The stability of portals in rock

Portals are frequently an exceedingly difficult area in terms of ground control due to the near-surface, weathered, and highly discontinuous rock mass conditions. Surface and subsurface failures involving portals were analyzed using over 500 case histories which were organized into a database. Critical factors contributing to both stability and instability were isolated, and failures were classified according to location. Correlations between rock mass classes and types of portal failure were made and a four step stability analysis methodology defined. To determine critical sections of portal approach cuts for stability analysis, the Geomechanics Classification System was appended with discontinuity orientation adjustments. The most common type of failure for active portals, that of 'Crown Face Overbreak' failure, was investigated and modelled for design and support purposes. Results are confirmed using case study data. Excavation and support guidelines, based on database information the predicted failure zone from the 'Crown Face Overbreak' model are provided.