Trenchless Repair of Critical Sanitary Force Main

The Norwich Public Utilities (NPU) combined sewer system dates to the mid-19th century. Deferred maintenance, infiltration, grit, and aging infrastructure have contributed to much of the system becoming physically and functionally obsolete. This presentation provides a discussion about the repair of a segment of compromised 24-inch ductile iron force main located beneath the Yantic River in Norwich, CT and the collaborative effort between Norwich Public Utilities and Vortex Services to complete the repair. The force main conveys more than half of the City's wastewater to the Norwich wastewater treatment plant, including large volumes of combined sewer flows during wet weather. Based on observed sanitary discharge to the tidally influenced Yantic River in January 2020, NPU and Jacobs Engineering began the investigation of the apparent force main failure. It was determined that the 24-inch force main had developed a leak near the west bank of the east channel of the Yantic River. NPU took emergency action by installing a dewatering well adjacent to the broken pipe and pumping directly into the force main downstream of the break. This temporary measure mitigated discharge to the river and allowed the development of a plan to install over 700 feet of HDPE sewer bypass and isolate the damaged section of force main. After bypassing flows from the damaged section of force main, it was determined that the leak was below river surface elevation, evident by the significant amount of infiltration seen entering the force main. NPU hired Vortex Services to clean and inspect the pipeline using CCTV and sonar technologies. By tethering foam pigs, camera, and sonar, the approximate location of the leak was determined. Using data from the sonar and CCTV inspection in addition to available record information and field measurements, construction documents were drafted, and the work was put to public bid. The proposed work would consist of the installation of one or two internal pipe seal sleeves to slow or stop infiltration followed by installation of a glass pressure liner system through the 150-foot length. Despite reasonable contractor interest in the construction documents, response to the bid was limited and considerably higher than expected. The bid was not awarded because of the high cost for the proposed work. Most of the overpriced work was associated with excavation and dewatering, which was not surprising given the limited information available about subsurface conditions. Jacobs and NPU worked together to restructure the construction documents and allocate the portions of work that appeared to be overpriced to NPU underground crews. The construction documents were developed in a manner that would allow the NPU to complete excavation, dewatering, and disassembly of the force main to the fittings just below the river elevation at low tide. Installation of two sacrificial lengths of C900 PVC coupled to the existing ductile iron force main allowed the excavations to be backfilled to grade, so that most of the remaining repair and lining work could occur without excavation, at ground surface. This feature saved countless hours of excavation, re-excavation, and dewatering during the remaining phases of the work. Jacobs reconfigured the construction documents to reflect a scope of work associated with the repair of the leaking force main with internal pipe seal sleeves. There were several issues with the installation of the internal pipe seal sleeves, the most impactful being the extensive deterioration of the ductile iron pipe. Based on the CCTV and sonar data, it was assumed that the damaged section of force main was limited to a single point of failure just above the lower fitting. During the installation of the internal pipe seal sleeve, it was determined that the actual extent of the damage was more involved than initially thought. Working with the contractor, a series of internal pipe seal sleeves were installed in an overlapping/shingled pattern. While this slowed the infiltration, the volume of infiltration was still too high to allow for the installation of a CIPP liner as planned. A plan was then made to stop the gushing Infiltration using Cured-in-Place (CIP) segmental liners installed on top of the internal pipe seal sleeves. The CIP segmental liners were installed beyond the limits of damaged pipe to prevent infiltration from following behind the liners. After the infiltration was stopped, the contract documents were revised to show a scope of work limited to the installation of the Cured-in-Place Pipe (CIPP) liner and two end seals with a required test pressure of 75 psi. Excavation and dewatering for the installation of the end seals would be completed by NPU in collaboration with the lining contractor's schedule. The first attempted lining was aborted when defects in the liner material were noticed by the contractor just prior to installing the liner. The second attempt ended with a cured liner installed in place. Pressure testing of the liner revealed minor pressure losses. The pressure loss issues are being resolved at the time this abstract is submitted. The presentation will expand on the most interesting aspects of the work: -The cost savings associated with bifurcating the work to move the more uncertain work efforts associated with excavation and dewatering to the NPU underground crew. -The advantages associated with public utilities having capable and competent work crews. -The financial benefit associated with rehabilitation over replacement-based alternatives. -The value realized when there is collaboration between owner, contractor, and engineer.