Case Study: Challenges to Install Vertically Curved HDPE Casing Pipes for 36-Inch Large Diameter Water Mains Crossing Highway

The proposed case study is a part of a state highway capital improvement project. In order to meet future growth, large diameter water mains were proposed to cross a proposed depressed freeway prior to freeway construction. The original design proposed a conventional casing method using welded steel casing utilizing a straight vertical profile. Considering all existing topographical features and design elevations for the roadway, the original design proposal was expected to require a significant amount of earthwork, deep connection to future water mains, and deep trenching adjacent to state lands that were not to be disturbed. Design alternatives were evaluated and proposed to reduce construction project costs, optimize project schedule, and avoid the need to extend construction limits onto state lands. Two alternatives were proposed to install the high-density polyethylene (HDPE) casing (1) with the straight profile under 40 ft from the existing ground and (2) with rolling curve profile under 10 to 20 ft from the existing ground. Design Alternative 2 was selected and saved approximately $4 million as compared to the original design in addition eliminating the need to work in state lands and facilitating operations and future connections. This paper describes technical challenges of the original design, reviews design alternatives, and evaluate value engineering implementations, which included cost, schedule, risk, and benefit analyses. This case study also summarizes lessons learned from the new design and construction practice to overcome the challenges.