We expose the principles on which is based the failure of anisotropic soils provided with one or two systems of diaclases, analysing the influence of the shear strength of the matrix rock, and the one of the different families of joints. Then we study the plastic equilibrium of a rocky dihedral of this kind when uniformly distributed forces are applied to its surfaces, and its application to dam foundation on stratified rocks. We make an analysis of the concept of safety factor in which strength is a function of two, four or six variables and the criteterium embraced on this respect by the Laboratory of Transport and Soil Mechanics. We make some considerations about the progressive failure and its influence on the election of the strength parameters.


Sont presentes les principes qui gouvernent la rupture des terrains possedant un ou deux systèmes de diaclases, tout en analysant la resistance de la roche matrice et celle des differentes families de diaclases. On etudie tout à l'heure l'equilibre plastique d'un dièdre rocheux de ce type lorsqu'on applique sur ses faces des forces uniformement repandues et l'application à la fondation de barrages sur terrains fissures. On fait aussi l'analyse du concept de coefficient de securite de ces terrains dont la resistance est fonction de deux, quatre ou six variables, et le critère adopte par le Laboratorio del Transporte y Mecanica del Suelo. Quelques considerations sont faites sur la rupture progressive et son influence sur le choix des paramètres de resistance.


Es werden die Grundlagen gegeben im Zusammenhang mit dem Bruch von Gesteinen mit einem oder zwei Kluftsystemen. Der Einfluss der Festigkeit im Muttergestein und in den verschiedenen Kluftsystemen wird untersucht. Weiterhin wird das plastische Gleichgewicht eines Keiles studiert, wenn an dessen Seiten eine gleichförmige Kraft ausgeuebt wird, sowie seine Anwendung bei der Gruendung von Talsperren auf unguenstigen Schichtgesteinen. Es wird eine Untersuchung gemacht ueber den Begriff Sicherheitsfaktor bei Gesteinen, in welchen die Festigkeit eine Funktion von zwei, vier oder sechs Veranderlichen ist. Die Stellungnahme des Laboratorio del Transporte y Mecanica del Suelo zu diesem Problem wird bekanntgegeben. Abschliessend werden einige Betrachtungen ueber den progressiven Bruch und dessen Einfluss in der Wahl der wiederstandsfahigen Parameter gezogen.


The foundation of hydraulic works, and specially the foundation of dams, on rock media can present difficulties when the rock is jointed or when there are special directions, like bedding or schistosity planes, in which the bonds between the particles of the rock mass are weaken. Few problems can occur, from the geotechnical point of view, when the foundation is constituted by a massive rock without faults or other defects. But it is much more frequent that the work has to be founded on a jointed rock. The great variety of these defects in its morphology and strength originates a great diversity of problems. The resistance of a rock media is determined, fundamentally, by these faults. Therefore, during the last years, the Laboratorio del Transporte y Mecanica del Suelo has made a great effort to improve the laboratory and «in situ» test methods for determining the strength of these joints and, on the other hand, the research on mathematical and scale models, which will permit us to foresee with a greater exactness the safety of the work. We have already demonstrated (see ref. 1) the apparent strength variation shown by a diaclased rock when the direction of the major principal stress changes. (See fig. 1). When two joint systems appear in the rock, the theoretical layout obtained is that of fig. 2. As it may be seen, there are very few directions in which the failure appears in the matrix rock. In these cases, the rock strength has very little influence on the behaviour of the whole foundation, and therefore, it is of great interest to determine the shear strength of these faults or these joints, in order to evaluate the stability of the work. The different types of laboratory and «in situ» tests for this purpose have been already described (see ref. 1, 2 and 3). Now, we will only refer to the theoretical research of stability of dams founded on these kind of soils.

Safety factor concept

After the rock characteristics are determined through tests, there exist two possibilities to compute the safety factor of the foundation. Until now the most common procedure employed was the first one, which allows the reproduction of the irregular boundaries that normally appear in the works. Actually, and due to the help of electronic computers, it is possible to pass the complexity of these problems to the numerical field. The first two concepts are based on a variation of the stresses applied by the dam on the rock, and the third one is based on the variation of the soil properties.

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