Concept of safety evaluation of the arch dam of the Inguri hydroelectric station

Problems of the reliability and safe operation of the arch dam of the Inguri hydroelectric station, a record dam in height and unique in many parameters and construction conditions, were decisive at all stages of its design, construction, and temporary operation. An extensive combination of surveying, research, design, and experimental field works not having analogues in Soviet dam construction was per formed to substantiate the reliability. New methods of calculation and experimental investigations were improved or created, making it possible to study with the necessary accuracy the main characteristics of the behavior of the arch dam--rock foundation system. A shape of the dam optimal in the entire possible range of usual and unusual loading combinations and effects, including seismic, was selected. To increase the reliability, antiseismic reinforcement of the dam was carried out, large-scale works on restoring the continuity of the rock foundation were performed, a perimeter joint reducing the tensile stresses in the contact zone in the event of an earthquake and thereby preventing possible damage of the grout curtain was constructed; high-reliabil i ty watertight seals were installed in the joints between sections and in the perimeter joints, the upstream face was covered with rubber--epoxy waterproofing and the dam--foundat ion contact was protected by mastic asphalt; in the body of the dam and foundation was constructed a branched system of galleries and adits enabling repair and restoration works if necessary, a system for repeated and multiple grouting of the dam and foundation was provided for. Numerous on-si te observations are provided for during operation for checking the safety of the arch dam, about 3000 embedded instruments were placed in the structure and foundation. The results of the on-si te observations are the actual basis for evaluating the technical state of the structure and safety of its operation. However, it should be noted that in the design, the reliability of the structure, predetermining the safety of its operation, is provided through the fulfi l lment of a number of conditions (with respect to strength, stability, formation and opening of cracks, etc.) guaranteeing the nonoccurrence of limit states, whereas the on-si te observations do not directly make it possible to check the fulfi l lment of these conditions, since the characteristics being measured, henceforth called the indices of the state and operation of the structure, as a rule, do not figure in the indicated criterion relation; the situation is complicated also by the fact that the operating conditions of the structure constantly change in time with fluctuations of the upper pool level (UPL) and ambient temperature, as a consequence of a change in the physical and mechanical properties of the concrete and rock foundation, etc.* From world practice are known two methods of overcoming this contradiction and solving the problem of evaluating the safety of hydraulic structures on the basis of data on on-si te observations. The most prevalent is the method based on analyzing statistical series f rom the results of on-si te observations the method of constructing statistical mathematical models of the behavior of a structure, making it possible to single out and describe the effect of individual factors on the indices being measured. The safety criterion in this case is not so much a check of the conditions of nonoccurrence of limit states as the absence of deformations in the dam--foundation system, piezometric heads, seepage discharges, and other indices of the operation of the structure which do not diminish in time and are irreversible under cyclic effects. This method is effect ive for structures operating for a rather long time and with the correct selection of the parameters of the statistical models. The essence of the second method is a comparison of the results of the on-si te observations with the results of calculations and model investigations, i.e., a comparison of the on-si te evaluations of the state and operation of the structure with the design one. This method has a more distinct physical basis of safety evaluation, but its use runs into