Two diaphram walls are frequently used in the rock-filled wall with a high core on the rock cover. Referring to the condition of a certain high rock-filled core dam with a nearly 50 m deep gravel-sand overburden

the steady seepage fields of the dam base in 6 set-up schemes of diaphram walls are analyzed using the finite element software developed by the first author

and the ratio of the hydraulic head acting on the main wall

secondary wall and gravel-sand layers in each scheme is obtained. The hydraulic head acting on the gravel-sand layers is less than 3 percent of the total in these schemes. In the schemes in which the water-tight curtain under the secondary wall is the same as that under the main wall and the round seepage between the two walls is blocked by the side water-tight curtains

the hydraulic head acting on the secondary wall is only slightly less than that on the main wall. If the side curtains are omitted

the hydraulic head on the secondary wall is only 20 percent to 30 percent of the total. If the curtain under the secondary wall is further canceled

the hydraulic head on the secondary wall will be decreased to 8 percent to 11 percent of the total. The hydraulic head on the secondary wall in the upstream side of the main wall is less than that in the downstream side. The hydraulic head ratio of the two walls is not sensitive to their thickness.