This paper is dedicated to examining the impact of fine particles, specifically stone dust (passing 600 microns), on the shear strength, friction angle, and dilation angle of a subbase mix. To assess these properties, a large-scale direct shear test employing a 300 mm x 300 mm x 230 mm box was conducted. The subbase mix consisted of well-graded aggregate with varying proportions of fines, ranging from 1 to 15% by mass of the mix. The direct shear test was performed at 49.03 kPa, 98.06 kPa, 147.10 kPa and 196.13 kPa of normal stress across different densities. The findings revealed that the inclusion of 15% fine particles in the mix led to an 18% reduction in the friction angle for the loose mix and a 10% reduction for the compacted mix. Notably, the friction angle of the subbase mix proved to be influenced by factors such as normal stress, density, void ratio, and stone dust content. In compacted subbase mixes, the friction angle was predominantly influenced by variations in the mix's void ratio. The average dilation angle was determined to be 7.73 degrees for the loose mix and 16.36 degrees for the compacted mix. The analysis indicated that alterations in the dilation angle were impacted by normal stress, density, and the mean grain size of the mix. Furthermore, statistical analysis underscored the significant influence of the proportion of stone dust particles on the peak shear stress of the subbase mix. These findings collectively contribute to a comprehensive understanding of how fine particles, specifically stone dust, affect crucial mechanical properties in subbase mixes.