CHARLESTON, W.Va. -- The investigation into a West Virginia slurry pond collapse that swallowed a bulldozer operator is in the early stages, but people familiar with the construction process say it's likely that someone pushed an expansion project too far, too fast.
The unidentified worker remained missing Thursday, six days after the accident, as the federal Mine Safety and Health Administration issued a preliminary report. It did not cite a possible cause for the failure at Consol Energy's Robinson Run impoundment near Lumberport, and Consol has declined to speculate.
But media outlets say the workers were pushing coarse mine refuse toward the upstream side of the dam to expand its foundation and increase its height.
Dennis Boyle of the federal Office of Surface Mining Reclamation and Enforcement in Charleston compared it to pushing material into a swimming pool. Move too far too fast, he said, and the material becomes saturated.
Geologists say there's a narrow range of stability in the process, and Jim Pierce, a dam safety engineer with the state Department of Environmental Protection, said it appears the stability of either the embankment or the pond's underlying foundation was lost.
"In my 30 years of experience I haven't seen anything like that-the size of the failure, the mass of material that moved all at once," Pierce said.
So far, Pierce said his review of compaction testing and pressure readings at the pond have revealed no problems or violations of federal construction standards.
DEP spokeswoman Kathy Cosco had also previously said that "overall compliance for the site has been satisfactory," and inspections in the weeks before the collapse mentioned no problems.
Impoundments are designed to hold liquid, silt and fine coal debris, the byproducts of washing coal to make it burn more cleanly.
The Robinson Run impoundment encompassed about 78 acres and held the equivalent of 2,500-2,900 Olympic-sized swimming pools.
State and federal records show there were leaks at the impoundment last year, and Consol had expressed concern that the expansion the DEP had approved in 2009 wasn't moving fast enough to keep up with the generation of waste.
It's unclear whether those leaks are related to the eventual failure.
But earlier this year, CONSOL told the DEP it had been hindered by a wet fall and mild winter, conditions that "made it difficult to maintain the extended haul road and proper placement of embankment fill."