Case Study of Bad, to Worse, to Success: Emergency Investigation and Trenchless Repair of a 48-in. Diameter Sanitary Sewer Interceptor Crossing at Interstate I-49

In 2021, the City of Fayetteville (the City) performed a remote inspection of the 48-in. gravity sewer from Gregg Avenue to Sunshine Road. Installed in 2006 via hand tunneling, the pipeline is a 48-in. diameter centrifugally cast fiber-reinforced polymer mortar (CCFRPM) pipe inside a 72-in. diameter steel tunnel liner, originally designed with a grouted annulus. The tunneled segment is 913-ft long with fill cover up to 24 ft and runs under Interstate 49 and the Southbound Highway 112 Exit Ramp. The City's inspection revealed a 100-ft section of localized cracking, damage, and high deflections under the interstate exit ramp, which required trenchless repair. The City retained the investigate-design-build team of Structural Technologies and their subconsultant, Simpson Gumpertz & Heger Inc., to investigate and repair the tunnel. This case study highlights the numerous challenges and multidisciplinary efforts of the project team to successfully implement trenchless repair, including setting up a bypass for the root cause investigation, discovery of unstable tunnel conditions, design of custom shoring solutions, and a three-stage tunnel stabilization effort to allow for installation of a CFRP liner. Thermal sensitivity of the carrier pipe also required a thermal control plan to limit the heat of hydration during grouting.