Model experiment on surface subsidence induced by excavation of shallow small-spacing tunnels

This study proposes a model test system to study the evolutionary process of surface subsidence under the excavation disturbance of shallow depth and small-spacing soil tunnels. During the excavation of urban shallow small-spacing tunnels, the stress state of the middle soil pillar is complex, which further affects the deformation of the overlying strata. Based on the Yuhan Road underground tunnel, a new soil material of shallow depth stratum is developed using a large number of material ratios and physical and mechanical performance parameter tests. The evolutionary characteristics of the stress around the tunnel and the surface subsidence during different stages of tunnel excavation are analysed. The results show that the surrounding soil stress of the tunnel continues to decrease with an increase in the excavation stage. The stress of the middle soil pillar demonstrates the most rapid decrease, and the surrounding soil is close to an unstable failure state. With the instability of the middle soil pillar, the surface subsidence displacement also increases sharply, and the peak subsidence gradually shifts from the vault to the middle soil pillar. The model test results align with the existing subsidence prediction methods for single-tunnel prediction. However, this test method is more accurate in predicting the surface subsidence caused by the excavation of a small-spacing tunnel, thus contributing to further understanding the stability of the middle soil pillar, which must be ensured to avoid large-scale subsidence or collapse.