Published June 07 2012
Landslide moves Fargo road 13 1/2 feet, breaks water main (with video)
“It was something very strange for our neck of the woods,” said Ben Dow, Fargo Public Works director. “It’s about the largest one we’ve seen. But our soils are always doing funky things up here in clay country.”
City employees became aware of the problem on the morning of May 31 when the water pressure dropped in a north-side tower because of the water main break, Dow said.
Engineers found that a section of 32nd Street North measuring more than 100 feet long had shifted 13 1/2 feet to the west, causing cracks in the gravel road and large ruts at each end. The road and an adjacent drain field had slid toward Legal Drain 10, pushing up the bottom of the drainage ditch and blocking the main channel.
Chad Engels, an engineer for the Southeast Cass Water Resource District, which owns the drain, said it appeared that the ground shifted under the weight of a stockpile of aggregate sitting along 32nd Street on the site of Border States Paving Inc.’s asphalt production plant.
Such landslides are technically referred to as rotational slides and commonly known as slumping. They happen when one layer of soil rotates under pressure while the layer below it stays in place, causing what’s known as a slip plane, Engels said.
Slumping often occurs along drainage ditches when they are being excavated and the spoil is piled next to the channel, but Engels said this is the largest slide he has seen.
“Because here we have a case where the bottom of the channel has actually been pushed up something around 8 to 10 feet, the channel was actually raised in elevation,” he said. “So it’s pretty impressive what forces can do.”
Engineers immediately surveyed the drain to make sure water could still pass over the plugged section without flooding surrounding land, said Engels, a senior project manager with Moore Engineering of West Fargo.
The water district has hired a contractor to clear the drain after Border States finishes moving the aggregate away from the road to prevent more slumping, Engels said. He expects the drain to be open by this weekend.
Braun Intertec, a geotechnical engineering firm in West Fargo, has been hired by the district to determine what caused the landslide and develop recommendations for repairing it.
Engels said he expects the area to be repaired in four to six weeks, but it’s unclear who will foot the bill.
“There’s an ongoing discussion between the water district and Border States,” he said.
Dan Thompson, part-owner of Border States, said the asphalt plant has operated in its current location since 1994 or 1995, and the aggregate pile comes and goes with the seasons. Nothing was done differently this year that would have triggered the landslide, he said.
Engels speculated that, in addition to the stockpile, fluctuations in soil wetness may have played a role in the landslide.
“They’re very unpredictable,” he said.
Dow said it’s similar to what happened with Mount Fargo, a ski hill created in the 1980s from earth excavated for the Bluemont Lakes development. The hill eventually began sinking into the soft, wet clay below and pushed up the ground around it. It was trimmed down in the early 1990s, and the clay was used to build dikes.
Thompson said no one was on the aggregate pile at the time of the landslide, which buckled a section of Border States’ fence.
“When we came out here, you could actually hear the fence popping,” said Dean Riemer of Cass County Electric Co-op, which had an electrical line broken by the landslide.
Officials said the affected water main had been capped off for future development and wasn’t serving anyone. Crews recapped it at the point of breakage and will likely have to replace the entire section that shifted, they said.
The damaged road has been barricaded, temporarily cutting off access to one building at the end of it.
Thompson said Border States plans to park equipment on the area where the aggregate was stockpiled.
“The soils around here … they aren’t real supportive,” he said.
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