Land rebound after banning deep groundwater extraction in Changzhou, China
- Authors: Wang, Guang-ya , Zhu, Jin-qi , You, Greg , Yu, Jun , Gong, Xu-long , Li, Wei , Gou, Fu-gang
- Date: 2017
- Type: Text , Journal article
- Relation: Engineering Geology Vol. 229, no. (2017), p. 13-20
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- Description: More than 30 years groundwater overdraft had resulted in hydraulic head declined from near the ground surface to 85 m 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 37.6 m 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 37.22 mm (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. © 2017 Elsevier B.V.
Effect of negative excess pore-water pressure on the stability of excavated slopes
- Authors: Ghadrdan, Mohsen , Shaghaghi, Tahereh , Tolooiyan, Ali
- Date: 2020
- Type: Text , Journal article
- Relation: Geotechnique Letters Vol. 10, no. 1 (Mar 2020), p. 20-29
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- Description: Generation of negative excess pore-water pressure (NEPWP) due to the excavation of saturated soils under undrained conditions and the dissipation that follows over time may result in different short- and long-term slope instability. The NEPWP generated due to excavation gradually decreases towards equilibrium or, in some cases, steady seepage. Hence, total pore-water pressures immediately after excavation are lower than the ultimate equilibrium values, leading to a reduction of the average effective stresses in the slope and subsequently threatening stability in the long term. In this research study, the stability of three benchmark civil and mining excavations has been studied, considering the effects of the generation and dissipation of NEPWP. A series of numerical simulations are conducted to determine the role of in situ stresses and time in NEPWP dissipation as well as the consequent effects on the stability of the excavated slopes. To conduct a realistic time-dependent transient analysis, fully coupled hydro-geomechanical formulation has been employed. Results show that in general, higher removal of stress levels lead to higher NEPWP generation and higher factor of safety values in the short term. Thereafter, the dissipation of NEPWP threatens the long-term stability of the excavation.