A　series　of　model　tests　were　conducted　to　simulate　the　active　failure　of　narrow　cohesionless　backfills　behind　retaining　walls　rotating　about　the　base　(RB　mode).　The　tests　aimed　to　investigate　the　effect　of　wall　displacement　magnitude,　backfill　widths,　and　the　inclinations　of　retaining　walls　and　existing　structures　on　the　failure　mechanism　and　earth　pressure.　The　test　results　revealed　that　the　rupture　propagation　follows　a　progressive　top-down　failure　pattern　and　does　not　extend　to　the　base　of　the　wall　under　RB　mode,　contrasting　with　the　assumptions　of　the　existing　theoretical　solution.　Notably,　a　narrower　backfill　exhibited　multiple　parallel　shear　bands　in　contrast　to　the　semi-infinite　backfill,　highlighting　the　significant　impact　of　backfill　geometry　on　the　orientation　of　these　shear　bands.　Furthermore,　the　active　earth　pressure　distribution　under　RB　mode　displayed　an　approximately　linear　trend,　slightly　reducing　earth　pressure　near　the　base.　The　development　of　earth　pressure　suggested　that　the　backfill　reached　the　active　limit　state　after　the　wall　had　experienced　a　displacement　equal　to　0.35%　of　its　height　(H).　It　was　observed　that　the　active　earth　pressure　for　a　backfill　width-To-height　ratio　(B/H)　of　0.5　closely　corresponded　to　values　obtained　through　the　Coulomb　method.　Moreover,　the　results　indicated　that　the　active　earth　pressure　increased　proportionally　with　an　increase　in　the　B/H　ratio　and　a　decrease　in　the　inclinations　of　both　the　retaining　structures　and　existing　structures.　©　2024　American　Society　of　Civil　Engineers.