The generalized procedure for the calculation of the pressure drop along tunnels is by using Atkinson’s equation. The friction factor in Atkinson’s equation is determined from measured or computed values of airway dimensions and tunnel interior finishing according to mining method and ground support type. This paper re-visits the friction factor according to Colebrook´s relationship and the Darcy-Weisbach equation, which are widely used in mechanical engineering. This method of calculating the friction has the advantage of a more reliable determination since the size of the airway is implicit in the method and only a representative absolute roughness has to be selected to determine the friction factor. Measurements of pressure drop performed at Codelco’s El Teniente mine, Chile, permitted the determination of the absolute roughness for different tunnel sizes and wall finishing.
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