The use of biochar in earthwork and slope engineering has gained significant interest due to its water and nutrition retention capacity that gives buffer against extreme wetting and drying, as well as helping vegetation growth. Recently, biocharmixed soil has been proposed as an alternative cover material for embankments and cut slopes for roads in tropic regions. Designing the hydraulic barriers with biocharmixed soil in earthwork systems needs clarification of water infiltration behavior into the soil. However, the physical and hydro-mechanical properties needed for application and assessment of hydraulic barrier by biochar-mixed soil are not yet fully understood. In this study, the impacts of biochar amount and types of biochar used on the hydraulic and mechanical characteristics of biochar-mixed compacted clayey sand were investigated by testing two different biochars deriving from rice husk and woodchip. The soil compaction test, permeability test, soil water characteristic tests and numerical study for seepage analysis were conducted for studying the biochar-mixed soil compared with mother soil. The microscopy and mercury intrusion porosimetry provided pore size distribution of biochars to deepen the understanding of mechanisms of water retention in biochar-mixed soils. The results showed that biochar addition increases the micropore which is expected to have higher water capacity, owing to the porous nature of the biochars. It is also shown that the biochar-mixed layer could delay water infiltration when biochar amount addition is large and sufficient. With the same biochar content, rice husk can help to reduce water infiltration more than woodchip.