Study On The M-method Design Parameter Of Pile-Internal Bracing Strut Support System In Subway Station Pit

With the rapid development of China's Metro lines, effects on surrounding environments and safety of subway station deep excavation are paid more attention. In deep foundation pit design, M-method is widely used to determine the working state of pit retaining structures in China, however, the value of M varies significantly when deduced by different approaches. How reasonable and accurate selection of the m-value, and accurately calculate in elastic foundation beam method, is an important topic in academic and practical problem that cry for settling of the project group.In this paper, an analysis unit is chosen in the subway station pit in Zhengzhou. The combined retaining structure consists of reinforced concrete piles and steel pipe struts. The horizontal displacements of retaining piles and the axial forces in pipe struts are monitored in-situ. Then, calculation assumption is taken as that during the elastic deformation stage, the value of M for each soil keeps constant. Then a series of equilibrium equations of forces acting on retaining structure are set up to determine the values of M for each soil corresponding to deferent stages of excavation. When the identified values of M are applied in the later projects, it shows the validity and accuracy of the method. Precise values of M are applied in the similar geological sites to optimize the retaining structure design. The main contents and results are as follows:(1) This paper describes the development status of foundation at home and abroad, describes several computing excavation method of internal force and deformation. This thesis analyzes the sub-step excavation and level-plus stays on the supporting structure of the work of traits, and give the actual figures of the existing norms,procedures.(2) Through monitoring the retaining structure's force, deformation and changes in the environment surrounding the entire pit when subway station during excavation, analyzed and summarized the supporting piles in horizontal displacement and pipe support axial force changes during the excavation, and through the field experiment summarizes tube temperature of support. Examples of engineering degree steel support temperature increases, the axial force will increase 10-15kN.(3) Combined with Qilihe subway station of Zhengzhou No.1 metro excavation monitoring data, based on static equilibrium back analysis of the m-value of the soil layers, this method is simple and easy projection, and the closer practical engineering situation.(4) By using the inversed m-value of the foundation pit to determine the actual structural deformation, combined with finite element numerical simulation results and experimental results are compared and analyzed, three showed good consistency, verify the m-value of this paper, proved the feasibility of this method. And by ANSYS simulation, obtained during the excavation of foundation pit retaining structure and the changes of the surrounding soil and trends, in order to grasp the extent of excavation to provide a design basis.