Experimental and Numerical Determination of the Head Loss of a Pressure Driven Flow through an Unlined Rock-Blasted Tunnel

The friction loss in a part of the rock-blasted unlined tunnel of the Litjfossen hydropowerplant in Norway was determined from experimental and numerical studies. Remote sensing datafrom the prototype tunnel provided the input data for both the numerical model and the constructionof a 1:15 scale model with an innovative milling approach. The numerical simulations were based onthe solution of the Reynolds-averaged Navier–Stokes equations using the CFD program OpenFoam.Head loss measurements in the scale model were carried out by means of pressure measurements fora range of discharges and were compared against the results of the numerical model. The measureddata were used to determine the Darcy–Weisbach and Manning friction factors of the investigatedtunnel reach. The high-resolution remote sensing data were also used to test the applicability ofexisting approaches to determine the friction factor in unlined rock blasted tunnels. The results ofthe study show the usefulness of the chosen hybrid approach of experimental investigations andnumerical simulations and that existing approaches for the determination of head losses in unlinedtunnels need to be further refined.