Abstract: For the high gravel-mixed clay-cored rockfill dams, a pure clay layer is needed to be placed between the core wall and the dam foundation.This paper discusses the stress state on the core-foundation interface and the shear strengths of the clay layer.3D finite element calculations are performed, considering the effects of width-height ratio of the dam.Stress distributions on the interface are obtained and shear strengths of clay layer are evaluated based on the Mohr-Coulomb criterion.The following results are achieved from numerical analysis:the shear and normal stresses on the bank slope interface are influenced significantly; the normal stress on the slope interface increases almost linearly with the decreased height and reaches the maximum value near the bottom corner; the shear stress first increases with the decreased height and then decreases with the peak value at around 1/5 of the dam height above the bottom.The peak shear stress is the largest when the slope angle is about 45°.According to the shear strength criterion, the failure will not occur in the bottom clay layer.In the slope clay layer, the strength is lower in the upper part.Furthermore, the shear failure will occur in the upper slope clay layer for the steeper slopes with an angle close to or greater than 45°.Compared with the symmetric model, the clay layer of an actual engineering model is safer on the lower part of the left bank slope as well as the shear strength coefficient is approximate on the upper part and the right bank slope.According to the results mentioned above, the critical bank slope angle of 45° is obtained.It is advisable to take some engineering measures to prevent large shear deformation and shear failure in the corresponding parts of the clay layer when the high core wall rockfill dam is built in a narrow valley with a slope angle close to or greater than 45°.