Report exposes the dirt on geologic hazards at Meadows
Post Independent Staff
GLENWOOD SPRINGS – From dangerous debris flows to sinkholes, a geologic hazards review from the Colorado Geological Survey cites a laundry list of hazards facing development at Glenwood Meadows.
The review was performed by CGS senior engineering geologist Jonathan L. White and sent to Glenwood Springs city planner Patti Haefeli. It outlines debris flow hazards, the possibility of ground subsidence and compressible soils, hydrocompactive soil hazards and the likelihood of sinkholes on the property. The report also gives suggestions for irrigation and landscaping.
“The site is located across the Colorado River from West Glenwood on a large alluvial fan,” the review said. “In past reviews of this property, the CGS has commented on the potential geologic hazards that may impact this property. . These hazards are well known for the property and have been covered in-depth by various geological, geotechnical and/or drainage reports.”
Debris flow activity and risk has increased since the Coal Seam Fire, the report said. It will be most pronounced at the city’s golf course – if built – and in residential neighborhoods, which are planned uphill from the commercial development.
“As such, we suggest the original comprehensive mitigation plan be re-examined or possibly redesigned to reflect the changes in basin characteristics,” the report said.
While debris flows could cause problems at the development, White’s report calls “the threat of ground subsidence the most problematic geologic hazard concerning the long-term performance of structures in the development.”
Ground subsidence could result in sinking foundations, or even sinkholes.
Hydrocompactive soils – or soils that compress when water is introduced – are listed as another problem at the Glenwood Meadows site.
“The consultant has made it clear that wetting of the bearing soils may result in large settlements,” the report said.
To avoid this problem, the original consultant suggested building an elaborate system of drains and landscaping in such a way that keeps the ground as dry as possible.
“We cannot stress enough how important these recommendations are, and how carefully drains must be installed,” White’s report said.
Sinkholes are another problem that have occurred and are expected to occur again.
According to records from the Wulfsohn family who formerly used the land to cattle grazing, there were “piping voids, sinkholes and washouts in the irrigated fields that were subsequently filled with trash and buried,” the report said.
The locations of these sinkholes and washouts are not known.
The report also points out that a sinkhole just opened just across Midland Avenue from the property.
“We consider the risk of sinkhole-type subsidence to be higher than what is stated in the consultant’s geologic hazard review section of their report,” White’s report said.
The last section of the report deals with the prevention of deep wetting by creating a landscape and irrigation system that drains water quickly and efficiently.
“Heavy and wide expanses of irrigation should be avoided,” the report states. “Xeriscape landscaping is strongly recommended.”
In his closing comments, White said while the development plan is designed to lessen some risks, it “does not fully mitigate them.”
White also insisted in the report that future residential property owners need to have full disclosure of the risks involved with buying property at Meadows.
“We are understandably more concerned about residential areas where risks may not be disclosed to home buyers, or when re-selling occurs and normal homeowners insurance does not cover damage due to ground movements,” White wrote in a supplemental e-mail sent to Haefeli.
In that e-mail, White provided some additional suggestions for testing and mitigating the potential hazards. These include shafts, columns or piles under foundations that are drilled down to the river gravel or bedrock below.
He also suggested “plate load tests,” which involve pouring a small footer and weighing it down with concrete blocks or bags of fill to approximate the planned bearing load.
“Similar testing was done on alluvial fans for the Highway 82 Snowmass Canyon project,” White wrote.
He also suggested using ground-penetrating radar to search out areas that could be prime sinkhole locations.
“Since we know sinkholes occur at this site, it might be wise to run this survey along the location of the building foundation footprint and wet utilities before the thick wedge of controlled fill is placed,” White wrote.
Contact Greg Masse: 945-8515, ext. 511
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