Characterization of land subsidence induced by groundwater withdrawals in Su-Xi-Chang area, China

Su-Xi-Chang area is one of the typical regions in China which suffers from severe land subsidence. Various tools of field monitoring were integrated to study the characteristics and mechanisms of land subsidence in this region. The occurrence and the development of the land subsidence in this region are strongly related to the groundwater pumping both in time and space. The main consolidation layers are the soft mud layers; however, the compressibility of the confined sandy layers should not be ignored. The second and third confined aquifers contributed more than 30% of total subsidence. Meanwhile, irrecoverable deformations were also observed in the sandy layers. Different sandy layers deform diversely under different stress conditions. Some have the elastic feature. But the soil strata, including both sandy layers and clayey layers, located in the center of the groundwater level depression cone exhibited obvious viscous mechanical behavior which caused the common lag phenomenon. The sand composition (mingled with small clay particles or interbeds) and sand rheology are the two main reasons for the lag phenomena in sandy layers. A series of laboratory tests for modeling the effective stress changes due to groundwater withdrawals, were conducted to investigate the mechanism of the lag phenomenon. Based on the test results, the relationship of stress-strain-time for saturated sands is obtained; and it could be expressed as power functions. The results also showed that the compression of the sandy layers was time dependent, and its deformation could be remarkable. When establishing land subsidence model, the deformation for the similar soil formation could be elastic, visco-elastic and even visco-elastic-plastic, because of the different groundwater level fluctuation experienced.