| With the rapid development of urbanization in China,urban traffic has become an important problem that restricts the sustainable development of cities and promotes the vitality of social economy.It is an important means to solve the traffic problems in big cities that develop underground space to build the underground and ground integrated transportation systems.The resulting adjacent construction problems between urban underground expressways and subway have become new challenges for geotechnical and underground engineering field.At present,the research on the influence of foundation pit partial excavation and unloading on the stress and deformation of subway tunnel segment structure is not perfect,so it is of great engineering value and scientific significance to study the influence of upper excavation and unloading on the lower existing tunnel segment structure.This paper takes the reconstruction project of Huaxia Avenue Expressway in Zhengzhou as background,where a part of the underground expressway construction is parallel with the Metro Line 17.The upper expressway is constructed by open excavation and the lower subway tunnel has been completed by shield tunnel in advance.Based on the numerical analysis model of assembled tunnel segment structure established by the finite element method,this paper simulates the response law of the stress and deformation behaviour of the assembled segment structure when the local unloading of the lower existing tunnel segment structure caused by the excavation of the upper strip foundation pit,and research the change law of the stress and deformation of the tunnel segment structure when the local unloading position changes.Then forward some suggestions on the protection scheme of the segment structure of the lower tunnel when the upper foundation pit is partial excavated,unloaded and approached.The main research contents and conclusions are as follows:(1)A finite element model of prefabricated tunnel segment structure considering the contact relationship between bolt and segment is established,and the correctness of the model is verified by relevant experiment.Based on this model,the response law of segment internal force and bolt stress in the failure process of tunnel segment structure is analyzed.The results show that the bolt stress is approximately positively correlated with the load under symmetric load,but the stress growth rate of bolts in different positions is quite different.The stress failure process of concrete segment structure can be divided into three stages: steel bar and concrete are in elastic stage,segment crack development stage and segment ring failure stage.(2)The finite element numerical analysis model of the foundation-fabricated tunnel segment structure is established to analyze the mechanical response law of the lower fabricated segment structure when the upper foundation pit partial excavation and unloading position change.The results show that the displacement of the segment structure of the existing tunnel at the lower part is affected by the partial excavation and unloading of the upper strip-shaped foundation,and the deformation is mainly the whole floating up,which the maximum vertical displacement occurring at the top of the tunnel.As the tunnel moves to the side of the foundation pit,the vertical displacements on both sides of the tunnel are asymmetrically distributed.The vertical displacement on the side near the center line of the foundation pit is relatively large,and the maximum vertical displacement often occurs at the segment joints.The internal force response of segment structure is mainly affected by its assembly mode and unloading position of foundation pit.The discontinuous of bending moment and axial force at segment joint may be exist,the variation of the bending moment and axial force of the segment structure is especially complicated when the middle line of the foundation pit is far away or under the supporting pile of the foundation pit.The connecting bolt mainly bears the pulling force,and its axial force is closely related to the changing trend of the segment joint and the bending moment at the segment joint.In the process of local unloading process in excavation of upper foundation pit,the opening amount of the segment joint is changing,and the segment dislocation is not occurred.(3)In order to solve the problem of large uplifting deformation of segment structure caused by upper foundation pit excavation unloading,the technical effects of common deformation control measures in tunneling are analyzed by numerical simulation.The results show that the portal reinforcement together with back pressure is the most effective measures to control the vertical and horizontal displacement of the segment structure. |
