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South Platte Interceptor

Adams County, CO

Project At-a-Glance

  • 6.8-mile interceptor that crosses beneath 3 major roadways and through 3 slurry wall-lined gravel pit reservoirs
  • Slurry Pressure Balance MTBM and open face rotating wheel TBM used to mine tunnel
  • Rural to Suburban Project Area
  • Subsurface conditions range from claystone to sand and gravel with cobbles
  • Contaminated groundwater encountered during construction.
  • Alignment is parallel to South Platte River and crosses beneath the river via a microtunnel


  • National Recognition Award (2017)” – Engineering Excellence Awards, American Council of Engineering Companies
  • Honor Award (2017)Engineering Excellence Awards, American Council of Engineering Companies of Colorado

Project Narrative

In order to convey flows to Metro Wastewater Reclamation District’s new wastewater treatment plant, a new gravity interceptor pipeline to redirect flows from the existing facility to the new plant was constructed. The 6.8-mile interceptor follows the South Platte River, crossing it once, as well as crossing under three major roadways and cutting through three slurry wall-lined gravel pit reservoirs.  In total, seven tunnels were constructed, the longest of which was over 3,700-ft with an excavated diameter of 78-inch. The largest diameter tunnel is near the northern end of the pipeline with an excavated diameter of 96-inch. The 992-ft crossing of the South Platte River  consisted of a 66-inch diameter microtunnel. Additionally,  construction of over 5,700-ft of the new slurry wall was completed. Project challenges included high groundwater inflow rates for open-cut sections and tunnel shafts along the South Platte River, crossing and repair of slurry walls lining gravel pit reservoirs, and contaminated groundwater in specific areas of the alignment.

Brierley Associates served as project geotechnical and tunnel engineer. Geotechnical investigations consisted of over 100 borings and three aquifer pump tests. Brierley provided geotechnical design recommendations including investigation reports and a Geotechnical Baseline Report for the pipeline. Brierley Associates was actively involved throughout the design of the project with close collaboration between the client and the owner. Brierley prepared and presented two workshops to the client and owner about tunneling methods and the uses of Geotechnical Baseline Reports.


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