Reduction of Head Losses in Tunnels by Means of Shotcrete

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by Laes Hammar,

Document Type: Proceeding Paper

Part of: Shotcrete for Underground Support V:

Abstract: Head losses in blasted rock tunnels conveying water to hydropower stations often significantly reduces the energy production in the plant. Typical values on the head losses in tunnels are 1-2 m/km tunnel length. The head loss in a tunnel is often described by the Darcy-Weisbach's formula which constitutes a relationship between the loss, a friction coefficient, a discharge and the hydraulic geometry of the tunnel. The friction coefficient is dependent on the values of two dimensionless variables, namely the Reynoldsnumber and the relative roughness of the tunnel. The application of shotcrete in tunnels is usually justified by reinforcement demands on the rock. However, if the application is performed in a hydraulically beneficial way the reduction of the head loss may also be substantial. The shotcrete must, however, be applied in a way that reduces the friction coefficient, i.e. the relative roughness, more than the impact reduced cross sectional areas has on the head loss. As the relation between head loss and cross sectional area is proportional to the power of 2.5, i.e. D5, an actual reduction of the head loss is not self-evident. The paper presents the results and a discussion around a shotcrete application, with the sole purpose of reducing the head loss, in the headrace tunnel to Sillre hydropower station located on a small tributary to the river Indalsalven in the middle of Sweden.

Subject Headings: Tunnels | Head loss (fluid mechanics) | Shotcrete | Hydro power | Power plants | Friction | Rocks | Europe | Sweden

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