Research On Deformation And Supporting Technology In Deep Foundation Pit Of Composite Loess Strata

The article take the deep foundation pit engineering of the fish village station in Xi ’an Metro Line 3 as the research object, through the methods of theoretical analysis and MIDAS-GTS three-dimensional numerical simulation, the horizontal displacement of the pile, the uplift of the bottom of pit and ground settlement deformation law of the surface near the deep foundation pit engineering with the bored piles and steel pipe support retaining structure in subway station deep foundation pit are studied. And the same time, the supporting structure key parameters which affect the stability of the deep foundation pit are optimized.(1) Study the engineering project profiles and engineering environment of xi ’an metro line no. 3 fish village station deep foundation pit and seriously select the supporting structure system of the deep foundation pit. Decided to use the support form of the bored piles with the steel pipe support. And design the preliminary supporting structure parameters of this station deep foundation pit according to the specification requirements.(2) Simulate all conditions of excavation and supporting progress to the fish village station deep foundation pit which considering the factors such as the physical and mechanical parameters of soil layer, supporting system, boundary condition and construction overloading, self-respect and excavation process, and study the deformation law and cause of the horizontal displacement of the pile, the uplift displacement of the bottom and ground settlement of the surface near the deep foundation pit engineering. With the excavation of the foundation pit, the horizontal displacement of supporting pile body increases gradually, and the development trend of maximum displacement is down. In the completion of the first and second excavation, the curve of the horizontal displacement is of belly shape, this is due to the first and second pipe supports and ring beam, the supporting system limits the pile body to move. After finished the third and fourth excavation, the curve of the horizontal displacement is of linear shape, this is due to the length of pile is not long enough to lead to formation, supporting pile, and the support system can not play a supporting role effectively, displacement effect of the bottom of pile to pit direction is enhanced. With the excavation of the foundation pit, the uplift displacement of the bottom increases gradually, The change laws of the uplift deformation keep consistent after all the working conditions, the larger is in the middle of the bottom and the smaller is in the two sides. After the condition one and two, the depth of the pit is only 8m and the supporting system limits the bottom to uplift which leads to the small deformation of the bottom. After the condition three and four, the uplift displacement of the bottom increases quickly, the maximum is of 64.97 mm and 112.5mm,this is because of the shortage of the pile which lead to the supporting system can not support the pit effectively, and the piles squeeze the bottom to uplift. Deformation of the ground around the foundation pit were presented the law of uplift to subsidence until the settlement deformation remain the same, and with the excavation of the foundation pit, the ground subsidence value increases gradually. Uplift of the soil near the top of the retaining piles is due to the big stiffness of the first steel brace and retaining pile, deformation of the top of the pile to the soil lead to the uplift deformation.(3) Study the deformation characteristics of deep foundation pit in the single loess strata and the composite loess strata. With the increase of depth in the scope of the pile length, the horizontal displacement of the pile increase gradually in the single loess strata and the the change curve is smooth. This is due to the linear continuous distribution from top to bottom of the change law of the soil pressure. In the composite loess strata, the horizontal displacement of the pile also showed a trend of increase gradually. The difference is that the change curve is periodic, and made up by some smooth curves. This is due to the different change laws of the soil pressure caused by the soils with different properties.(4) Take numerical simulation research of optimization for the key parameters(pile diameter, pile length, horizontal spacing of the steel pipe support) to the fish village station deep foundation pit supporting structure system by the MIDAS-GTS three-dimensional numerical simulation software, obtain the optimization results by compare and analysis the displacement deformation curve. The optimization result of pile diameter is 0.8 m, pile length is of 26 m to 28 m, horizontal spacing of the steel pipe support is 5 m.