Permanent tunnel support with reinforced shotcrete arches and forepoling in over-consolidated till and weathered rock. A case study based on the construction of the Bergås tunnel, Norway

Abstract

Ground support in tunnels excavated through weak ground conditions in Norway has normally consisted of reinforced shotcrete methods. However, in cases where exceptionally poor ground or soil tunnel conditions were expected, final support consisted of cast-in-place solutions designed in advance. Although reinforced shotcrete methods have proved satisfactory for a wide range of weak ground conditions, the adequacy of such support method solely to act as the final, permanent support and fully substitute cast concrete liner in soil tunnels has not been realized until the recent construction of the Bergås highway tunnel in northern Norway. In this tunnel, an unexpected portion of 70 linear m of the tunnel consisted of sedimentary deposits of glacial origin (tills) and a weathered, soil-like rock mass. In the present study, the ground behavior and the performance of reinforced shotcrete arches as permanent, final support in this soil portion are investigated. The findings suggest that the compact and over-consolidated tills pose favorable excavation conditions as evidenced by the monitored ground stability under full face excavation passes, and the low support loading during the advance through the soil zone. As such, a pragmatic design considering a permanent functional and stable ground support made of reinforced arches in the soil section was possible from the combination of the elements of the Scandinavian hard rock tunnelling with the principles of weak ground engineering, and pre-stabilization methods with grouted forepoling. The permanent tunnel support finally consisted of ribs of reinforced shotcrete (RRS), fiber reinforced shotcrete (FRS) and fully grouted steel rebar rock bolts, which substituted the pre-determined cast concrete lining that is traditionally used in soft tunnelling conditions. The approach is therefore innovative as it considers a cost-effective alternative with significantly lower concrete consumption, on the basis of a shotcrete-based support solution designed from a combination of elaborated design work, monitoring, ground characterization, and advanced numerical analyses.