- Associated Press - Tuesday, June 3, 2014

CHEYENNE, Wyo. (AP) - Options for the town of Jackson to permanently shore up a massive, slow-moving landslide that destroyed one home and evacuated dozens of others earlier this spring will be complicated and cost anywhere from $8 million to $25 million, a geologist told town officials Tuesday.

Dirt and rock should be removed from the top of the slide zone on East Gros Ventre Butte just south of downtown Jackson, George Machan said. In addition, Machan suggested installing massive barriers or underground cables at the base of the hillside to hold the earth in place.

“It is a big landslide. As deep as it is, as heavy as it is, it’s a big challenge. It’s going to take a fair amount of force to keep it from moving,” said Machan, who has consulted town officials on the landslide since the crisis began.

The landslide began in early April. A crack opened at the edge of a neighborhood on the hillside and the ground moved slowly at first, no more than an inch a day.

Still, the movement worried town officials enough to evacuate more than 50 homes and apartment units in the area and several businesses, including a newly built Walgreens store, at the foot of the slide zone.

On April 17, the ground suddenly dropped 10 feet, shearing one house in half and raising concern that the hillside could come down all at once in a catastrophic collapse.

That didn’t happen. Instead, efforts to slow the movement by piling 8,000 tons of rock along part of the base of the landslide appear have paid off.

Ground movement since then has slowed down to less than an inch of movement per week, and nearly all residents have returned home.

The landslide measures 450 feet wide, 300 to 400 feet long and up to 140 feet deep, Machan said, and contains an estimated 500,000 tons of rock and dirt.

In Tuesday’s presentation streamed online, Machan outlined landslide-stabilization projects he has overseen and options to stabilize the Jackson landslide.

The options included several scenarios for building a thick wall, 15 to 35 feet high, of wire-retained stone. The wall would be built above an area at the base of the landslide where soil would be removed and replaced with rock.

The town also could drill rows of holes into the landslide and fasten cables to sturdier ground beneath the shifting earth. At the surface, 10-by-10-foot concrete plates would be attached to the cables to anchor the hillside in place, he said.

Jackson is located near fault lines that geologists say have the potential to cause massive earthquakes. An earthquake could severely damage a cable-retention system, Machan said, while a rock-filled retaining wall could shift some without being severely compromised.

Town officials didn’t need to act this year, but in the meantime they might consider adding rock along the base of the landslide, he said. Machan suggested they calculate the landslide’s potential risk to life and property in evaluating their long-term stabilization options.

“I don’t think we have a simple way to build this one,” Machan said. “It’s going to be complicated.”

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