FACTORS CONTRIBUTING TO THE 2005 TAUM SAUK UPPER RESERVOIR FAILURE, REYNOLDS COUNTY, MO

The failure of the remotely-operated AmerenUE Taum Sauk Pumped Storage Powerplant upper reservoir on the morning of Dec 14, 2005 occurred due to the uplift failure of the anchors for water stage instrumentation conduits and human decisions regarding the severity of the instrumentation problem, which was initially discovered during a previous overtopping incident, three months before the failure. The reservoir’s instrumentation ducts continued to deflect during the interim between discovery and failure, and this, combined with differential settlement of the rockfill dike, lead to erroneous stage level readings of as much as 6 feet by the time catastrophic failure occurred. Water was normally pumped into the upper reservoir at a rate of 5,000 cfs.

In September 2005 the pumps had been re-programmed to shut down when the pool level was within 4 feet of the assumed crest elevation of the parapet wall. When the pumps failed to shut down around 5 AM on the morning of December 14^th the pool began overtopping the parapet wall, principally, in three different locations, where differential settlement of the rockfill dike had been most severe (the reservoir did not have a spillway). Post-failure forensic assessments revealed that cylindrical instrument ducts housing the water pressure transducers had detached from their anchors, attached to the upstream face of the reservoir.

The parapet wall created a thin crested weir overflow condition with the water initially free-falling 10 feet, onto an unprotected rockfill bench, adjacent to a downstream face of 1.3:1 inclination. Post-failure observations of the undermined parapet wall a few hundred feet south of the main breach suggest a plunge pool ~30 feet deep was likely excavated along ~ 500 feet of the parapet wall on the northwest side of the reservoir, before it gave way, via overturning.

Post-failure assessments also revealed that most of the rockfill was end-dumped without mechanical compaction during construction in 1962-63 and that the fill contained a surprising quantity of fines, which likely exacerbated hydrocompaction-induced settlement of up to two vertical feet (2.44 % of embankment height). The breach zone had experienced the greatest volume of settlement (though not the maximum settlement).