NASTT Abstract: Tredyffrin Township faced three catastrophic failures of a 30-inch Prestressed Concrete Cylinder Pipe (PCCP), the Wilson Road Sewage Force Main (WRFM), between 2012 and 2014.. The majority of force main pipe is located within the national park, and the pipe alignment also parallels Valley Creek, which is an exceptional value trout stream. The compromised pipe alignment is also adjacent to George Washington’s Headquarters during the American Revolutionary War, which is one of the most culturally sensitive and valuable areas of the park. Tredyffrin proactively entered into a voluntary consent decree to ensure a solution was reached in a timely manner given the complex stakeholder environment and threat of legal action from environmental groups. The engineer sought a trenchless rehabilitation pipe lining solution that was cost competitive and maintained or improved flow capacity of the line. Methods considered included cured-in-place pipe (CIPP) lining, conventional slip lining and compressed fit HDPE lining.
Assessment of the 18,000LF force main, determined that external corrosion had degraded the strength of the wire rope inside the 30-inch PCCP, rendering the pipeline unable to handle the system’s operating pressure. Working with the engineer on the project, CH2M Hill (now Jacobs), the method of repair chosen was a compressed fit HDPE pipe lining solution.
A high-density polyethylene (HDPE) pipe liner system can provide a fully-structural solution when pulled into place inside the existing pipeline. Modeling showed that the trenchless system would increase capacity of the line due to its improved Manning’s n-value. The modeling results and the fact that HDPE is widely accepted in the industry for force main replacement and rehabilitation made it the obvious choice.
The compressed fit HDPE pipe lining solution was engineered and custom made to be slightly larger than the host pipe and installed utilizing a roller compression system. The trenchless pipe lining solution helped minimize disruption within the Park’s limited jobsite footprint, while also offering low material costs and long installation sections. The longest pull topped 1,500 feet, with pulls averaging 600 feet. The lining portion of the project was completed over 3 months in 31 separate installations – on time and on budget – with zero safety incidents or wetlands impacts. This paper will highlight the trenchless rehabilitation method selection, project planning and installation.
Susanne Lockhart, CH2M Hill’s project manager/engineer of record contributed, “Everyone worked together as a team and Aegion did an exemplary job. 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/erosion control by PACT ONE, eight miles of difficult bypass constructed by Sunbelt Rentals and specialty subcontracting by Aegion, not to mention support by PA DEP and other environmental agencies during the permitting processes. The Township Engineer provided much needed coordination with regulatory agencies. Permits that would typically take months to obtain were pushed through in a matter of weeks. While a project of this size and scope would normally take years of permitting and planning, construction was successfully completed in less than one year.
Although compressed fit HDPE trenchless technology had not previously been used by the Township, the engineer was comfortable recommending the methodology for this application 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 Wilson Road Force Main, the project was not only the largest project ever undertaken by Tredyffrin Township, but also the largest-ever compressed fit HDPE force main rehabilitation project in history.
To purchase the full version of this case study from NASTT, please click on the below link:
NASTT: Emergency Sewer Force Main Rehabilitation in Valley Forge National Historic Park
NASTT: Trenchless Rehabilitation Project of the Year: Honorable Mention