Under　weak　geological　conditions,　soil　deformation　and　surface　settlement　are　the　key　factors　affecting　the　success　of　shallow-buried　rectangular　excavation.　To　investigate　this　issue,　an　underpass　of　Zhongxiao　East　Road　in　Taipei　City　was　used　as　a　case　study.　The　surface　settlement　and　lateral　deformation　of　an　underground　diaphragm　wall　caused　by　the　excavation　of　a　rectangular　pedestrian　underpass　using　the　pipe-roof　preconstruction　method　(PPM)　were　investigated　by　3D　finite　element　analysis.　The　numerical　analysis　results　showed　that　the　constructed　numerical　analysis　model　had　considerable　accuracy.　The　use　of　PPM　combined　with　a　box　culvert　structure　to　form　a　pedestrian　underpass　could　effectively　control　the　surface　displacement　above　the　box　culvert.　Under　the　condition　of　the　same　sectional　area,　the　smaller　the　width　of　the　pipe-roof　structure,　the　more　the　impact　on　the　ground　surface　was　reduced.　The　maximum　positive　bending　moment　and　maximum　negative　bending　moment　on　the　pipe　roof　produced　by　excavation　at　each　stage　were　roughly　inversely　related　to　the　height　per　the　width　of　the　cross-section　of　the　pipe　diaphragm　structure.　The　results　showed　that　the　pipe-roof　structure　was　suitable　for　underground　excavation　with　shallow-buried　depth　in　the　soft　soil　of　the　Taipei　Basin.　Moreover,　the　shallow-buried　box　culvert　was　more　sensitive　to　the　subsidence　caused　by　construction　than　the　deep-buried　box　culvert.