As explained in statement of problem, Arjo Dedessa rock fill dam have wide vertical core. However, the effectiveness of a wide embankment core acting as a seepage barrier should not be underestimated. In wide core the water flow or hydraulic gradient is so low that means the amount of water passed is goes to be minimized. Wide cores of relatively impervious soils lead to significant head losses as the leakage run through a long path. However, in this wide core, there is a development of pore water pressure through the core because water cannot dissipate in a short period of time (USBR, 2014). Accordingly, previous Reclamation guidance typically has been bound the width of the core to be greater than one-fourth of the height of the dam. However, thinner cores lead to higher gradients through the core and place an even greater dependence on the filter compatibility of adjacent filter or drain and transition zones.