Coal Bank Pass rockfall, wall work planned through November

Colorado Department of Transportation crews are racing to complete rockfall mitigation and replace a retaining wall on U.S. Highway 550 at Coal Bank Pass before snow arrives this fall.

Crews are working on slopes between 45 and 90 degrees with boulders perched on a ledge at mile marker 59.3, near Deer Creek. CDOT Project Manager Jeff Reichle said the plan is to excavate from the top down, reducing slopes to about 35 degrees so rocks slide to a stop rather than rolling downhill.

After heavy excavation, CDOT will bring in rope crews to perform lighter excavation, he said. A rockfall catchment wall will be built that will gradually fill with rocks and sediment, eventually becoming a retaining wall.

At mile marker 56.9, crews will replace a failing timber crib wall with a mechanically stabilized earth retaining wall on the northbound downslope. Reichle said drivers will see centerline barriers, with lanes narrowed to about 10 feet, as crews dig into the road and install wires and cribbing.

“We’re going to cut the road in half, basically right at center line, and dig a hole in the road,” he said. “That’s really the only way to do the work because of the steepness.”

The rockfall work will remove about 12,000 to 14,000 cubic yards of rock and dirt, which will be stockpiled at Andrews Lake for future use in road shouldering and other projects, he said. That’s enough to fill 1,300 dump trucks or nearly four Olympic-sized pools.

The work will require patience from motorists, with the rockfall mitigation and wall replacement scheduled to be completed by Oct. 1.

The projects have been on CDOT’s list for about 10 years, Reichle said.

The retaining wall at mile marker 56.9 is in poor condition. If it fails or a major rockfall hits, CDOT could face an emergency project and extended full closures.

Falling rocks near Deer Creek dwarf the current Jersey barriers, Reichle said. Some boulders are as large as 10 cubic yards – comparable in size to a small dump truck.

Rockfall and wall work on Highway 550 between the summits of Coal Bank and Molas passes will pause through Sunday for the holiday weekend.

However, drivers should still expect delays at two traffic signals at mile markers 56.9 and 59.3, according to CDOT.

Reichle said wait times will be about two minutes, but could be longer depending on traffic volumes and the automated signals.

“Peak periods before and after the Silverton fireworks show will have longer waits,” he said.

Reichle, a geologist, said he often wonders how these slopes formed.

The first and foremost answer is the road itself cuts across the slope, he said. If a road had never been built, there wouldn’t be a falling rock hazard. But it’s also possible a landslide occurred thousands of years ago, or that it and boulders are remnants of glaciers that moved through the canyons thousands of years ago.

“It could have been a lateral moraine or a terminal moraine, or something like that, that deposited the large stones as it traveled through this area,” he said.

cburney@durangoherald.com