After only 10 years of service, the roof of a 15-million-gallon reservoir in Greeley, Colorado was near collapse. The reservoir was originally designed and built as a partially buried tank with an exposed post-tensioned concrete roof. The concrete had been exposed to Colorado's extreme temperature cycles and subjected to repeated volume changes and freeze-thaw cycles. The 6-inch-thick post-tensioned roof slab had deteriorated significantly and was in imminent danger of collapsing. The Denver engineering firm of Richard Weingardt Consultants Inc. (RWC) was called in to evaluate the situation and recommend a solution. After reviewing a number of plans for repairing and strengthening the roof, the consultants decided the only viable solution was to replace the old roof entirely.
The hundred columns spaced 30 feet on center as supports for the original roof remained as supports for the new roof. A 6-inch-thick layer of gravel that had been placed under the floor slab had to be drained so the bearing soils could dry out and stay dry. This restored the structural integrity of columns and footings in the reservoir. The consultants also developed an ingenious column support system that reduced soil pressure under the column footings and distributed the increased loads. Before the concrete could be removed, extensive shoring was installed to support the deteriorated roof when tension in the tendons was released.