Ground Subsidence Mechanism Causes Beijing Typical Regional Variation Analysis

In Beijing, two-thirds of water supply comes from groundwater, over pumping groundwater for nearly fifty years have cause regional land subsidence and the trend is growing. The maximum cumulative subsidence beyond1000mm in subsidence funnel, the area that cumulative land subsidence beyond300mm up to1300km2, and expand as a rate of30-60mm/y, which is a serious threat to urban safety. Especially for uneven land subsidence, pose a great threat to the municipal infrastructure, buildings and people’s lives and property safety. Uneven land subsidence, which also calls for differential land subsidence, is refer to uneven loss of land elevation over a relatively short distance. Due to the long-term overexploitation of groundwater and complex geological background, the problem of regional uneven land subsidence become more prominent in the typical subsidence region in Beijing. However, the study of formation mechanism for uneven land subsidence remains uncertain.In view of the above problem, this paper combined InSAR technology, GIS spatial analysis, numerical simulation, with means of hydro-geological exploration and data mining. The research focus on extraction of uneven land subsidence information and mechanism formation, the mainly research and conclusions as following:(1) Extraction of uneven land subsidence based on InSAR technology:multi-scale transformation method and time series InSAR analysis method. Multi-scale transform method apply space interpolation with the land subsidence rate of permanent scatterers, extract land subsidence slope using slope analysis in GIS spatial analysis tools, through transform scale to analysis uneven land subsidence distribution characteristics. The result shows that the uneven land subsidence occurred at the edge of land subsidence funnel or along the fault line zone. Time series InSAR analysis method select permanent scatterers nearby to analysis the dynamic changes, identify differential subsidence in typical areas. Xiwanglu level measurement stations data were used as the validation, and the result found that the average annual land subsidence rate of PS points nearby are consistent with the monitoring station data.(2) Analysis the causes of the uneven land subsidence, the falling groundwater level, the structural differences of Quaternary sedimentary and the load of building group. Over-exploitation of groundwater caused regional groundwater level falling, on the both sides of the uneven land subsidence area the hydraulic connection dynamic change are consistent with the diving and pressurized water level, but the hydraulic gradient is greater than other areas, obviously the main reason for uneven land subsidence. The structural differences of Quaternary sedimentary, due to the thickness of the clay layer formed by the fault of the space distribution differences in artificially pumping and other factors lead to differential compaction subsidence, geological background for uneven land subsidence condition. The load of building group as external incentives to some extent affected the uneven land subsidence distribution characteristics, new high-rise buildings in the primary consolidation stage have greater impact on uneven land subsidence, while the rural areas the main houses structure are one or two story brick, the load have less affect on uneven land subsidence.(3) Analysis the formation mechanism of uneven land subsidence, we select the Beijing Northern Plains uneven land subsidence area, Quaternary aquifer is divided into five aquifer layers group in the vertical based on the information gathered, and then establish hydro geological conceptual model in the study area, set hydro geological parameters based on the hydro geological exploration, pumping test and soil test data, the finite element software ABAQUS was selected for numerical simulation, InSAR and monitoring station data was used to verify the reliability of the result of subsidence with numerical simulation.The result of numerical simulation shows that:a) at the early stage of groundwater extraction, the hanging wall and foot wall differential subsidence is not obvious, but with the continued groundwater mining, the vertical deformation gradually formed differences in the surface at the hanging wall and foot wall, the fault as the control borders, deformation at hanging wall greater than that at foot wall, thus form uneven land subsidence. At the end of the numerical model, the difference of the underground water level on the hanging wall and foot wall is about8meters, which may result of fault changed groundwater flow conditions, b) Vertical deformation and horizontal deformation has a greater change at the fault zone, the maximum horizontal surface deformation occurs near the foot wall at the fault zone, as the distance away from the fault, the deformation decreases, the fault act as a control boundary that effect difference deformation characteristics induced by pumping. The numerical result shows that compression strength concentration on the deep fault zone while tensile strength formation in the near-surface sediments, with the development of uneven land subsidence may induce cracks.