Study On The Adaptability Of Construction Method Of Underground Excavation Subway Station In Upper-Soft And Lower-Hard Stratun

The rapid development of urbanization in our country has brought enormous pressure to urban traffic,and the urban underground rail transit system can effectively alleviate the traffic pressure.Therefore,more and more cities gradually begin to build a large number of subways.During the construction of underground structures such as subways,a ubiquitous upper-soft lower-hard composite stratum was gradually discovered.The upper-soft and lower-hard stratum has the distinctive features of“soft on top”and“hard on the bottom”.Due to the existence of the soft-hard interface,it brings huge challenges to the construction of underground structures such as subway stations.In order to reduce the impact on the ground during the construction period,the construction of urban subways is still mainly based on the underground excavation method,while for the underground excavation of subway stations in the upper-soft and lower-hard stratum,the construction methods such as the arch cover method,the double side wall guide pit method and the hole pile method are mainly used for excavation..However,through the investigation of a large number of engineering cases,it is found that there is a phenomenon of confusion in the selection of construction methods,and there is a lack of a recommended selection system for construction methods suitable for underground excavation of subway stations in upper-soft and lower-hard stratum.In response to this phenomenon,through the combination of literature research,numerical simulation,model test,field application and actual measurement,a research on the adaptability of construction methods is carried out,and a new construction method was innovatively proposed which suitable for underground excavation of subway stations in upper-soft and lower-hard stratum.The main work and conclusions are as follows:(1)The stability classification of the underground excavation subway station in the upper soft and lower hard strata is studied.Through a series of numerical calculations,the stability classification of the thickness of the overlying rock mass is determined.The stability is divided into W-Ⅰ,W-Ⅱ,W-Ⅲ,W-Ⅳ,W-Ⅴ five grades.(2)On the basis of stability classification,research on the adaptability of construction methods was carried out.Mainly,the mainstream construction methods such as the step method,the initial support arch method,the double-layer initial support arch method,the double side wall guide pit method and the hole pile method were studied.It can serve as a reference for similar projects in the future.(3)Based on the stratigraphic conditions of the Xiaogang Station of Qingdao Metro Line 2,the Xiaogang Station is divided into W-III grades.The applicability of the recommended construction method is verified through model experiments and field tests.(4)An idea of a new construction method for underground excavation subway stations in the upper soft and lower hard stratum is innovatively proposed,named "Drawing Arch Method",and its feasibility is studied through numerical simulation and model test.