Cave-in site longstanding issue

Graham Jaehnig/Daily Mining Gazette While ground above a damaged stope of the old Petherick Mine continues to subside, excavations by a local contractor have been in order to locate the precise location of the caving in of the ceiling (hanging wall) of the stope, so that it can be permanently capped. Mining consultant, John McKana, said it was the excavating and not the cave-in that damaged a section of the road from the Central Mine Location to the Copper Falls Mine Location.

KEWEENAW COUNTY — The section of the Copper Falls mine that recently caved in has been a source of curiosity and irritation for more than three decades, said John Cima, Keweenaw County mine inspector.

“This site has had several minor subsidences over the last 30-plus years,” he said, “and for years (it) was nothing more than a wet area approximately 30 feet in diameter with snow fence around it.”

The owner of the property decided last fall to excavate the area to determine why the area kept caving in, and place a permanent cap on it, he said.

A few weeks ago, a contractor started excavating at the site under the guidance and supervision of John McKana.

John McKana works as a consultant for the land owner, Cima said, and had soil borings done at the site to determine how far down solid rock was located, since that will be the depth the concrete cap will be placed.

“Initially, we dug a test hole, and at 14 feet, we didn’t hit bedrock, I was worried,” McKana said. The test hole was then dug deeper, and hit rock at 15 to 20 feet deep, McKana said.

He emphasized that it was not the cave-in that damaged the road, it was the excavating. Once the testhole had determined the depth of the bedrock below the surface, in order to find the precise location of the cave-in, the contractor had to cut into the road. The actual cave-in, he said, occurred some 20 feet from the road.

“The initial hole was only maybe eight or 12 feet in diameter, but of interest is the fact that it was not caving into the bank at all,” McKana said. “The original fence post and snow fence were up from the 1970s, or whenever.”

McKana said at some point, the stope had come from the surface, but it is not known when. He said it could have been during mining, or any time thereafter.

“You have to remember that back in those days, if they saw copper, they mined it and they just threw a timber in it,” said McKana. “So, they may have just broken through, put a little hole, and put a timber across, driven a couple wedges in, and kept mining right across its length.”

McKana said while talking a local resident, he was told that the hole had opened up again in 1994, and somebody started dumping refuse into the hole.

“Washing machines, carpeting,” he said, “we found all sorts of things in there.” He added that the site was good until about a year ago, when it subsided again, this time with water draining through it.

Excavations have been ongoing, and a steel-reinforced, concrete cap will be constructed over the cave-in,

“This cap,” said Cima, “will basically act as a cork in the hole to keep the ground from above it from continually falling into it.”

He said the large pit at the site, however, is not the result of the cave-in, but is the result of excavating the ground to locate the cave-in.