Page - 1001 - in Book of Full Papers - Symposium Hydro Engineering

- Probability of the forming a roof of a pipe (PR); the roof of a pipe can be formed by layers of soil in the foundation which are cohesive or have high fines content. The erodible soil in foundation has been classified as SM, ML and SC with fine content more than 15%, thus, the probability of forming a roof of a pipe has been calculated as 0.7. - Probability for no limitation of the flow (PL); the flow limitation can be performed by zoning within the dam, cutoff wall or other sealing elements in dams or foundation. In Ahmadbeiglu dam, the sealing element in right abutment is key trench. Therefore, the probability of no limitation has been considered as 0.01. Thus, the probability of progression of erosion through the layered foundation has been calculated as 7e-3 (PP =PR x PL). 3.1.1.1.3. Probability of detection, intervention and repair fail Determination of water level at the downstream of dam by using readout of observation well and open standpipe piezometer during construction show that the water level is normal. For more safe, four open standpipe piezometers in the downstream shell and right abutment and eight observation wells in downstream of dam body and right abutment have been installed in this stage besides the other instrumentations of dam body and foundation. By considering the installed instruments in dam body and foundation in right abutment and frequency of inspection (weekly or every two weeks), it is likely that the intervention on dam and right abutment would be successful. The probability of the intervention and repair not to be successful is considered unlikely (P=0.1). 3.1.1.1.4. Probability of breach in the layered soil foundation Dam breach could be occurred by the following mechanisms in this dam; - Breach by instability of downstream slope; since there is a blanket drain in downstream and downstream shell is free-drain material, so, the pore pressure will dissipate and the probability of breach by instability of downstream slope is estimated 0.02. - Breach by sinkhole development; for this purpose, the loss of freeboard due to the settlement of embankment induced by the loss of foundation material should be determined. Since the settlement of dam is less than 1% dam height (according to the stress-strain analysis results), and freeboard is equal to 6 m, the freeboard at the time of failure is calculated 5 m, so breach by sinkhole develop is estimated 0.01. The probability of breach in layered soil foundation due to the above mechanisms and engineering judgments has been estimated as 0.03. 1001