Design of Rehabilitation of the Failed Masonry Sewer Wye Intersection

This paper discusses the design challenges of repairing the failed, non-redundant, masonry sewer wye intersection, where flow could not be diverted or stopped. The Northwest Boundary Trunk Sewer (NWBTS) is a 19th-Century brick sewer, which serves as a critical infrastructure component of Downtown DC's sewer system. During inspections, it was discovered that the gravity sewer exhibited significant signs of structural damage, including approximately 10 ft by 5 ft section of missing bricks at the crown, a hole above with eroded soil up to the pavement level, and adjacent hollow sounding and delaminated regions of masonry. The geometric and environmental constraints included complex geometry at the intersection of two circular sewers, hydraulic considerations of the rehabilitation, strong continuous flow, and unexpected pressurization during storm events, which posed major design and constructability challenges. Potential internal repair options such as carbon fiber-reinforced polymer, steel plates, arched concrete shells, and full circumferential shotcrete repair were considered. Each repair option was evaluated for constructability with access through a single manhole and periods of low and high flow, impact on flow, construction sequencing, and required time for completion. A LiDAR scan was utilized to determine the geometry of the failed section and eroded soil above. Archival research was performed to understand the structural behavior of the original masonry wye section. The primary design loads considered were self-weight, weight of fill, live load, and internal pressure due to storm events. The final rehabilitation scheme comprised a full circumferential reinforced shotcrete repair with localized reinforcing steel detailing that recreates the lost arch mechanism of the original masonry sewer. The design factored in critical construction activities such as filling of the missing bricks, removal of temporary shoring, and void stabilization, which enabled the rehabilitation of the wye from inside the sewer while maintaining continuous sewer operation.