Selection Of Foundation Pit Enclosure And Treatment Measures For Foundation Pit Groundwater In Lanzhou Metro Station

With the development of urban rail transit,subway has become the main means of transportation in large and medium-sized cities.The construction of subway is mostly located in the center of the city.In the process of subway construction,it is inevitable to encounter various foundation pit projects.Compared with general foundation pit engineering,subway station foundation pit has the characteristics of large excavation depth,complex surrounding environment and many underground pipelines.All these characteristics have brought great difficulties to the support and excavation of the subway foundation pit.Therefore,the research on the selection of foundation pit retaining structure in subway station is of great significance.Groundwater is an important issue affecting the stability of foundation pit excavation.Lanzhou is also located in special water-rich sand and gravel strata,red sandstone strata and collapsible loess strata.In some areas,the water level is shallow and the water inflow is large,which makes foundation pit dewatering difficult.Therefore,the foundation pit groundwater treatment is also a major focus of research.In order to study the selection of retaining structure and groundwater treatment for foundation pit excavation of lanzhou subway station,and to achieve the purpose of guiding construction,taking lanzhou subway station as the engineering background,this paper studies and analyzes the enclosure structure selection of subway station foundation pit and groundwater treatment by combining numerical simulation and engineering monitoring.The main contents are as follows: 1.Selection of groundwater treatment measures for typical construction sites under different hydrogeological conditions.It is concluded that under the premise of ensuring technical feasibility,combined with safety,environment,economy and other factors,the groundwater treatment measures need to determine that the sand pebble stratum in Lanzhou area generally adopts the precipitation outside the foundation pit,the red sandstone stratum adopts the retaining structure to stop water,and the water level adopts the open drainage under the structure floor.2.Construction difficulty of precipitation and its impact on the surrounding environment.Improper precipitation will cause excessive ground settlement around the foundation pit,which will seriously affect the safety of surrounding construction environment risks.It is concluded that the additional ground settlement caused by the measured underground precipitation in practice is generally only 0.035-0.10 times of the theoretical calculation value.3.Under different stratum conditions,the selection of typical construction site foundation pit retaining structure type.According to different stratum conditions,it is concluded that drilling pile is generally used in pebble stratum of Lanzhou metro,and diaphragm wall or bite pile is generally used in red sandstone stratum.4.Sensitivity analysis and selection of design parameters of deep foundation pit retaining structure.It is concluded that the calculation of the internal force and deformation of the retaining structure and the deformation of the foundation pit will involve the calculation of the strength,deformation and stability of the soil mechanics,as well as the collaborative deformation of the slope and the retaining structure.Therefore,this study analyzes the sensitivity of the design parameters of the enclosure structure of the typical construction site,and determines the key parameters such as C,j value,embedment depth and their values of the water and underwater correction.5.Risk assessment of deep foundation pit excavation.The risk of deep foundation pit excavation is very high,but the construction company often does not estimate the risk.This time,the risk assessment of foundation pit is carried out,and the corresponding preventive measures are proposed for the emergency state of foundation pit.