Northeast Boundary Tunnel - Mount Olivet Road Drop Shaft-Technical Paper Upload

Technical Paper Upload

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The Mount Olivet Road Drop Shaft (MOR-DS), part of Washington, D.C.’s Northeast Boundary Tunnel project, is a 120.5-ft-deep circular shaft built using slurry-wall methods. Nearby approach-channel and ventilation-vault excavations reached depths of 75 ft and 30 ft and required slurry-wall support with multiple levels of internal bracing.<br /><br />The original construction sequence required completing and backfilling the shaft before excavating the adjacent structures. To accelerate construction, the contractor proposed excavating the approach channel and ventilation vault concurrently, while the shaft’s permanent structure was still incomplete. This raised concerns that excavation-induced forces could overstress or deform the shaft.<br /><br />Engineers used three-dimensional finite-element modeling to assess the revised sequence. The analyses compared short-term undrained soil behavior with conservative drained-soil scenarios. The undrained model, which also accounted for later changes in soil properties, best represented the expected construction conditions. Consolidation estimates indicated that the cohesive soils would remain largely undrained during excavation. This model predicted shaft movements of about 0.1 in. A drained-soil model predicted greater movement and substantial overstress, while a third model showed that additional internal bracing and a diaphragm slab could protect the shaft if drained behavior occurred.<br /><br />The analyses informed monitoring thresholds and contingency plans. Shape arrays measured shaft deformation, and strain gauges tracked loads in the approach-channel braces. After accounting for temperature effects on brace readings, instrumentation generally supported the undrained model’s predictions.<br /><br />The study concluded that concurrent excavation could proceed safely with monitoring and predefined response limits. The conservative drained analyses were not considered representative of actual conditions, but they provided contingency designs to manage the unlikely event of greater loads or movements.

Keywords

Mount Olivet Road Drop Shaft

Northeast Boundary Tunnel

slurry-wall construction

concurrent excavation

finite-element modeling

undrained soil behavior

drained soil scenario

internal bracing

shaft deformation monitoring

Washington D.C. excavation