Model tests on characteristics and evolution of tunnel collapse in soil-sand interbedded strata

As a major safety risk in the construction of tunnels in soft strata, collapse deserves special attention in the construction of tunnels in layered strata. In order to prevent and control tunnel collapse, it is necessary to fully grasp the characteristics and evolution law of collapse. A tunnel in the Quaternary soil-sand interbedded strata is taken as the research object. By using model tests and digital image processing technology, the collapse characteristics and evolution laws of the tunnel in soil-sand interbedded strata are studied for the first time. The results show that: (1) The failure of the surrounding rock is not instantaneous, but gradual. It starts from the cracks in the surrounding rock at the foot of the arch and expands upward, and local falling of the surrounding rock is induced. Then the surrounding rock above the vault of the arch collapses in a small range, the crack expansion and instability occur step by step, and the surrounding rock above the vault of the arch collapses in a large range and eventually breaks away from the stratum contineously. (2) The collapse of the surrounding rock above the vault of the arch of the tunnel in the soil-sand interbedded stratum is characterized by "shear failure of the surrounding rock in the stratum and bending failure of the surrounding rock in the interbedded stratum". The interbedded characteristics have a significant influence on the collapse. The larger the ratio of span to depth and the larger the ratio of sand to soil, the more obvious the collapse failure. (3) The main influence range of the collapse on the settlement of the surrounding rock above the vault of the arch is 1 times the tunnel span, and the reinforcement of the surrounding rock should be strengthened in the above area during tunnel construction. The results may provide reference for the prevention and control of the collapse in the process of the tunnel construction in soil-sand interbedded strata.