Abstract The present study investigates the effects of clay core thickness and cutoff wall depth on some of the design parameters in the Sabalan rock fill dam by the finite-element method using the SEEP/W software. Clay cores with various thicknesses and cutoff walls in different situations of dam foundations were tested. The results revealed that (1) the maximum hydraulic gradient in Sabalan dam foundation occurs in the cutoff wall; (2) the minimum hydraulic gradient can be found by changing the location of the cutoff wall, under the center of clay core; and (3) discharge reduction depends on the optimum position at heel of the core. As the permeability decreases and the depth of the cutoff wall increases, the hydraulic gradient in the cutoff wall increases. As the thickness of the clay core increases, hydraulic gradient and the seepage discharge decreases and the uplift pressure increases.