Research On Fuzzy Risk Analysis Method Of Gravity Dam Foundation Instability

In the safety evaluation system of gravity dam, the stability against sliding has been a very important part. Currently, calculating the safety factor by rigid equilibrium method is generally used to evaluate the stability of gravity. But in the actual engineering, the uncertainty of variables affecting the safety factor is not taken into account in conventional safety factor method, in which the evaluation of guillotine to its stability is not reasonable and usually has limitations. Following are the main contents of this paper.Firstly, the traditional methods in the calculation of the stability against sliding were presented in this paper. Among them, rigid equilibrium limit method, FEM method and partial coefficient method were stated in detail, which were combined with engineering example and coincident conclusion was reached. Based on that, the uncertainty of dam instability was studied and the view that risk analysis of dam instability should be carried out was proposed.Secondly, random risk analysis of dam foundation instability was presented on the basis of the randomness in risk analysis, meanwhile, the correlation and sensitivity of the variables were studied. Combined with engineering example, some conclusions were obtained. Risk rate calculated values were higher and safer without the negative correlation between friction coefficient and cohesion. Through sensitivity analysis, main sensitive factors to reliability index and risk rate could be obtained, for example, the friction coefficient, the cohesion, headwater level, and other non-sensitive factors. This can provide guidance on design and construction of gravity, and the study to the calculation of fuzzy risk rate could be simplified with the non-sensitive factor as constant.This paper focused on the following aspects. Three kinds of mathematical models for fuzzy risk analysis of gravity dam foundation instability were established through fuzzy mathematics theory, based on the randomness and fuzziness of the factors influencing dam foundation stability against sliding and the fuzziness of instability criterion. Theα-level-cut method was stated in detail which is commonly used to solve the models. Fuzzy-random limit state equation was established on the basis of the safety factor, and improved fuzzy processing method was proposed and applied in the calculation and study of fuzzy risk rate, consideringα-level-cut method in which the design parameters of the model was fuzzily processed and lower semi-trapezoid membership function which was used to fuzzily process the instability criterion. In the calculation, the fuzzy interval on which the fuzziness of the factors and instability criterion had the same influence degree on fuzzy risk rate was searched, and the risk rate on this fuzzy interval was regarded as the final fuzzy risk rate to evaluate the stability against sliding. By the analysis of an engineering example, the method given in this paper was more reasonable than the method in which the design parameters and the instability criterion were both fuzzily processed byα-level-cut method with the results of fuzzy risk interval value. Dam safety evaluation conclusion by fuzzy risk analysis was also corresponding with that by the deterministic approach in current Design Code.