The Study Of Cracking Mechanism Of An Existing Shield Tunnel Segment Induced By An Adjacent Pit Excavation

As the construction of the subway can drive the development and appreciation of residential and commercial areas along the line,the subway is often a prime location for various commercial developments.In order to maximize the attraction of the passenger flow brought by the subway and facilitate the travel of the people,many commercial buildings or residential houses are close to the existing subway lines.The foundation of these high-rise buildings needs to excavate the foundation pit during the construction process.Therefore,the influence of the foundation pit excavation on the surrounding underground structure has become a problem that geotechnical engineers have to face in the design and construction process.Aiming at the current situation that the cracking failure mechanism and failure mode of adjacent shield tunnels are less induced by foundation pit excavation,this paper uses finite element software to construct a finite element for the existing foundation pit around the existing shield tunnel.The model,by simulating the generation and expansion of tunnel segment cracks during the excavation of foundation pit,studies the mechanism of cracking of adjacent shield tunnel segments and the law of crack propagation.The main research contents of this paper are as follows:Triaxial shear test on the soil layer in the Shangyuanmen area of Gulou District,Nanjing,to study the stress-strain characteristics of the soil,analyze the soil information and hydrological conditions,and obtain the Mohr-Coulomb constitutive model of each layer of soil and Constitutive model parameters such as elastic models(1)By constructing a three-dimensional finite element model of foundation pit excavation around the existing shield tunnel and simulating the generation and expansion of cracks in the tunnel segments during the excavation process,the location and morphological characteristics of the adjacent shield segments induced by foundation pit excavation were obtained;(2)Through analysis and study of foundation pit,foundation pit excavation depth,distance between both the tunnel excavation process,the excavation of the underground continuous wall built in depth,thickness and foundation pit excavation of underground continuous wall relative to the change of the parameters such as the space position of the tunnel that forms and characteristics,the influence of cracking of shield tunnel segment to obtain the parameters on the influence law of tunnel segment crack morphology;.(3)A three-dimensional finite element model is built for the isolation pile between the existing shield tunnel and the new foundation pit.By analyzing the location and morphological characteristics of adjacent existing shield pipe cracking induced by foundation pit excavation in the presence of isolated piles,the blocking effect of isolated piles on adjacent existing shield pipe cracking induced by foundation pit excavation is studied.(4)Based on the excavation of the surrounding foundation pit of an existing shield tunnel in Nanjing,a three-dimensional finite element model is established based on the actual conditions of the project and the geological survey report,and the adjacent shield pipe is analyzed during the excavation process.The morphological characteristics of the deformation and cracking of the sheet provide a basis for the optimization of the treatment scheme for the cracking of adjacent tunnel segments during the excavation process.