Force main repair increases flow capacity at Valley Forge
When Tredyffrin Township, Pa., faced three catastrophic failures on the Wilson Road Force Main (WRFM) between 2012 and 2014, they knew an innovative fix was in order. Operated and maintained by Tredyffrin Township, the WRFM is a 30-in. prestressed concrete cylinder pipe sanitary sewer force main and part of the Valley Creek Trunk Sewer (VCTS). Owned by the Tredyffrin Township Municipal Authority (TTMA), a group of five municipalities that discharge into the VCTS system, much of the main is located within the Valley Forge National Historical Park. Totaling more than 18,000 ln ft, the pipe follows Valley Creek to its confluence with the Schuylkill River and across to the Pawlings Road Wastewater Treatment Plant.
History of Failure
The first failure in March 2012 resulted in a wastewater spill to Valley Creek. Second and third failures occurred only 42 days apart in spring 2014. This resulted in additional spills into the creek, both affecting the same major intersection near Washington’s headquarters. To mitigate any further issues, Tredyffrin proactively entered into a voluntary consent decree with the Pennsylvania Department of Environmental Protection (PADEP) to ensure a solution was reached in a timely manner given the complex stakeholder relationships and involvement from environmental groups.
Due to the physical location, social and constructability constraints, it was determined the ideal approach would be a rehabilitation solution utilizing the existing force main. The engineer sought a structural solution using a “tried-and-true” method that was cost competitive, trenchless and maintained or improved capacity of the line. Methods considered were cured-in-place pipe lining, slip lining and compressed fit lining.
Increasing Flow Capacity
Increasing flow capacity also was a priority. With this in mind, the Tite Liner system—a close-fit high-density polyethylene (HDPE) lining system— was chosen for the project. Installed by United Pipeline Systems, an Aegion company, the system has the ability to provide a fully-structural solution when pulled into place inside an existing pipeline. The engineer’s modeling showed that the system would increase flow capacity of the line due to the improved flow coefficient of HDPE pipe. The modeling results and the fact that HDPE is accepted in the industry for force main replacement and rehabilitation made it a suitable choice.
In addition to the improved flow, the installation method would help minimize disruption within the park’s limited jobsite footprint, while also offering low material costs and long installation sections. Prior to installation, DR 26 and DR 32.5 HDPE liner was engineered and custom-made to be slightly larger than the host pipe. The liner was later installed using United’s proprietary mechanized roller box. The longest pull topped 1,500 ft and pulls averaged 600 ft due to the number of bends on the existing alignment. The lining portion of the project was completed over three months in 31 separate installations—on time and on budget—with no safety incidents or wetlands impacts. Not only was the existing pipeline successfully rehabilitated to prevent future wastewater spills, but it also expanded system capacity from 20 to approximately 28 million gal per day.
Complex Stakeholder Environment
The project faced a combination of unusual technical challenges in the highly sensitive national park setting with proximity to sensitive rivers and streams. These challenges were heightened by emergency-type work conditions, making project team communication and community coordination essential.
Tredyffrin Township engineering staff led the commitment from the project stakeholders, which included the five TTMA communities, state and local politicians, engineering, park staff, the contractor and specialty subcontractor. This commitment was integral to successful project completion. Weekly communication with the core team (township engineer, consulting engineer, contractor and specialty subcontractor) to manage outward expectations of the other stakeholders was critical so that coordination could be made with park staff. Road and bridge closures were timed and permitted with the transportation agency since road closures were a major inconvenience to constituents. The work was subject to intense scrutiny.
“Everyone worked together as a team and United Pipeline did an exemplary job,” said Susanne Lockhart, project manager for CH2M and engineer of record on the project. “All parties realized how important this project was to Tredyffrin Township and the park. We were standing shoulder to shoulder working toward a common goal.”
Examples of that teamwork include site work and erosion control by Pact One LLC (the prime contractor), 8 miles of difficult bypass constructed by Sunbelt Rentals and specialty subcontracting by United Pipeline Systems, as well as support by the Township, PADEP and other environmental agencies during the permitting processes.
“United Pipeline was a pleasure to work with—they came in with their equipment and got busy fusing pipe,” Lockhard said. “They worked in a methodical, collaborative and efficient manner along with Pact One that facilitated the positive project outcome.”
The township engineer provided much needed coordination with regulatory agencies. Permits that normally would take months to obtain were pushed through in a matter of weeks. While a project of this size and scope usually would take years of permitting and planning, the emergency nature of the project made it essential for the project to be completed within 18 months.
Although the township had not previously used compressed-fit HDPE technology, the engineer was comfortable recommending HDPE as it has been used for force main rehabilitation industry-wide for more than 30 years.
The township and engineer felt that the WRFM was the perfect application for this project due to the trenchless installation, maximization of cross-sectional area, potential for long pull lengths and the ability to provide a structural solution. Additionally, the increased hydraulic capacity of the rehabilitated force main will enhance the capacity of the main for another 25 years. After successful completion of the emergency rehabilitation of the WRFM, the project was not only the largest project ever undertaken by Tredyffrin Township and TTMA, but also the largest-ever compressed fit HDPE force main rehabilitation project in United’s history. “
This project has been a career highlight due to the many complexities—so many environmental and archeological constraints, emergency design and construction schedule, fabulous collaborative client and contractors,” Lockhart said. “To successfully complete the project on time and on budget feels like a real accomplishment.”