Supporting Design And Numerical Simulation Of Deep Foundation Pit In Hexi Public Service Center Of Gulou District,Nanjing

With the continuous development of society as well as economy and technology,the number of population is increasing,and the scale of the city is also expanding.In order to alleviate the limited available land resources in cities,the number of urban buildings has been increasing,which has led to the excavation of a large number of deep foundation works.With the deepening of foundation pit excavation,a series of safety problems arising from excavation of deep foundation pit have become a very attractive problem.Based on the deep foundation pit project of Hexi public service center in Nanjing Gulou District,through detailed engineering geological investigation,combined with the actual excavation and support process of the foundation pit,the supporting scheme of reinforced concrete internal support system and pile retaining and support used in this foundation pit is carried out by using the software of deep foundation pit and MIDAS GTS software.Through numerical simulation analysis,the stress of foundation pit,horizontal displacement of supporting pile and axial force of supporting structure are calculated and analyzed.And compared with the monitoring data in the construction process,the accuracy of the three-dimensional finite element model established in this paper is verified.The study in this paper shows that:(1)In the process of excavation,with the continuous excavation of the excavation,the stress conditions in situ are constantly changing,and the displacement of soil will occur in the direction of the surface due to unloading,and the cumulative displacement increases with the depth of the excavation.And the supporting system of foundation pit can significantly reduce the deformation of foundation pit.(2)According to the calculation results of correction and MIDAS,the maximum deformation of the bored pile occurs at the top of the pile.As the foundation pit deepens,the bored pile’s horizontal deformation is characterized by the "spindle shape",that is,the displacement of the pile is small and the middle and lower parts move larger.(3)According to the numerical simulation results,with the excavation of the foundation pit,the supporting role of the bored pile is more and more obvious,so the obvious stress concentration will be produced in the pile body.The vertical stress mainly depends on the weight of the upper load,and the overall shape is still consistent with the topography.Horizontal stress will cause a sudden change near the pile,and its influence area will be about 1 times the depth of foundation pit.(4)In the process of excavation,there will be obvious uplift at the bottom of the foundation pit,but the range of foundation pit uplift will be significantly reduced by the cast-in-place pile,and the bottom uplift of the foundation pit shows the characteristics of "big middle and small sides".(5)According to the calculated results of the rock and soil,the maximum axial force of the supporting system appears on the bottom of the reinforced concrete support system,and with the erection of the second and third RC internal support system,the axial force of the first reinforced concrete internal support system decreases,and the second RC support system is the first one.The erection of three reinforced concrete supporting systems makes the axial force value decrease.The maximum value of the supporting axial force increases with the increase of excavation depth.(6)According to the calculated results of the rock and soil,according to the relevant regulations of the technical specification for monitoring of building foundation pit(GB50497-2009),the deformation of the foundation pit and the axial force of the supporting system are all meet the requirements of the standard.According to the numerical simulation results,the deformation of the foundation pit and the deformation of the supporting structure are far less than the vigilance value.It shows that the supporting system of bored pile + internal support can effectively control the deformation during the excavation of the foundation pit,and maintain the stability of the foundation pit,and it also plays a certain guiding role in the similar foundation pit engineering.