In-situ monitoring and analysis of mechanical characteristics and deformation of bottom structure of tunnels

By means of the in-situ monitoring method, the deformation of bottom structure of tunnel and the stress of concrete and rebars in its inverted arch are tested and analyzed. Under the action of unloading of surrounding rock, the loads of tunnel linings, and the upper surrounding rock auxiliary facilities and train load, the bottom structure of the tunnel witnesses a repeated process of uplift-subsidence-uplift-subsidence during the whole course of tunnel construction, and the concrete and rebars in the inverted arch have a process of tension-compression-tension-compression as well. The better the condition of the surrounding rock, the smaller the values of the deformation and stress and their change ranges. During the whole course of tunnel construction, the maximum subsidence of the bottom structure of the tunnel is less than 5.0 mm, and the maximum tensile and compression stresses are both less than 2.0 MPa, thus high-speed trains can run safely. The inverted arch of the tunnel is in the worst conditions before the tunnel linings are constructed. So the tunnel linings must be constructed in time to make sure that the inverted arch is under better conditions. The redistribution time of the stress on the surrounding rock after tunnel excavation is generally more than two years.