Experiences from the planning,construction and supporting of a road tunnel subjected to heavy rockbursting

Summary A 5360 m long road tunnel along a fjord with mountains rising steeply up to 1500 meters above sea level has recently been completed in Norway. 3 km of the tunnel was subjected to rockbursting caused by high and anisotropic stresses in the Precambrian gneisses. During the construction it was necessary to install 15 000 rock-bolts. Steelfibre reinforced shotcrete was successfylly applied for the first time to help stabilize the spalling rock. Supporting measures were only needed for the half of the tunnel closest to the fjord. 2600 m³ shotcrete was used.The paper outlines the preinvestigations and the design approach used for tunnels subjected to high and anisotropic stresses, and gives information about the construction and the support works carried out. A tunnel mapping system is described together with a classification of rockbursting activity in tunnels. Stresses in the rock were calculated by the use of finite element models and measured with doorstopper cells and the CSIR three-dimensional gauge. Comparisons of calculated and measured stresses are carried out.Rockbursting activity as observed along the tunnel is related to the estimated and observed stresses as well as to the properties of the rocks. Simple classification systems are used to correlate the results. Experiences from the use of the support are given.