Study on the macro-micro influence of gradation area on the maximum shear stiffness of coarse-grained soils

The accurate determination of the maximum shear stiffness of coarse-grained soils is critical for engineering safety design yet remains challenging due to its complex dependence on gradation. This study investigates the macro- and micro-scale effects of the gradation area, a comprehensive parameter characterizing the entire gradation curve, on this stiffness. Cyclic shear tests were conducted on coarse-grained soils with varying gradation areas. Concurrently, a discrete element method numerical model was developed to elucidate the underlying micromechanisms. Experimental results demonstrate that, under constant normal stress, the maximum shear stiffness decreases with an increasing gradation area, following an inverse relationship. A predictive formula incorporating the gradation area was thus proposed and validated against independent data. The micromechanical analysis reveals that a reduction in the gradation area corresponds to a higher proportion of coarse grains, which leads to a wider shear zone, more irregular particle motion, and the formation of a robust, high-strength force-chain network. The synergistic interaction of these mechanisms explains the observed increase in maximum shear stiffness for soils with smaller gradation areas. This study provides a macro-micro integrated understanding of how gradation regulates shear stiffness, offering theoretical support for the optimized design of granular materials in engineering applications.