The design-build team of our firm and Garney Construction, with Burns & McDonnell serving as owner’s representative, evaluated project options and sought to minimize the impact on the community, cost, and schedule. We capitalized on a progressive design-build delivery method to slipline the tunnel, saving the district tens of millions of dollars and expediting a potential multi-year timeline, all while maintaining continuous utility service.

Our approach had rarely been used for water and wastewater projects, and never for a project as ambitious as the Colsman Tunnel, which required a nearly 1.5-mile existing tunnel pipeline pull-in. Construction was completed in just six months. The fused high-density polyethylene (HDPE) pipe was fed into the tunnel at the western portal, with a machine set up at the eastern portal for the pull-in. Grouting the annular space between the tunnels was a critical phase of the project, involving a barometric loop to keep the pipeline full and mitigate thermal expansion and pipeline buoyancy. Garney Construction oversaw the drilling of injection points over stretches of 600 feet, up to 90 feet underground, and verified the grout placement with specially built cameras. Together, we addressed many other challenges, including determining the exact geometry of the existing tunnels without reliable records. We also determined pipe loading/stresses with the long HDPE pipeline installed in a 1.5-mile-long tunnel with no intermediate access in live flow.