Northeast Boundary Tunnel - Mount Olivet Road Drop Shaft-PPT Presentation

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The Mount Olivet Road (MOR) drop shaft was built as part of Washington, D.C.’s Northeast Boundary Tunnel (NEBT), a 26,700-foot combined sewer overflow storage and conveyance project. The MOR site included a 122-foot-deep drop shaft, a 73-foot-deep access chamber, and a 39-foot-deep ventilation structure, with several excavation-support walls shared between structures.<br /><br />Design requirements included a 100-year service life for permanent works, compliance with ACI 350 for underground concrete structures, and protection against groundwater pressures during construction. The design used hand calculations for excavation support, supplemented by finite element modeling (FEM) to assess three-dimensional loading, predict ground movement, develop groundwater control measures, and evaluate partially completed permanent structures.<br /><br />FEM analyses compared short-term undrained and drained soil conditions. Instrumentation data showed that the undrained model best represented excavation-support loads and deformation of the permanent shaft structure. Drained-condition modeling was not considered representative of actual construction behavior, but helped prepare contingency measures for unexpected ground conditions.<br /><br />Monitoring, with established action and maximum thresholds, reduced risks during near-surface excavations. Contingency measures were prepared in case monitored movements or bracing loads exceeded action levels.

Keywords

Mount Olivet Road

Northeast Boundary Tunnel

combined sewer overflow

drop shaft

excavation support

finite element modeling

groundwater control

undrained soil conditions

instrumentation monitoring

ACI 350