In this study, soil samples with varying P₅ content were prepared for the impervious gravel soil of a 300 m ultra-high earth-rockfill dam. Tests were conducted to investigate the influence of P₅ content on the compaction, strength, deformation, and permeability characteristics of the soil samples. The characteristic values of the primary and secondary gravel content for the project were determined as 37% and 60%, respectively. With increasing P₅ content, the maximum dry density gradually increased while optimum moisture content decreased. When P₅ content ranged from 60% to 75%, the maximum dry density remained stable and did not decrease until the P₅ content reached 80%. This stable stage can be attributed to particle breakage. Additionally, as P₅ content increased, the compression modulus, shear strength parameters, deformation parameters, and permeability coefficient of the soil gradually increased, while the characteristic hydraulic gradient gradually reduced, and consequently changing the seepage failure mode of the soil.