Analysis of water ingress, grouting effort, and pore pressure reduction caused by hard rock tunnels in the Oslo region

Abstract

Many subsurface projects in Scandinavia face challenges with settlement risk from water ingress into tunnels, due to challenging ground conditions featuring hard bedrock underlying soft, marine clay deposits. Pore pressure reduction in these clay-filled depressions can cause damage to nearby buildings and is one of the main risks associated with infrastructure development in Oslo. This paper presents an extensive database from 44 tunnels in the Oslo region, constructed between 1975 and 2020. The data consists of measured water ingress after pre-excavation grouting, reduction in pore pressure, pre-excavation grouting effort, and geological parameters. The data is analysed to identify trends and relations between key parameters, such as expected pore pressure reduction for a given water ingress rate and necessary grouting effort to obtain a given hydraulic conductivity of the grouted zone. The analysis shows that it is necessary to focus on pore pressure monitoring in future projects, rather than water ingress, to reduce the risk of unacceptable pore pressure reduction. Suggestions are given on how to optimise monitoring and follow-up of pre-excavation grouting to ensure that required watertightness is met.