Influence of Minor Coarse Aggregate Fractions on Hardfill Strength: A Case Study of Mallegue-Amont Dam

This study examines the influence of low proportions (< 2.5% by mass) of 40/63 mm gravel on the shear strength of hardfill used in the Mallegue-Amont Dam (Tunisia). To address the coarse nature of the material, a custom medium-scale direct shear apparatus was developed, despite its nonstandard dimensions. Nine mixtures with varying sand-to-gravel ratios were prepared to evaluate the effect of fine content. Experimental testing was supported by statistical analysis and validated through numerical simulations using FLAC3D. Results indicate that the 40/63 mm fraction has a negligible effect on shear strength parameters. Instead, the mechanical response is predominantly controlled by cementation and particles smaller than 40 mm. Numerical modeling confirmed the reliability of the experimental findings and reinforced the validity of the adapted testing approach. The study demonstrates that representative shear strength parameters can be obtained using non-standard equipment, provided mixture preparation and mold dimensions are carefully controlled. These insights contribute to cost-effective hardfill design and improved durability of dam and infrastructure projects.