Study On The Influence Of Excavation And Unloading Of Foundation Pit On Working Characters Of Lateral And Adjacent Tunnel

With the acceleration of urbanization in China,the number of urban foundation pit projects has increased rapidly in recent years,and the density of subway tunnels has also increased.The conflict of building space between the new foundation pit project and the existing subway tunnel has gradually become prominent.The impact of foundation pit excavation construction on the adjacent subway tunnel has become an important research issue to ensure the safety of urban subways.At present,the research mainly focuses on the influence of foundation pit excavation on the lower shield tunnel,but the research on the change of the working characteristics of the lateral and adjacent subway tunnel is relatively insufficient.This paper focuses on the problem from three aspects: model test,numerical simulation and theoretical analysis.In this paper,based on the common displacement pattern of foundation pit support structure,an indoor model test is established.The deformation of the supporting structure caused by the excavation of the foundation pit is simulated by using two deformation modes of the rigid retaining wall(foundation pit retaining structure).The deformation of the supporting structure causes the stress release of the soil behind the wall,forming a corresponding unloading effect.During the test,the soil pressure distribution and the evolution of cross-section deformation around the lateral and adjacent tunnel of the foundation pit are mainly monitored.The model test results are verified by three-dimensional finite element simulation.And through the parameter analysis,the influence of the important factor of the distance between the tunnel and the retaining wall on the working characteristics of the lateral and adjacent tunnel caused by the excavation and unloading of the foundation pit is deeply explored.Based on the SSPM calculation theory and superposition principle,the displacement of the soil at the cross-section of the lateral and adjacent tunnel under the excavation and unloading conditions of the foundation pit is derived.The tunnel is simplified to the circular elastic foundation beam model in the passive state,and the "two-stage" idea is adopted.By applying the soil displacement at the tunnel position to the calculation model,the deformation of the tunnel is calculated.Through the above three aspects of research,the main research results are as follows:(1)In the translating or rotating mode of the foundation pit retaining structure,as the amount of translation or rotation increases,the soil pressure at the side of the tunnel near the enclosure and at the end of the side remote from the enclosure is rapidly attenuated and then remains unchanged.And the soil pressure around the top of the tunnel increases as the displacement of the enclosure increases.When the enclosure structure is translated,there is a small attenuation of the soil pressure around the tunnel side which is away from the enclosure,but when the retaining structure is rotated,the soil pressure around the tunnel side which is away from the retaining structure does not substantially change.(2)In the translational or rotating mode of the foundation pit retaining structure,the displacement of the tunnel cross-section is closely related to the displacement of the foundation pit retaining structure.As the displacement of the foundation pit retaining structure increases,the cross-sectional displacement increases gradually,and the increasing rate decreases slowly.At the same time,as the excavation process progresses,the horizontal displacement of the tunnel point which is nearest the enclosure is always the largest.The amount of vertical settlement occurring at the top of the tunnel is second.The vertical settlement at the lowest of the tunnel is minimal.(3)In the translational or rotating mode of the foundation pit retaining structure,as the amount of translation or rotation increases,the bending moment at the two side points of the adjacent tunnel near and away from the foundation pit changes little.However,the bending moment at the remaining points of the tunnel cross-section increases significantly.And the largest additional bending moment at the lower left end of the tunnel near the side of the foundation pit is produced by the factor of translation or rotation of enclosure structure.(4)In the translational or rotating mode of the foundation pit retaining structure,the displacement of the cross-section of lateral and adjacent tunnel is closely related to the distance between the tunnel and foundation pit.Increasing the distance between the tunnel and foundation pit retaining structure can reduce the displacement of the cross-section of lateral and adjacent tunnel.However,as the distance between the tunnel and the enclosure increases,the rate of decrease gradually decreases and tends to zero.In addition,even if the distance between the tunnel and foundation pit retaining structure is increased,the tunnel cross-sectional displacement caused by the translational deformation of the retaining structure is not significantly reduced when the lateral and adjacent tunnel is outside the slip surface of foundation pit.(5)In this paper,two basic foundation pit deformations which is the retaining wall translating or rotating are used to replace foundation pit excavation and unloading.And based on the theory of SSPM,the displacement of the retaining structure and the model of the elastic foundation beam,the theoretical analysis method of the cross-section deformation of the lateral and adjacent tunnel under the excavation and unloading of the foundation pit is obtained.Through the comparison with the experimental and finite element calculation results,the theoretical analysis method proposed in this paper is reasonable and effective.