A reanalysis of the collapse of the Heidegroeve: subsidence over an abandoned room and pillar mine due to previously unknown mine workings underneath

In the region of Maastricht, both in the Netherlands and in Belgium, about 400 room and pillar mines have been excavated in weak Upper-Cretaceous limestones. Pillar instability has resulted in a number of large-scale collapses and serious surface subsidence with faulting and sinkhole formation. The Heidegroeve used to be a very stable mine for more than 50 years, until pillars started to fracture and spall unexpectedly in the summer of 1987. The collapse of the abandoned mine occurred in June 1988, and was initially detected when faults and sinkholes had formed at the surface. Originally it was postulated that just creep deformation inside this mine was the main cause. However, a stability analysis revealed that all pillars inside the collapse area showed sufficient safety factors and should still be intact, while the weakest part, with several pillars of insufficient strength, had been fractured but is still standing. In the vicinity of the collapse area mines have been excavated at a lower level. Therefore it was postulated that the collapse of the Heidegroeve was related to an unknown and inaccessible continuation of these mine workings underneath. Indeed, recent, rather adventurous fieldwork revealed a downward collapse-induced fault giving access to open galleries and collapse structures about 3.5 m below the Heidegroeve. Inside the collapse area of the Heidegroeve itself, accessible through openings between the debris fragments, severe tilting of gallery floors was observed, which was probably brought about by punching of pillars of the lower mine. This case study with an unusual result shows that great care must always be taken in the analysis of the stability of mines and the assessment of the risk of surface subsidence.