Rare rock cut comes tumbling down
Wednesday, April 24, 2013
The tallest rock cut along U.S. 63 between the Capital City and Columbia is being blasted by heavy equipment this month.
A rock slide in late March — caused by numerous weaknesses in the rock face — compelled geologists and engineers to address the problem once and for all, said Kevin McLain, director of the Missouri Department of Transportation’s Geotechnical Section.
He noted four primary sedimentary layers — shale, limestone, sandstone and dolomite — are visible in the cut. The key problem is that one of those layers — shale — is weaker and more prone to crumbling than the others. Not only does the shale absorb water, it degrades when wet.
“It’s a problem material,” said Sheri Lamberson, senior geotechnical specialist for MoDOT. “There are a lot of things going on in that rock cut.”
The messy stack of rust, cream, grey and gold layers reveal hundreds of millions of years of geologic history. But Missouri’s freeze-and-thaw weather cycle has worked the layers over since they were exposed in the 1970s.
As the shale is whittled away, blocks of limestone and dolomite come to protrude. Eventually, those ledges and hanging blocks collapse under their own weight falling to the shoulder and pavement below.
The cut has always been a curiosity for geologists. Years ago, when dynamite and earth-moving equipment first carved it into the earth, it became famous when the Journal of Sedimentary Petrology featured the exposed rock in an article.
“It shows, clearly, two unconformities in geological history,” MoDOT geologist George Davis said.
An “unconformity” is a large gap or break in the sedimentary geologic history, the result of no sedimentation or no erosion happening on the surface of the earth.
“Basically, no deposit and no removal,” he said.
It leaves a question mark in geologists’ minds, Davis said.
The rock cut exhibits multiple faults, fractures and the reactivation of a few sinkholes. Those sinkholes have been especially problematic because the surrounding rocks layers have been packed tightly over time. The sinkholes, in comparison, collapsed and were filled with all kinds of sediments, loosely packed. So when water courses through the layers of rock, the looser sediments tend to wash away, leaving holes.
Although there aren’t any real caves in the rock cut, there are a few smaller cavities, Lamberson said.
Patches of white-colored rock are evidence of a coral reef that existed in the Devonian era, about 420 million years ago. The rock cut also shows evidence of sedimentary layers laid down in the Mississipppian and Ordovician eras, roughly 419 to 485 million years ago.
To fix the problem, employees with Boone Construction Co. built a new road to get on top of the rock face. A blaster from Hollister, Explosives Contractors Inc., has been setting off charges to dislodge some of the bigger limestone boulders. Contractors are also creating “steps,” so that when rocks and boulders fall in the future, they’ll likely only tumble to the next-lower step, rather than plunging toward the northbound lanes of U.S 63.
So far, crews have used a rock-chipper to break up large boulders and a track hoe with a large, articulated arm to scoop up loads. Massive dump trucks have been ferrying away the loose rock to a new pile near Westbrook Drive. Smaller bobcat excavators have been used to help level and smooth the piles.
Davis said he is proud at the speed at which the MoDOT team assembled document to bid the project out; it only took about 12 hours to assemble the contract documents. Ultimately, a $558,000 contract was issued to Boone Construction Co.
“They’ve done a good job of stabilization,” Davis said.
The work is expected to be finished by the first week in May.
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