Study On Spatial Effect Of Stepped Sectional Excavation Of Deep Foundation Pit Of Subway

The subway deep foundation pit project is a complex and large-scale comprehensive project with high risk,which has obvious spatial effect.In order to accurately study the spatial effect of the subway deep foundation pit engineering and reasonably avoid its adverse effects,this paper systematically studies the spatial effect of the subway deep foundation pit by basing on a deep foundation pit project of Changsha Metro Line 5,which are detailed as follows:(1)Based on a large number of literatures and books on the spatial effects of deep foundation pit engineering,this paper reviews the research status of deep foundation pit engineering and its spatial effects.It also raises the deficiency of the commonly used planar strain simplification design method in spatial effects.(2)A large number of on-site measured data were collected based on the subway deep foundation pit project of Changsha Metro Line 5.Systematic analysis of the surface settlement,the horizontal side slope of the retaining structure,and the supporting axial force show that the spatial effect in the deep foundation pit engineering of the subway needs special attention.(3)A finite element modeling method of stepped and layered excavation is proposed based on the spatial effect of the subway deep foundation pit engineering.The calculation results are compared with the measured data,which verifies the effectiveness of the modeling method.Abnormal results are also studied.(4)Two kinds of spatial effects in the deep foundation pit engineering of the subway are discussed in this paper based on the proposed finite element model,which includes: the pit corner effect at the corners of the foundation pit,which is caused by the long-shaped foundation pit,and the different spatial distribution of foundation pit deformation,which is caused by different excavation methods.The results show that the reinforcement at the pit corner will reduce the deformation of the foundation.Therefore,the foundation pit retaining support can be reduced in the design.However,the impact of reinforcement at the pit corner is limited in a certain range,which needs to be accurately determined to avoid unreasonable design.The stepped excavation method can effectively reduce the deformation of the foundation pit structure.Therefore,this construction method should be given priority under appropriate construction conditions.(5)This paper also studies the causes of abnormal results and the foundation pit over-excavation of foundation pits in three-dimensional space.The main research contents include: the effects of over-excavation under different layers of foundation pits,the effects of over-excavation under different over-excavation depths and widths,and the effects of over-excavation under different excavation methods.The results show that over-excavation of the soil layer around the maximum lateral slope of the foundation pit retaining structure has the greatest influence on the deformation of the foundation pit.In addition,with the increase of over-excavation depth and over-excavation width,the deformation of foundation pit also increases.Compared with the direct layered excavation method,the stepped excavation method can reduce the harm caused by over-excavation,and should be given priority in practical engineering.