Stress conditions in rock foundations of concrete gravity dams

Abstract

1. The results of theoretical and experimental investigations and data from full-scale observations indicate that even in the case of sufficiently high safety factors in dams and absence of tensile stresses in the concrete-rock interface, a zone of tensile stresses below the interface is almost always developed, which at some dams leads to damage of the impervious curtains and to other adverse phenomena capable of disturbing the normal operation of the structure and sometimes causing failure. 2. It is necessary to introduce into the design methods of analysis and investigation of the stress-strain conditions of rock masses below the lower surface of dams, in order to establish the zones of tensile and larger comptessive stresses. 3. The analyses and investigations carried out reveal that it is practically impossible to prevent tensile stresses in the foundations of concrete dams. However, it is evidently unnecessary to prevent these stresses. The overwhelming majority of dams are being successfully operated under these conditions. Nevertheless, our knowledge about the operating conditions of foundations without analyses of their stress conditions is insufficient. Such analyses are necessary in order to develop constructional measures which, by taking into account the specific geologic conditions, will ensure normal operating conditions of the dams even with zones of tensile stresses in their foundations. These measures include: rational location of the impervious and drainage curtains, in particular, if possible, by transferring the impervious curtains to the upstream side; compression of the rock foundation by means of anchors; second-stage grouting after filling of the reservoirs, etc. 4. The presence of tensile stresses in the foundations of almost all concrete dams indicates that there is no need for introducing design limitations prohibiting the formation of tensile stresses in the concrete-rock interface, which in many cases determine the overall dimensions of the dams. In this lies the possibility of substantial lightening of dams and design of economical sections. By applying special constructional measures which recognize the presence of tensile stresses, the reliability of such dams could be made even higher than that of dams of the conventional design, the stress conditions in whose foundations have not been studied.