A rockfall occurred at this site off RM 2222, near the Paradox Cove and Westslope Drive bend, on Jan. 2, resulting in the Texas Department of Transportation closing one lane for a couple of days. Experts say the timing of such rock slides can’t be predicted. LESLEE BASSMAN
By LESLEE BASSMAN, Four Points News
Following a Jan. 2 rock slide off RM 2222, on the sharp bend between Paradox Cove and Westslope Drive in northwest Austin, the Texas Department of Transportation closed the roadway’s westbound right lane to protect motorists from any additional slides, TxDOT Public Information Officer Bradley Wheelis said. The route reopened Jan. 3, after the agency’s crews spent most of the day clearing rocks that had fallen and taking down any loose boulders from the cliff to limit future rock slides, he said.
According to Wheelis, recent rains in the area loosened the soil around the rocks, causing them to fall and staff waited until the day after the rocks fell to remove them due to the continued wet weather.
“Part of the reason people move to this area of Austin is its natural beauty, including the trees and cliffs that make RM 2222 a scenic drive,” Wheelis said. “Short of a retaining wall that would obstruct views, there’s not a lot more that can prevent such slides. Signs have been in place for years warning of fallen rock because of this naturally occurring event.”
However, he said there had been a rock slide on the roadway several years ago. Loop 360 fell victim to a slide in September near the Pennybacker Bridge.
“There are no plans for additional measures,” Wheelis said. “This is a naturally occurring event that happens on cliff sides. This happens all over the country, not just Texas or Austin.”
The January rock slide did not come as a surprise to Joel Johnson, associate professor of geological sciences at the Jackson School of Geosciences at The University of Texas at Austin, whose area of specialty includes the study of landscapes and erosion processes such as rockfalls and landslides
“When you have cliffs or steep slopes made of rock, rockfall is inevitable and is part of the natural process and part of the natural world,” Johnson said. “The road planning and engineering is done to minimize that risk.”
However, certain factors may make the area more sensitive to a rockfall, he said,citing how strong the rock is, how fractured it is and the nature or character of the rock. Much of the rock along RM 2222 is likely to be limestone and the more fractured this rock is makes it more erodible or prone to collapse, he said. The orientation of the fractures, or its angles, also make a difference, he said.
Additionally, there are a large number of outside variables that influence the likelihood of rockfall, Johnson said. Precipitation and climate can affect the possibility of rockfall, he said.
“Precipitation is a big (factor),” Johnson said. “Adding water to the rock in the pores of the rock — the little void spaces of the rock — and also in the fractured spaces of the rock adds mass, it adds weight to the rock that pulls down (the rock) more because of gravity.”
Water can act as a lubricant to the rock and, in many to most cases of rock fall, an incident will happen just after a big rainstorm, he said.
“(A rockfall) can happen during a storm or after a storm but there is a period of time where the water, even if it stops raining, is still seeping into the rock and part of it is seeping through the soil over days and weeks,” Johnson said. “It doesn’t mean that anyone necessarily did their job badly. It just means there are a lot of different variables and factors. Some of them you can’t see inside of the rock.”
There are engineering methods used to determine the stability of the rock but those are only estimates done from the outside physical rock attributes, he said.
As far as solutions for potential rockfalls, Johnson said redesigning or excavating part of the cliffs is a possibility but it’s expensive. Covering cliffs with concrete is done in various parts of the country such as California but is also expensive and not attractive, he said. In Central Texas, outside of the Longhorn Dam, some sides of the river are covered in concrete to control erosion, Johnson said.
Constructing netting over the rocks on a cliff is a good choice to help remedy the issue of falls and isn’t as costly as some other fixes, he said.
“Nets that are made of woven metal and designed to hold impressively large rock falls are a possibility,” Johnson said, adding that he is not an engineer and focuses on earth science.
He said netting does not obscure the view of the cliff and provides a continued reminder for passing motorists or pedestrians that a potential for rockfall exists at the site. Signage also helps identify the area, he said.
Aside from being dangerous and a concern, rockfall doesn’t happen too often, Johnson said, noting the local rockfall issue is “not that bad.” Although it makes sense for motorists to be more aware of their surroundings while driving on cliff-lined roadways after a rainfall, rockfalls can occur when the weather is dry as well, he said.
“(A rockfall) is such a rare and random event that, frankly, it’s hard to give a good prediction,” Johnson said.