Research On Stability And Optimization Design Of Foundation Pit Enclosure In Soft Soil Area

With the rapid development of urban construction in the Pearl River Delta region,the support technology of deep foundation pit engineering has been unprecedentedly developed,especially when excavating foundation pits in soft soil areas,due to the rheological and thixotropic properties of soft soil,for The deformation of the enclosure structure has a great influence,and the design of the foundation pit has gradually changed from strength control to deformation control.As one of the primary tasks of construction and construction,at present,there are comparative and systematic studies on the deformation and parameter values of the envelope structure in the soft soil area,mainly in the Shanghai area.However,due to the vast territory of our country,there are great differences in the distribution of strata in different regions.On the one hand,there are few studies on structures related to deep foundation pit engineering in other soft soil areas.At this stage,the theoretical calculation of deep foundation pit is not mature enough,and the specifications of each foundation pit have not formed a unified structure for the deformation and parameter values of the enclosure.On the other hand,during the excavation process of deep foundation pits in soft coastal areas,the enclosure structure is often deformed too much.Steel sheet piles are often used for excavation of deep foundation pits in the Pearl River Delta region where silt or silty soil is dominant.In order to ensure the stability of the supporting structure,the passive area at the bottom of the foundation pit needs to be reinforced.On the basis of ensuring stability,the support cost is reduced,and the lateral resistance of the passive area of the foundation pit is seriously insufficient.The passive area reinforcement technology has undergone a lot of engineering practice and theoretical analysis and research,and comprehensively considered factors such as foundation pit deformation and engineering cost.There are numerous studies,but most are relatively single forms of support In view of the above two problems,the specific research work and some conclusions of this paper are as follows:(1)First,the existing calculation methods in various codes and the differences in soil parameter values are summarized.Secondly,the calculation results of three different soil layer models in the engineering case are carried out using the classical method and the elastic foundation beam method.Through the comparative analysis of the calculation results of three different soil layer models,the differences between the methods in various codes and their inducing causes are comprehensively discussed.The differences in the depth of the pile,the length of the pile and the horizontal displacement of the pile top.(2)Use PLAXIS 3D software to establish a small-strain constitutive model of hardened soil for cement-soil mixing piles and bored piles in passive zone reinforcement combined steel sheet piles.By establishing a passive zone without reinforcement,cement-soil mixing pile reinforcement and drilling Three-dimensional numerical model of the reinforcement depth and soil parameters of cement-soil mixing piles under three conditions of cast-in-place pile reinforcement.The influence degree and law of passive zone reinforcement on the overall deformation behavior of the foundation pit.(3)Judging from the calculation results of the horizontal displacement of the pile top,the calculation results of the elastic foundation beam method combined with the heterogeneous soil layer model are more reasonable with the actual values.The calculation results of the elastic foundation beams in the same soil layer with the same specification are larger than those calculated by the classical method.In practical engineering,the homogeneous soil layer model is used to check the stability of foundation pits in soft soil areas.Due to the diversity of soil quality and differences in calculation methods and other factors,the calculation error between the calculated value and the design value may be further expanded.(4)When the excavation depth of the foundation pit is about 6m,the supporting effect of the cement-soil mixing pile is the best.The horizontal displacement of the pile bottom reinforced in the passive area of the cast-in-place pile is 65.1%smaller than that of the cement-soil mixing pile in the passive area.(5)From the perspective of controlling the horizontal displacement of the top of the pile and the horizontal displacement of the deep soil,the reinforcement thickness of the cement-soil mixing pile in the passive area should be 0.3 times the excavation depth of the foundation pit.Combining with the regression curve,it can be seen that when the consolidation fast shear parameter design is adopted,the soil layer parameters should be corrected,the cohesion remains unchanged,and the internal friction angle is increased by 1.3 times.Experience is considered to meet the design requirements.