| In recent years,the acceleration of urbanization process in China,greatly promoted the development of underground engineering.In order to meet the needs of high-rise buildings and subway systems,both scales and depths of foundation pits are getting larger.At the same time,the foundation pit constructions in urban areas are facing a series of complex problems.In excavation processes,the foundation pits not only require to meet its safety and stability,but also to ensure the safety of nearby buildings and structures.Moreover,the shapes of the foundation pits are becoming more and more diverse,and the space-time effect in excavation processes is gradually focused on.This paper takes a triangle foundation pit project No.2 in the core area of Hongqiao Business District in Shanghai as background,comprehensively adopts various means,such as theoretical analyze,on-site monitoring,numerical simulation and optimization comparison,to study the mechanical properties and deformation law of deep and large foundation pit in soft soil area systematically.The main work of this paper is as follows:(1)Based on the flow theory,and the foundation pit is assumed to be a large well,the groundwater level at any distance beyond the foundation pit was calculated considering water insulation effect of retaining structures during the dewatering process.The vertical displacement of the soil outside the pit is obtained by calculating the additional stress of the soil.A calculation method of pile settlement due to reconsolidation of soil around the pile is deduced,in which the pile-soil interaction is considered by shear displacement method.The influences of insulation effect of retaining structures and the distances from the foundation pit on pile settlement are analyzed.(2)The two-stage simplified method was adopted to develop an approach for the energy pipe gallery settlement induced by excavation.Firstly,an analytical method for the soil deformation was proposed to calculate the additional deformation caused by the excavation.Then,by assuming that the pipelines can be simplified as a beam on the surface of a Winkler foundation,an equilibrium equation for the interaction of pipelines and soil under additional deformation was deduced.The finite difference method was adopted to solve the differential equation.Through the comparison with the monitored data of the excavation,the validity of the proposed method was analyzed and verified.(3)To investigate the performance of the deep excavation and associated effect on the gallery,utility pipelines and piers,a long-term comprehensive instrumentation program was conducted during the construction.Deflections of diaphragm walls,vertical wall movements,ground settlements,vertical column movements,as well as the settlements of adjacent pipelines and energy pipe gallery were observed.The field data was analyzed and compared with other excavations of metro stations and rectangular excavations in Shanghai.(4)Taking into account the characteristics of site environment and influence of excavation process,this paper carries out a three-dimensional elastio-plastic finite element analysis.The influence of foundation pit excavation on the soil settlements outside the pit,enclosure wall vertical displacements and vertical props settlements are studied by comparison of the field monitoring results.(5)Three kinds of excavation optimization design,including layered excavations,block excavations,partition excavations,were analyzed.The deformation regularity of the retaining structure deformations and surrounding strata settlements were sum up,and finally,corresponding optimization technologies of foundation pit construction are put forward. |
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