Study On Deformation Laws And Deformation Control Of Subway Deep Foundation Pit In Tertiary Weathering Red Sandstone Formation

In order to solve the problems such as traffic congestion 、 environmental degradation,subway construction has become an inevitable trend, and many big cities around the world take it as one of the basic measures to ease traffic pressure. The foundation pit project of Wulipu metro transfer station of LanZhou subway line 1 is located in tertiary weathering red sandstone formation with high water level, the characteristics of red sandstone are poor diagenetic grade, easy weathering and collapse under the influence of water and construction. It has important theoretical significance and application value to study the choice of support measures、deformation prediction and deformation control technology.The monitoring data during the process of foundation pit excavation are studied, the finite difference software(FLAC3D) is used to simulate the process of deep foundation pit excavation, deformation laws and deformation control techniques for deep foundation pit in tertiary weathering red sandstone formation with high water level are studied, hoping to provide reference for engineering design and construction of foundation pit which under the same conditions.The major work and conclusions of the present thesis are as follows:(1)Several kinds of common foundation pit supporting structures are introducted, their respective application conditions and Advantages are also compared.The feasibility of Wulipu metro transfer station supporting structure(secant piles+steel support) are demonstrated and it is more economical and reasonable.(2)The internal force and deformation calculation theory of foundation pit support structure are introduced, then the stability of Wulipu station foundation pit of LanZhou subway line 1 is checked. The results show that the supporting structure(secant piles+steel support) can satisfy the requirements of foundation pit stability and deformation control.(3)The foundation pit retaining pile deformation appeared in two forms when the excavation finished. One type is that the pile deformation is larger at the top of the pile;another is that the pile deformation is larger near the surface of excavation. The different influence of construction conditions are the main reasons lead to the difference of the pile deformation. If the excavation progress immediately slow down when the soil around the pile top is excavated, and the outburst of pile above the bottom need to be chiseled too much, the deformation of pile top is larger; if the soil around the pile is excavated close to the bottom of the foundation pit quickly, and also the pile has no obvious part of outburst, the pile deformation is large near excavation surface.(4)If there is no water and sand leakage phenomenon of the retaining pile, the final surface subsidence shape is groove shape behind the retaining pile; if there is water and sandleakage phenomenon of the retaining pile, the final surface subsidence shape is triangle shape behind the retaining pile.(5)Numerical results show that by increasing the pile diameter and embedded depth could reduce the deformation of retaining structure and surface subsidence to some extent, but,the effect of controlling the deformation of foundation pit is no longer obvious when the pile diameter is 1.4m or embedded depth reached 12 m, thus through the pile diameter and embedded depth to control the deformation of foundation pit should be considered from the aspects of technology, effect and economy.(6) The specific parameters of the supporting structure are optimized which based on the numerical results, and the following schemes are proposed:(1)Retaining structure of foundation pit and steel vertical spacing of support is the same as the original scheme, the first and second steel support which design parameters is Φ800mm,t(wall thickness)=16mm, the second to the forth steel support horizontal spacing adjusted for all 6m.(2) The structure of foundation pit use Φ1000@1600mm reinforced concrete pile andΦ1000@1600mm plain concrete pile bite with each other, the first and second steel support which design parameters is Φ800mm, t(wall thickness)=16mm, the design parameters of the other steel support is the same as the original scheme.