This experimental analysis aims to develop a fundamental understanding of the geotechnical properties of soils treated with an organic material named biochar. In this context, an experimental protocol was set using the Casagrande shear box apparatus to effectively demonstrate the combined influence of the biochar fraction (Fbio=0%, 5%, 10% and 15%), the initial relative density (Dr = 20%, Dr = 54%, and Dr = 90%), and the initial normal stress (σn = 100, 200, and 300 kPa) on the mechanical behavior of Chlef sand. The obtained results clearly show that biochar has a significant influence on the mechanical behavior of the tested materials. Additionally, the shear strength decreases with increasing biochar content up to a threshold value of (Fbio = 10%). Beyond that, the shear strength increases with the increase of biochar content up to the value of (Fbio = 15%) for all the considered parameters. The observed trend of increasing shear strength with the addition of the biochar content (Fbio = 15%) can be attributed to the amplification of the particle interlocking due to the presence of biochar particles between the large particles of Chlef sand, inducing an increase in the dilative character of the tested materials; offering implications for various geotechnical applications. Moreover, pertinent correlations were obtained between the residual shear strength and the grain size characteristics of the sand-biochar mixtures.