Roller-Compacted Concrete Delivers Rapid, Efficient Dam Rebuild
The rebuilding of Taum Sauk Upper Reservoir is a project of many superlatives. The upper bowl of the 440-MW pumped-storage system sits on top of Missouri’s highest peak, 1,590-ft Proffit Mountain. It is believed to be North America’s largest roller-compacted concrete dam. When the original earth-and-rockfill dike was over- topped and failed in December 2005, Missouri Gov. Matt Blunt (R) called the damage done by the 4,365 acre-ft of water it released “the worst man-made disaster in the history of Missouri.” And the $10-million civil penalty imposed on St. Louis-based utility AmerenUE by the Federal Energy Regulatory Commission for the failure was the largest FERC penalty ever in a hydroelectric case.
Today, the reservoir is nearly complete, and preparations are under way to start commercial operation. Roller-compacted concrete (RCC) placement was completed in November, and the new dam was topped out. Engineer-of-record Paul C. Rizzo Associates Inc. will begin a refill-testing program required by FERC in the first week of February and expects to complete it by March 1. “We could start commercial operation April 1,” says Carl M. Rizzo, vice president of construction management services.
A pumped-storage system takes advantage of differential electricity rates to store power generated when rates are low, then sells it when rates are higher (see graphic). The Taum Sauk system pumps water overnight from the lower reservoir, which is at an elevation of 730 ft, through the 6,300-ft-long penstock and 26-ft-dia vertical shaft to the upper reservoir at 1,500 ft. The pumps are later reversed and draw the water into the shaft and penstock to the powerhouse at 767 ft, where it drives two 220-MW hydroturbines and is discharged to the lower reservoir.
The original 4,365-acre-ft Taum Sauk Upper Reservoir in Reynolds County breached shortly before dawn on Dec. 14, 2005, emptying out in a 20-ft-high wave down the side of Proffit Mountain, through a state park and the system’s lower reservoir and into the Black River. No deaths resulted, but the flood devastated a popular recreation area, aroused fears for safety and took the Taum Sauk system off-line.
AmerenUE took full responsibility for the failure after its preliminary investigation pointed to an instrumentation failure. The utility retained Paul Rizzo, head of the Monroeville, Pa., firm that bears his name, to investigate. Rizzo’s April 2006 report cited stability failure stemming from poor-quality material, which deviated from the original design specifications, and poor construction practices as root causes of the failure. Construction practices were consistent with early 60s standards but not with construction practices used today. Contributing factors of human error and inadequate instrumentation led to the overfilling of the reservoir and the overtopping of its dam.
In a series of settlements with state and federal authorities, AmerenUE cleared the way to rebuild the upper reservoir. But the effort was costly: In addition to the $10-million FERC penalty, AmerenUE settled for $180 million in cash and property with the state. In November 2007, AmerenUE announced the selection of the team that would rebuild the upper reservoir: Rizzo Associates was named the engineer-of-record and project manager, and Ozark Constructors LLC, a 50-50 joint venture of dam-builder ASI Constructors Inc., Pueblo West, Colo., and St. Louis-based Fred Weber Inc., was named construction contractor. Rizzo already had designed the replacement reservoir, so work was set to begin immediately.
The reservoir “bowl” occupies 40 acres on the leveled mountaintop. In addition, 35 acres were cleared and...