Cohesion (c) and internal friction angle (phi) are fundamental indicators of soil shear strength and are strongly influenced by the grain-size distribution (GSD). While numerous studies have correlated c and phi with various GSD descriptors, the diversity of parameters employed hinders the comparison of results and limits the general applicability of these correlations. This study proposes a novel framework to express c and phi using a pair of GSD parameters derived from a unified GSD function, the fines-control exponent mu and the coarse-characteristic diameter Dc. A series of undrained direct shear tests were performed on remolded wide-graded soils under constant dry density and varying initial water contents. Experimental results demonstrate that c exhibits a nonmonotonic dependence on mu, whereas phi increases monotonically with Dc. A normalization framework is proposed to remove the effects of water content, converging the laboratory data onto a simple relationship. Application to a controlled rainfall experiment demonstrates the efficacy of the functions in explaining contrasting failure modes between coarse- and fine-dominated slope zones. The proposed unified variables and functions provide a tool for slope stability analysis in geohazard-prone regions.