Wait, We’re Really Considering a Tunnel? A Case Study on When Open Cut Was Infeasible

The City of Boulder, Colorado, is in need of a new 2.5-mi sewer transmission main to convey the majority of the city’s wastewater flows to the Boulder Water Resource Recovery Facility. On the upstream end of this project, a 2,000-ft section of the proposed pipeline was planned to be constructed via open cut methods beneath a major arterial road and adjacent to several rural residential properties. The project team investigated the impacts of open cut pipeline construction in the area. Key areas of the investigation included: (1) hydrogeologic modeling of the construction dewatering and its influence on existing domestic wells; and (2) construction phasing requirements needed to limit traffic disturbances, adhere to work hour limitations, and maintain access to the residences. After the team analyzed the cost, schedule, and public impacts of open cut construction, the design team developed a trenchless alternative for comparison. The trenchless alternative is further complicated by the likely presence of boulders, potential mixed-face conditions, and the possibility of a basalt dike along the pipeline alignment. Ultimately, the team pivoted the design philosophy toward the trenchless option and focused on gathering additional field data and refining the vertical alignment to mitigate risks associated with tunneling this section of the project. This case study reviews how the team weighed multiple approaches to mitigate the unique design challenges for this project, ultimately leading to the design of a three-shaft trenchless installation that includes a nearly 1,500-ft long continuous section. The presentation includes a description of the constructability methods reviewed and implemented that ultimately provided value to the project.