Study On Unloading Bias Characteristics And Stability Of Supporting System Based On Spatio-temporal Effect In Deep And Shallow Adjacent Foundation Pits

With the acceleration of urban construction,there are more and more cases of adjacent pits due to the narrowness of the central area.On the one hand,the foundation pit projects have time and space effects.On the other hand,each adjacent pit has an independent supporting system and excavation scheme.Therefore,the internal forces and deformations caused by the excavation of adjacent pits would affect each other and overlap each other.The problem of the protection system stability is also reletively significant.As a result of that,It is very important to study the space-time effect of excavation unloading of adjacent foundation pits and the stability of support system,which can instruct the safe and stable excavation of adjacent foundation pits.In this dissertation,Chose the deep and shallow adjacent foundation pits of the Qiaojiahu station of Xuzhou Metro Line 1 as engineering background and the partial bias pressures formed by the excavation unloading of two foundation pits are studied.Emphasized on the analysis of features of the space-time effect of the unloading bias of the two foundation pits and support system’s stability.The calculation method of the support structure under asymmetrical load which is based on the theoretical calculation method of the elastic fulcrum method has been discussed.Finally,the space-time effect and support system stability of the adjacent foundation pit under different unloading bias conditions and under different excavation conditions have been investigated.Some research results has been achieved.(1)Analyze and summarize the basic theory of spatio-temporal effect of general excavation unloading,including the common deformation forms of the supporting structure and soil outside the pit,the space-time effect mechanism caused by the excavation unloading of foundation pit and its influencing factors.At the same time,the existing deformation control standards for foundation pits are described.(2)By establishing a finite element analysis model,the space-time effect of the unloading bias of the supporting structure and the soil outbside the pit of adjacent pits under various excavation conditions was explored.It is concluded that a "kind of pit-like effect" at both ends of the deep pit shifting section and three typical spatial effect characteristics of supporting piles are presented.In the direction of paralleled and vertical pits,there are different combinations of subsidence characteristics of the soil outside the pit in the biased and symmetrical sections.(3)In terms of the stability of the support system,the maximum value of the internal forces of the supporting piles in the bias section does not exceed its bearing capacity,but the deformation is too large during the excavation of the deep foundation pits,which may causeinstability of the support structure.When the shallow foundation pit is excavated to the end,if the maximum axial tension of the concrete supports exceeds its carrying capacity,this condition may lead to the structural failure and cause construction risks.(4)Based on the basic principle of elastic fulcrum method,several forms of support stiffness under asymmetrical loads are discussed.By using iterative calculation methods,the equivalent support stiffness of each fulcrum on both sides of the asymmetrical foundation pit is solved.The theoretical calculation results of the internal force deformation of the retaining structure on both sides are obtained and compared with the numerical simulation results.(5)Through the establishment of different analytical models,the space-time effect characteristics of the deformation of the adjacent foundation pit supporting and the soil settlement outside the pit under different unloading bias conditions were discussed in terms of the depth and width of the shallow foundation pit.When the depth of the shallow foundation pit is small,that dominated effect is the overload of the bottom of the pit,and the south side is biased when the depth is large.For shallow foundation pit widths,the smaller the bias ratio,the greater the effect of the cofferdam on the deformation of the foundation pit and the more obvious the space-time effect of the foundation pit.(6)Based on the basic principles of hierarchical block excavation,several schemes for deep excavation are proposed.Through analysis,it can be concluded that the partial excavation of the bias section can ensure that the magnitude of the internal force deformation of the entire foundation pit and its supporting system can be controlled within the specification and design allowable limits.The analysis of on-site monitoring data confirms the reliability of the numerical simulation results and shows that this kind of excavation scheme can effectively resolve construction risks.