Stress-strain measurements of deforming aquifer systems that underlie Shanghai, China

This article describes aquifer system deformation in Shanghai, where land subsidence, caused by excessive groundwater withdrawal, has caused considerable economic loss during the past 40 years. The article investigates the relationship between the amount of groundwater pumped and land subsidence and the relationship between layers with the most significant deformation and the main exploited aquifers. Based on monitoring data, including compaction of individual strata documented by extensometer groups and the groundwater levels from observation wells, the characteristics of aquifer system deformation are studied. When the groundwater level fluctuates within a certain range or the average declines slowly above the previous lowest value, sand layers exhibit elastic or elasto-plastic behavior, whereas soft clay layers exhibit visco-elastic-plastic behavior. The softer the clay, the more the viscous deformation. When the average groundwater level decreases below the previous lowest value, both sand and clay layers exhibit visco-elasto-plastic behavior. Although the sand layers have less plasticity and creep than the clay layers, sand deformation can be significant, and a sand layer may become a primary compaction layer if the sand is thick. In the period 1981-1990, soft clay layers one, two, and three were the major compaction layers, but in the period 1991-2000, sand layer five was the major compaction layer that contributed more than 50 percent of the total subsidence.