Deformation characteristics of ground with voids induced by shallow metro tunnelling

The existence of the ground voids makes the ground conditions around the tunnel more complex, which may increase the ground surface settlement and even induce collapse during shallow metro tunneling. Concerning the metro tunnels in the surrounding rock grade V, a series of model tests and 3D numerical simulations are conducted to investigate the characteristics of the void deformation and the impact of different void positions on the ground surface settlement. Besides, an improved Peck formula is proposed to perform the curve-fitting analysis on the model test results. The main conclusions are as follows: the ground voids increase the ground loss, depth and width of settlement trough of the ground surface, and the settlement trough moves towards the ground voids. The curve-fitting results of the improved Peck formula correspond well with those of the model tests and numerical simulations. Furthermore, from an overall perspective of the settlement trough shape of the ground surface, the ground surface settlements are relatively larger within a certain range above the void, and a basin-shaped groove occurs on the ground surface. Considering the overall influences of the ground loss, depth and width of settlement trough, the void locating on the inclined top of the tunnel is the worst position for the tunnel construction, in which case the engineers should pay more attention to monitoring of the ground movement. According to the results of the numerical simulations, the void deformation modes are different with different positions of the void, and the main deformation modes are vertical ellipsoid and horizontal ellipsoid.