Safety control and mechanical response of existing underground structures induced by excavation of new tunnels under construction

In crossing projects, the sections and construction methods for new tunnels are numerous, the geological conditions are complex, and the structural forms of the existing tunnels are various. There is a complex relationship among them, which has not been deeply studied till today. Based on the measured data from 13 projects in Beijing, the structural forms and the construction methods for the new tunnels are summarized, the deformation laws of the existing tunnels are analyzed, and the mechanical response of the existing tunnels is investigated by using the two stage method. The results show: (1) The new tunnels have three types of municipal pipelines, subway stations and metro tunnels. Their construction methods include guiding hole method, bench method, cavern-pile method and center drift method. The corresponding relationship between the construction methods and the excavation area is clear. (2) The probability distribution of the maximum settlement of the existing tunnels accords with the normal one, with the mathematical expectation being 4.89 and the variance 16.4. The average maximum settlements of the existing tunnels caused by excavation of municipal pipelines, metro tunnels and subway stations are 2.56, 3.82 and 11.07 mm. (3) The crossing projects can be summarized into 7 combinations by the spatial relationship between the new and existing tunnels. According to the deformation and stress characteristics, there are three modes of W, U and V in the mechanical response of the existing tunnels. (4) In order to reduce the disturbance of the existing tunnels, the excavation area of the new tunnels should be strictly controlled, the soil between the roof of the new tunnels and the floor of the existing tunnels should be less or not left, and the preferable crossing mode should be W-type.