Jun 192017
 

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.

During the wet spring of 1995, rockfall and landslides incidents increased throughout Colorado, some resulting in fatalities. In Manitou Springs, a fortunate set of circumstances occurred before the Memorial Day holiday weekend when local residents observed the movements of a large, dangerous block of rock before it actually could fall. The observation set into motion an emergency declaration by the town, resulting in a compulsory evacuation of homes located below the rocky slope, the closing of the road in the area, and an immediate rock stabilization project. During this emergency situation, the Colorado Geological Survey was asked to provide expert assistance to help stabilize the rock. The emergency evacuation decree remained in effect until the rock was stabilized and the area subsequently declared safe.

The ledge of jointed sandstone along with several large displaced blocks is seen at the center of the image. Photo credit Jon White.

The ledge of jointed sandstone along with several large displaced blocks is seen at the center of the image. Photo credit Jon White.

A prominent 12-foot-thick ledge of strongly-jointed sandstone forms the rim of this slope. Two essentially vertical and intersecting joint sets produce large orthogonal sandstone blocks that are being undermined by the more easily weathered mudstone beds below the ledge. The blocks begin to topple as the underlying rock that supports them erodes, creating dangerous overhangs. At the time of discovery, this particular block had moved 5.5 feet from the back face of the sandstone ledge and tilted precariously over the next sandstone ledge below. Had the 70-ton block fallen, it would have certainly crushed a home below.

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.

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.

The extremely unstable, tilted, rock could not be removed due to the proximity of homes directly below, so high-strength steel cables were wrapped around the rock and anchored to the surrounding ledge. Once the block was safely restrained, additional cables were physically attached to the top of the block at anchor points that were cemented into drill holes to provide an additional level of support for the block and safety for the homes below.

After the rock was stabilized, additional cables were physically attached to the top of the rock block and secured to surrounding stable rock. Photo credit Jon White.

After the rock was stabilized, additional cables were physically attached to the top of the rock block and secured to surrounding stable rock. Photo credit Jon White.

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