The　inner　core　of　traditional　stiffened　deep　mixed　piles　is　typically　a　prestressed　concrete　cylinder,　relying　solely　on　friction　and　bonding　between　the　inner　and　outer　cores　to　transmit　load.　Field　investigations　reveal　that　the　bonding　force　between　the　soil-cement　pile　and　the　tubular　pile　interface　is　minimal.　When　the　design　is　carried　out　according　to　specifications,　the　shear　strength　of　the　inner　interface　is　often　significantly　low.　To　address　this　issue,　the　inner　core　prestressed　concrete　cylinder　is　replaced　with　a　steel　pipe　pile,　and　the　shear　strength　of　the　interfece　between　the　inner　core　prestressed　concrete　pipe　pile　and　the　outer　core　cement-soil　pile　is　investigated.　Additionally,　a　steel　pipe　pile　with　spiral　blades　on　its　outer　surface　is　proposed　as　a　new　type　of　inner　core　pile.　Shear　tests　on　the　internal　interface　were　conducted　on　pile　section　samples　with　inner　cores　of　non-ribbed　steel　pipes　and　spiral　blade　steel　pipes.　The　factors　influencing　the　ultimate　shear　force　of　the　internal　interfece　were　analyzed,　and　the　quantitative　data　were　characterized.　The　shear　failure　mode　of　the　inner　and　outer　cores　of　the　new　composite　pile,　along　with　the　evolution　law　and　calculation　method　of　the　shear　strength　parameters　of　the　inner　interface,　were　determined.　The　shear　strength　parameters　of　the　inner　interfece　in　the　bearing　capacity　calculation　formula　of　the　pile,　as　specified　in　the　Technical　specification　for　strength　composite　piles　(JG/T　023—2007),　were　modified.　©　2025　Biodiversity　Research　Center　Academia　Sinica.　All　rights　reserved.