To meet the increasing demand for potable water in the Denver area, the Foothills Water Treatment Plant was significantly expanded. A unique feature of this expansion included a circular post-tensioned potable water storage tank approximately 390 feet in diameter and 32 feet in height. Post-tensioned water tanks have been efficiently constructed in sizes from 250,000 to 25,000,000 gallons in capacity with the structural details and construction methods being generally the same. The Foothills Tank is notable for its scale alone (25,000,000 gallons).

The membrane floor was cast in a single pour with no construction or control joints, requiring crews to place approximately 1,700 cubic yards of concrete and finish 122,000 square feet without interruption. Post-tensioning tendons were placed orthogonally and jacking forces applied in two stages, the first stage within 48 hours of concrete placement to help minimize or eliminate plastic shrinkage cracking. A highly prestressed floor with no joints is important with respect to maintaining watertightness and eliminating construction and maintenance costs associated with construction joints. Construction of footings and columns followed the completion of the floor post-tensioning and was concurrent with the wall construction.

The wall was comprised of a total of 18 wall segments each approximately 67 feet in length. Of the 18 segments, 6 were pilaster segments that allowed for insertion and stressing of the horizontal strands after the final segment was cast. Segments were constructed consecutively with the concrete placed full-height. Each typical wall segment required approximately 170 cubic yards of concrete with an additional 25 cubic yards for the pilaster segments. Vertical wall tendons were prefabricated and stressed prior to stressing the horizontal tendons.

The roof slab consisted of a 10-inch thick post-tensioned two-way flat plate. Typical spans of 36 feet were chosen to minimize the quantity of columns that interfere with the periodic washdown of the tank interior. A flat roof allowed the placement of soil on the roof which provided many benefits, including the reduction of thermal stresses on the structure due to environmental heat gain. Due to the large quantity of shoring required to support the roof, a single construction joint was required. Upon stressing the first pour tendons, the shoring was moved to the second half of the roof.

Unbonded monostrand tendons were used in the bottom and top slabs and bonded multistrand tendons were used both vertically and horizontally in the wall. All tendons incorporated encapsulation details for long-term durability. The post-tensioning provides durability to the structure by bi-axially compressing the concrete members so that either tension is eliminated or limited to below cracking limits, depending on the design requirements of the member.