Study On The Influence Of Rock Foundation Pit Excavation On The Structure Of Lower Subway Station

Because of its convenience,subway stations often attract a lot of tall buildings to be built near them.In order to make use of underground space,more and more deep and large foundation pit projects,which is bound to cause the structure of the lower or adjacent subway station to produce displacement,deformation and internal force changes,may affect the normal use of the subway station,it is necessary to accurately grasp the relevant change rules.For this reason,this paper combined with the excavation of Nanping Station of Chongqing Metro Line 10 and its upper foundation pit,to study the influence of rock stratum foundation pit excavation on the lower subway station.Firstly,the influence of tunnel depth and excavation method on subway station is analyzed through model test.Then,the displacement,deformation and internal force of the subway station under different excavation depths and different levels of surrounding rock are analyzed in detail by two-dimensional numerical model.Finally,a threedimensional finite element model is established to study the influence of upper foundation pit excavation on the structure of Nanping subway station,which provides a reference for actual engineering construction.The main work of this paper is as follows:(1)Through the indoor model test and the corresponding numerical model,the influence of the upper foundation pit excavation on the tunnel is analyzed in the case of different tunnel burial depth and different excavation methods of foundation pit.The results show that the greater the depth of the tunnel is,the less the influence of the excavation on the tunnel is.Different excavation methods have different effects on the tunnel below,so the island excavation scheme is recommended for this kind of foundation pit engineering.(2)Through two-dimensional numerical simulation,the deformation characteristics of the underground subway station under different excavation depths and surrounding rock levels are studied.It is found that with the increasing of the depth of the upper foundation pit,the deformation response of the subway station is mainly vertical uplift,accompanied by vertical stretching and horizontal convergence of the cross section.When the depth of the upper foundation pit is large,the deformation of the primary lining and secondary lining will appear inconsistent,and the deformation of the primary lining structure is generally greater than that of the secondary lining structure.The grade of surrounding rock has a great influence on the deformation of subway station.In Ⅴ level under the condition of surrounding rock,the upper excavation deformation caused by the underground station,far more than Ⅲ level in the surrounding rock deformation of the subway station.(3)Through two-dimensional numerical simulation,the internal force variation characteristics of the underground subway station under different excavation depths and surrounding rock levels were studied.With the increase of the depth of the foundation pit,the arch and side wall of the primary lining and secondary lining of the station will produce tensile stress areas,and some of the stress values exceed the tensile strength of the concrete.In addition,the changes of internal forces in subway stations are also quite different under different levels of surrounding rock.Generally speaking,the worse the quality of the surrounding rock,the greater the stress of the subway station,and with the excavation of the upper foundation pit,the greater the range of stress change.(4)Combined with the supporting project,a three-dimensional numerical model is established according to the unfavorable conditions of the excavation of the upper foundation pit of Nanping Station,and a case study of the influence of the excavation of the foundation pit on the lower subway station is carried out.The analysis results show that the structure of Nanping subway station is mainly affected by the excavation of the upper foundation pit.The deformation is low on both sides and high in the middle,and the local vertical displacement of the secondary lining exceeds the deformation control value specified in the code.The excavation of the upper foundation pit will change the stress state of the primary support structure and the secondary lining structure of Nanping Station,so that the tensile stress of the arch foot and side wall area of the initial support exceeds the tensile strength of the concrete,resulting in tensile cracks.The compressive stress of the arch and side wall area of the secondary lining is reduced,and the tensile stress is generated,but it does not exceed the tensile strength of the concrete.