Threat of rockslide closes 82 |

Threat of rockslide closes 82

Tim Mutrie

An enormous slab of rock broke free of Shale Bluffs and threatened to collapse onto Highway 82 Thursday morning, forcing closure of the highway in both directions for nearly six hours.

Traffic was diverted onto McLain Flats and Owl Creek roads from about 10:45 a.m. until 4:30 p.m.

Only small amounts of shale rock actually slid onto the upvalley section of the highway – closest to the shale wall and presently under construction – but the slab was large enough to cover all four lanes of the highway had it fully released. Highway construction officials feared that had traffic not been diverted, vibration caused by passing vehicles might have triggered a more substantial slide.

None of the rocks reached the downvalley section of the highway – the two lanes presently in use for both directions of travel. No one was hurt when the slab came loose.

By 4:30 p.m. yesterday, road crews had completed a six-foot-high, 200-foot-plus-long earthen berm along the outer edge of the under-construction upvalley lanes to protect motorists in the downvalley section from any rockfall, according to Joe Bair, assistant project engineer. Bair said lights had been installed in the area and that spotters would be monitoring the area all last night.

“If there are any indications that it’s getting less stable, we’ll shut traffic off again,” said Roger Pihl, project geologist for the Highway 82 corridor.

Early reports from authorities Thursday indicated that scheduled blasting for ongoing highway construction work triggered the rockfall, but construction officials later said that wasn’t the case.

“This had nothing to do with any blasting,” said Bair. “This was a portion of the mountain that we never even touched, this was just a slab release. It took us all by surprise.”

However, Bair said that the area where the slab – measuring some 200 by 45 feet – came loose had been “trimmed” as part of the construction process.

“We were putting a trench in for drainage below the slope that fell, and people that were putting in the pipe noticed that it started to dribble rock a little bit, so they all got out of the way and then it released,” Bair said. “I was standing there when it initially slid. The slab just moved away from the wall, and it’s weird that it slid like that, because it was just a really slow slide.”

Pihl explained the geological phenomena which triggered the slide.

“One of the shale layers in the slab formation was weak enough that it crushed under the weight of the overlying shale, changing the geometry of the slope,” he said. “I guess what slid was something in the range of 20,000 to 30,000 cubic yards. Cutting the drainage pipe trench was probably the straw that broke the camel’s back, but there was also a lot of moisture in the rock and that may have contributed to it … the area ended up being weaker than we thought.”

“There’s a tremendous amount of variation in the different layers of shale, and in this area, we had some in the 500 PSI (pounds per square inch) range and other layers that were in the 14,000 PSI range, which is up in the range of granite and very hard, while other parts are very soft,” Pihl said.

Pihl said yesterday evening that road crews were “scaling some of the slab material off with a crane and a drag line,” focusing on the upper sections of the slab. “What we’re doing now is trying to stabilize it so that it’s safe to work on and safe for the public.”

In the long run, he said crews would blast away top sections of the slab, “rock bolt” middle sections of the slab, and then “regrade” lower portions.

“We won’t get rid of the whole slab, because we want to leave its weight against the slope, so we’ll just regrade it, but we won’t remove it,” he said. “We’re trying to buttress it against the bigger wall.”

Bair said barring any future unexpected events like yesterday’s, blasting work will continue to force periodic traffic delays of five minutes or more.

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