Study On The Stability And Deformation Characteristics Of Deep Foundation Pit Near Existing Line

In order to reduce traffic pressure and to solve the problem of city traffic jam, so as to adapt to the needs of the development of national economy, urban rail traffic has developed rapidly. And then the station foundation pit engineering becomes some new characteristics, such as more larger, deeper, closer and adjacent. Because of the multiple characteristics and serious consequences of the accidents about deep foundation pit engineering, deep foundation pit engineering, as an old structure and geotechnical engineering problems, has some new characteristics, then it gradually cause more and more attention. This paper is finished based on the major project supported by China Railway Engineering Corporation, named Research on Design and construction Key Technology of Foundation Pit near Existing Rail Line (No.200902). The foundation pit of Minzhi station and Wuhe station in Shenzhen subway No.5line, which longitudinal side of foundation pit parallels the existing railway or road line with high embankment. According to the complex engineering background of the foundation pit, research are carried out by field test, laboratory test, theory and numerical analysis, and the characteristics of the stability and deformation features of the foundation pit close to existing lines is studied. The main research contents and results are as follows:(1) According to the asymmetric overload of deep foundation pit of Minzhi station and Wuhe station, and its stability influenced by vehicle dynamic load induced by the high embankment existing lines, plenty of dynamic load tests are carried out. Based on the dynamic test results, the static or dynamic response of the supporting structure system for foundation pit are studied by the static and dynamic analysis method. The rules of the internal force, settlement and deformation, horizontal displacement of the supporting structure system, and the settlement of existing line under different conditions are obtained. The rationality and adaptability of the design program about supporting structure of deep foundation pit under action of asymmetric overload is evaluated. (2) The parameter study about the stability of supporting structure system of foundation pit under the five conditions, such as the equivalent static load, no locomotive dynamic load and locomotive speed of60km/h,80km/h,120km/h is carried out. when locomotive running speed is120km/h, dynamic load effect coefficient reaches1.2, its effects on the deformation and internal force of supporting structure system of foundation pit is prominent. But when the Locomotive speed is less than60km/h, dynamic load has little influence, and the dynamic load influence coefficient is less than5%. During the design analysis, locomotive dynamic load can be simplified as equivalent soil column static load when the Locomotive speed is less than60km/h.(3) Based on the results of test and theoretical analysis, the safe standards of the60km/h limited speed is put forward to meet the requirement of the settlement deformation standard for the existing line and the stability of supporting structure, and to ensure the safety of the foundation pit excavation construction and the existing line.(4) Relying on the foundation pit project of Minzhi station and Wuhe station, site tests including the displacement and internal force of the diaphragm wall or the braces, and settlement of the existing line. The characteristics of internal force and deformation for supporting structure system of deep foundation pit with asymmetric overload is summarized by the tests of the typical section. Based on the test and theoretical analysis results, the load and displacement model of the retaining structure of the foundation pit with asymmetric overload are proposed. Under action of the asymmetric overload, supporting structure system will appear the overall reverse trend when the foundation pit excavation depth reaches a ritical depth, the internal forces and deformations of the diaphragm wall at overloading side is significantly greater than the results of no overloading side. After the excavation depth reaches a ritical depth, the deformation direction of the upside of the diaphragm wall at no overloading side will point to outside of foundation pit. At the same time, the original active earth pressure in this region of diaphragm wall, which deformation direction pointing to outside of foundation pit, changes to passive earth pressure. (5) Based on the great difference such as the stress of supporting structure and the deformation direction of retaining structure system points to no overloading side of foundation pit, the asymmetric design method of the foundation retaining structure and the technical measuresis, such as take the initiative reinforcement for The overload side, etc, are put forward to balance asymmetric overload induced by high embankment.