Reliability of rock foundations of concrete dams and some recommendations on their assessment

Abstract

The requirement of assurance of the operating reliability of hydraulic structures and economical expenditure of construction materials leads to the need for a critical examination of experience gained in their design and operation. The operating reliability of a dam, including the reliability of its foundation at the contact with the dam and at depth, can be evaluated on the basis of criteria characterizing the final state of the structure as a result of its interaction with the environment. Such a criterion of the final state of the structure can be either fa4Zu/~e, by which is meant that the structure is put out of operatio n with unregulated emptying of the reservoir (as a result of which loss of life and material losses are possible), or c~a~, in which case the structure operates with deviations from the operational requirements (which is wxhibited in a change in the permissible seepage, increase of the uplift pressure of water, nonuniform settlements, and formation of cracks and leads to the necessity of investigating the damages and subsequent repair works). These two criteria In general descriptions can be united by the common term "incident."Concrete dams on rock foundations are reliable structures whose probability of incidents during the period from 1900 to 1970 is statistically estimated to be 3.0%, including the probability of failure 0.3% (99.7% reliability) and probability of damage 2.7%.Safety of the structures is provided for by calculations. Their reliability depends on the correct selection of the calculation scheme.It should correspond to the real conditions of operation of the structure, but be so simple that methematical treatment of the parameters of the design checked by observations can be carried out. Unfortunately, no calculation scheme can completely reflect the behavior of a structure in reality. One of the reasons for this is related to the conditionalities of schematization, another depends on the random character of the effect of various factors. It is possible to take into account the actual interaction of various elements of the "dam-foundation" system under external effects with the use of methods of reliability theory.Therefore, reliability should be estimated on the basis of analysis of a multlfactor model, which permits both consideration of the effect of external factors and estimation of the structure of relisbillty of the "dam foundation" system (Fig. i). An analysis of known cases of incidents with the use of such a model made it possible to distinguish groups of significant external factors. It was established that: 78% of incidents of rock foundations occurred during filling of the reservoir, i.e., under the effect of forces which the structure should successfully withstand; 16% of the incidents occurred as a result of floods exceeding the design flood with respect to discahrge; 2% were related to earthquakes (other than induced ones); 4% occurred as a result of the action of other factors, mainly in the construction period.In this article we will examine problems of the...