Study On Retaining Effect Of Semicircular Cross-sectional Bored Pile In Open Cut Foundtion Pit Adjacent To Existing Urban Road Tunnel

The southwest side of a station on Chengdu Metro Line 6 is close to the existing urban road tunnel,and the minimum net distance between the two is 0.8m～1.55m.In order to meet the needs of passenger evacuation design in the public area of subway station,the side wall of the public area of subway station needs to occupy the part of the pile body of the foundation pit.Therefore,the semi-circular cross-sectional bored cast-in-place pile is used as the retaining structure of the foundation pit in the subway station near the side of the urban road tunnel.In this thesis,based on the engineering support of the open pit near the existing urban road tunnel of the subway station,the retaining effect of the bored piles with semicircular cross section near the existing tunnel in the foundation pit is studied.The research work carried out in this paper and the research conclusions are as follows:（1）The calculation method of internal force and deformation of bored pile with semicircular cross section was deduced.Based on the beam method of elastic foundation,the numerical solutions of horizontal displacement,bending moment and shear force of bored pile with semicircular cross section were derived by power series method.（2）The three-dimensional finite difference method is used to simulate the construction process of foundation pit excavation and transverse bracing,and the variation characteristics of horizontal displacement,bending moment,uplift at the bottom of the foundation pit and surface settlement outside the foundation pit of two different kinds of retaining piles with semi-circular and circular cross-sections are compared and analyzed.The difference in the retaining effect of the foundation pit of bored piles with semi-circular cross-section and bored piles with circular cross-section is analyzed by using the combination of calculation conditions of different pile diameters,pile spacing and embedded depth at the bottom of the pile.The suggested values of reasonable pile diameters,pile spacing and embedded depth of the semi-circular cross-section retaining piles are obtained.（3）Combined with the structural design characteristics of the open excavation foundation pit of a subway station,a three-dimensional numerical model of the deep foundation pit near the existing urban road tunnel was established by using FLAC^3Dfinite difference method.The settlement of the retaining pile,foundation pit,existing road tunnel and the ground surface around the foundation pit during the foundation pit excavation and transverse bracing under the protection of the bored piles with semi-circular cross section was analyzed.The effects of five improvement measures on the stability of the foundation pit and adjacent existing tunnel were compared,including adding transverse bracing in the foundation pit,reinforcing the stratum with grouting and increasing the diameter of the retaining pile,reducing the spacing of the retaining pile and increasing the embedding depth of the pile bottom.（4）Combined with the on-site monitoring data of open-pit excavation of a subway station,the characteristics of horizontal displacement of pile body,horizontal bracing axial force,surrounding surface settlement and deformation of existing urban road tunnel during foundation pit construction are analyzed,and the calculation results are compared with those of elastic foundation beam method and finite difference method to verify the accuracy of the calculation results.