The hydraulic roughness of unlined and shotcreted TBM-bored tunnels in volcanic rock: In situ observations and measurements at Kárahnjúkar Iceland

The hydraulic roughness of igneous rock in unlined and shotcreted TBM-bored tunnels is presented in the paper. This is derived from in situ observations and direct measurements of tunnel wall roughness made during the construction of the Kárahnjúkar Hydroelectric Project in Iceland, to reliably estimate the headlosses of the 39 km long headrace tunnel. The tunnel is to a large extent excavated by TBM and is mostly unlined. The detailed assessment of the wall roughness combined systematic and comprehensively recorded visual inspections of the tunnels and accurate laser scans of a representative fraction of the inspected areas. The measurement program has shown that surface roughness of TBM-bored tunnels through volcanic rock such as basalt, is in general larger and the tunnel walls are more heavily jointed than in sedimentary rocks and coarse grained igneous rocks, such as granite. A verification of the methodology used to determine hydraulic friction factors was obtained by direct measurements of actual headlosses in the headrace tunnel system. These have shown an overall agreement with the methodology adopted for the project.