Page - 1002 - in Book of Full Papers - Symposium Hydro Engineering

3.1.1.2. Initiation of erosion by suffusion/internal instability in a cohesionless layer of foundation Suffusion, or internal instability, involves the erosion of finer particles from the matrix of coarser soils, under the seepage gradient in the soil. For assessing the suffusion/internal erosion of soils, the particle size distribution and hydraulic gradient are the main factors. By using Fell and Wan (2004) method, and considering the particle size for D15 (0.08mm), D60 (0.3mm), D90 (0.8mm), the erosion initiation by internal instability is not possible in right abutment. 3.1.2. Assessment of the probability of dam failure due to internal erosion from embankment into foundation (PF2) 3.1.2.1. Initiation of erosion from embankment into foundation In order to assess the erosion initiation from embankment into foundation, the existence of a coarse layer in the base or sides of key trench, initiation of scour or initiation of core material erosion by hydraulic fracture due to arching in a narrow key trench should be evaluated. The probability of existence of a coarse layer in the base of key trench has been estimated as 0.5, by using the geology studies, mapping and photographs taken during construction. According to the results of laboratory tests on samples of the fine borrow area (clay core material), the material is not susceptible to erosion. The results of seepage analysis in a section located in right abutment showed that the maximum hydraulic gradient in the core-foundation contact is 1.4 that is less than allowable value. Therefore, the probability of initiation of scour in core-foundation contact in spite of existence of coarse layer and unfiltered exit is estimated as 0.01. In addition, according to the results of static stress-strain analysis, occurrence of hydraulic fracture due to arching in key trench with slope 2H: 1V is very unlikely (p=0.005). However, the probability of initiation of erosion from embankment into foundation has been calculated 0.01. 3.1.2.2. Continuation of erosion from embankment into foundation In order to evaluate the probability of erosion continuation from embankment into foundation, the presence of filter/transition zone in downstream of core or filtered and unfiltered exit should be determined. As previously mentioned, the exit point in Ahmadbeiglu dam is unfiltered but there is a very low permeable layer between erodible layer and blanket drain. Also, there is a filter in downstream of clay core which the D15 filter ranges between 0.2 to 0.7 mm. According to reference [5] for some, excessive and continuing erosion, D15 filter should be between 0.7 to 5, 7 to 12 and 12 to 33 1002