Dual stage pre-excavation grouting methodology with a completely dry tunnel as target. Experiences with an innovative approach during the construction of the Hestnes railroad tunnel, Norway

The construction of the Hestnes railroad tunnel in southern Norway, used an innovative pre-excavation grouting (PEG) methodology to achieve a functionally dry tunnel surface of the rock support lining. The final tunnel design was reconsidered, omitting the originally planned cast-in-place concrete lining with a final shotcrete lining. This enabled the achieved result of the PEG works to act as the permanent measure for water ingress control. A dual stage PEG methodology was used. The first grouting stage was based on microcement and targeted a homogenisation of the rock mass. The second stage used a low-viscous mineral based grout, colloidal silica, which successfully enabled the sealing of the finest joints. After completing the excavation of the tunnel, the larger portion of the tunnel was sealed using two grouting rounds only. Some portions required some post-grouting. A reduction the total investment cost and a significant reduction of the CO₂-footprint was achieved.