Hoping to avoid a repeat of the rock slide that shut down U.S. 250 on Afton Mountain last summer, Virginia Department of Transportation crews will divert traffic on the highway to bolt steel mesh to a section of mountain that could fall to the road below.
The loosening rock is located above the highway, also known as Rockfish Gap Turnpike, and below the scenic view overlook near mile marker 100 on eastbound Interstate 64.
Officials will begin their effort April 11.
“During work, VDOT will close access to both scenic overlooks to allow for equipment and work crews,” officials said in a statement. “At the onset, U.S. 250 will be reduced to one lane with alternating traffic controlled by flaggers. Once concrete barriers are in place, VDOT will open that section of road to two lanes for eastbound and westbound traffic.”
Officials said engineers first identified the area as a possible problem spot during the spring of 2021. VDOT monitors rock outcroppings near roadways by watching for rock movement and comparing photographs, taken over time, to detect changes.
The process to stabilize the mountainside involves drilling deep into the rock with bolts to secure a special mesh which will keep the outcropping in place. It will cost about $197,000, VDOT officials said.
Early last May, a nearby outcropping of rock began dribbling a few rocks onto U.S. 250. Within days the outcropping let loose tons of soil, rock and debris to shut the road down.
The rockslide occurred in an area of geologic instability where a past rockslide already had occurred. According to geologists who studied the rockslide, the rock along the highway is unstable due to the cuts that were made into it in order to put I-64 over the top of U.S. 250 in the early 1970s.
Cuts were also made into the mountain in the 1930s to put U.S. 250 over the mountains.
The rock is schistose greenstone, a fine-grained foliated metamorphic rock that formed in sheets as it went through hundreds of millions of years of continent creation and tectonic processes.
Greenstone rock lies in layers, or foliation, which is its weakness. Hard impacts can make the rock split along its layers, making it prone to slides once it has been exposed for a period of time.
Geologists say that the rock rots when exposed to weather and elements. Rain, snow and sun slowly break down minerals, leaving the rock to decay and crumble. For greenstone, that creates something akin to clay between the rock layers, which make it even more susceptible to sliding.
VDOT officials said that during the stabilization effort, U.S. 250 will remain open. The work should take about a month, if the weather cooperates.
Source: www.dailyprogress.com
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