ST. LOUIS – Inspectors were shocked to discover that the collapsed portion of a mountaintop reservoir was made of rocky "fill" instead of the granite that was assumed for decades to be the main material, the state's chief reservoir inspector said Thursday.
James Alexander, director of the Missouri Department of Natural Resources' Dam and Reservoir Safety Program, said the broken portion of retaining wall — 70 to 80 feet high and about two football fields wide — appeared to consist entirely of soil and smaller rock.
"We were shocked," he said, to see the "rubble material."
Alexander inspected the dam after the accident Wednesday that released a billion gallons of water down the side of Proffit Mountain.
The 50-acre upper reservoir of the Taum Sauk Lake hydroelectric plant in southeast Missouri breached shortly after 5 a.m. The torrent of water ripped through a state park, then down along the Black River, knocking cars and trucks off a rural highway.
The water tore from its foundation the home of park superintendent Jerry Toops, his wife, Lisa, and their three young children. All five survived, but the children were being treated at a St. Louis hospital for hypothermia.
A hospital spokesman said 5-year-old Tanner was in critical condition. His 3-year-old sister Tara and 7-month-old brother Tucker were upgraded Thursday night to fair condition.
The breach apparently occurred after an automated system malfunctioned and pumped too much water into the reservoir. A backup system that should have caught the problem also apparently failed, said Gary Rainwater, chairman and chief executive of St. Louis-based utility AmerenUE.
Inspectors from AmerenUE and the state were assisting the Federal Energy Regulatory Commission, which oversees the plant, with the investigation.
FERC officials did not return phone calls seeking comment. Asked to respond to Alexander's comments, AmerenUE issued a statement that read, "We will address this as part of our investigation."
If a large part of the retaining wall was mostly soil and smaller rock, it was likely doomed once too much water was pumped into the reservoir, said Charles Morris, an environmental engineering professor at the University of Missouri-Rolla. Soil-based retention walls will erode when overtopped, he said.
Over the years, leaks have occurred at the reservoir, which was completed in 1963. AmerenUE installed a plastic liner about a year ago to limit the leaks.
While inspectors might not have been able to predict the collapse, they would not have been as dismissive of the leaks if they knew part of the wall was made of fill, not granite, Alexander said.
"If I would have known that, yeah, that would be more of a concern," he said.
Information provided by AmerenUE said six million tons of granite was removed to level the top of 1,590-foot-tall Proffit Mountain, and workers used the removed stone to build a sloping retaining wall 90 feet tall and covering an area the equivalent of 30 football fields.
The company said that in addition to granite, the reservoir was lined with concrete and asphalt.