Water saturating ash linked to Oak Creek bluff's collapse
Removal of vegetation also looked at as possible contributing factor
Water takes the easy way, the path of least resistance, as it flows over the surface or underground.
Water was doing just that when it became a player in Monday's dramatic bluff collapse at We Energies' Oak Creek Power Plant on the Lake Michigan shoreline, a geologist said.
The easiest path available to water seeping out of the bottom of a hilltop unlined storm water retention basin at the property was the adjacent coal-ash filled ravine, said Doug Cherkauer, emeritus geosciences professor at the University of Wisconsin-Milwaukee. Ash is less compact and less stable than the clay rich glacial soils in the rest of the hill or the bottom of the ravine, he said.
Monday's dramatic bluff collapse at the property likely will be linked to water gradually saturating ash at the bottom of the old ravine, essentially lubricating the area between the ash and the clay-rich till at the bottom of the ravine, Cherkauer said Wednesday.
Saturated ash would have started oozing out of the bottom of the old ravine causing the top of the bluff to collapse and fall down the hill, Cherkauer said.
Mud and ash cascaded more than 300 feet down the slope to the lake, destroying a temporary storage building and carrying a trailer, storage units, a pickup truck and construction equipment in its wake. No worker was injured.
Building the storm water pond so close to the old ravine and without a lining were a pair of bad decisions, Cherkauer said.
The ravine, even after it was filled with coal ash but before the pond was excavated, would have received just intermittent flows of storm water or melting snow.
But the pond became nearly a year-round, steady source of water, slowly draining between storms, he said.
"We don't know yet whether anything associated with that pond caused the bluff to fail," said Gale Klappa, the utility's chairman and chief executive.
Engineers and geologists with We Energies and federal and state regulatory agencies are investigating the cause of the bluff collapse, Klappa said in an interview with Journal Sentinel reporters.
Bizhan Sheikholeslami, an environmental engineer with the state Department of Natural Resources' waste management program, said there is at least one other possible contributing factor in the collapse. He has been at the property to view the damage.
Nearly all of the vegetation was removed from the hillside in preparation for construction activities, he said.
Shrubs, trees and grasses would have absorbed storm water in the past. In their place, much of the hillside was covered with an impermeable surface of crushed gravel to provide access roads and parking areas for vehicles and storage trailers.
The landslide occurred immediately south of a $900 million air pollution control facility under construction at the power plant. No damage was done to the structure and the project is on schedule and nearly complete, Klappa said. Remaining tasks on that project include connecting the facility one-by-one to four boilers at the original coal plant.
Ammonia storage tanks for the new facility were built on a thick concrete slab south of the building. The tanks are empty.
The landslide carried away some of the soil supporting the concrete so utility engineers set up lasers to detect whether the slab is moving, Klappa said.
The tanks might be moved to a different location if the site is no longer stable. The move would not delay start up of the pollution control equipment, scheduled for 2012, he said.
In anticipation of the steady rains hitting the area Wednesday, contractors built a gravel berm west and uphill of the bluff collapse.
The berm will divert storm water flowing down the hill from the landslide and prevent more ash and mud from being pushed into the lake. Storm water held back by the berm would be pumped to a retention pond south of the collapsed bluff, We Energies spokesman Brian Manthey said.
Representatives of the U.S. Environmental Protection Agency and DNR collected lake water samples Wednesday. Samples will be tested for heavy metals and suspended solids, such as coal ash, an EPA spokeswoman said.
On Wednesday, workers from URS Corp. and Edgerton Contractors will begin building a second berm close to the shore. This berm will act as a wall and block additional debris from sliding into the lake, Manthey said.
After that is in place, workers can begin extracting vehicles, storage units and equipment stuck in the wide plume of mud and ash on the beach.
A separate contractor, Clean Harbors, on Wednesday delivered two boats specially designed to skim fuel and other debris off the surface of the water.
The 34-foot-long skimmers were to begin work late Wednesday beyond a pair of booms on the water. The booms form semicircle barriers on the lake, blocking debris from floating farther from shore.