The use of weathered phyllite waste slags generated from the excavation of cuttings and tunnels as roadbed filler material can effectively address issues related to filler scarcity, environmental protection, and cost. This study focused on weathered phyllite obtained from a highway expansion project in the Longnan Area of Gansu Province, China. Various experiments were conducted in a laboratory setting, including compaction, unconfined compressive strength (UCS), California bearing ratio (CBR), permeability, and disintegration tests, to investigate the response of mixtures with different gravel contents (GCs), ranging between 30 %-70 % by weight of weathered phyllite filler (WPF). The test results indicate the presence of a critical GC threshold. At 55 % GC, the WPF exhibits optimal compaction, the highest UCS and CBR values, and the lowest permeability and disintegration rates. Upon reaching this critical GC threshold, the phyllite gravels contact each other to form a skeletal structure, while fine grains fill the gaps within this structure to create a denser skeleton configuration. Coarse phyllite gravels are more prone to fragmentation into finer grains, which can effectively occupy large, medium, and small voids between particles. Consequently, the WPF exhibits enhanced structural density and improved mechanical and hydraulic properties. These findings provide a theoretical reference for the engineering application of phyllite in mountainous projects.