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Severe land subsidence caused by groundwater exploitation is one of the anthropogenic environmental problems. Even when groundwater exploitation has been limited or prohibited in some areas, it will continue in some water-deficient areas for a long time. Therefore, it is a challenge to reasonably exploit groundwater and not to cause severe land subsidence. A physical model test was conducted in this paper to simulate the extraction and recharge of groundwater, and to observe the soil deformation characteristics and the change of the pore water pressure. Based on the test results, the mechanism of land subsidence was investigated and different pumping modes were involved including the single-well pumping, single-well pumping and recharging, and double-well pumping and recharging tests. The tests indicated that the sand layer and clay layer had different pore water pressure change and deformation characteristics. In the sand layer, the pore water pressure decreased when pumping and increased when pumping stopped. In the clay layer, however, pore water pressure first increased rapidly and then decreased when pumping, and it decreased rapidly and then increased after pumping stopped. The variation of pore water pressure in the clay layer was consistent with the Mandel's effect and was related to the distance to the pumped layer. The further the distance was, the longer the Mandel effect lasted. The sand layer mainly exhibited elastic deformation and the clay layer almost exhibited plastic deformation. The clay layer showed clear hysteresis. The hysteresis time was increased with the distance from the pumped sand layer. In addition, the results indicated that the three pumping modes could reduce land subsidence. With regard to the pumpage and land subsidence, the double-well pumping and recharging mode was better than the other two. With pumping or pumping and recharging together, the increasing rate of vertical displacement became smaller and gradually reached a stable state. In general, these three modes were reasonable for groundwater exploitation, and could effectively control land subsidence. [ABSTRACT FROM AUTHOR]

Summary: Shanghai is one of the cities suffering from land subsidence in China. Land subsidence has caused serious financial losses. Thus, artificial recharge measures have been adopted to compensate the drawdown in shallow, confined aquifers and thereby control land subsidence. In this study, a multi-cycle recharge–recovery field experiment was performed to investigate the response of a shallow, confined aquifer to artificial recharge through a well. In the experiment, a series of recharge–recovery cycles with different recharge volumes and durations, with and without artificial pressure, were performed. The water levels monitored in the recharge and observation wells indicated the response of the aquifer to the multi-cycle recharge–recovery process. Meanwhile, a finite-difference method (FDM) numerical model was established, and its parameters were obtained via a reversed numerical analysis on the experimental data. The responses of the shallow, confined aquifer to the multi-cycle recharge–recovery process were simulated in detail using the model. The calculation results showed that the water level dropped significantly when the recharge ended. Moreover, the efficiency of a multi-cycle recharge was found to be higher than that of a concentrated one under the same recharge volume and time. The relationship between recharge frequency and efficiency, expressed as H =0.29498 f 0.40163 and R 2 =0.97264, respectively, was obtained through the FDM numerical simulation. In the recharge intervals, the optimal recharge efficiency was achieved when the water level rose to 40% of the peak. [ABSTRACT FROM AUTHOR]