Investigation on tunnelling-induced soil pressure changes, a modified trapdoor test study

To investigate the failure mechanics and soil pressure redistribution in cohesive soil during the formation of the arching effect after tunnel excavation, a series of trapdoor tests were conducted on similarity material, and the tunnelling-induced soil movement was modeled. In addition, the moveable soil on the two sides of a tunnel was replicated by modifying the boundary condition of the classic trapdoor. Results indicated that as the trapdoor descended, the cracks extended outward to the ground when the cover ratio (depth to width) was 2, and extended upwards below the ground when the cover ratio was over 2. After the descent of the trapdoor, the unloading effect and arching effect were formed. Therefore, above the trapdoor, the vertical stress (radial stress for the trapdoor) decreased significantly while the tangential stress for the trapdoor increased. Due to the arching effect, the lateral pressure coefficient (K) at the central axis of the trapdoor increased, while it decreased above the two sides of the trapdoor. In the modified trapdoor device, the arching effect was weaker, and reduced the angle between the outermost crack and the horizontal plane, the tangential stress, and the K at the central axis of the trapdoor, and increased the K above both sides of the trapdoor and the range of the soil arch. In addition, in the classic trapdoor device, the arching effect was underestimated at small trapdoor displacements while it was overestimated at large trapdoor displacements. Therefore, the influence of boundary conditions in trapdoor devices should be considered when investigating underground excavation-induced soil deformation and stress redistribution. When calculating the soil pressure, the K should be determined according to the parameters of the soil, the span of the tunnel, and the deformation of the tunnel lining.