CHARLESTON, W.Va. -- Details began to emerge Wednesday of previous problems at a Harrison County coal-slurry impoundment, as CONSOL Energy continued its efforts to locate a coal miner missing and presumed dead following last week's collapse of an embankment at the facility.Federal and state records showed previous questions about stability and leaks at the Nolan Run impoundment, and outlined company concerns that construction to enlarge the dump had not been moving fast enough to keep up with slurry waste generated by the preparation plant at CONSOL's nearby Robinson Run Mine.The embankment that collapsed last week was part of a "saddle dike" that CONSOL was building to help raise the overall height of the facility to accommodate more slurry. CONSOL proposed the saddle dike on the opposite end of the facility from the main dam, to raise the embankment in a low spot, or "low gap", in the surrounding topography.State Department of Environmental Protection officials had approved that expansion in April 2009.But earlier this year, CONSOL told DEP officials that the construction had been hampered by "an unusually wet fall and a mild winter" that "made it difficult to maintain the extended haul road and proper placement of embankment fill during inclement weather."CONSOL was concerned that the timing of the project's completion "may be very close or even lag behind the filling of the pool with slurry," a company engineer told DEP in a March 2012 letter.The company submitted another plan for an "intermediate stage" impoundment expansion. DEP approved that proposal on June 18, 2012.Jim Pierce, a DEP dam safety engineer who is investigating Friday's incident, said such issues are fairly common at impoundments as coal operators try to ensure space to dispose of preparation plant wastes."They have to stay head of the slurry," Pierce said. "It's always a concern."CONSOL has not responded to questions about whether it was having problems at the site prior to the embankment collapse, and during a media briefing Wednesday, company spokeswoman Lynn Seay said it was too early for such inquiries."We have just begun the investigation into the cause of this accident and I do not have any initial information to share with you on the cause of the accident," CONSOL vice president for safety Lou Barletta said during a media conference call in which news reporters were not allowed to ask questions.The Nolan Run impoundment near Lumberport stretches about a half-mile across and currently holds about 2 billion gallons of water and slurry, a mixture of solid and liquid wastes from plants used to clean coal and prepare it for burning in power plants.This week, the search continued at the site for the missing United Mine Workers member whose bulldozer fell into the impoundment when part of the saddle dike collapsed shortly after noon on Friday. Two CONSOL engineers and their pickup trucks also ended up in the slurry, but the engineers escaped and were treated and released at local hospitals.CONSOL said it would first try sending divers into the impoundment through a 40-foot pipe to try to enter the dozer and recover the miner's body. If that doesn't work, CONSOL will try building a smaller dam around the dozer, pumping coal waste out of that area, and then sending divers into the water.All mining at Robinson Run was initially suspended over the weekend. Small-scale "development mining" resumed Monday morning and the mine's more lucrative longwall machine was running against Wednesday, officials said.CONSOL was also seeking approval from the U.S. Mine Safety and Health Administration to restart its coal-cleaning plant. Company officials released a diagram showing a large boom being placed across the impoundment, to isolate active slurry disposal areas from the section where the dozer is located and said the move "will not interfere" with recovery of the missing miner.The area of the saddle dike that failed has been described as about 200 yards long and about 200 feet wide, and is located on the opposite end of the site from the impoundment's main embankment."In my thirty 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 Wednesday. "Something went awry. There was a loss of stability either in the embankment material or in the underlying foundation."Construction of the embankment involved placing coal refuse on top of previously placed coal refuse, a process in which experts say it is key to allow enough time for wet material from the preparation plan to dry out, harden, consolidate and gain enough strength.Pierce said that so far, his review of compaction testing and pressure readings for the embankment that failed have not revealed any problems or any violations of regulatory standards.DEP spokeswoman Kathy Cosco said earlier that "overall compliance for the site has been satisfactory," and agency inspections in the weeks prior to the incident mentioned no problems related to the embankment that collapsed.Last year, U.S. Office of Surface Mining inspectors warned of water seepage and stability problems at the impoundment, according to the federal agency's records.OSM's May 2011 inspection report mistakenly refers to the "saddle dike" on the western side of the impoundment, where the collapse occurred, when federal officials were actually concerned about a similar embankment on the eastern edge of the site, closer to the main-stem dam at Nolan Run, officials said.In that instance, OSM officials were concerned that the main-stem dam appeared to have been built much steeper than the CONSOL construction plans approved by state and federal officials, records show."During the winter and spring, areas of this steeper slope have broken off (sloughed) and exacerbated the apparent unstable appearance," the OSM inspection report said.CONSOL officials assured OSM that the embankment just appeared to be steeper than the approved design."When the pool was down last fall, the portion of the earthen soil facing that is below the slurry level was visible and had the appearance and effect of a bench along the upstream slope," a company engineer told OSM, according to the federal agency's report. "I feel that the embankment remains in an uncompromised state and that the design plan is being followed."No citations were issued by either OSM or DEP.Reach Ken Ward Jr. at email@example.com or 304-348-1702.