Quantitative assessment of the reliability of dams

Abstract

The known statistical information about incidents ~ [3,4] makes it possible to analyze their causes, the relationship of various forms (types) of limit states, and also the relative damageability of components of the "dam--foundation--reservoir" system. About half of the incidents is related to random deviations of properties and effects in situations amenable to a probabilistic calculation. The remaining cases are the result of uncalculated situations, including design, construction, and operating errors. The average annual frequency of incidents.of modern large dams is (3-5) • 10 -3 . This value characterizes the level of damageability of dams allowed by modern society and can be used for substantiating the regulated level of reliability of s~ructures when calculating them on a probability basis. The role of such calculations is quite important, particularly in connection with the aforementioned considerable proportion of random deviations of effects and properties among the causes of incidents.The possibility of the indicated random deviations is partially taken into account by the current building codes [i] realizing the "semiprobahilistic"method of "calculated limit states" in the form of deterministic criterion relationships of the formwhere V i is the index of the state of the structure determined by the deterministic calculation scheme adopted for the i type of limit state; Vi* is its limit value~ In this case, the nonoccurrence of any type of limit states is guaranteed by introducing a system of coefficients to the left-and right-hand side of relationships (i).Violation of criterion conditions (i) is interpreted by the building codes as the occurrence of a limit state with unit probability, which does not correspond to reality. The system of separate standard coefficients used in relationships (i) can be based only on probabilistic calculations. These calculations should be performed when drawing up the building codes oriented mainly at massive structures. But the recommendations of the codes, strictly speaking, are not completely applicable to large dams, which are unique, as a rule, not only from the viewpoint of design but also operating conditions~ Further improvement of the design of hydraulic structures is possible on the basis of methods of probabilistic--economic optimization, for example, in conformity with the condition minimizing total discounted cost on construction C, annual outlays R, and possible loss (risk) from an incident ~'Pi Yi: i C+ P + ~VYt ~min'(2) i where Yj is the cost of loss from an incident of the j type, determined by the severity of the aftereffects of the incident; Pj is the probability of its realization in any ofi forms, i.e., Pi=UPii.(3) i An analysis of the experience of modern large dams [3,4] permits distinguishing three (i = i, 2, 3) main forms of incidents realized in the body of a dam, its foundation, or in their contacts: erosion during flow of water over the crest; disturbance of the strength (stability) or extreme movements; disturbance of the seepage strength o...