DC Water North East Boundary Trunk Sewer
In 2014, a five-story residence was constructed directly over DC Water’s North East Boundary Trunk Sewer (NEBTS) with only about 2 ft of soil between the bottom of the basement and the top of the 22-ft dia. masonry sewer. DC Water inspected the century-old NEBTS and found longitudinal cracks at the crown. SGH helped DC Water and the repair contractor by evaluating the structure before and after the repair, designing emergency repairs, and inspecting the finished work.
DC Water determined the cracks were growing under the load from the building above and needed to further investigate the condition of the sewer. SGH developed two- and three-dimensional finite element models and evaluated the stress state in the sewer. We performed a series of linear and nonlinear analyses to determine the risk for future damage considering the following:
- Soil excavation over the sewer in the area of the building footprint
- Cracking in the crown
- Building loads
- Water pressure
DC Water needed to stabilize the sewer and implement repairs to mitigate crack propagation. They evaluated several repair options and decided to use carbon fiber-reinforced polymer (CFRP) liners. SGH designed the CFRP repairs based on our finite element analysis results and developed special repair details for the project. We helped address field conditions, such as water infiltration during construction, and inspected the sewer after the contractor completed repairs.