A task force of independent investigators should begin work this week to determine the cause of the New Year's Day rupture of Quail Creek dike.
Richard B. Hall, directing engineer for dam safety, Utah Division of Water Rights, said Thursday that the blue-ribbon panel will be "a group of engineers that had nothing to do with the project."Whether geological hazards that engineers working on the project have known about for years were adequately addressed will be a major question facing the task force.
Geologic hazards at the dam site were listed in a memorandum by Utah Geological and Mineral Survey engineering geologist Bruce Kaliser on April 11, 1983, before the dike was built.
The report said gypsum in fill material intended for the dike's construction could be a problem, as gypsum dissolves when wet. "What assurance is there that the gypsum won't dissolve and create a reduced strength of embankment?" Kaliser wrote.
Also, he wrote, "The shallow subsurface beneath the (then proposed) dike contains gypsum beds at least a quarter-inch thick."
The Moenkopi formation at the reservoir basin seemed relatively impervious "except in those areas where it has been subjected to considerable weathering," the report said. It raised the possibility that deeply weathered rock might be common.
"It was part of our review of their dam," survey director Genevieve Atwood said Thursday. "Apparently, based on those comments, they (state officials charged with the safety of dams) did some additional drilling."
Atwood said she isn't trying to say, "We told you so." Instead, she said, the survey's job was to make sure that geological hazards were known.
"I don't think they would have continued if they hadn't thought they could engineer around the problems," she said.
"Virtually every dam site that we're looking at now has some kind of geologic hazards associated with it, and geological conditions that ought to be taken into account when a dam is designed and built."
One of the reasons that the failure of a dam or a dike happens every so often, she said, "is that the good dam sites have already been taken." Yet thirsty, expanding metropolitan areas like St. George need to expand their water sources.
"At Quail Creek, the geologic hazards were relatively well known," she said.
That doesn't mean no dam should be built if geological problems are discovered. "In general it's an engineering solution to a geologic problem," that determines if it's possible to build.
"The tentative list we have, which isn't confirmed, is that two are from the (U.S.) Bureau of Reclamation and then there are two out-of-state consultants."
The Bureau of Reclamation experts telephoned Dee Hansen, director of the Utah Department of Natural Resources, "and volunteered their services. So we just decided to incorporate them into the panel," he said.
The group may have a conclusion to report in March.
A state team will return to Quail Creek next week to map the exposed bedrock in the breech, he said. Whether additional drilling will be needed to sample the underground formation is "another decision the independent panel will have to make.
"If they don't feel they have enough data about why the dam failed, or they want to confirm their hypotheses, we will do some core drilling," Hall said. And if a decision is made to rebuild the dike, a great deal more drilling will be required to assure the new structure's safety.
Panel members will review the written record, examine the dam site, interview witnesses to the failure and talk with "engineers, inspectors and geologists who were involved with the project."
The contractor who built it may be interviewed, he said. Without question, the engineers who designed the project will be called before the group.
"I think for expediency's sake the committee's discussion will be behind closed doors," Hall said.