Thinking Outside the Box: Innovative Emergency Repairs on Twin Outfall Relief Sewers

A twin outfall relief sewer system for a major municipality is comprised of twin box structures approximately 11 LF in height and 10-12 LF in width, with average flows of 200-700 MGD. This sewer system is located below a military facility, making all inspections and repairs demand additional security considerations. During recent inspections of the twin outfall relief sewer system, CCTV footage showed areas of the sewer structure with extensive corrosion damage from microbial hydrogen sulfide (H2S). The damage caused by the H2S was so severe that the center wall between the twin box structures was missing for approximately 75 LF and much of the inner layer of reinforcement was also missing. The Owner's engineer performed structural analysis and determined the degraded portion of this structure was near collapse. Based on the as-found conditions and analysis performed, emergency remediation of approximately 910 LF of rectangular box structure was deemed necessary. The next step to minimize risk of collapse involved removal of the soil overburden in the region where the sewer system showed severe distress. The project team worked to develop a bypass system to reroute flows so permanent repairs could take place. Given the live flows within the structure, rehabilitation options needed to be selected prior to the bypass system being in place and the structure dewatered. This uncertainty regarding the condition of the structure required the repair system selected to be flexible to accommodate varying degrees of degradation. An innovative sewer rehabilitation technology was selected which involves installation of custom fabricated steel rings, HDPE interior protective layer, and a flowable grout to fill the space between the HDPE and the host structure. The design of the rehabilitation system can be adjusted based on as-found conditions, giving the project team the flexibility needed to move forward with repair design in parallel to installing the bypass and performing a more thorough condition assessment of the structure. This technology has a history of over 20 years of successful implementation in Japan with over 250,000 LF of successful pipeline repairs. This extended history along with extensive technical vetting allowed the project team to feel confident moving forward with a first-of-a-kind application in the United States. This paper will highlight the investigation, analysis, design, and emergent repair of this critical structure. Perspectives from the Owner, Engineer, and Contractor will be provided, along with insight on the options analysis process, and lessons learned throughout.