Abstract
More than 30years groundwater overdraft had resulted in hydraulic head declined from near the ground surface to 85m deep in the second confined aquifer (CA2) in Changzhou, and lead to regional land subsidence from 1970's to early 2000's. After banning deep groundwater extraction was banned in 2000, the hydraulic head of CA2 had recovered to 37.6m in Changzhou by the end of 2013. Based on several stages first and second order leveling results and long term monitoring data from the multi-strata borehole extensometer station (BES), it was revealed that the land subsidence is attributed to the compression of both aquifers and aquitards in the porous aquifer system. The spatial characteristics of subsidence are related not only to hydraulic head pattern in the area, but also to the thickness and compressibility of different soil strata, and distance from the aquifer. Since banning deep groundwater extraction, the ground uplifted 37.22mm (5.4% of the pre subsidence) at BES, Changzhou due to the hydraulic head recovering. Strata compression and rebound was recorded as: the upper most stratum, and the underlying aquitard of CA2, and the upper CA3 are still in the compression process, the lower CA3 layer and the upper most segment of aquitard of CA2 rebounded about 90% of the pre compression recorded since 1984, and the CA2 and its adjacent overlying aquitard rebounded 3.8%–9.7% of the pre compression.
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