In　engineering　applications,　narrow　backfills　are　often　squeezed　by　retaining　walls.　Insufficient　information　is　available　with　respect　to　the　passive　earth　pressure,　whereas　the　active　earth　pressure　of　narrow　backfills　against　retaining　walls　has　been　considerably　investigated.　In　this　study,　the　passive　failure　mechanism　of　a　narrow　cohesionless　backfill　under　translation　mode　has　been　investigated　via　a　finite　element　lower-bound　limit　analysis,　considering　the　effect　of　backfill　geometries　and　external　friction　on　the　failure　mechanism.　Multiple-slip　surfaces　could　be　observed　in　a　narrow　backfill　in　the　passive　limit　state.　Further,　we　summarized　the　number　of　slip　surfaces　under　various　conditions　and　proposed　a　fitting　function　for　the　inclined　angles　of　the　slip　surfaces.　In　addition,　a　multiple-slip　surface　calculation　mode　was　developed.　Subsequently,　a　method　was　derived　for　calculating　the　passive　earth　pressure　of　a　narrow　cohesionless　backfill　using　the　differential　slice　element　method,　the　wedge　limit　equilibrium,　and　the　finite　difference　method.　Parametric　studies　explored　the　effects　of　the　backfill　geometries　and　internal　and　external　frictions　on　the　passive　earth　pressure.　The　results　demonstrated　an　increase　in　the　passive　earth　pressure　and　the　number　of　slip　surfaces　when　the　backfill　space　decreased,　in　which　the　passive　earth　pressure　was　nonlinearly　distributed.　©　2020