Les fuites des galeries en charge en terrain sec. Rôle de revêtement, des injections, du terrain

Dry-ground leakage from power tunnels. Respective effects of the tunnel lining, grouting operations and ground features

Directeur de l'Ecole Nationale Supérieure d'Hydraulique de Grenoble

Abstract

Author's report is the latest in a series of research reports published in La Houille Blanche over a period of more than ten years in collaboration with other authors on the subject of unlined power tunnel equilibrium, among which notably on the theoretical and experimental research carried out on the unlined shaft at the Capivari power plant, which operates under a world record static pressure of 450 metres. In the most frequent cases, and as a general rule whenever possible, power tunnels are installed at sufficient depth underground to ensure that the groundwater pressure exceeds the tunnel pressure so that, if anything, the tunnel receives water from its environment when in service. Tunnels in a permeable medium not containing pressurized groundwater are a different case, however, as leakage is the only form of exchange that is liable to take place. This is the subject of Author's report, in which he attempts to calculate the possible leakage rates and establish how leakage can best be reduced in the first part of the report, the theoretical characteristics of flow originating in the assumed homogeneous and isotropic rock medium as a result of tunnel pressure are determined and several leakage rate formulae derived, which allow for permeability of the surrounding medium and the presence or absence of tunnel lining. Next, the forces causing leakage head loss are investigated, and it is shown that the resulting deformation of the tunnel lining will eventually crack it, thus considerably impairing its effectiveness in limiting losses (although this might be avoided by a very heavy lining, Author considers this to be a costly and somewhat doubtful solution). The only thing left, therefore, is to rely on ground impermeability to provide an effective barrier. This can be improved by grouting, but the effectiveness of present methods is restricted to fairly definite limits associated with operational difficulties or deformation of the ground, even after grouting. As a final conclusion, therefore, ideal impermeability of the considered type of structure would appear to be unattainable, except if there were a strict ruling that power tunnels only be sited in areas known to contain a rock aquifer, i.e. beyond the stope of the present study.