Page - 1000 - in Book of Full Papers - Symposium Hydro Engineering

- Estimation of heave occurrence probability at the toe of the embankment dam when reservoir is at normal level; The probability of heave occurrence should be considered in areas of a thinner confining layer with a distance of 2 x dam height at downstream of dam toe. In order to estimate the probability of heave occurrence, the hydraulic gradient at toe of dam and critical hydraulic gradient should be determined. The hydraulic gradient at toe of dam can be calculated by using piezometers reading or results of seepage analysis and the critical hydraulic gradient are calculated by Terzaghi equation (1960). The gradient at the toe of dam in right abutment has been obtained from the results of seepage modeling (i =0.4) and the critical gradient is equal to 0.9. The probability of heave occurrence has been estimated as 0.01 by using the judgment probability table and hydraulic gradient at the dam toe. - Estimation of backward erosion occurrence probability with heave occurrence The initiation of backward erosion is depended on the hydraulic gradient under the main part of dam. The other factors are particle size distribution/uniformity coefficient, permeability of the soil and the geometry of dam and foundation. According to the results of seepage analysis, the gradient is 0.4 and the uniformity coefficient is more than 6 (Cu>6), thus, the probability of backward erosion with heave occurrence is estimated as 0.1. - Estimation of backward erosion occurrence probability without heave occurrence For this purpose, the main factors are average hydraulic gradient, uniformity coefficient and permeability of continues layer. By considering the gradient equal to 0.4 and Cu > 6, the probability of backward erosion without heave occurrence is estimated 0.02. From the results of the above assessment, the probability of initiation of backward is estimated 0.0208. 3.1.1.1.1. Continuation of backward erosion In order to evaluate the continuation of backward erosion, the filtered or unfiltered exit should be assessed. In Ahmadbeiglu Storage Dam, the exit point of seepage is unfiltered but a blanket drain and toe drain system is present. In this dam, a low permeability layer is existent between blanket drain and erodible layer. By considering the mentioned condition and using probability judgment tables, the probability of erosion continuation is estimated 0.25. 3.1.1.1.2. Progression of backward erosion In order to assess the probability of erosion progression, the following issues have been considered; 1000