no prior case studies to review and little in the way of industry guide- lines. The design-build approach, with construction beginning before the design was complete, created additional pressure but also yielded a clear benefit: the team had worked closely together for many years on design-build water/wastewater projects and had confidence in its ability to problem-solve on site. Challenges early on included determining the exact geometry of the existing tunnels without reliable records. The team also focused on identifying pipe loading and stresses that would occur as the HDPE pipeline was installed in a 1.5-mile-long tunnel with no intermediate access in live flow. The fused HDPE pipe was fed into the tunnel at the western portal, with an HDD rig set up at the eastern portal for the pull-in. The pulling of the new carrier pipe into the host pipe proceeded smoothly, as pull forces remained low, and this phase of the project was completed in just over two months. Fast-Paced Problem-Solving The most imposing challenge arrived with the next step: grouting the annular space between the pipe and the old tunnel. A barometric loop was used to keep the pipeline full during grouting to mitigate thermal expansion and pipeline buoyancy. The grout placement was verified with specially built cameras. As this step carried the risk of collapse The custom designed pull head is ready to be fused onto the new 48-inch HDPE pipe.
or water infiltration, the team selected a low-density cellular grout, a form of extremely lightweight concrete that had never been used for this application over such a long length before. As the grout was pushed through the tunnel, however, air bubbles began to pop, decreasing the volume of the grout. The team paused the grouting process and considered remedies, ultimately selecting a specialty drilling company with lightweight equipment to install new injection points at approximately 500-foot intervals. Although there were concerns about drilling the verticals and potentially puncturing the pipe, the drillers were able to stop within a foot of the pipe, and the grout was safely installed along the length of the entire pipe. The rehabilitation of the tunnel was completed in just six months with an $8 million price tag. The compressed construction timeline minimized the impact on the community, with just one temporary road closure. Although the project was complex from a construction standpoint, the approach offered a much smaller footprint and saved the district from the expense of boring a new tunnel and installing a redundant sanitary sewer line.
KONNOR BURSAW is a project engineer in Dewberry’s Denver office.
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csengineermag.com
march 2021
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