A Critical View on the Recommendation of Ribs of Reinforced Sprayed Concrete Support in the Q-System

Ribs of reinforced sprayed concrete (RRS) is a rock support measure used in weak rock mass and other challenging rock mass conditions where support that exceeds bolts and shotcrete is needed. RRS is typically constructed using four to ten rebars in one or two layers mounted on rock bolts along the tunnel profile and covered with sprayed concrete. In Norway, the tradition of using RRS started in the 1980s by experience based, on-site dimensioning as a support measure to avoid cast concrete constructions. Since then, RRS has completely replaced the use of cast concrete as a support measure in Norway. In 2002, the Q-system had its second update, where RRS dimensioning was incorporated into the Q-system support chart. This update introduced the arched load-bearing RRS, rather than the unarched rock reinforcing structure following the uneven tunnel profile that had been the common design up to then. This article reviews the technical basis for the calculations in the update, including the Q-system’s ability to be a technical assumption for the dimensioning of load-bearing support, and concludes that a further revision of the Q-system is needed. Measurements and parameter analysis performed in later years have suggested that RRS is over-dimensioned in most cases. In addition, a few RRS collapses suggest that RRS has weaknesses in its actual supportive design and may, in some cases, be under-dimensioned. A discussion on these matters is important both from a technical point of view and in terms of an economical/environmental aspect, as steel and concrete volumes can be reduced using leaner rock reinforcement than arched RRS.