Study On The Interaction Mechanism Of Adjacent Foundation Pit Construction Of Existing Underground Structures

With the booming construction of subway projects in recent years,the distribution of line networks in urban underground space has become increasingly complex.Due to the reasons of construction timing,early urban underground space design and planning lacked reservation for later interfaces.Newly-built foundation pits often encounter situations where they are closely attached to existing underground structures,share structures,or are added within the scope of influence of existing underground structures.The resulting adjacent additional construction will also have an impact on the envelope and existing underground structures.This thesis studies the distribution law of limited soil pressure by using theoretical analysis methods for the interaction problem of existing underground structures adjacent to foundation pits;Using the methods of indoor model experiments and classic cases,its rationality was verified,and the interaction mechanism between new and existing structures in the vicinity of additional construction projects was further analyzed.The mechanical characteristics of the foundation pit retaining structure and the deformation characteristics of existing structures during construction were studied.The main research contents and conclusions are as follows:(1)Using the thin layer microelement method,based on the different positional relationships between the new foundation pit and the existing underground structure,and considering the frictional action between the soil and the retaining structure,as well as the interface between the soil and the existing underground structure,a calculation formula for the finite soil pressure of the foundation pit adjacent to the existing underground structure under five soil failure modes is derived.Based on the location relationship of soil failure model 2,a simple and convenient method for calculating finite soil pressure is proposed.Verify the consistency of the formula based on the critical state relationship of soil failure modes.The rationality of the formula is verified by comparing and analyzing the calculation results obtained from literature cases,model tests,and the methods proposed in this paper.(2)Explore the effects of proximity,buried depth of existing underground structures,depth of foundation pits,and soil parameters on the strength of limited soil pressure,the resultant force of soil pressure,and the point of action of the resultant force.By calculating the combined soil pressure forces on the retaining structure of the foundation pit and the subway station under various working conditions,and drawing the combined soil pressure forces into isolines,the influence of existing underground structures on the retaining structure of the foundation pit is explored.The results indicate that the interface friction angle has a significant impact on the height of the resultant force point.As the wall soil friction angle increases,the height of the point of action increases almost linearly.When the friction angle between the wall and soil is 0,the point of action of the combined force is equal to 1/3.(3)Based on the finite soil pressure calculation method for five soil failure modes,the elastic fulcrum method is used to obtain the deformation and internal force analysis(deformation diagram,bending moment diagram,shear diagram)of the inner supporting foundation pit retaining structure.The mechanical characteristics of the supporting structure of the adjacent additional foundation pit are analyzed.The results show that the bending moment from the ground to the first support is very small,and then gradually increases along the depth,with sudden weakening at each support.The bending moment near the base reaches its peak and then continues to decrease.(4)Fully investigated domestic and foreign cases of adjacent construction projects,statistically analyzed a large number of measured data on the deformation of existing underground structures,and provided the deformation laws of existing underground structures with proximity distance.Using the finite element numerical simulation method,the effects of proximity distance and overburden thickness on the horizontal and vertical displacement analysis of existing underground structures,as well as the location of the maximum and minimum principal stresses,are analyzed,and relevant control measures for the construction of additional proximity projects are proposed.