures in different geological conditions. The results confirmed that the squeezing of rock depended upon a combination of the surrounding rock strength and the in situ stress field; fracture zones and water conditions reduced the integrity and mechanical properties of the surrounding rocks; topography and bedding structure can induce asymmetric mechanical behavior of the supporting structures during construction. Meanwhile, time-dependent rock deformation may have greater adverse impacts on the structural safety performance of tunnel during their operation.