Study on Settlement Characteristics of Small Clear Distance Tunnel Excavation and Optimization of Excavation Spacing

Clarifying the settlement characteristics of small clearance tunnel excavation in urban areas is the premise of ensuring construction safety. In this paper, based on a rail transit subway station in Chongqing, the settlement characteristics of small clear distance tunnels (clear distance of 0.57 m) under different excavation sequences were explored by combining on-site monitoring and numerical simulation, and the optimal tunnel excavation spacing was determined. The results show that: (1) The removal of the temporary support will aggravate the deformation of the surrounding rock, and the deformation increment of the surrounding rock is about 1.43 mm~4.00 mm; (2) The main tunnel is excavated first, and then the final settlement curve of the No. 6 entrance and exit tunnel is "W" shaped, and the No. 6 entrance and exit tunnel are excavated first, and then the main tunnel is excavated, and the final settlement curve is "U" shaped, and the influence of the latter tunnels on each other is strong. (3) With the increase of the excavation spacing, the settlement curve changes from "U" shape to "W" shape, and the influence of tunnels caused by excavation is weakening. The surface load can lead to greater settlement, and when the load is large enough to a certain extent, it will affect the form of the settlement curve. (4) Taking the maximum settlement of the main tunnel vault as the control index, the effectiveness coefficient k of the excavation spacing is defined, and the optimal tunnel excavation spacing to meet the on-site construction conditions is 1.5 m under the premise of ensuring construction safety.