Golden CO; USA IMWA 2013 “Reliable Mine Water Technology” Wolkersdorfer, Brown  Figueroa (Editors) 995 Introduction In February 2009, important ground subsi- dence suddenly occurred over an area of a di- ameter of about 50 m in the town of Saint- Vaast, near the city of La Louvière (Belgium; Fig. 1). Subsidence caused important cracks to several houses, to the sidewalks and to the road surface. A couple of days earlier, a torrent of mud erupted from the ground in a home garden near the Haine river, about 600 m south of the subsided area. It was rapidly found that an underground drainage tunnel ran at a depth of about 30 m under the sub- sided area and had its exit located near the eruption point of the mud torrent. Historic documents report that the con- struction of the drainage tunnel started around 1747 and lasted for more than a cen- tury. Tunnel construction was heavily compli- cated as it had to cross wealden-facies sedi- ments containing numerous pockets of quicksands. As tunnel construction progressed and reached such pockets, quicksands ofen burst into the tunnel, causing human losses and systematically flling the tunnel over sig- nifcant distances. Eventually, tunnel construc- tion was abandoned before it was connected to the mine works and used for drainage. The likely scenario of the Saint-Vaast acci- dent is that the drainage tunnel remained in- tact and water-flled over signifcant length portions but was plugged with debris materi- als near its exit. Water pressure acting on the plug eventually caused its failure, leading to a rapid emptying of the tunnel. This caused the collapse of the tunnel at one or several weak points, allowing the quicksands to fow into the tunnel and move towards its exit. The residual voids in the ground caused in turn surface subsidence. The goal of the study is to improve the un- derstanding of the causes of the collapse and, in particular, identify a potential accidental hy- drogeological trigger to the increase in water pressure suspected in the likely scenario. The accident occurred in a region where natural groundwater fows are potentially afected by two important hydrogeological artifacts: the drawdowns at the boat lif of Strépy-Thieu and the groundwater rebound in the coal mine of Non-conclusive time series analyses – what can we learn on the behavior of the groundwater system? Understanding what caused the collapse of an abandoned drainage tunnel Christophe Frippiat¹, Mathieu Veschkens¹, Daniel Pacyna², Luc Funcken³ ¹Institut Scientifque de Service Public, rue du Chéra 200, B-4000 Liège, Belgium, ch.frippiat@issep.be ²Service Public de Wallonie, Avenue Prince de Liège 15, B-5100 Jambes, Belgium ³Service Public de Wallonie, rue Côte d’Or 253, B-4000 Liège, Belgium Abstract Time series analysis was used as an attempt to identify a potential accidental hydro- geological trigger to the collapse of an abandoned mine drainage tunnel in Belgium. Correlation, spectral and wavelet analyses were applied to piezometric data in order to characterize regional- scale hydraulic connections between groundwater bodies, but failed to provide direct insight into regional hydrogeology. No correlation between data collected in the hydrogeological unit of the collapsed area could be identifed, showing that the piezometers were tapping isolated sand lenses. As a conclusion, the collapse was rather attributed to a slow increase in water pres- sure linked to a limited recharge. Keywords correlation analysis, spectral analysis, wavelet analysis, barometric efficiency