Mechanical modeling of aquifer sands under long-term groundwater withdrawal

Abstract Long-term excessive groundwater withdrawal causes severe land subsidence and considerable economic loss. During the period of land subsidence, aquifer sands present complex deformation characteristics. Field data in the Southern Yangtse Delta, China have shown that the deformation characteristics of aquifer sands are closely related to the changing patterns of groundwater level the aquifer units have experienced. When the groundwater level fluctuates yearly within a certain range, the aquifer unit behaves primarily as an elastic material. When the groundwater level rises and falls alternatively with the average decreasing but higher than the previous lowest value the unit has experienced, the aquifer unit behaves primarily as a visco-elastic and elasto-plastic material. However, when the average level decreases and is lower than the previous lowest value the unit has experienced, the aquifer unit behaves primarily as a visco-elastic, elasto-plastic, and visco-plastic material. The plastic and creep deformation of aquifer sands is also proved through laboratory tests. With the increasing cycle number of loading, the creep and plastic deformation decreases and the expansion under unloading is closer to the compression under loading in a single cycle. On the basis of field and laboratory data, a new mechanical model is constructed, which can describe various kinds of deformation characteristics under different changing patterns of groundwater level, including elastic, visco-elastic, plastic, and visco-plastic deformation. The model includes two yielding stresses and six parameters. When the appropriate field data are available, the six parameters of the model can be determined through graphic methods. Then the deformation of aquifer units can be calculated if the changing groundwater level is known.