Water will begin running uphill—and north—along Colorado's southern Front Range next year when the $841-million Southern Delivery System goes on line.
SDS is one of the largest water projects under construction in the Western U.S. and the biggest project undertaken by owner Colorado Springs Utilities in decades. The project will convey Arkansas River water stored in the Pueblo Reservoir to the cities of Colorado Springs, Fountain, Security and Pueblo West.
Program manager MWH of Broomfield, Colo., has teamed up with Colorado Springs Utilities to build the multi-phased SDS project. Phase 1 components under construction or completed since the project broke ground in 2010 include a reservoir connection at the North Outlet Works of Pueblo Dam, 50 miles of large-diameter water pipeline, three pump stations and a 50-million-gallon-per-day (mgd) water treatment plant.
This month, crews are nearing completion on the Central Pipeline project, which features one of the most complex portions of the SDS system. General contractor Garney Cos. of Littleton, Colo., and its design partner Dewberry Engineers Inc. of Denver took a proactive approach in dealing with the project's myriad environmental and logistical challenges. It runs along a mile-long stretch that interacts with Interstate 25, Fountain Creek and two railroad crossings.
Instead of laying pipe in an open-cut trench, the typical approach, the team designed a solution using a 105- in.-dia boring machine to drill a mile-long tunnel 85 ft below the surface. The tunneling method also proved to be $10 million cheaper than digging a traditional surface trench.
"We had a pretty standard construction process for pipelines—open cut. But as we got a little further along with the design portion, we realized that model didn't fit this specific project, and we conducted a RFP process for design-build," says Colorado Springs Utilities project manager Brian Whitehead.
"That critical decision [to use design-build] brought the very best ideas from private industry to the table," adds Ron Evans, MWH vice president and director of construction management. "The resulting tunneling method protected the interstate and the sensitive Fountain Creek corridor, reduced cost overall and minimized risk for the owner."
"Fountain Creek is a pretty environmentally sensitive waterway for this reach of Southern Colorado, with a 3,000-foot-wide floodplain at our crossing location," says Dewberry's principal-in-charge Randy Parks. "We quickly realized the groundwater and dewatering issues would communicate pretty seriously with that creek. The other issue was depth. It has a pretty major alluvial soil profile and sits atop of shale and clay stone bedrock. Dewatering and tunneling in that environment can be unstable. Saturated running soils can come into your tunnel. The thing that drove us deep into the bedrock was risk."
The tunnel depth for the boring machine was carefully studied to allow the machine to safely pass about 15 ft to 50 ft below the top of bedrock. As the main machine worked through the rock heading east from I-25, a second machine worked under the surface heading west from the east portal, 5,000 ft away. The two machines connected the tunnel in November 2014. Since then, more than 200 sections of pipe have been placed into the tunnel to link the pipeline through challenging terrain.
"The industry has a lot of 300-ft to 400-ft-long tunnels, but very few are this long," says Garney project manager Bill Williams. The team used a small train to bring in supplies and haul out muck while a crew of about 35 people worked to bore the tunnel and install a beam-and-lag system to provide structure and support to keep the tunnel open and safe for workers. The train was also used to install the 20-ft-long pipe segments of the 66-in. SDS water carrier pipe into the tunnel. Welders worked with a 15-in. clearance to weld the pipe together and fill the space between steel pipes and the tunnel walls with grout.
Full-scale demonstrations were performed on the surface to validate the assembly joints. Work on the Central Pipeline project started in November 2013 and was scheduled for completion in mid-April.
Under the Dome
A high-profile, 10-million-gallon precast, prestressed concrete water storage tank is also nearing completion at the SDS water treatment plant site on the east side of Colorado Springs. The largest of its kind in Colorado, this 60-ft-tall, self-supporting dome was constructed on top of a scaffolding system. Dallas firm DN Tanks designed and built the tank for McCarthy Building Cos. of Phoenix, the general contractor.
SDS officials say the storage tank is the largest free-spanning dome in Colorado, which means there are no interior column supports, according to Chris Brown, DN Tanks project manager. Fifty-five exterior wall panels were precast on site and erected in place with a crane to create the 900-ft circumference of the tank. Once the concrete cured, the scaffolding was disassembled and removed piece by piece through a few openings.
DN Tanks works around the country with a core crew of tank builders, but the contractor also tapped into the local labor pool on this project. Most of the materials for the tank were purchased locally to further contribute to the Colorado economy, Brown says.