Norwegian ground conditions with hard bedrock underlying soft, marine clay deposits are challenging with respect to the risk of settlements caused by water ingress to tunnels. Settlements in clay-filled depressions and damage to nearby buildings are one of the main risks associated with future upgrading of infrastructure. This paper presents a database and findings from 44 tunnels in the Oslo-region, excavated between 1975 and 2020. One of the main findings is that few of the tunnels in the database meet the strict leakage limits necessary to avoid settlements for future tunnel projects. Previously, water ingress of typically 3-7 l/min/100 m has been allowed to limit pore pressure reduction to 10-30 kPa (1-3 m water head), limiting settlements and building damage. In areas where the pore pressures already have been affected by water ingress to an existing tunnel, any additional leakage and pore pressure decrease due to a new tunnel will cause additional settlements. Previous reduction in pore pressure results in an even stricter water ingress limits for new tunnels. Hence, water control through improved preexcavation grouting (PEG) techniques and better monitoring of pore pressure development during excavation is needed. To ensure necessary data-collection for future research it is important to increase the quality of collected data.
KEYWORDS
Urban tunnelling; Water ingress; Pore pressure; Settlements; Pre-excavation grouting.
