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rockfall

A rockfall is the free falling of a rock from a cliff face or from the roof or walls of a mine or quarry. Mitigation techniques include passive drape nets, rockfall catchment fences, and diversion dams, as well as active rock bolts, shotcrete, and revegetation. Significant rockfalls in Colorado have occurred along interstate 70.

Avalanche debris in the runout zone taken from a helicopter on the morning after the avalanche occurred, 24 March, 2003. Photo credit: Xcel Energy.

Hazards

March 23, 2003

Case Study: Avalanche – Silver Plume

On March 23, 2003, a large avalanche occurred about one mile west of the Town of Silver Plume. The avalanche brought trees, rock, soil and snow to the valley floor, knocked down overhead utility lines, blocked the I-70 frontage road, damaged the town’s water treatment plant (WTP), and dammed Clear Creek. The dam was breached using explosives before the plant’s electric pump motors were flooded. With damage to the WTP’s chlorine contact tank and building, Silver Plume residents had to boil their tap water for over a month.
A precarious rock above Manitou Springs started to move in 1995 after a period of wet weather. As an emergency measure, high-strength steel cables were wrapped around the rock and anchored to the surrounding ledge to arrest the movement. Photo credit Jon White for the CGS.

Hazards

June 19, 1995

Case Study: Rockfall – Manitou Springs

[ED: This report from 1995 was written by Jon White, (Senior Engineering Geologist, Emeritus). It looks at a specific rockfall situation in the central Front Range town. ] Manitou Springs occupies a narrow valley where Fountain Creek emerges from the foothills northeast of Pikes Peak and west of Colorado Springs. The valley slopes are composed of interbedded resistant sandstone and conglomerates (i.e., gravelly sandstone), and weaker mudstones and shale. The outcropping sandstone is most prevalent on the steeper slopes on the north side of the valley.