Abstract: Gravelly soil has been mostly used as the core material of the high earth-rock dam. When the coarse-grained content of a gravelly soil (referring to the mass fraction, the same below) may exceed 50%, the coefficient of permeability of a gravelly soil is highly likely to fail to meet engineering specifications or standards. This is deleterious to the seepage stability of a dam. Three-dimensional finite element numerical simulation was conducted to study the influence of a gravelly soil having a coarse-grained content of greater than 50% on the seepage through a high earth-rock dam. The coefficient of permeability of elements having a coarse-grained content of greater than 50% is assigned a different value compared with those elements having a coarse-grained content equal to 50% in the simulation. The results show that the gravelly soil in the different parts of the core having a coarse-grained content of greater than 55% had little effect on the seepage field and the seepage discharge through the dam, but exerted a significant influence on the seepage gradient. The maximum seepage gradient occurred at the element having a coarse-grained content equal to 50%. For those cases having a coarse-grained content of less than 55% in the lower and middle parts of the core, the increase in the maximum seepage gradient was 36.4%. For those cases having a coarse-grained content of greater than 55% in the lower and middle parts of the core, the increase in the maximum gradient was even higher, and might exceed the allowable seepage gradient, with adverse effects upon dam safety. Therefore, in practice, the downstream filter should be designed based on the grading and the fine particle content of such gravelly soils. Besides, the relative density of such gravelly soil should be controlled to ensure the safety of the body of the dam.