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Abstract. In the region of Maastricht, both in the Netherlands andin Belgium, about 400 room and pillar mines have been excavated in weakUpper-Cretaceous limestones. Pillar instability has resulted in a number oflarge-scale collapses and serious surface subsidence with faulting andsinkhole formation. The Heidegroeve used to be a very stable mine for morethan 50 years, until pillars started to fracture and spall unexpectedly inthe summer of 1987. The collapse of the abandoned mine occurred in June1988, and was initially detected when faults and sinkholes had formed at thesurface. Originally it was postulated that just creep deformation inside this minewas the main cause. However, a stability analysis revealed that all pillarsinside the collapse area showed sufficient safety factors and should stillbe intact, while the weakest part, with several pillars of insufficientstrength, had been fractured but is still standing. In the vicinity of thecollapse area mines have been excavated at a lower level. Therefore it waspostulated that the collapse of the Heidegroeve was related to an unknownand inaccessible continuation of these mine workings underneath. Indeed,recent, rather adventurous fieldwork revealed a downward collapse-inducedfault giving access to open galleries and collapse structures about 3.5 mbelow the Heidegroeve. Inside the collapse area of the Heidegroeve itself,accessible through openings between the debris fragments, severe tilting ofgallery floors was observed, which was probably brought about by punching ofpillars of the lower mine. This case study with an unusual result shows that great care must always betaken in the analysis of the stability of mines and the assessment of therisk of surface subsidence.

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