Author(s): Yu-Yun Lin; Yin-Jen Chen
Keywords: Subsidence; Aquifer; Aquitard; Pumping; Perturbation
Abstract: The land subsidence caused by excessive extraction of groundwater is a complex phenomenon due to the complexities of soils involved. The extraction of groundwater reduces the interstitial water pressure (i. e. drawdown) ,which means a transference of stress to the soil skeleton and its subsequent volume reduction (i. e. subsidence) .The coupled theory of Biot provides a full three-dimensional model of mutually interacting drawdown and deformation of soils. However, the calculation of subsidence based on the full three-dimensional theory will be very time-consuming, especially for a complex soil system. Our research extends the perturbed equations, which simplifies the coupled equations of Biot to a quasi threedimensional model, to groundwater flow in a multi-aquifer system. The analysis is carried out based on two small parametersand, which are the ratio of aquitard to aquifer in hydraulic conductivity, and that in constrained modulus, respectively. The model considers the effect of gravity and the phreatic aquifer on subsidence. The computation efficiency of the prediction model is greatly improved by incorporating Laplace transform and numerical Laplace inversion. The calculated results have been validated by finite element simulations based on Biot’s theory. Using this model, we analyze an unconfined two-aquifer system, in which the top aquifer is the phreatic aquifer. Although the pumping locates at the bottom aquifer, the water releases from the decline of water table in the phreatic aquifer is much more than from the aquifer and the aquitard. The other study case is a confined four-aquifer system. Our results show that the subsidence can be largely affected by the slow drainage of a soft aquitard underlying the pumping aquifer in long time.