Bridge　pier　failures　from　granular　flow　impacts　are　common.　Installing　defense　piles　upstream　is　an　effective　mitigation　strategy,　yet　their　protective　mechanisms　and　standardized　design　guidelines　are　unclear.　This　study　employed　3D　discrete　element　method　to　analyze　the　influence　of　defense　pile　size　and　placement　on　its　performance　across　219　scenarios,　providing　a　detailed　examination　of　their　protective　mechanisms.　Results　show　that　optimizing　these　factors　can　reduce　the　maximum　impact　force　on　bridge　piers　by　up　to　94%.　In　terms　of　size,　a　critical　height　threshold　is　identified,　beyond　which　increasing　pile　height　does　not　enhance　protection.　This　threshold　depends　on　the　movement　height　of　granular　particles　at　the　slope　base.　Protection　effectiveness　varies　with　pile　size:　when　H　≤　0.05　h　(H　is　the　height　of　defense　piles,　h　is　the　height　of　bridge),　protection　marginally　improves　with　increasing　height　and　diameter;　for　0.05　h　＜　H　＜　0.15　h,　protection　strongly　correlates　with　both　parameters;　for　H　≥　0.15　h,　diameter　becomes　the　dominant　factor.　In　terms　of　placement,　an　optimal　longitudinal　distance　exists　between　the　defense　pile　and　the　bridge　pier.　The　larger　the　diameter,　the　greater　the　optimal　longitudinal　distance.　However,　the　transverse　distance　is　inversely　related　to　protection　effectiveness.　Mechanistic　analysis　shows　that　defense　piles　are　more　effective　at　redirecting　particles　to　prevent　direct　collisions　with　the　pier　(contributing　100%　impact　energy　reduction　before　the　non-dimensional　travel　time　t*　=　7.01　and　63%　–　100%　afterward)　than　at　reducing　particle　velocity.　This　study　provides　insights　into　the　protective　mechanisms　of　defense　piles　and　informs　strategies　for　optimizing　bridge　pier　protection　in　granular　flow-prone　regions.　©　Science　Press,　Institute　of　Mountain　Hazards　and　Environment,　CAS　and　Springer-Verlag　GmbH　Germany,　part　of　Springer　Nature　2025.