A fibre optic monitoring system has been installed in two limestone quarries in Valkenburg aan de Geul, The Netherlands, to monitor pillar stability and to give early warning of locations at risk of collapse. The system is highly sensitive and can detect creep deformation of the pillars as small as 1 micrometer. Measurements are displayed via a web-based data portal and alerts are triggered if the creep velocity exceeds pre-determined thresholds or if creep accelerates.
We present the design of the monitoring system and results from the first three years of operation. We show examples of the natural behaviour of the pillars and we present a number of deviating patterns that sparked particular interest. We also share some lessons learnt that can be used when designing and installing similar systems in other quarries, caves or mines.
In the Dutch and Belgian provinces of Limburg the Maastrichtian bedrock contains some layers of calcarenite which are virtually free of flint. These layers have been mined by room and pillar methods since the Middle Ages for the production of building stone. The quarries are situated at depths of 50 m at most and range in size from a few galleries to labyrinths of 85 ha. The now abandoned quarries are of great historical and cultural value, often containing paintings, drawings and sculptures. Their touristic exploitation (at least 500 000 visitors per year) is economically important for the region.
In many of the quarries, pillars have become fractured and unstable, finally resulting in several large-scale pillar collapses extending over up to 8 hectares (e.g. Bekendam 1998, Bekendam 2020). Pillar shortening due to creep deformation plays an important role and is still going on. The hazards of falling rock and air blasts inside the quarries are particularly serious for the mines with touristic exploitation. In addition, several quarries are located below buildings, roads and other surface structures, which would potentially be catastrophically damaged by surface subsidence.
To monitor the safety of the Gemeentegroeve (40 ha) and the Sibbergroeve (85 ha), near Valkenburg aan de Geul, a fibre optic monitoring system was installed in the autumn of 2019 in areas of deteriorated pillars. The Gemeentegroeve in particular is a popular tourist attraction, with guided tours throughout the year and an annual Christmas market attracting more than 100,000 visitors. Both quarries are also frequented by bat environmentalists or (mining) historians. The purpose of the monitoring system is to provide continuous, real-time measurements of shortening of the deteriorated pillars. The system provides automatic early warning of pillars at risk of collapse, so that for example reinforcements can be provided in time.
