This　paper　presents　an　experimental　investigation　into　the　interaction　mechanism　between　aqueous　foam　and　unsaturated　granite　residual　soil　during　conditioning.　Contact　filter　paper　tests　and　undrained　shear　tests　were　used　to　analyze　foam＇s　effects　on　soil　water　retention　and　shear　behavior,　while　surface　tension　tests,　capillary　rise　tests,　and　microscopic　observations　examined　the　role　of　soil　particles　in　foam　stability.　The　findings　demonstrate　that　foam–conditioned　granite　residual　soils　exhibit　three　distinct　saturation–　dependent　phases　(soil–only,　transition,　and　soil–foam　mixture)　governed　by　foam＇s　gas–liquid　biphasic　nature,　with　foam　injection　effectively　reducing　matric　suction　in　unsaturated　conditions.　Increasing　foam　injection　ratio　reduces　shear　stress　while　enhancing　pore　water　pressure,　with　vertical　displacement　transitioning　from　contractive　to　expansive　behavior　at　low　shearing　rate.　Effective　cohesion　stress　varies　with　gravimetric　water　content　via　a　rational　function,　while　other　effective　cohesion　stress　and　friction　angles　with　respect　to　foam　injection　ratio,　shearing　rate,　and　gravimetric　water　content　obey　exponential　relationships.　The　probability　distribution　function,　cumulative　distribution　function,　and　decay　pattern　of　bubbles　in　foam–only　systems　and　soil–foam　mixtures　all　exhibit　exponential　relationships　with　elapsed　time.　Furthermore,　a　new　water–meniscus　interaction　model　was　established　to　characterize　rupture　and　stabilization　mechanisms　of　foam　in　unsaturated　granite　residual　soils,　with　particular　emphasis　on　capillary–dominated　behavior.　Saturation–dependent　particle　contact　modes　were　identified　for　foam–conditioned　unsaturated　granite　residual　soils,　offering　valuable　guidance　for　enhancing　soil　conditioning　protocols　in　earth　pressure　balance　shield　tunneling　operations.　©　2024