FAIRBANKS -- A contractor needed 100 tons of gravel, sand and rock to fill a 75-foot hole in the ground that mysteriously opened up in the front yard of a hillside house after a deluge last week.
“We filled up the hole and got rid of the danger,” Gary Powell of B&B Excavating said Monday.
The driveway of Al Schultz’s house off Gold Hill Road is now flanked by a large dirt pile that covers the short-lived open shaft. He lives on a part of Gold Hill that was never dredged, a neighborhood that remains atop millions of tons of loess -- material that consists mainly of glacial silt.
Less than 1,000 yards to the west of the neighborhood, the industrial and large-scale dredging operations of the 1930s and 1940s excavated the valley down to near-bedrock. With water pumped to the site from the Chena Pump House on the Chena River, the miners used hydraulic pressure to excavate the loess.
In this case, the water pressure occurred when the rains came last week and the small culvert in front of Schultz’s house overflowed at the end of his driveway.
The summer rainstorm, the heaviest to hit Fairbanks in decades, appears to have eroded the piece of earth that formed a bridge over the chasm, about 3 to 4 feet wide.
The theories on what happened to create the cavity were based on ancient history and more recent events. The explanation based on the recent past is that it was an abandoned mining shaft that everyone forgot about 100 years ago.
But the lack of timber framing inside the shaft -- the use of timbers to keep a shaft from collapsing was standard practice by early miners in this area -- lends more credence to the ancient explanation: that ice once occupied that spot and melted long ago, leaving an underground hole in its place.
The ice wedge theory would also explain why there had been no sign on the surface that anything was amiss down below.
Tom Bundtzen, a veteran Alaska geologist and mining historian, said he can’t say that a drift-mining shaft was never built without timber framing in this area, but miners would have worried about cave-ins without that support. He suspects this hole was a natural phenomenon, not a mining venture.
“I think an ice wedge melted away,” he said. “This happens all the time in permafrost country.”
Before the hole became history, a Fairbanks Daily News-Miner reporter lowered a camera into the shaft, using a small flashlight to capture a video that showed the walls were straight and uniform most of the way down. Schultz said that the camera and the 82-foot rope that he lowered into the hole did not come back wet, so he suspects there was no water at the bottom.
On Monday, two permafrost scientists from the University of Alaska Fairbanks, Yuri Shur and Matthew Sturm, visited the site. The first thing Shur asked was, “What happened to the water?”
Shur said he believes a chunk of ice once occupied the hole and while it probably disappeared long ago, the heavy runoff found the weak spot and cracked it open.
At the base of Gold Hill, far below the Parks Highway, there was evidence that water was continuing to drain from the hillside, flowing from many locations underground, but it wasn’t clear exactly where the water went.
The other side of Gold Hill, where the land was dredged and flattened, has “one of the longest records of permafrost history and climate change found anywhere on earth,” according to a 2008 permafrost guidebook.
“The loess here is rich with clues to the last several million years of Alaska’s geological history,” the authors of the guidebook said.
For Shultz, the homeowner, the history of the hole is a down-to-earth matter.
“I just wanted it buried before someone slipped in it or fell,” he said. "I wanted it plugged."