Roofing slate was traditionally quarried in Spain for roofing purposes since the Middle Ages. 40 years ago, and due to an increasing market demand from European countries, convenient prices and depletion of old quarries, producers envisaged the underground exploitation of these deposits. Nowadays, there are some underground room and pillar roofing slate mines operating in NW-Spain, and some projects under assessment. In this study, we analyze, from a geomechanical point of view, a wedge failure in a large 50-m high and 30-m wide room in one of these mines. Most of these rooms were observed to be stable in the mines, but for a particular case of a not so deep room, a relevant structural wedge instability problem took place. It was the case of a shallow room with its longitudinal axis normal to a river valley. The geomechanical features of slate rock mass and their discontinuities are briefly presented to explain the reasons behind this failure in relation to previous rock mechanics studies. With the idea of learning from errors, some considerations are finally discussed regarding rock mass characterization and design decisions for this type of applications.

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