Page - 1004 - in Book of Full Papers - Symposium Hydro Engineering

The probability of breach in embankment or from embankment into foundation due to erosion has been calculated as 0.03 by using the results of above assessment. Finally, the probability of dam failure by erosion from embankment into layered soil foundation is 1.35 e -8. 3.1.3. Assessment of the probability of dam failure due to internal erosion through embankment (PF3) 3.1.3.1. Initiation of piping through embankment According to the results of laboratory tests on samples of fine borrow areas, especially clay size fraction and plasticity index, the probability of erosion initiation by backward or suffusion through embankment is negligible. But, erosion can initiate by concentrate leakage in low stressed zones which may be caused by differential settlement and arching or high permeability zone due to poor compaction. According to the static stress-strain analsis of Ahmadbeiglu dam, evaluation of arching and hydraulic fracturing, and construction quality, the probability of erosion occurrence in this dam is estimated as 0.02. 3.1.3.2. Continuation of piping through embankment As it was mentioned, a protective filter in downstream of clay core is present with D15 between 0.2 and 0.7 mm. According reference [5], the probability of continuing erosion has been considered as 0.01. The probability of progression, failure to detect, breach is the same as for erosion from embankment into foundation. Therefore, the probability of failure due to erosion through embankment has been estimated as 5.4e-9. 4. CONCLUSION According to the geology and geotechnical studies, the results of field and laboratory tests and the dam site investigation, the right abutment of this dam site has been considered as layered soil foundation and the internal erosion was the most important factor in dam failure. In order to assess the probability of dam failure due to erosion, three failure modes have been evaluated as failure by erosion through layered soil foundation, erosion from embankment into soil foundation and erosion through embankment. All failure modes have been assessed (reservoir is considered to be in normal level) and the probability of their occurrence have determined. In order to categorize the internal erosion risk, f-N event chart has been used. On the f-N 1004