ABSTRACT: Weak and slaty rocks are susceptible to hydraulic fracturing in both testing and grouting. In order to determine the critical pressure which initiates fracturing it is required not only to measure the pressure at the top of the borehole but to determine the pressure being effective in the section tested. At Albarellos Dam (NiW Spain),it was difficult to establish a direct relationship between permeability and groutability. The example of the grouting programme realized at Albarellos Dam shows that grouting pressures should be kept below critical pressures.
RESUME: L"analyse de la permeabilite de la fondation de la barrage d"Albarellos, on est reali se avec l"evaluation du pressions reels au moyen d"une programme qu"il voiloi toutes les factures, depuis la situation de la nappe pheatique jusqu"a la perte de lespressions par la friction dans le sondage. Finalement, il est. tress difficile de trouver une relation direc parmi la permeabilite et la injectabilite, lorsque est In schistosite la discontinuite principal du massif rocheux.
ZUSAMMENFASSUNG: Wenn das Gebirge aus weichem und geschiefertem Gestein besteht, kommt es wahrend der Versuche und der Injektionen haufig zum Aufreiβen des Gebirges.Um den kritischen - Druck zu bestimmen, bei dem das Gebirge aufreiβt, ist es erforderlich, nicht nur den Druck am Bohrlochkopf zu messen sondern den effektiven Versuchsdruck. Das Beispiel der Albarellos Tals perre durchgefuehrten Injektionen fuehrt zoo deer Schluβfolgerung, daβ man verpreβdruecke anwenden sollte, die unter dem kritischen Druck liegen. Endlich, die Bestimmung der lugeon-Einheit im gekluefteten Festgestein ist nur möglich, wenn man den Wasserdruckversuch zur Bestimmung der Gebirgsdurchlassigkeit in Ubereinstimmung den Normen durchfuehrt, die M.Lugeon 1933 aufgest hat.
1 INTRODUCTION The geology of Albarellos Arch Dam foundation, Orense (NW Spain), consist in a sequence of gneisic granites and gneis with a important development of the cleavage, as the main structural discontinuity. Geologically, that sequence is included in the Centro-Iberica Zone of the Hercinic Mountains. Photo.1. Of the results of permeability tests realized in the foundations of some Spanish Dams, Foyo, A.& Cerda,J. (1990), we conclude that the analysis of foundations characteristics has to be done by using the "Total Test Pressures". These "Total Test Pressures" are evaluated by means of the INJEK Program, developed in the Paderborn University, Germany Ewert F.K.(1985). The program takes into account all factors, from water table level to the pressure loss due to friction. Before of the grouting treatment of Albarellos Dam foundation, some traditional Lugeon Permeability Tests were realized. The results, analyzed by means of the INJEK Program, permitted the determination of the Critical Permeability, defined as water absorption at Critical Pressure. This Critical Pressure shows the presence of Hydraulic Fracturing during the Lugeon Permeability Test. Fig. 1.
2 ANALYSIS OF GROUTING TREATMENT The grout curtain of Albarellos Dam was built by means of injection boreholes of 50 meter depth. We investigate the relation between the Critical Permeability, water absorption at Critical Pressure, and the cement absorption, groutability at different pressures. The Albarellos Dam foundations is divided in five zones referring to the permeability and groutability characteristics, with a total of 78 grouting boreholes.
2.1 The socle characteristics. That is the only foundation zone where there is not any Hydraulic Fracturing during the previous permeability test. It"s a zone of very low permeability,Fig2. Also, the groutability or cement absorption capacity could be considered low. The Fig.2 shows that in spite of high Grouting Pressures, the cement absorption was practically nothing.
2.2 Right abutment. Low zone In this foundation zone, there is Hydraulic Fracturing during the previous permeability test until 22.50 metre depth.