Over the last decade, large rock caverns have been developed for storing vast volumes of hydrocarbons and in particular crude oil. In this context, large rock caverns have light support made of grouted steel or fibreglass rock bolts. Reinforcing cables are strictly prohibited to avoid leak paths for the product. Therefore, the risk of mega-wedge occurrence is high since the size of the potential mega-wedges is proportional to the cavern size. Some geometrical situations are more detrimental than others and the possibility of a local or extensive collapse be very high, including in rock masses which can be ranked as good and above. Such an apparent paradox could have been detrimental to the concept of large caverns because mega-wedges are extremely difficult to stabilize once discovered because rock bolts are generally too short to stabilize them. Anticipation is therefore the key. First, we clarify the term mega-wedge and then, analyse the various possibilities of occurrence in large caverns, as well as classical tunnels. Guidelines are proposed to identify whether the conditions are met for experiencing mega-wedge failure at hand. Two main geological structures, shear fractures and smooth-persistent-planar-spaced (SPPS) joints, are favourable for mega-wedge formation. These two fracture types are analysed and criteria are given to ensure a quick and efficient determination procedure. The field approach is synthesised by a decision chart, to be used at site, during the excavation works.
Large caverns do exist for several decades, essentially in dam engineering for placing generators. Large underground caverns are also required for storing hydrocarbons. They look very similar in shape but their design is very different [1, 2, 3]. The most important difference is the way to achieve a stable excavation: hydropower caverns can benefit, if required by the rock mass conditions, of heavy support and reinforcement, using cable bolt and concrete lining for example, which is not the case for hydrocarbon rock caverns that remain unlined. This design is imposed by the hydrodynamic containment principle and the avoidance of gas migration along the cables which represents a potential risk for the tightness of these caverns [1]. Thus, hydrocarbon caverns use only rather small rock bolts (4 to 6 metres typically) which, in addition, are fully cemented, avoiding any gas migration.