Although located at the base of Pikes Peak, the Colorado Springs region of Colorado draws much of it water from mountain sources located over 100 miles away. A significant portion of the city’s raw water transmission system dated to the 1960s and reliability was becoming a concern. Better protection against drought as well as an expanded water supply to meet future regional growth was also needed. A solution that could provide “water for generations” became the Southern Delivery System (SDS).
Water for Generations
SDS conveys raw water from an existing storage reservoir on the Arkansas River outside Pueblo, Colorado through a 45-mile 66 inch diameter steel pipeline to a new 50-MGD water treatment plant on the outskirts of Colorado Springs. SDS also includes 3 pump stations and 5 additional miles of large-diameter finish water pipeline. A mile-long tunnel under Interstate 25 and sensitive wetland area was also needed.
Initial SDS planning started in the early 1990s. A 6-year permitting and environmental review process preceded program authorization in 2009. Primary construction commenced in spring 2011—nearly two decades after the project was conceived.
Steel Pipe: Engineered for Value
In addition to moving water 45 miles, a 1,000 ft gain in elevation also had to be overcome. Following pressure class design principles, multiple steel pipe wall thicknesses were specified. Thicker wall pipe—up to 0.734 inches– was generally used for lower reaches of the pipeline where working pressures reach 450 psi. Upper reaches of the pipeline used wall thickness as low as 0.187 inch. In deep cover areas–in some locations as deep as 25 ft.–flowable fill was also utilized as a means of controlling pipe deflection instead of increasing pipe wall thickness.
Bell and spigot lap joints with a single fillet weld were also used throughout much of the project. Weld-after-Backfill (WAB) was also incorporated with the use of single lap joints. With WAB, the pipe joint is assembled, coated with a heat shrink sleeve and backfilled. Lap joint welding from the interior is then undertaken, with the heat from welding dissipated by the surrounding embedment. Because pipe welding can be performed away from an active open trench area, WAB affords faster pipeline installation rates. Also, less open trench is needed, reducing construction impacts on the public. WAB is estimated to have saved SDS nearly $1.4 million.
In keeping with the high design standards set for SDS, each installation contractor had to first demonstrate its ability to successfully perform WAB, matching as close as possible to actual lap weld installation conditions using the same diameter and wall thickness pipe, heat shrink sleeve, buried condition, and field welders. Northwest Pipe partnered with each installation contractor to provide test pipe and on-site expertise to ensure each WAB demonstration was successful.
The early completion of the raw water pipeline was key to the overall SDS construction schedule. Pipe was manufactured mostly in 50 foot nominal lengths and shipped one pipe per truck to the jobsite. During peak construction, Northwest Pipe shipped up to 30 loads of pipe per day, serving three installation contractors and five construction headings. Nearly 7,000 loads of pipe were ultimately required for SDS. In addition to raw water pipeline, Northwest Pipe also furnished 90-inch diameter pipe for the outlet connection at Pueblo dam; 54, 42 and 36-inch diameter pipe for finished water transmission and 84, 60 and 42-inch diameter yard pipe at the new WTP. In total, Northwest Pipe performed under 15 separate SDS constructions packages.