Page - 999 - in Book of Full Papers - Symposium Hydro Engineering

consequence d) Risk prioritization and risk control. It should be noted that, in this analysis, only life loss has been considered as dam failure consequence. The probability of dam failure due to internal erosion has been determined based on event tree methods. In this method, all failure modes due to internal erosion are broken down in two four phases; initiation of erosion, continuation of erosion, progression of erosion and detection and finally breach. Thus, the probability of dam failure has been obtained by using below equation [1]; PF = P Load x P Initiation x P Continuation x P Progression x P Detection x P Breac [1] 3.1. IDENTIFICATION OF FAILURE MODE The failure of embankment dams by internal erosion or piping has been evaluated in three different modes including piping through embankment, piping through foundation and piping from embankment nto foundation. 3.1.1. Assessment Of The Probability Of Dam Failure Due To Internal Erosion Through Soil Foundation (PF1) In order to evaluate the internal erosion through soil foundation, determination of soil classification and existence of continues layer in dam foundation is the first step. As it was mentation, in feasibility and detail design study, the foundation soil is classified as CL, SC, CH and layers of SC-SM, SM and GC-GM. In this stage, three bore hole which are located in upstream, axis and downstream of dam body has been drilled for determining of the soil classification, existence of continues layered soil and also permeability of the soil. According to the results of field and laboratory tests, the foundation soil is often in CL classification; soils with classification of SC, SM, and ML have been observed as layer between soils with CL classification and these layers are approximately continues layer. In addition, the plasticity index (PI) of these layer is less than 7.0 (PI<7). Thus, internal erosion in soil foundation with continues cohesionless layer may initiate and progress due to backward erosion or suffusion (erosion of fine soils into adjacent coarse soils). The evaluation of internal erosion has been presented as following; 3.1.1.1. Initiation of backward erosion In order to estimate the initiation of internal erosion by backward, the following issues have been evaluated; 999