An extensive programme of instrumentation and monitoring was carried out during construction of the Ingula hydro power caverns given the size of these excavations in Karoo mudrocks, for validation of assumptions made during design. In this paper, some of the monitoring results are compared to predicted convergence and an incident is reviewed where excessive overbreak in one of the side headings of the machine hall top heading resulted, locally, in undue roof deformation which necessitated remedial action to stabilize this part of the crown.