City fills sinkhole off Midland Ave. | PostIndependent.com
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City fills sinkhole off Midland Ave.

GLENWOOD SPRINGS – City workers here used 12 dumptruck loads of fill Tuesday to close a sinkhole that formed Friday afternoon along Midland Avenue.

The hole developed alongside the westbound lane of Midland Avenue between the Community Center and the city’s Municipal Operations Center.

The oval-shaped sinkhole measured 16 by 20 feet at its widest points.



“It didn’t affect traffic,” city engineer Larry Thompson said. “We barricaded off the shoulder; we wanted to keep traffic off the very edge of the shoulder.”

Those barricades were left in place over the weekend.



Thompson said the cause of the sinkhole has not been determined.

“I initially suspected when I received the call that it was going to be associated with the rain that fell Friday morning,” he said. “But when I got there, it didn’t look like it was caused by rain.”

Unstable soils are not a new or unknown phenomenon in Colorado, and the problem is known to be especially damaging in the lower Roaring Fork Valley. From Basalt to Glenwood Springs, there is a high susceptibility to collapsible soil and shallow evaporitic bedrock, the Colorado Department of Natural Resources reports.

Collapsible soils and evaporitic bedrock can create two types of hazards. Collapsible soils, also known as hydrocompactive soils, may settle when they become wet. This settlement can be quite rapid, resulting in soil collapse, the department said.

Evaporitic bedrock is composed of evaporite minerals (gypsum and salt) that may dissolve when wet. The dissolution creates voids, fissures, and caverns in the bedrock that can collapse and cause ground subsidence, sinkholes, subsidence troughs and localized depressions.

But Thompson said a sinkhole as large as the one that opened up Friday most likely took years to form.

“It could have formed over a few years or hundreds of years,” he said. “Twelve dumptruck loads of dirt didn’t just disappear in a matter of just a rainstorm.”

City workers used a road base material to fill in the depression.

“That way water will drain through it easily,” Thompson said.

The hole is not the first to form in the old Wulfsohn Ranch area, which is currently being developed, Thompson said.

“Obviously there is some level of concern that it could happen elsewhere,” he said, noting that there have been reports of other sinkholes on the alluvial fan at the base of Red Mountain.

“It’s known that it’s happened at other times on the ranch,” Thompson said. “I’m not panicked over it, but the city and people developing it need to know it has the potential to exist.”

Workers from the city’s engineering department plan to investigate the sinkhole to try and determine the cause.

“They’ll try to determine if the subsidence extends into the gravel alluvium or if it’s just in the surface clay,” he said. “We’re going to do whatever we can to determine the cause of it, and we may or may not be successful.”

Contact Greg Masse: 945-8515, ext. 511

gmasse@postindependent.com


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