Dewatering Excavation And Numerical Simulation Analysis Of Deep Foundation Pit Of Subway Station In Water-rich Soft Soil Layer

Since the reform and opening up,metro construction has developed rapidly in China’s cities and has become a powerful network hub for transportation in various cities.However,station construction is one of the most important aspects of metro construction,especially underground construction in water-rich and soft soil areas,and deep foundation pit precipitation excavation brings new major challenges to project safety and quality.This paper takes the deep foundation pit project of Guangmingang Station of Fuzhou Metro Line 4 as a case study,and through theoretical analysis,on-site monitoring and numerical simulation,focuses on the research and analysis of the deformation law of deep foundation pit and support structure by precipitation excavation based on relevant literature and engineering geological conditions,and analyses the influence of different support structure design parameters on the deformation of foundation pit.The main contents and research results of this paper are as follows,which can have certain reference value for similar foundation pit projects in the future.(1)Through literature reading,the support mode and deformation mechanism of deep foundation pit engineering in water-rich and weak soil layer are summarized and elaborated;the factors mainly affecting the deformation of deep foundation pit in water-rich and weak soil layer are summarized and analyzed;the general change law of deep foundation pit precipitation excavation on horizontal displacement of underground diaphragm wall,surface settlement outside the pit,vertical displacement of ground connection wall and axial force of support is summarized and discussed.(2)By collating the data from the on-site dynamic monitoring during the excavation of the foundation pit,the changes in the deformation of the foundation pit and the force of the supporting structure were analysed.It was found that the support axial force would change suddenly before the ground settlement before the pit was in danger,and the signs were obvious.(3)Using FLAC3 D finite element analysis software,a three-dimensional model of the deep foundation pit based on the Mohr-Coulomb principal structure was established,and the general laws of change generated by the horizontal displacement of the underground diaphragm wall,the settlement of the ground surface outside the pit,the vertical displacement of the ground connection wall and the support axial force were studied and analysed.The numerical simulation results were compared and analysed with the actual monitoring results,and it was found that the laws between the two were basically consistent.(4)On this basis,the effects of steel bracing stiffness,thickness of the underground diaphragm wall and bracing arrangement sequence on the deformation of the foundation pit and the enclosure structure were further simulated and analysed.The results show that the influence of the thickness of the diaphragm wall on the deformation of the foundation pit is very obvious;the effect of increasing the steel support stiffness on limiting the deformation is not obvious;it is reasonable to adjust the installation sequence of the internal support to meet the requirements of deformation restraint as well as to achieve better economy.(5)The simulations show that the surface settlement without considering fluid-solid coupling is smaller than that with fluid-solid coupling,and the maximum surface settlement value outside the pit is 17.3 mm,which is 32.4% smaller than the maximum value of surface settlement with considering seepage.However,the actual monitoring results are closer to the calculated results considering fluid-solid coupling,indicating that the role between the pit stress field and the seepage field needs to be considered when carrying out simulations for deep foundation pit precipitation excavations.It also shows that numerical simulations can be carried out to theoretically predict the effects of pit precipitation excavation on the pit and support structure.(6)The calculation of the foundation support structure was carried out in accordance with the elastic resistance method using the Rizheng Deep Foundation Pit Geotechnical Design Software,and compared with the calculation results of the numerical simulation to further verify the feasibility of the numerical simulation calculation model.