K+A performed a geotechnical engineering study and provided construction-phase services for Clearwater Reservoir No. 3, a 25 million gallon (MG) buried concrete tank, at the Foothills Water Treatment Plant in Douglas County, Colorado. The circular tank is a post-tensioned concrete structure supported on a prepared subgrade, and is the largest tank of its type west of the Mississippi River.
The foundation was excavated into expansive claystone bedrock, and overexcavation and placement of a structural fill layer beneath the tank was recommended to mitigate heaving risks. Although located within the steeply dipping bedrock area of Douglas County, which is known for containing areas of very high foundation heave risk, the tank site was located above the Hygiene Sandstone member of the Pierre Shale, which is characterized as having only moderate heave risk. By evaluating the site swell potential, we were able to limit the depth of overexcavation to 3 feet, rather than the 10-foot depth frequently used on high heave risk sites in areas of steeply dipping bedrock. Additionally, a two-tiered underdrain system was recommended: one around the base of the structure to collect any tank leakage, and another keyed into the bedrock at the base of the structural foundation fill layer to intercept potential perched water flowing above the bedrock surface from outside of the project area.
Construction phase services included review of plans for steep temporary excavation slopes in the steeply dipping bedrock materials, and supplemental staffing for Denver Water materials lab for some of the large concrete placements, in particular the tank base slab which was required to be placed in one 3,200 cubic yard pour, one of the largest single-day concrete placements in the history of Colorado construction.