Pit-in-pit　(PIP)　excavations　in　an　aquifer-aquitard　system　likely　undergo　catastrophic　failures　under　the　hydraulic　uplift,　the　associated　undrained　stability　problem,　however,　has　not　been　well　analyzed　in　the　past.　To　this　end,　a　hypothetical　model　of　PIP　braced　excavation　in　typical　soil　layers　of　Shanghai,　China　is　developed　using　the　finite　element　limit　analysis　(FELA)　tool.　The　FELA　solutions　of　safety　factors　(FSs)　against　hydraulic　uplift　are　verified　with　the　results　from　the　finite　element　analysis　with　strength　reduction　technique　(SRFEA)　and　existing　design　approaches.　Subsequently,　FELA　is　employed　to　identify　the　triggering　and　failure　mechanisms　of　PIP　braced　excavations　subjected　to　hydraulic　uplift.　A　series　of　parametric　studies　considering　the　various　geometric　configurations　of　the　PIP　excavation,　undrained　shear　strengths　of　aquitard,　and　artesian　pressures　are　carried　out.　The　sensitivities　of　relevant　design　parameters　are　further　assessed　using　a　multivariate　adaptive　regression　splines　(MARS)　model　that　is　capable　of　accurately　capturing　the　nonlinear　relationships　between　a　set　of　input　variables　and　output　variables　in　multi-dimensions.　A　MARS-based　design　equation　used　for　predicting　FS　is　finally　presented　using　the　artificial　dataset　from　FELA　for　practical　design　uses.