About 300 m of tunnels have been constructed in 2012 at ca 400 m depth to expand SKB’s Hard Rock Underground Laboratory. The main design constraints for the project were: (1) the hydraulic head above the tunnel must be maintained to at least 350 m at the end of construction, and (2) as little grouting as possible should be used in order to deliver undisturbed conditions for some experiments that will take place in some of the excavated tunnels. The most practical means to meet the first constraint was to plan for tunnel pre-grouting as part of the excavation cycle. To meet the second constraint the spread of the grout must be controlled by optimizing the grouting strategy. This paper describes how the project met these constraints by: characterizing the fracture flow system prior to start of excavation work, establishing correlations between predicted hydraulic aperture and grout takes; and conducting the design and execution of the grouting methodology using the Observational Method.

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