Frost heave in highway tunnels in Nordland County, Norway

Abstract

Two highway tunnels opened in the years 2005-2006 in Nordland County, Norway has suffered from frost heave and formation of lumps of ice on the road surface up to 3 kilometres into the tunnels. Norwegian Public Roads Administrations (NPRA) started an investigation program to find the reasons for this. This program included sampling of materials in the road structure, measurements of frost indexes and frost penetration into the tunnels, registration of the drainage system and studies of the construction plans and performance of the construction. These investigations showed that errors where made both in planning and construction of the tunnels leading to the problems. The reasons for the frost heaves were mainly that the frost affected areas was larger than estimated partly because of forced ventilation in one direction, inadequate drainage system because it was placed too shallow compared to the formation rock-bed, inadequate cleaning of the frost susceptible fines in the tray after blasting of the mica-rich bedrock and slightly frost susceptible materials in the sub-base where brought in. These tunnels are now being rehabilitated based on these findings by replacing the all the frost susceptible soil and placing of a new drainage system below the level of the formation rock-bed while also ensuring frost protection of the pipes by using XPS-plates. New tunnels in the northern region are now being built more robust with longer frost zones, cleaning of the fines in the tray and deeper drainage systems within the frost zones.