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For a change, we have a pretty good idea when a very large piece of sandstone is going to go tumbling off the Rims.

If all goes as scheduled, a contractor from Vancouver, Wash., will nudge a 340-ton chunk of stone off the Rims at 8:30 a.m. Friday.

The slab in question sits above and just east of a house at 1313 Granite Ave. that was destroyed last Oct. 9 when a rectangular slab of sandstone estimated at 1,000 tons broke away and fell off the sheer face of the Rims, breaking into smaller boulders that crashed downhill. Granite Avenue is a little north of 13th Street West.

The rock to be removed Friday is just to the east of the slab that fell away, and it is already separated from the Rims by a large fissure that filled with rock debris during the Oct. 9. event.

Eight houses will be evacuated during the operation Friday, but the hope is that most of the rock knocked loose will land in a large natural depression under that stretch of Rims and stay put.

"The contractor is fairly confident that the main mass of stone isn't any danger to the houses below," said Jon Thompson, parks superintendent for the city Parks and Recreation Department. The department is involved because most of the Rims north of the city is park land, including the Rims above 1313 Granite.

Accurate Concrete Cutting, the contractor, hopes to be done with the Granite Avenue job Friday morning and then move five or six blocks east to remove another dangerous rock, this one hanging over Shady Lane. If there is more work than expected above Granite, the crew won't move to the Shady Lane project until Saturday morning.

Sometime Wednesday, a short section of Granite Avenue between Smokey Lane and Cactus Drive will be closed and crews will begin setting up a work and safety zone around the project site.

On Thursday morning, a big crane will be set up on Granite to lift equipment, including a small Caterpillar, up to the work site. The caterpillar will be used to break up and remove boulders and rock fragments that fell on the house and yard at 1313 Granite, including the 60- to 65-ton boulder still wedged inside the house, which has settled and moved nearly 10 feet since Oct. 9.

Thompson said the work in and around the house will be done after the rock is dropped from the Rims and could take until next Wednesday. Accurate Concrete originally planned to secure the loose slab to the Rims and remove the boulders from behind and inside the house first.

But Vern Balkowitsch, a co-owner of Accurate Concrete who arrived in Billings on Monday, said he determined it was too dangerous to have his crew working below the rock, and too difficult to secure it to the Rims.

He had a big job in Montana last summer, he said -- "pinning" a 4.5-million-ton piece of rock to the side of a mountain to keep it from falling on the Madison Dam near Ennis -- but that involved granite, not unstable, relatively soft sandstone.

Starting Friday morning at 7, barricades will go up at Smokey Lane and Cactus Drive where they meet Rimrock Road, and another at Placer Drive and Granite Avenue. That will keep traffic out of the "security area" that will be established around the project area.

Mike Whitaker, director of the Parks Department, said personnel from the police, fire, parks public works departments will be working together to keep people out of the security area.

Balkowitsch said he will drop the rock above Granite Avenue using two 200-ton hydraulic jacks that will be set in place by workers rappeling down the face of the Rims. He plans to put the jacks on the west side of the slab, slowly moving it out and to the east, like a swinging door, until it breaks free and falls.

He said his main concern at Granite Avenue is not rolling boulders but "fly rock," smaller pieces that break loose and go sailing through the air. The slab is only 130 feet from the house at 1313 Granite. If the slab doesn't stop in the depression below the Rims, Thompson said, all indications are that it will fall toward the already-ruined house.

The house belongs to Jon Lodge and Jane Deschner, who filed a lawsuit against their insurer in late October, naming the city of Billings and the state of Montana as possible co-defendants. Their suit says the city has some liability because it owns the Rims and that the state may have contributed to rock fall by allowing a culvert under Highway 3 to become plugged, directing huge amounts of water directly into the section of Rims that broke away on Oct. 9.

The City Council voted to pay Accurate Concrete $139,000 for the Granite Avenue project, with the option of seeking reimbursement later from the homeowners or their insurer.

The council voted against funding the project above Shady Lane, so the the homeowners most in danger there, Dale and Margaret Vermillion and Kevin and Erica Bruen, are directly paying Accurate Concrete $50,000 for its work above their homes.

The rock above Shady Lane became a danger in February 2010, when the bottom half of a pillar of rock broke loose and fell, leaving the top half hanging precariously above.

Balkowitsch said he will use also hydraulic jacks above Shady Lane. He said he intends to let that 300-ton rock fall down the slope, since it is 400 feet from the closest house and should follow a path already taken by other naturally falling rocks.

He expects that slab to break into at least four smaller pieces, none of which he believes will penetrate a natural barrier of boulders firmly anchored into the soil at the bottom of the slope. 

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Contact Ed Kemmick at ekemmick@billingsgazette.com or 657-1293.

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