Experimental study on ground subsidence caused by pumping and recharging

The natural flow field of groundwater is slow at Binhai New Area in Tianjin, which leads to the decrease of heat exchange efficiency of ground source heat pump. To solve this problem, pumping wells around the buried pipe were arranged to accelerate the flow rate of groundwater. Pumping would improve heat exchange efficiency and make ground subsidence. Our aim is to find a method to improve heat exchange and reduce ground subsidence. Based on previous researches, the periodic alternately pumping and recharging method was designed. Through field test and physical model test, the water level in pumping well, recharging well, observation holes, and pore water pressure and subsidence at different points were monitored and analyzed. The results show that the periodic alternately pumping-recharging method makes smaller subsidence than that of conventional pumping methods. It is feasible to apply it to ground source heat pump project in Tianjin. At a reasonable flow rate, period, and pumping-recharging mode, subsidence is small or even negligible. In addition, the pore water pressure and subsidence in aquifer and aquitard have different characteristics. The clay layer is mainly plastic deformation. With the process of pumping and recharging, the plastic deformation becomes smaller and smaller. The sand layer presents both elastic and plastic deformation. With the process of pumping and recharging, the plastic deformation becomes smaller and smaller, and tends to have elastic deformation gradually. Additionally, when the flow rate is large enough, soil particles in aquitard below the aquifer move to the aquifer with the seepage force caused by drainage.