The presence of fines in granular materials significantly affects their shear strength, particularly under low confining pressure conditions. In this study, drained triaxial compression tests were simulated using the discrete element method on spherical granular assemblies with varying fines content \(\:{f}_{c}\) and confining pressures \(\:\:{\sigma\:}_{c}\) to investigate the influence of \(\:{f}_{c}\) on the macroscopic mechanical response and underlying micro-mechanical mechanisms. Macroscopic results show that the global void ratio exhibits a non-monotonic trend, first increasing and then decreasing whereas the skeletal void ratio monotonically decreases with increasing \(\:{f}_{c}\) . The peak stress ratio \(\:{\eta\:}_{p}\) rapidly increases initially and then stabilizes above a certain critical confining pressure \(\:{p}_{1}\) . Intriguingly, this \(\:{p}_{1}\) is found to approximately decrease as the \(\:{f}_{c}\) increases. At high fines content, many fine particles initially act as rattlers under low confining pressure but progressively become incorporated into load-bearing force chains as the confining pressure increases. This mobilization enhances coarse–fine contacts, thereby contributing additional shear strength to the assembly. Based on these findings, an improved failure criterion is proposed, which accurately predicts the shear strength of granular materials across different fines contents and effectively captures its nonlinear variation under low confining pressures.
