A Study On The Land Subsidence Effect After Prohibiting Extraction Of Groundwater In Suzhou-Wuxi-Changzhou Area

Long-term excessive deep groundwater withdraw has led to serious geological hazards of regional land subsidence and ground fissures in Suzhou-Wuxi-Changzhou area. In order to slow-down and control the regional land subsidence, the provincial and local government of Jiangsu had taken countermeasure of reducing groundwater pumping quantity by 20% each year since 1996, and the strict ban of the deep groundwater withdrawal has been undertaken within a given period since 2000. By the end of 2005, it has been finished to prohibit the deep groundwater withdrawal in Su-Xi-Chang area. Long term monitoring data show that significant achievement has been made on control of land subsidence from macro viewpoint in the research area, however, further and systematical research work has not been carried out on analyzing the relationship between banning of groundwater pumping and land subsidence, and evaluating the land subsidence effect and subsidence mechanism after Prohibiting Extraction of Groundwater. Based on large number of land subsidence measuring data, the author chooses the land subsidence effect after prohibiting extraction of groundwater in Suzhou- Wuxi- Changzhou area as dissertation research topic to study in detail the mechanism and developing law of land subsidence before and after the prohibition of groundwater withdrawal. Some valuable results are summarized as follows:1. It is analyzed and summarized in detail of the groundwater pumping, groundwater level, and the characteristics and trend of land subsidence before and after the ban of groundwater withdrawal. Based on the large amount of monitoring data and analytical results, it is proved that the relationship between the groundwater pumping and land subsidence is fully consistent in time and space. It is found that there exhibited obvious regionalization of the land subsidence development after ban of groundwater withdrawal. In the area that the groundwater table going up rapidly, the rate of land subsidence slowdown faster, and the time delay of land subsidence is shorter. Even the ground rebound occurred in some site, such as the area in east of Suzhou and near Yangtze River, in the area of better groundwater recharge. While in the area that groundwater recharge condition is bad, the groundwater table rises slowly, which delay is apparent, typically that in west region of Wuxi and south part of Jiangyin, so that the rate of land subsidence can obviously be controlled.2. The relationship between groundwater withdrawal and land subsidence in Su-Xi-Chang area is comprehensively and systematically analyzed. It is further proved that the critical groundwater table of land subsidence exists, and put forward that critical groundwater table is related to the pre-consolidation stress of soil layers, The existence of critical groundwater tablel of land subsidence has great significance in scientific and rational development of groundwater resources and land subsidence controlling.3. The mechanism of land subsidence in Su-Xi-Chang area is further discussed and studied after banning of groundwater pumping. On the basis of relationship analysis between land subsidence and geological structure and hydrogeological condition, and stress history of the strata, and the piezometric head of groundwater, the concept of ground rebound is put forward, which rebound occurred generally in the area of groundwater table rapidly rising, and the groundwater level of which is higher obviously than the critical groundwater level of subsidence.4. A large amount of measuring data in Su-Xi-Chang area is utilized to modify the existing coupled mathematical model of land subsidence and groundwater flow, and the prediction of regional land subsidence is given respectively by two plan of pumping-banning and appropriate pump of the groundwater. Taking land subsidence control as the constraint condition, the allowable groundwater resources of the area is calculated through the numerical modeling. Finally some suggestions and countermeasures have been proposed such as a scientific development scheme of groundwater, zoning of the reasonable groundwater pumping area such as limited extracting zone and prohibited extracting zone. The research achievement of this dissertation provides a scientific basis for the rational development, utilization and management of groundwater in Su-Xi-Chang area.