Reliability Analysis Of Deep Sliding Stability Of Gravity Dams

With regard to the design of gravity dams, DL 5108-1999 Design specification for concrete gravity dams is based on probabilistic limit state design method and partial factor design expression, while SL 319-2005 Design specification for concrete gravity dams is based on single safety factor method. In the field of hydraulic structure, it is critical to study the relation between safety factor and failure probability, probabilistic limit state design method and single safety factor method. Reliability of deep sliding stability of gravity dams is analyzed, and the relation between reliability analysis and deterministic design method is discussed. The approach is searched to combine the probabilistic limit state design method and deterministic design method.(1) System reliability is studied for wedge stability against deep sliding of gravity dams. To the wedge stability against deep sliding of gravity dams, the reliability indexes of the two blocks are calculated by means of equal safety coefficient method and first-order second-moment method. The system reliability index is calculated by means of equivalent linearization method. The results show that the two blocks with the equal safety factors has two different reliability indexes. When the safety factors of two blocks meet the requirements, the reliability indecis may not meet the demand. If the probabilistic limit state design method is used to compute the stability against deep sliding of gravity dams, two limit state equations should be established to analyze two blocks’relibaility indecis respectively and their system reliability.(2) The tradional safety factor has been endowed with general menaing K= F (x1, x2,…, xn), and the reliability of wedge stability against deep sliding of gravity dams is calculated using the method of Monte Carlo and limit state equation of K-1.0=0, which shows the reliability index of the same level with system reliability. It’s a new approach to study the system reliability for wedge stability against deep sliding of gravity dams, and established a linkage between probabilistic limit state design method and safety factor method.(3) System reliability is studied for stability against deep sliding of gravity dams with various sliding surfaces. Various sliding surfaces should be considered in the stability of gravity dam against deep sliding when there are several faults. The strength reduction factor method and FEM method of stress analysis of sliding plane are adopted to solve the safety factor of stability against deep sliding. Based on the weighted regression response surface method and gradual equivalent linear method, the system reliability is calculated to set up the relation between safety factors and reliability indecis of the faults,and the link bewteen the strength reduction factor and system reliability index. Because of the system in stability against deep sliding of gravity dams with various sliding surfaces, the result indicates that the strength reduction factor is bigger than the safety factor of all the faults. Based on FEM method of stress analysis of sliding plane, safety factors of the faults don’t have the same regularity with their reliability indecis. By means of the strength reduction factor method, the results, such as failure mode and safety level, are consistent with the conclusiong by system reliability analysis.(4) Fuzzy reliability is studied for stability against deep sliding of gravity dams based on fuzzy response surface method. There are several destabilization criterions in the stability of gravity dam against deep sliding, and fuzziness exists in the failure criterion. Principal ultimate state and auxiliary ultimate state are put forward, and fuzzy response surface method and reduced fuzzy response surface method are brought forth to analyze stability against deep sliding of gravity dams. The results show that fuzzy response surface method is reasonable it’s more rationale to use fuzzy response surface method in the analysis of gravity dam against deep sliding.(5) According to the result of reliablity analysis for stability against deep sliding of gravity dams, Some proposals are put forward for the revise of Design specification for concrete gravity dams and the engineering design, which can be used for reference revise of Design specification for concrete gravity dams. The suggestions are listed as follows.1) The slide body of ABD is analyzed along the horizontal direction in the code of DL 5108-1999 Design specification for concrete gravity dams, which is not reasonable and should be revised.2) SL 319-2005 Design specification for concrete gravity dams, based on single safety factor method, can’t accurately calculate the safety level of the stability against deep sliding of gravity dams, so it’s recommended that the reliability theory is drawn into the deterministic design method. The tradional safety factor can be endowed with general menaing K= F(x1, x2,…, xn), and the reliability of stability against deep sliding of gravity dams is calculated using the limit state equation of K-1.0= 0, which established a linkage between probabilistic limit state design method and safety factor method. General safety factor, K= F(γO,γG,γQ,γm,…x1, x2,…, xn) can be adopted in the design of stability against deep sliding of gravity dams, but the partial factor and the general safety factor must be thoroughly studied.3) Compared to FEM method of stress analysis of sliding plane, the more precise result can be achieved by means of the strength reduction factor method. The potential sliding surface and safety level can be obtained by means of the strength reduction factor, and the results are consistent with system reliability analysis. It’s recommended that the strength reduction factor method should be used to assess the stability of gravity dam against deep sliding, and reliability theory should be combined with deterministic design method so that partial coefficient FEM can be popularized to improve the accuracy and rationality of the stability analysis of deep sliding of gravity dams.4) Because of the fuzziness of failure criterion in stability analysis of deep sliding of gravity dams, fuzzy reliability should be computed for stability against deep sliding of gravity dams.5) Conventional reliability index calculated by strength reduction factor method is bigger in gravity dam against deep sliding due to the fuzziness of failure criterion. The fuzziness should be taken account of, and safety level can be improved when the strength reduction factor method is used to assess the gravity dam against deep sliding.