The behaviour of rock abutments, where geological discontinuities are present, can now be analyzed far better by geological investigations than by mere methods of determination of the modulus of elasticity. To this purpose, abutments are schematized as systems of rock blocks with delimiting surfaces corresponding to the fractures located, which cause deviations in the applied forces, whenever the friction between separation surfaces is exceeded. Beyond that limit, unfavourable dynamic movements may occur in consequence of unbalanced forces arising along the surfaces, particularly when the vectors of the forces are directed downstream. The determination of the influence of a rock mass on the statics of a dam is uncertain, but can be clarified by proper model tests. An account is given of the behaviour of a particular dam foundation (Caprile), ascertained by geomechanical studies and tests on two different models, including or excluding some of the located disturbances, and the influence on such behaviour of mere faults, even if of limited importance, is determined.
En presence de discontinuites geologiques, la geomecanique, à la difference des seules methodes de recherches du module elastique, est aujourd'hui capable d'analyser d'une façon meilleure le comportement des flancs, consideres comme un ensemble de gros blocs de roche dont les surfaces de delimitation sont constituees par les fractures relevees. Ces dernières provoquent des deviations des forces appliquees, quand le frottement entre les surfaces de separation est depasse. Au-dela d'une telle limite, le long des dites surfaces de discontinuite, surgissent des forces non equilibrees, qui peuvent provoquer des mouvements dynamiques defavorables quand les vecteurs des forces sont diriges en aval. L'influence du massif rocheux sur la statique du barrage est d'une determination incertaine mais peut être eclaircie par des essais sur modèle. Un exemple est donne du comportement de fondation du barrage (Caprile), releve par une etude geomecanique et par des essais sur deux modèles avec et sans la reproduction d'accidents localises; la determination de l'influence de seules failles, même si elles ont une importance modeste sur le comportement lui-même, est mise en evidence.
Angesichts der geologischen Zusammenhangslosigkeiten ist die Geomechanik, im Unterschied zu den einzigen Nachforschungsmethoden des elastischen Modulus, heute imstande die Reaktion der Abhange naturgetreulicher zu pruefen, sie zu schematisieren als Gesamtheit der grossen Gesteinsblöcke, deren Flachenbegrenzung auf die abnehmenden Brueche zusammengesetzt sind: diese letzten rufen Ablenkungen der anwendbaren Krafte hervor, wenn die Reibung der Trennungsflache ueberwunden wird. Der Einfluss des Gebirges auf die Statik des Damms ist von ungewisser Bestimmung aber kann mit Pruefungen am Modell geklart sein. Ein Beispiel des Betragens des Fundaments des Damms (Caprile), geklart vom geomechanischem Studium und von Pruefungen an modellen, mit und ohne Darstellung einiger bekannter Störungen, wird gegeben, Die Bestimmung des Einflusses der alleinigen Spruenge, auch wenn von geringer Wichtigkeit, auf das gleiche Befragen wird gemacht.
The behaviour of rock masses regarded as foundations for large structures depends not only on their own nature and intrinsic characteristics but, obviously, also on the shape of the particular structure and on the external stresses the rock masses are able to withstand. The knowledge of their behaviour is particularly important where the rock masses are used as dam foundations, and it becomes determinant in the case of arch dams, in which heavy and concentrated stresses are transmitted to the abutments. In this case, the two elements - structure and foundation rock - constitute a whole that cannot be disjoined when the stability of the dam is investigated, and a precise determination of the characteristics of the rock becomes increasingly important. These characteristics can be determined not only employing new methods (static and dynamic) for the study of the deformability of the rock, but also, and particularly, conducting an extensive geomechanical study, after the geological survey has been completed, to ascertain the suitability of the rock and its degree of strength. The behaviour of dam rock abutments can now be determined by geomechanical studies, which can also ascertain how tectonic disturbances deviate and modify the flux of forces to which abutments are subjected, and consequently affect the flux of forces of the dam body. Therefore, the rock abutments can be schematized according to a system of big rock blocks whose delimiting surfaces correspond to the disturbances. When the applied forces are transmitted from one block to the other, the transmission is complete only if the angle they form with the normal is smaller than the angle of shear strength of the infilling material or the angle of friction between the separation surfaces of the blocks (Fig. 1). More precisely, whereas the normal components (N) of the forces that are re-transmitted are always entirely absorbed by the part beyond the geological disturbance (second body), the tangential components (n are fully re-transmitted only if the angle formed by their resultant with the normal is smaller than the maximum angle of friction.
