Study On The Influence Of Metro Construction On The Surrounding Environment Under The Complex And Sensitive Geological Conditions

Nanjing is located in the middle and lower reaches of the Yangtze River,the positioning of the national new district and the Yangtze River Delta megalopolis has brought strong impetus to the development of Nanjing.Nanjing has developed from the ages of river-side and river-crossing to that of river-around,a large number of metro lines are inevitably located in the floodplain area of the Yangtze River.The upper part of the Yangtze floodplain is filled with soft clay,the lower part is dominated by sand and gravel,which has a typical dual structure of river alluvium.The soft clay has the properties of low structural strength,strong thixotropy,high sensitivity,high water content and high compressibility,and is at mostly soft and flow plastic state;The gravel layer has large permeability coefficient,strong permeability,rich groundwater,which lead to poor engineering characteristics.As shown in the early monitoring data of subway constructions,the deformation of surrounding soils in river floodplain area,caused by subway construction,greatly exceed the limit value,which could endanger the security of the surrounding environment.Therefore,it is particularly important to study and summarize the deformation laws of surrounding environment and relevant controlling methods in the river floodplain construction area.Because the design for the foundation pit of subway station,shield construction methods and soil layer conditions are similar in this area,the regional research results are more important to guide the design of the subsequent subway and construction in this area.Through the study on a series of difficult problems such as excessive deformation of surrounding environment under complex and sensitive geological conditions,the empirical support for the subway construction in this area is provided,the deformation of surrounding environment could be predicted,and the corresponding monitoring and protection methods could be further put forward.The main contents of this dissertation are:(1)Based on the deformation data of the deep foundation pit engineering from several subway stations in floodplain areas,the theoretical analysis,empirical formula and finite element numerical simulation method are adopted,the subsidence deformation range of surrounding surface,settlement curve forms,positions of maximum settlement point,settlement amount ratio caused by excavation and water level declining,horizontal displacement of underground continuous wall,which are caused by excavation under suspended curtain foundation pit and dewatering fluid-solid coupling in floodplain area,are respectively discussed.A deep foundation pit for a subway station with suspended curtain is selected,and a three-dimensional groundwater seepage model is established through MODFLOW,the relationship between the water drawdown outside the pit and the penetration depth of underground continuous wall is discussed.(2)Based on the measured long-term settlement data of adjacent shallow foundation buildings caused by the construction of underground continuous wall in soft soil of river floodplain,the settlement characteristics and deformation law of buildings are summarized.Several mathematical models are used to predict the long-term settlement of buildings,a mathematical prediction model for the long-term settlement and deformation characteristics of buildings caused by the construction of underground continuous wall in soft soil is put forward,after comparing of the prediction accuracy among each mathematical model.The finite element software is used to simulate and predict the long-term settlement deformation of buildings caused by the construction of underground continuous wall.A constitutive model for the long-term settlement characteristics and deformation law of buildings is proposed through comparing with the measured values.By analyzing the change of internal force caused by uneven settlement,the structural safety of buildings could be judged.(3)A numerical analysis model to estimate the influence of subway station deep foundation pit excavation on the adjacent shallow foundation buildings in deep soft soil of river floodplain is established.The variation law of settlement and uneven settlement of adjacent shallow foundation buildings are simulated and calculated in the foundation pit excavation stage after the construction of underground continuous wall in the cases of different pile lengths,pile diameters,pile positions,whether the pile top is restrained or not,whether the influence of isolation pile construction is considered or not.The optimum design parameters of isolation pile are put forward in order to provide quantitative guidance for the design of isolation pile in soft soil.Through adopting the optimized isolated pile design parameters,three-dimensional fine numerical model in ABAQUS is built to simulate and then used to compare with the results of the two-dimensional model,in order to verify the protection effect of the isolated pile on the shallow foundation.At the same time,the settlement and deformation of two rows of buildings adjacent to the foundation pit under the combined reinforcement methods of isolation pile and anchor static pressure pile are simulated numerically,and the effects of the two measures are compared.After the deep foundation pit being strengthened by anchor static pressure pile during excavation stage,the settlement and deformation of the first row of houses,the deep horizontal displacement of soil before and after the first row of buildings,the horizontal displacement of underground continuous wall,the displacement of surrounding soil mass,and the change of soil properties are discussed,respectively.To find out the changing law,the measured data are compared with the calculated ground subsidence values.(4)A statistical analysis is carried out for 12 sets of measured ground settlement data for 7 shield construction projects in floodplain areas.The adaptability of peck empirical formula in the area of river floodplain is discussed preliminarily.The regional empirical values are suggested for two controlling parameters of the peck empirical formula,the width coefficient of settlement trough i and the soil mass loss rate ?.The surface subsidence area affected by shield tunneling construction in the floodplain area is partitioned.(5)Taking the pile-box foundation building under the double-line shield tunnel as the research object,a two-dimensional numerical model of the pile-box structure under the shield tunneling is established.Through the numerical simulation and analysis,the influences of pile length,pile diameter and soil loss rate on the deformation of adjacent pile group and box foundation floor are explored,which provides the basis for optimization design and construction parameters.