Research on the Influence of Foundation Pit Excavation Unloading on the Pressure Arch of Underlying Tunnel

With the increasing development and utilization of urban underground space, it is inevitable to excavate around the urban operational tunnels. The excavation and unloading of the foundation pit above the tunnel has a great influence on the formation of the pressure arch of the existing tunnel. Based on previous studies, three unloading ratio models for excavation of foundation pits are summarized, and a method for judging the inner and outer boundaries of the pressure arch is provided. The effect of excavation and unloading of foundation pit on the formation of pressure arch of underlying tunnel is studied by numerical simulation. The results show that: The tunnel vault rock and soil are subjected to the combined effect of the lateral pressure of the rock and soil on both sides of the foundation pit above. Compared with the one-dimensional unloading ratio N₁, the correlation between the two-dimensional unloading ratio N₂ and the boundary of the pressure arch is more significant. Based on the analysis of the impact of foundation pit excavation on the formation of the pressure arch of the underlying tunnel, the engineering impact zones of foundation pit excavation and unloading are proposed, which are divided into general impact zone, secondary impact zone, significant impact zone and strong impact zone. When the excavation width L of the foundation pit is not greater than D and the excavation depth h of the foundation pit is less than 0.15 H, it is regarded as the general influence area; When the excavation width L is not greater than D and the excavation depth h is between 0.15 H and 0.225 H, it is regarded as a significant influence area; When the excavation width L of the foundation pit is greater than D and its two-dimensional unloading ratio N₂ is less than 0.33, it is regarded as a secondary affected area; when the two-dimensional unloading ratio N₂ of foundation pit excavation is not less than 0.33, it is regarded as a strong influence area.