Critical Thinking and Swift Mitigation in Saving a Century-Old Masonry Trunk Sewer in Washington, DC

During DC Water’s multiple-camera entry inspection for an in-depth condition assessment of the Northeast Boundary Trunk Sewer (NEBTS), an abnormal condition with severe invert heaving, crown sagging, longitudinal fracture, and up to 10% ovality in a century-old 72″ brick sewer trunk sewer triggered DC Water Engineering attention. DC Water engineers coordinated with DC Water’s permitting and construction management teams to conduct a thorough investigation and review all adjacent construction activities in the damaged area. The investigation found that a tieback from the recently completed building was placed too close to the invert of the brick sewer. The pressure grouting in the tiebacks heaved the brick invert causing the crown and invert to deform and crack. The risk assessment revealed that there was a potential high impact sinkhole in the narrow street congested with a gas line, two other sewer lines, two water lines, and multiple duct banks above the 72″ brick sewer. To prevent a catastrophic sinkhole, an immediate “no-dig” zone in these blocks was enforced, and DC Water engaged a sole source high confidence specialty contractor to perform carbon fiber reinforced pipe (CFRP) laminating to structurally replace the compromised brick sewer without compromising hydraulic capacity. This paper will present how DC Water performed every critical step to manage the risk and successfully averted a catastrophic failure, handled design and construction challenges, and lessons learned.