Squeezing deformation in layered surrounding rock and force characteristics of support system of a tunnel under high in-situ stress

The layered soft rock is widely distributed in Western China, such as slate, carbonaceous slate, phyllite, and so forth. During the tunnel excavation, geotechnical problems frequently occur in the surrounding rock, such as excessive overbreak and intense deformation associated with asymmetric squeezing, because of low strength, poor self-stability and intense anisotropy of structural strength in layered surrounding rock. Large deformation can result in intense damage to the primary support, and even splitting of secondary support, significantly influencing the construction and security of tunnels. At the site of Liangshui tunnel in Lanzhou-Chongqing railway, a series of real-time contact pressure monitoring tests is conducted at different positions in section between surrounding rock and support system. Mechanical responses of the support system are analyzed in time and spatial domains using the monitored contact pressure. In-situ monitoring and numerical inversion analyses of displacement are also performed. The results indicate that the contact pressure of support system shows irregular distribution in space, which agrees with the concentrated deformation position of the surrounding rock; and its variation often lasts for a long period of time due to the influence of excavation method. The failure of the steel arch frame takes place in the weak axis plane because of its smaller lateral anti-bending rigidity. Meanwhile, the monitoring data show that their occurrence significantly lags behind the stabilization of convergence deformation. Based on the force characteristics of the support system, a more reasonable design scheme is proposed for tunneling in such a kind of rock.