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<br /> <br /> <br />J;J-':::: <br />'0' ~,;a. <br />, ~t <br />. . 1iI- <br /> <br />FIGURE 18. - Rockfall on eastbound lane of Interstate 70. 2 miles west of Bergen Park interchange (El Rancho). <br />Failure was along joints (fractures) and foliation planes (layering) in Precambrian gneiss. Rockfall ob- <br />structed all of eastbound lane and median and part of westbound lane before cleanup began. Estimated weight <br />of largest boulders exceeded 200 tons. Photographed on May 8, 1973. <br /> <br />ROCKFALLS <br />Rockfalls were triggered by the storm on the <br />steeper hillsides and canyon walls west of Den- <br />ver when rainwater and snowmelt percolating <br />into cracks loosened rock fragments and sent <br />them plummeting down the slopes. Loose man- <br />tle rock behaved similarly. Rockfalls were abun- <br />dant in the canyons as far west as Berthoud <br />Pass along U.S. Highways 6, 40, and 285, Inter- <br />state 70, and Colorado Highway 74. Most of <br />these were quickly removed by highway main- <br />tenance crews. <br />By far the largest single rockfall collapsed <br />onto Interstate 70 about 2 miles west of Bergen <br />Park interchange, near El Rancho (fig. 18), <br />completely blocking the eastbound lane. Some <br />rock bounded across the median and into the <br />westbound lane. Failure along steeply dipping <br />foliation planes (layering) and joints (frac- <br /> <br />tures) involved thousands of tons of rock, in- <br />cluding giant boulders weighing hundreds of <br />tons each that required drilling and blasting <br />before they could be removed. The hillside above <br />the fall was left in a precarious state, also, and <br />much loose rock had to be drilled and blasted <br />before cleanup work could start on the rockfall <br />itself. <br />South of the metropolitan area in suburban <br />Douglas County, rockfalls collapsed down the <br />flanks of several steep-sided buttes south and <br />west of Plum Creek, particularly Dawson Butte, <br />Raspberry Mountain, and Bald Mountain (John <br />O. Maberry, written commun., 1973). Rockfalls <br />at these places resulted when failure of the soft <br />claystones and sandstones in the Dawson For- <br />mation removed support from the overlying <br />caprock. <br /> <br />17 <br />