In this paper, the results of permeability test obtained in two large dam foundations in weak and impervious rock, Tepoa Dam (Mexico) and Ponga Dam (Spain), are analysed, and the use of the Low Pressure Test to determine the permeability in terms of real pressure and the use of Hydraulic Monitoring or seepage control by means of a drainage courtain for this kind of foundations, is proposed.
In diesem Artikel werden Permeabilitat Testen von zwei verschiedenen Talsperren mit sanfte und undurchlassige Felsfundamente, Tepoa (Mexico) und Ponga (Spanien), analysiert. Der LPT, um die Permeabilitat mit der Anwendung von dem realen Druck zu messen und die Filtrations Kontrolle durch einen Dranageschirm werden fuer diese Fundamente vorgeschlagen.
Dans cette communication, ils sont analyse les resultat de la realisation de les essais de permeabilite realise dans la fondation sur roches meubles de deux grandes barrages, le Barrage de Tepoa (Mexico) et Ie Barrage de Ponga (Espagne). Nous proposons l'utilisation de l'essai LPT, Low Pressure Test, pour la determination de la permeabilite, et l'Auscultation Hydraulique pour le control de les filtrations dans cette classe de fondations meubles.
In the XVII ICOLD, (Vienna 1991. Q.66) and in the 7th IRSM Congress, (Aachen 1991. W3), we have done the definition of the characteristics of the Low Pressure Test or LPT Test, after one consideration: "In weak and impervious rock foundations, the conventional Lugeon Test, WPT Test, are ill suited for the definition of permeability, as they produce hydraulic fracturing in the test zone". Furthermore, when hydraulic fracturing occurs, the results of the test they must be interpreted in terms of the Total Pressures, taking into account the influence of the depth of the section tested, the water level depth and the head loss along the pipes. The Real or Pressure Pc, determinated expression: Total critical it could be by means the Pc = Pcm + Ph - Pw - Pr Where: Pc is the Critical Pressure in the test zone. Pcm is the Critical Pressure measured in the surface manometer. Ph is the Hydrostatic Pressure function of the depth of the test zone. PW is the Pressure due to the water level depth. Pr is the head loss along the pipes. Taking into account these considerations, the Low Pressure Test, LPT Test, is specified as follows:
Three metres for the test section length.
Pressure level sequence between 1 and 5 bar.
Five minutes are enough for load time for every pressure level. with these specifications, under conditions of hydraulic fracturing, the Critic Pressure will be defining and the corresponding flow will be the real permeability in the test zone, called critic Permeability Unit or ULC.
In the Ponga Dam foundation, North of Spain, we have had the first experience with the use of the low pressure tests for the determination of the permeability of weak and pervious rock foundations. The dam foundation is made of a sequence of Carboniferous slates and sandstones, with the lamination or cleavage as the main structural discontinuity. The realisation of 11 conventional WPT Test preceded by a low pressure test in the same section, they confirmed the capacity of resolution of the low pressure test to detect the hydraulic fracturing phenomena. Consecutive Test consist in a low pressure test, followed by a conventional WPT Test in the same section, and with the next specifications:
Five metres for the tests section length.
Pressure levels in the low pressure test: 0–1-2–5-2 bar.
Ten minutes for each pressure level.