Seepage Phenomenon for Wadi Megenin Dam

Most of the recorded dam failures in different parts of the world were related to seepage problems. Avoiding earth dam's failure due to seepage usually requires evaluating settlement, and piping, through field monitoring. The problem of dam seepage was experienced in The 38 m high rock fill Megenin dam is a at located at Souk AL khamis area 75 Km south of Tripoli. The dam was constructed in 1972, for two main purposes; to provide flood controls of the downstream areas, the cultivated fields and Tripoli city, and also provides water for irrigation purpose . The rock-fill dam was supported with an upstream reinforced concrete membrane on the upstreamface consisted of a number of concrete panels, linked together with central rubber water stops, and surface-sealed with a bituminous sealant. A concrete cut-off wall and grout curtain were constructed below the dam body to prevent reservoir seepage through the foundation. It was observed during the 3rd filling of the reservoir at the reservoir storage water level at +271.86 m on 15thMay 2003, seepage occurred at the wadi bed downstream of the main dam. The seepage occurred at a piezometric water level higher than the previously measurements during the period of 1973 to 1974. Seepage problem was also encountered in previous reservoir operation. To assess the seepage phenomena and it's impact on the dam safety, a study was carreied out through appliaction of t ground penetrating radar survey (GPR), piezometric analysis, and temperature field evaluation techniques as well as the construction of a flow net model for the dam. The results of the study showed that: the GPR profiles have delineated the critical zones in the dam body ,identify the water seepage paths from the reservoir to the down stream side of the dam, estimated the seepage rate through the dam foundation as 0.08 Ɩliter / ssecond and suggested measures to avoid problem that may occur in the future in Megenin dam due to seepage problems The study revealed the needs to design and implement effective monitoring program ,establish surface water gauging for any sudden increase in the seepage discharge in bed wadi at downstream , groundwater water levels fluctuation in the observation and seepage water beyond the dam. Maintain the upstream concrete facing joints and in case of alert condition occurred undertake additional raw of grouting curtain along the full length of the dam to protect dam foundation from flow.