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Case Study: Clear Creek Canyon rockslide

2005-06-21 | Dr. John Hopkins

Rockfalls and rock slides are common along transportation corridors in the Rocky Mountains. Clear Creek Canyon just west of Golden is one of the most active rockfall areas in Colorado. The canyon has been cut into Precambrian schists and gneisses by Clear Creek, one of the primary drainages in the Denver area. Rockfalls occur every year in the canyon in response to freezing and thawing, snowmelt, and intense or prolonged rainfall. Historical rockfalls have ranged in size from small (less than an inch (several cm) in diameter) individual rocks to large boulders up to 10-13 ft (3-4 m) in diameter.

A high-profile rockslide event occurred on June 21, 2005 along U.S. Highway 6 in Clear Creek Canyon, approximately 10 miles (16 km) west of Golden, CO. Around 11 AM, 2,000 cubic yards (1500 m3) of rock slid from a pre-existing road cut on the north side of the road and completely covered the road. Two tractor-trailers were caught in the rockslide and were pushed off the road by the debris. The tractor-trailers were themselves totaled but the drivers sustained only minor injuries.

One of two semi trucks caught in the catastrophic rockfall in Clear Creek Canyon, about 10 miles west of Golden, Colorado. Photo credit: Colorado Geological Survey.
One of two semi trucks caught in the catastrophic rockfall in Clear Creek Canyon, about 10 miles west of Golden, Colorado. Photo credit: Colorado Geological Survey.

The geology at this location consists of Precambrian metamorphic schist and gneiss, which has been subsequently intruded (cut through by molten rock) by granitic pegmatite dikes. Unfortunately, one of these thin pegmatite dikes that had intruded into the metamorphic rocks was steeply inclined toward the roadway. When the magma intruded the metamorphic rocks and solidified into the granitic pegmatite, the contact between the two rock types became “baked” and the mineralogy and texture of the rock was changed. This “baked” contact zone weathered to produce a transition of clay-rich material. The clayey zone was structurally weak, providing a plane for the rocks above to detach from the underlying rocks and produce this large rock slide.

Jon White, Senior Engineering Geologist, examines the failure zone between the granitic pegmatite and the surrounding metamorphics. Photo credit: Colorado Geological Survey.
Jon White, CGS Senior Engineering Geologist, examines the failure zone between the intrusive granitic pegmatite and the surrounding metamorphics. Photo credit: Colorado Geological Survey.
View of the slip surface looking north (i.e., the surface of a pegmatite intrusion). The installation of wire mesh to control the inevitable small rocks falling. Photo credit: Vince Matthews for the CGS.
View of the general plane of the slip surface (that is, the surface of the pegmatite intrusion). The wire mesh was in place prior to the June 2005 rock fall to control the inevitable shedding of smaller rocks. Photo credit: Colorado Geological Survey.
Aerial view of the rockslide site eight days after the event, the initial slide has been cleared from the road. Mitigation work would proceed for the next ten weeks until the road was reopened in September 2005. Photo credit: Colorado Geological Survey.
Aerial view of the rockslide site eight days after the event, the initial slide has been cleared from the road. Mitigation work would proceed for the next ten weeks until the road was reopened in September 2005. Photo credit: Colorado Geological Survey.

To mitigate the unstable rock slope remaining after the slide, approximately 35,000 cubic yards (26,800 m3) of rock had to be excavated by blasting. The slope was laid back to an angle of 45 degrees, and rock reinforcement anchors were installed into the slope to enhance stability. Wire mesh was then draped over the slope to help control any small rocks that will inevitably get loose. The cost to remove the rock and repair the road was 3.2 million dollars, more than the typical annual state budget for rockfall mitigation at the Colorado Department of Transportation (CDOT)! By the middle of September 2005, after the longest full road closure in Colorado’s history, the road was reopened to traffic.