The　girder,　abutment　and　pile　of　integral　abutment　jointless　bridges　(IAJB)　are　integrated.　In　IAJBs,　the　backfill　soil　behind　abutments　is　directly　subjected　to　the　horizontal　cyclic　displacements　originated　by　the　expansion　and　contraction　of　girders　under　thermal　loadings.　Meanwhile,　the　monolithic　behavior　of　the　abutment　and　pile　of　IAJBs　causes　more　complexity　in　calculating　the　backfill　earth　pressure　in　IAJBs　as　compared　with　other　bridges　and　earth　retaining　structures.　In　this　paper,　it　is　shown　that　the　existing　earth　pressure　calculation　methods　are　inaccurate　for　IAJBs;　therefore,　a　modified　method　for　calculating　the　backfill　earth　pressure　behind　the　abutment　and　the　internal　forces　of　the　piles　of　the　IAJBs　is　proposed.　For　this　purpose,　a　quasi-static　cyclic　test　was　performed　on　a　scaled　specimen　to　understand　the　behavior　of　backfill　earth　pressure　and　internal　forces　of　the　integral　abutment　and　the　pile　during　expansion　and　contraction　of　the　bridge　considering　the　soil-structure　interaction.　The　test　results　including　bending　moment　and　shear　force　of　the　abutment　and　pile　were　briefly　discussed.　In　addition,　the　external　forces　at　the　bottom　of　the　abutment　calculated　by　the　existing　earth　pressure　methods　were　compared,　and　the　internal　forces　of　the　pile　based　on　the　elastic　foundation　beam　method　were　discussed.　The　analysis　results　showed　that　the　bending　moment　distribution　of　the　pile　under　the　positive　displacement　loads　(during　the　expansion　when　the　abutment　was　pushed　toward　the　backfill)　was　significantly　larger　than　that　under　the　negative　displacement　loads　(during　the　contraction　when　the　abutment　was　pulled　away　from　the　backfill).　The　internal　forces　of　the　pile　calculated　by　the　existing　earth　pressure　theories　based　on　the　elastic　foundation　beam　method　were　much　different　from　the　test　results.　Meanwhile,　the　internal　forces　of　the　pile　calculated　by　the　proposed　method　were　more　accurate　and　can　be　used　as　a　reference　for　the　design　of　IAJBs.　©　2022,　Wroclaw　University　of　Science　and　Technology.