The Sogn–Ulven Tunnel Project
Together with AF Gruppen (contractor), Statnett is currently building a tunnel for new cable interconnectors between Sogn and Ulven substations in Oslo. NGI and Norconsult are responsible for daily tunnel follow-up and engineering geological assessments during the construction phase. NGI holds the technical responsibility for pre-grouting and rock support.
Tunnel Section and Status as of February 7, 2025 (Statnett and Norconsult, see source reference at the bottom of the page) ( )
Mapping (Rock Mass Classification and Rock Support)
Engineering geological mapping of rock mass quality (using the Q-system) and the required extent of rock support is carried out and documented on tablets using the software Bever Mapping. This system also provides access to relevant MWD data (Measuring While Drilling), supporting decision-making at the tunnel face. Installed rock bolts are also logged in the same system for documentation, inspection, and approval.
The rock overburden along the tunnel varies from approximately 5 to 80 meters. There are several significant weakness zones and varying rock mass quality, meaning that in some cases Reinforced Ribs of Shotcrete (RRS) are necessary for rock support, in addition to fibre-reinforced shotcrete and bolts.
Mapping at the tunnel face using a tablet.
From Bever Mapping. MWD data from long-hole drilling, where red areas indicate interpreted weakness zones.
Installed rock bolts. Metadata such as length, bolt type, and diameter are recorded for each individual bolt. Here we see bolts three meter long (red), 2.4 meter (black) and four meter (purple).
Pre-grouting Under Varying Ground Conditions
The tunnel location is in an urban area containing thick clay deposits and a high potential for damage to buildings and other infrastructure. This results in extremely strict water tightness requirements. At its strictest, the requirement for groundwater leakage into the tunnel is limited to 2 liters / minute / 100 meters of tunnel. This calls for extensive and time consuming pre-grouting efforts. Before each pre-grouting fan, the engineering geologists prepare a detailed procedure for how it should be carried out.
The rock types in which the tunnel is being excavated consists of Cambro-Silurian sedimentary rocks with occurrences of magmatic intrusions (formed through volcanism), typical for the Oslo region. The varying geology often leads to significant differences in hydraulic conductivity within the rock mass addressed by one pre-grouting fan. As seen in many previous projects, achieving sufficient watertightness in such conditions has proven to be difficult. In these cases, divided/segmented grouting has generally proven to be effective, both by increasing the number of pre-grouting fans from the same tunnel face, and by using colloidal silica after pre-grouting with cement, for sealing very fine joints.
Setup for pre-grouting work.
