Gap-graded　soil,　characterized　by　the　absence　of　certain　particle　sizes,　is　commonly　used　in　infrastructure　projects　such　as　dams　and　roadbeds.　A　comprehensive　understanding　of　both　the　macro-　and　micro-mechanical　behaviors　of　discontinuously　graded　soils　is　essential　for　their　effective　use　in　engineering　applications.　In　this　study,　drainage　triaxial　compression　tests　were　conducted　on　four　gap-graded　soil　samples　with　different　fine-grain　contents　mainly　using　the　DEM　method,　whereas　the　flexible　boundary　part　was　performed　using　the　FDM-DEM　method.　The　contacts　were　classified　based　on　the　magnitude　of　contact　forces　between　coarse　and　fine　particles,　considering　the　coordination　number　of　the　particles　involved　and　the　normal　angular　distribution　of　these　contacts.　This　classification　enabled　a　detailed　analysis　of　how　fine　particles　contribute　to　stress　transmission　and　structural　evolution　during　shearing.　The　fabric　tensor　for　these　contact　types　provided　further　insights　into　the　anisotropy　of　samples　during　shearing.　On　the　microscopic　scale,　the　evolution　of　contact　numbers　was　found　to　closely　align　with　the　observed　stress–strain　behaviors.　Increasing　fine　particle　content　significantly　altered　the　role　of　fine　particles　in　the　stress　transmission　process.　With　low　content　of　finer　particles,　initially,　fine　particles　were　situated　within　the　voids　formed　by　coarse　particles,　and　the　fine　particles　are　gradually　embedded　into　the　coarse　particles　during　the　loading　process.　With　the　increase　of　fine　particle　content,　fine　particles　constantly　aggregate　to　block　coarse　particles　and　become　the　main　medium　of　stress　transmission.　©　The　Author(s),　under　exclusive　licence　to　Springer　Nature　Switzerland　AG　2025.