Ground Deformations Related to an Old Drainage Adit in The Abandoned Coal Concession Around Saint-Vaast (Wallonia, Belgium) Analysed Using PS-InSAR and Piezometric Wells Time Series

The exploitations of the coal mines in Wallonia were ceased progressively till the end of the last century. Groundwater pumping’s were stopped inducing a groundwater rebound with different rates but still visible today with radar interferometry. To mitigate the threats to the environment during the post-mining period, flooding and outbreak risks, surface ground deformations and the stability of civil engineering infrastructures in the vicinity of those mined areas need to be monitored. The persistent scatterer radar interferometry technique (PS-InSAR) has revealed ground displacements during three decades in Saint-Vaast (Wallonia, Belgium). Under this residential area, an old drainage adit at a depth of approximately 30 m allows mine water discharge to the Haine river. A confined aquifer lies in this Wealdian sandy layer characterized by a weak mechanical resistance and frequent loose sandy pockets. The Saint-Vaast area has so far suffered twice from a clogging rupture in the drainage adit. The clog has induced an underground water pressure increase in the confined aquifer. This water outbreak resulted in a stream of mud, sand and water flowing out during a few days in 2009. This event has induced a displacement at the surface with cracks observed in the walls of several houses nearby. 236 interferograms were produced using images acquired by the ESA’s satellites (ERS1/2, ENVISAT and Sentinel-1A). A time serie of surface displacements from 1991 to 2022 is calculated. Land surface displacements trends before and after both events were recognized. During ERS1/2 (1992–2002) an uplift has a rate exceeding 3 mm/year. The saturation of the Wealdian aquifer sands reduces the effective intergranular stress leading to an uplift. Groundwater recharge slowed down after the uplift and turned to almost stability during ERS (2002-2006) and ENVISAT (2003–2010). The sudden water outbreak in 2009 has resulted in a localized land subsidence. Before and after the second outflow associated to a clog rupture in early 2018, a similar pattern of land surface deformations was observed. After 2018 (Sentinel-1A), the average LOS velocity values are -1.8 mm/year where the 2009 event occured. Piezometric wells drilled after the 2009 event are monitoring the groundwater levels in the Wealdian aquifer. Analysis of piezometric data combined with PS-InSAR allowed to compare the water table evolutions in the confined aquifer and the ground deformations observed at the surface.

Abstract of an Oral Presentation or a Poster

Belgium, InSAR, (15) géologie