Research On The Characteristics Of Foundation Pit With Inclined Brace And Pit-in-pit Combined Support In Deep Soft Soil Site

With the continuous development of underground space,foundation pit engineering is developing in a larger and deeper direction,and there are more and more pit-in-pit foundation pits.For deep soft soil areas,due to the high rheology of soft soil,the bottom of the pit is often located in deep soft soil,whether it is a normal foundation pit or a pit-in-pit foundation pit.Long-term exposure of the soil will inevitably cause large deformation of the soil,which will have a greater impact on the retaining structure of the foundation pit.The traditional pile-anchor system is limited in the construction of super large foundation pit,while the inclined brace support system has the characteristics of flexible arrangement,convenient earthwork excavation,short construction period and low cost.On the basis of fully summarizing the previous research results,this paper uses the finite element numerical analysis software MIDAS GTS NX as a research tool to comprehensively analyze the influencing factors and supporting effects of the combined support system of inclined brace and pit-in-pit in deep soft soil sites and propose its design control points.Combined with the three-dimensional finite element simulation analysis results and related monitoring data of the inclined brace support system project used in a pit-in-pit foundation pit in Hengqin New District of Zhuhai,the supporting effect of the peripheral large foundation pit under the condition of pit-in-pit excavation is verified.At the same time,the matters needing attention in the design,construction and monitoring of the combined support system of inclined brace and pit in pit in deep soft soil area are put forward.The following are the main research contents and results:(1)According to the classical earth pressure theory,the finite earth pressure theory and the stress field of pit-in-pit foundation pit are briefly introduced.The finite earth pressure and soil failure angle of pit-in-pit foundation pit are studied.The deformation mechanism of foundation pit is briefly introduced and the common analysis and calculation methods of foundation pit engineering are expounded.(2)Based on the engineering example of a foundation pit supporting project in Hengqin New District,Zhuhai,Guangdong Province,this paper analyzes the geotechnical geological characteristics of the area,focuses on the monitoring project results of the surrounding surface settlement,deep horizontal displacement and inclined brace axial force,compares the supporting section with or without inclined brace and analyzes the deformation and internal force law of the peripheral large foundation pit under the excavation of the pit in the pit.Based on the elastic method,the combined support model of inclined support and pit in pit is established,which provides reference for engineering design.(3)The general geotechnical engineering finite element analysis software MIDAS GTS NX is briefly introduced.The basic method of Midas GTS NX simulation of foundation pit engineering is discussed from the aspects of selection of geotechnical constitutive model,calculation range of model,determination of material and establishment of model.Finally,based on the survey,design,construction and monitoring data of a pit-in-pit soft soil foundation pit project in Hengqin New District of Zhuhai City,a three-dimensional finite element numerical analysis model is established and numerical simulation is carried out.The monitoring results and numerical simulation results are compared and analyzed to further verify the rationality of the numerical simulation method and the oblique bracing scheme.(4)Numerical simulation method is used to analyze the influence of several main parameters on the surrounding large foundation pit in the case of pit-in-pit excavation.The main parameters include the different depth of the pit in the pit,the width of the pit in the pit,the distance between the pit and the pit in the pit,the reinforcement method of the passive zone,etc.The law is summarized from the numerical simulation results,which provides support for the design and optimization of practical engineering.