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| | Wednesday, March 10, 2010
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New bolting system for the I-40 mountainside
After consulting with its specialty contractor and federal highway officials, the North Carolina Department of Transportation has decided to use a different, thicker type of rock bolt to stabilize the site of the I-40 rockslide, just east of the Tennessee state line. The interstate has been closed since the 800 foot high, rock mountainside collapsed onto the interstate on October 25. The new “passive” rock bolt system uses the mountain itself to create the downward pull that stabilizes the rock. Under the “tensioned” system that was used previously, workers employed a hydraulic jack to create a downward pull in the steel rod rock bolts and cable strands anchors. The elimination of this step, which took upwards of an hour for each rock bolt or anchor, is the primary advantage of the new system. The new rock bolt system is as safe, if not safer then the previous system, according to Jon Nance, the department’s chief engineer of operations . Because it requires less time to install each bolt, the switch will not affect plans to reopen I-40 by late April, weather permitting. And the move will not add to the cost of the $10.1 million project. “After working on the mountain for several months and analyzing its geology, we realized that this system, which we have used in other projects, would be effective here,” said Nance. “I would not say this is a better plan, just a more efficient one that will allow us to produce similar results in less time.” Crews will still drill 590 holes into the mountainside. Now, however, they will install thicker steel rock bolts into those holes. The new rock bolts have a diameter of 1 3/4 inches, compared to the 1 3/8 inch steel rock bolts that had been used. The increased strength of the new bars is what allows crews to adopt the “passive” system. About 125 tensioned rock bolts and cable anchor strands have been installed at the site. All 30 of those that have been tested have passed, withstanding pressure at 133 percent of the design load. Crews began working with the new rock bolts on Monday. As clear skies dominated the weekend weather, workers drilled more than a dozen new holes in the mountainside and tested six more rock bolts. Taking advantage of their first sustained stretch of good weather in more than a month, crews at the I-40 rockslide have intensified drilling on the mountain. Crews are operating five to six drill rigs around the clock and have completed 270 of the holes required for the rock bolts. That number should increase more quickly in coming weeks, because as crews move down the mountain the depth of the holes they must drill should decrease. “A lot of the holes higher up are 100 to 120 feet in depth,” said NCDOT project inspector Mike Patton. “As we move down, more of them will be in the 70 to 90 foot range.” Patton added that crews have drilled about 31,000 linear feet, more than half of the total 50,000 linear feet they expect to drill.
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Return to Today's News Stories - Front PageWednesday, September 08, 2010
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