LUMBERPORT, W.Va. -- Investigators on Tuesday began looking for the cause of an embankment collapse at a massive CONSOL Energy coal slurry pond in Harrison County last week, as crews continued their search for a dozer operator who has been missing since his vehicle fell into the impoundment Friday afternoon.

MSHA spokeswoman Amy Louviere said that "informal interviews" were conducted as the investigation of the incident at CONSOL's Nolan Run slurry impoundment, located near the company's Robinson Run underground mine.

Four days after the accident, the company, MSHA and state officials were slowly moving toward the recovery of the dozer from the soupy wastewater. The dozer operator, who has not been identified, is missing and presumed drowned, though federal officials continued to refer to a "rescue and recovery" operation.

MSHA used flatbed trucks to bring barges to the recovery site as crews dredged around the shoreline so the barges can be launched once they're assembled.

Small boats will maneuver the barges into place, and a certified marine surveyor will confirm the loads they can hold. The barges are about 12 feet wide and 36 feet long, like those that move coal along rivers, and they'll serve as the platform for the operation, Louviere said. Fifty-foot pipes will anchor the recovery platform.

"A 25-foot buffer zone will be established from the shore, which will be illuminated," Louviere said. "Anyone in this buffer zone will be required to have a spotter and be wearing a life jacket."

Louviere said the plan is to sink sheet pilings into the pond around the dozer, walling it off.

The dozer is believed to be stuck 25 to 35 feet in the slurry, the refuse and solid material that sit beneath about 10 to 12 feet of dirty water. Slurry is the waste created when coal is washed to help it burn more cleanly.

Crews will pump the slurry out from around the dozer, Louviere said, and replace it with water so divers can enter and search for the missing operator. It's unclear how the bulldozer will be removed. Pittsburgh-based CONSOL has scheduled a technical briefing for the news media for Wednesday afternoon, but said the telephone event would last only 15 minutes and not include an opportunity for reporters to ask questions.

Louviere said there was no risk to the public from the failure of the 200- by 200-foot section of the embankment because the structural problems were inside, not outside, the impoundment.

The pond encompasses about 78 acres and is estimated to hold between 1.6 billion and 1.9 billion gallons of wastewater, said state Department of Environmental Protection spokeswoman Kathy Cosco.

That's the equivalent of about 2,500 to 2,900 Olympic-size swimming pools, each holding about 600,000 gallons.

The impoundment is permitted to hold 3.4 billion gallons, Cosco said, but typically operates well below that volume.

The mine was idled over the weekend, and production has not yet resumed.

Two engineers whose pickup trucks also went into the slurry suffered no life-threatening injuries.