A Study On The Land Subsidence Characteristics After Controlling Extraction Of Groundwater In Shanghai

Long-term excessive extraction of groundwater has led to serious regional land subsidence in Shanghai, In order to control the land subsidence in Shanghai, many measures, such as limiting groundwater extraction, adjusting the exploited aquifers and carrying out artificial recharge, have been taken since the mid-1960s. As a result, land subsidence in Shanghai has been effectively controlled. According to the filed data obtained from extensometer groups and observation wells in Shanghai, the changes in groundwater levels in aquifer units and the characteristics of soil deformation were analyzed during the period2001-2012. It is found that in recent years, the water level in phreatic aquifer and the first confined aquifer approximately remained stable, while the water level in the second to the fifth confined aquifers had an obvious trend to rise. Moreover, three soft layers were still in compression whereas the deformation of the third soft layer was effectively controlled and rebound occurred in some areas. Under the condition of overall recovery of groundwater, the first to the fourth confined aquifers showed clear plastic and creep deformation. The deformation of aquifer sand lags behind the variation of water level. With groundwater level rising continuously, the soil layer was still compacting, while the fifth confined aquifer behaved primarily as an elastic material, and the sand layer rebounded constantly albeit the lag is not so obvious. Four modes were classified based on the relationships between the deformation and the groundwater level of aquifer under the pump-limit condition during the period2001-2012.Based on the laboratory compression tests, the relationships between the deformation and time of the soils in different loading-unloading were obtained. The results show that saturated sand had similar creep characteristics with saturated clay soil, presenting a significant nonlinear characteristic. Its stress-strain relationship can also be described by the power function. Finally, a newly developed visco-elasto-plastic model was adopted to fit the relationship of deformation and time of the sand under loading. The four creep parameters of this model changes with loading.