Effect of Gradation on the Compactability of Coarse-Grained Soils

The optimal gradation for coarse-grained soil used in earth-rockfill dams is an essential design index when determining the compactability of coarse-grained soil. With the optimal gradation, coarse-grained soil can be rolled to the maximum of dry densities among all gradations under the same compaction effort. Accordingly, this research introduced a gradation equation with the gradation parameters b and m and the maximum particle size dmax to establish a quantitative description of the gradation curve. From the results of the maximum dry density tests, the maximum dry density ρmax is found to show a parabolic curve relationship with a parameter called gradation area S, which is defined as the area surrounded by the gradation curve. If the maximum particle size is a constant, the optimal gradation area, S0, has the same value for coarse-grained soils with different shapes or different parent rocks, whereas S0 linearly decreases with the increase in ln dmax. Furthermore, if dmax is known, the optimal gradation area S0 can be evaluated by the proposed linear equation. Findings support a feasible method for designing the optimal gradation distribution in laboratory tests and in-situ.