Investigation and assessment of pre-grouted rock mass

Abstract Pre-grouting is a technique for reducing water ingress into tunnels and caverns by grouting fractures and joints prior to excavation. This study investigates pre-grouted rock mass to evaluate grout penetration in fractures and transmissivity of water in the rock mass surrounding the built tunnel, with the use for core drilling, OTV, high-precision water injection tests and core logging. The study was performed in three tunnel localities, in tunnels excavated in connection with the Follo Line project in Norway, where pre-grouting was performed using cement-based grouts. It was found less cement than expected in fractures with small apertures, compared with results of grout penetrability in laboratory studies of similar grouts. Further, it was found that fractures in coarse-grained rock types had rougher fracture surfaces and higher hydraulic apertures, than fractures in fine-grained rock types. It was also found that fractures with smoother surfaces had smaller hydraulic apertures in general. Hydraulic jacking was evidenced during the pre-grouting in this area, which is likely to have contributed to unnecessary high grout consumption.