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ON UNSOLID GROUND: Colorado Springs commissions study of massive landslide complex

September 24, 2017 Updated: September 25, 2017 at 1:59 pm
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Colorado Springs is undertaking a $500,000 study of the enormous Cheyenne Mountain landslide complex, nearly 1½ years after being urged to do so quickly by the Colorado Geological Study. In this photo, Judith Myklak walks out to the landslide area below her home on Friday, July 22, 2016. Myklak's property, sitting above the Broadmoor South Golf Course, has been affected by a landslide after soils were saturated last spring and summer. Photo by Stacie Scott, The Gazette

Colorado Springs is undertaking a $500,000 study of the enormous Cheyenne Mountain landslide complex, nearly 1½ years after being urged to do so quickly by the Colorado Geological Study.

The study will be conducted by Yeh and Associates Inc. of Denver, and the city will split the cost with Colorado Springs Utilities, said city Councilman Don Knight.

The landslide complex runs from the base of Cheyenne Mountain to the east almost 2 miles, according to State Geologist Karen Berry, director of the Colorado Geological Survey.

"Little is known about the depth of the landslide, its rate of movement, the aerial extent of movement, or the toe of past and current slope movement," Berry advised the city in April 2016.

But its current movement "may be deep-seated and extensive," she wrote.

She had urged the city to hire a geotechnical engineering firm with extensive landslide experience, and Yeh and Associates appears to fit the bill as a "full service firm specializing in geotechnical engineering, engineering geology, and construction management," its website says.

The CGS suggested the study use monitoring instruments and a detailed subsurface investigation of slope stability, "to understand the current and future risks posed to city infrastructure and residents."

The Broadmoor South Golf Course, also known as the Mountain Golf Course, is on the landslide complex, as are parts of surrounding development. The golf course has been closed for more than two years, another victim of the 500-year record rains of May 2015 that triggered landslides on the city's western half.

In the Cheyenne Mountain scarp area, "The risk of landslides and ground instability will always be present," CGS Senior Engineering Geologist Jonathan White said during a presentation last week at the convention of the Association of Environmental & Engineering Geologists.

"These are risk areas," White said. "Higher standards of care (i.e., higher costs) are needed in geotechnical investigations in landslide susceptible areas. In sensitive slopes, one must discern stealthy landslides, disturbed claystone vs. intact bedrock, and what are the fluctuating groundwater levels during periods of high precipitation. Consider proactive measures to safeguard property valuations."

The landslide under the Mountain Golf Course was reactivated in 2015, he noted, creating fresh scarps above and extensional ground cracks within the golf course and toe damage to homes below it.

"The Colorado Geological Survey earlier recommended no homes be placed on ancient landslides. ... The slope edge recommendations were not underrated," White said.

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