Study On Pile Foundation Underpinning Technology For Open-excavated Underpass Bridge And Optimization Analysis Of Intercity Railway

The construction of rail transit in our country is in the golden period of rapid development.The construction of urban rail transit often needs to build tunnels.The construction of tunnels inevitably encounters the problem of crossing existing buildings or structures.This paper takes the Songshan Lake Tunnel open-cut section underpass the Guangzhou-Shenzhen Expressway viaduct project as the background.In order to ensure that the superstructure and underground structure of the bridge are not disturbed by the underpass of open-cut tunnels,underpinning construction of the foundation pile foundation of the original structure is carried out.Based on the engineering practice,through on-site monitoring,theoretical analysis and numerical calculation,the influence of bridge pile foundation underpinning and foundation pit excavation on adjacent pile foundation is studied.The following works are mainly carried out:(1)In pile foundation underpinning construction,through MIDAS-Civil numerical simulation and related formula calculation,the deformation law of bridge superstructure is obtained and the safety of actual project is verified.At the same time,the finite element simulation is carried out under different load modes and different load conditions.The results show that N76 beam is more affected by construction in pile foundation underpinning project.Under the two kinds of vehicle loads,the moment and deflection of underpinning beam and diaphragm beam are the largest under the ultimate combination of bearing capacity,while the moment and deflection of diaphragm beam and underpinning beam are the smallest under the vehicle loads only.The bearing capacity of underpinning beams under ultimate state is checked according to relevant codes,which proves the safety and reliability of actual construction.(2)The interaction between foundation pit excavation and adjacent pile foundation is studied by using MIDAS GTS NX software.The finite element analysis is carried out on the basis of changing pile stiffness,diaphragm wall stiffness,distance between pile foundation and foundation pit and pile top load from three angles of pile displacement,pile bending moment and pile side friction.The results show that the vertical load can effectively improve the horizontal bearing capacity of piles.At the same time,with the increase of the distance between the pile foundation and the edge of the pit and the stiffness of the continuous wall,the lateral displacement of the pile top,the bending moment of the pile body and the maximum friction resistance of the pile body gradually decrease,and the position of the neutral point of the friction resistance of the pile foundation gradually moves down;the rigid bridge pile is more likely to be damaged before the upper structure,whereas the flexible bridge pile will be damaged later than the upper structure.(3)Stress-seepage coupling analysis of excavation process is carried out by using MIDAS GTS NX software.The results show that with the increase of excavation depth,surface settlement and lateral displacement of pile foundation increase continuously,and plastic deformation will occur in the soil around the foundation pit and the soil around the pile.According to the results of calculation and analysis,the foundation pit support is optimized.The results show that when the first cross brace is reduced by 2 and the second cross brace is reduced by 4,the safety of open-cut construction process can be guaranteed.At the same time,the material and process can be saved,and the construction progress can be accelerated.