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 d_max to establish a quantitative description of the gradation curve. From the results of the maximum dry density tests, the maximum dry density ρ_max 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 d_max. Furthermore, if d_max 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.

Using polypropylene (PP) fiber and cement to modify iron-ore tailing and applying it to road engineering is an effective way to reuse iron-ore tailing. The compressive properties and deformation characteristics of PP-fiber-and-cement-modified iron-ore tailing (FCIT) under traffic load were studied by the unconfined-compressive-strength (UCS) test and the dynamical-triaxial (DT) test. The test results indicated that the UCS and residual strength both increased with increasing PP-fiber content, and tensile and toughness properties were positively correlated with PP-fiber content. Moreover, the dynamic elastic modulus and damping of FCIT both showed a negative linear relationship with cycle time. It can be found from the test results that 0.75% was the best PP-fiber content to modify iron tailing sand in this work. Lastly, a prediction model was developed to describe the relationship between the cumulative plastic strain, PP-fiber content and cycle time, which can effectively capture the evolution law of the cumulative plastic strain with cycle time of FCITs at different PP-fiber contents.