Compaction of aquifer units under complex patterns of changing groundwater level

Long-term groundwater withdrawal has caused severe land subsidence in Shanghai, in which compaction of aquifer units has taken an important role. The compaction characteristics of aquifer units are closely related to the changing patterns of groundwater levels the units have experienced. They may be elastic, elasto-plastic, visco-elastic, or visco-elasto-plastic. Several compaction models are discussed and used to calculate the compaction of the aquifer units in Shanghai. Compared with other models, it is found that double-yielding visco-elasto-plastic model can capture the compaction characteristics of aquifer units and more reasonably simulate the compaction of aquifer unit under complex changing patterns of groundwater level. In particular, the double-yielding model can describe the plastic compaction when the average groundwater level declines but is higher than the historically lowest level and the obvious visco-plastic compaction when the groundwater level is lower than the historically lowest level, which is a popular deformation characteristic for aquifer units in Shanghai.