Interaction Mechanism Among Metro Underground Space And Groundwater Environment In Xi'an-Xianyang Region

The shortage of urban construction space resources,most of the domestic subways are built underground,and a large part of the subway lines are deeply buried in the groundwater environment,which greatly increases the difficulty of subway space construction and also affects the natural groundwater environment.The subway space operates for a long time in the groundwater environment,and the changes in the groundwater flow field may cause a series of adverse effects on the subway space,structural floating problems,leakage problems,high water pressure and ground subsidence.The subway space and the groundwater environment influence each other,making the relationship between the two more complex and closer.Therefore,clarifying the interaction mechanism between the urban subway and the groundwater environment and giving targeted regulation suggestions are of great significance to the sustainable use of urban underground space.This paper takes the main urban areas of Xi'an and Xianyang cities as the main research area.Based on extensive literature review,reliable three-dimensional hydrogeological numerical models are established to study the mutual feedback mechanism of subway space and groundwater environment during subway construction and operation.And correspondingly propose novel and efficient construction precipitation methods and control measures.The research conclusions are as follows:1.Establish a three-dimensional groundwater unsteady flow model,simulate and predict the impact of foundation pit dewatering on the seepage field during the subway space construction process,propose several different foundation pit precipitation methods,and evaluate and study from the aspects of precipitation efficiency and seepage field disturbance.The conclusion shows that: In the process of foundation pit dewatering,compared with the traditional tube well dewatering method,the use of radiation wells for foundation pit dewatering is more efficient,has a smaller impact on the seepage field,and has less engineering and economic investment,which can be given priority.After the curtain is added to the foundation pit dewatering project,the water barrier effect of the curtain greatly improves the working efficiency of the foundation pit dewatering.At the same time,it can effectively control the total amount of drainage of the foundation pit dewatering and the disturbance to the seepage field.Using of radiant wells and water-proof curtains together can make the foundation pit dewatering project more efficient and economical.2.By simulating and calculating the drainage efficiency of several construction precipitation methods,quantitative comparative analysis is made from the drainage time of foundation pit precipitation,the total amount of groundwater pumped by precipitation,and the impact on the seepage field.And comprehensively consider factors such as dredging efficiency,engineering investment,and disturbance to the seepage field.Optimize parameters such as the arrangement method and number of pumping wells,the pumping volume of a single well,the vertical drawdown of the radiant well,the diameter of the radiant tube,the permeability performance,and the number of radiant tubes.3.The simulation predicted the distribution of groundwater flow field during the construction and operation of some subway lines in the second phase of the Xixian New District Metro Line 1,and the results showed that: Due to the large thickness of the aquifer around the subway,strong permeability,and small hydraulic gradient of the groundwater flow field,the disturbance of the seepage field by the subway line is concentrated near the subway space,and the seepage field in the area far away from the subway space basically does not produce disturbance.When the subway space is buried in the groundwater environment as a whole,the hydrostatic pressure generated by the groundwater environment on the side walls of the space will cause safety hazards such as deformation of the walls of the space and leakage of groundwater.The bottom of the space is subject to large buoyancy for a long time,which can easily cause the space to float.Instability and other issues.4.Research on the influence law of seepage field during subway operation in different subway conditions and different groundwater environments,and the conclusions are as follows:The deeper the subway is buried in the groundwater environment,the smaller the impact on the phreatic flow field.However,the pressure on the sidewall of the subway space and the buoyancy at the bottom increase with the depth of the subway,which is more likely to cause safety hazards.The subway space is buried in aquifers with different permeability.When the permeability coefficient of the aquifer is small,the subway disturbs the seepage field greatly;as the permeability coefficient of the aquifer increases,the influence of the subway space on the seepage field decreases.When the hydraulic gradient of the groundwater flow field is small,the subway space has less disturbance to the seepage field;the hydraulic gradient gradually increases,and the influence of the subway space on the seepage field increases.5.Based on the mutual feedback mechanism between the subway space and the groundwater environment in the study area,several reasonable and efficient anti-floatation and decompression schemes are proposed and calculated and optimized.The conclusions are as follows: Compared with the anti-floating and decompression scheme of tube wells,the decompression of radiation wells is more efficient,has less disturbance to the groundwater flow field,and has less economic and engineering investment.The curtain is applied to the antifloating and decompression project,which plays a role of water isolation and pressure control,which can not only control the disturbance range of the seepage field during the decompression process,but also reduce the economic investment of the project.At the same time,the choice of filter layer is also very important.The better the permeability of the filter layer,the better the decompression effect.However,if the permeability coefficient of the filter layer is too large,it will cause safety hazards such as quicksand and piping.The research results of this article provide a scientific theoretical basis and important reference significance for the planning and construction of subway projects in the study area and the selection of construction methods and structural parameters during the construction process.At the same time,it can also provide empirical guidance and research ideas for related research in this region or areas with similar conditions in the future.This research puts forward a reasonable and feasible technical plan based on comprehensive consideration of the project's economic investment and environmental influence factors,which has certain economic and environmental value.