This paper reports the case histories of foundation deterioration in the Portuguese Varosa and Venda Nova arch dams. The main characteristics of the works, the geological conditions, the observation systems, the detection of the abnormal behaviours, and aspects of the repair works are referred to.
Ce travail presente les cas de deterioration de fondation dans les barrages-vôûte portugaises de Yarosa et Venda Nova. Les caracteristiques principales des ouvrages, les conditions geologiques, les systèmes d'observation, la detection des comportements anormaux et des aspects des travaux de reparation sont cites.
Dies Referat stellt die Pallstudien des Felsgruendungsverfalls in den portugiesischen Varosa und Venda Nova Bogenstaudammen vor. Die Hauptmerkmale der Bauten, die geologischen Beschaffenheiten, die Beobachtungssysteme, die Entdeckung der ungewöhnlichen Verhalten und Eigenarten der Reparaturarbeiten werden besprochen.
Seepage through the rock mass foundation of concrete dams is of major importance to the uplift set up, velocity and the corresponding rates. Uplift in the foundation is one of the main actions on those structures, due to its permanence along the life time and to the forces it originates, which may condition the stability of the structure. Seepage velocities in the discontinuities of the foundation may give rise to erosive actions, or else they may produce deposition of materials and so contribute to changes in the hydraulic and mechanical characteristics of the medium. Physical-chemical actions also occur, which are related to the rock mass properties and of treatment works. These actions may alter hydraulic and mechanical properties of the rock mass during the exploration. Seepage control is achieved by means of the foundation water tightening and drainage works. These make it possible a limitation of uplift and see page velocities, avoiding excessive gradients, and preventing the occurrence of excessive flow. Though in the last years there has been significant progress in the hydraulic characterization of rock mass, the hydraulic conductivity of rock media still has to overcome some difficulties (Mascarenhas 1979). In addition to the anisotropic and heterogeneous characteristics of the permeability, account should be taken of its variation with the state of stress. This is particularly important in the case of fissured media. The variation of the permeability with the state of stress, particularly when this results from failures affecting the grout curtain and the drainage system, may be associated with a marked decrease of the efficiency of the works (expressed by increase of uplift at the level of the drainage curtain and of drainage seepage). This phenomena frequently detected during the first filling has been observed in numerous arch dams. There is also evidence of reduced efficiency of those works when they are located in highly compressed zones of the rock mass. In this case a marked decrease of permeability is expressed by a decrease in the drainage capacity. The cases of Varosa and Venda Nova arch dams (Portugal) are examples of deteriorations in water tightening and drainage systems which evidence the need of more adequate studies on the hydromechanical behaviour of foundations.
Deterioration of dams (accidents and incidents) is usually concerned with the dam body and its foundation, appurtenant works, and slopes downstream of the dam and of reservoirs. In the present paper, we are mainly concerned with deterioration occurring in the rock mass foundation of concrete dams, though the behaviour of whole dam-foundation complex has to be taken into account. Concrete dams have different structural types, each one demanding the contribution of the rock mass foundation in a specific way.