Highly plastic clay is usually used to control the cracks produced by the stress concentration around the concrete structure in the rock fill dam. Numerical analysis is preformed on different highly plastic zones in a rock fill dam using the finite elements method. The FEM calculation results have shown that the installation of highly plastic soils on the top of gallery and major diaphram wall can better improve the structural stress of concrete than the full-length installation of the soil along the core bottom. Too large area installation of highly plastic soils in the core bottom will make the originally dispersed loads on the dam body caused by the local installation of soils readjusted

thus increasing the stress of the gallery and the major diaphram wall.