The developer at Gold Hill Mesa has commissioned more testing on a plot where dozens of homes are planned after concerns were raised by state geologists last fall about whether the site is safe and stable for construction.
The new report, due in late February, will address issues raised by the Colorado Geological Survey about slope stability, settlement and liquefaction potential of the soils atop the century-old mine tailings pile, said representatives of the developer, Gold Hill Neighborhood, LLC. Liquefaction is a form of destabilization that could be set off by seismic activity from nearby faults.
The Geological Survey said in its Oct. 19 report that it couldn’t recommend approval of the proposed expansion without more information about the land, citing concerns that the agency has said also apply to another plot nearby.
Meanwhile, city planners and engineers have put construction on hold at the plot, until the additional test results are available.
Whether the additional tests on the plot, known as Filing 10, will be enough to answer state geologists’ questions remains to be seen.
Approval to build homes in another area of the development, known as Filing 11, was also put on hold in April after Jonathan Lovekin, a senior engineering geologist for the Geological Survey, asked the developer to test the plot for its settlement potential – a recommendation he made again in his report on Filing 10, in addition to calling for more slope stability analysis.
“There are minimal records to indicate how the mill tailing slimes were placed but we know of no compelling reason to expect the tailings were placed carefully or in the type of controlled conditions that would be conducive to residential development,” he wrote of Filing 10.
A regional engineering firm the developer has hired, CTL Thompson, is testing soil samples from the areas of both Filings 10 and 11 for a variety of metrics, including moisture content, dry density, swell potential and strength, said Hayden Fischer, chief technical officer at Bryant Consultants, a Denver firm hired to peer-review the results.
Seismic geophysical testing is also being done on the plots, said Fischer.
Through another form of testing, international geotechnical contractor ConeTec is using an instrument with a cone-shaped tip to measure parameters such as soil saturation at various depths beneath the plots, Fischer said. Saturation testing is important because state geologists have expressed concerns that further developing Filing 10 could weigh down soils and increase the saturation level, which makes soil behave like liquids and could cause slope failure.
“We are gathering additional data that’s going to supplement what was already collected previously,” said Bill Hoffmann, the senior principal engineer for CTL Thompson. “All of it together — it all builds upon itself.”
Hoffmann has disagreed with the Colorado Geological Survey’s assessment of Filing 10, asserting that his firm has already completed a proper slope analysis, along with other work, that shows the site is safe and stable for construction.
Lovekin wrote in an October letter that earlier tests didn’t provide enough information to thoroughly understand the tailings’ physical and engineering properties. Details offered about the soils were based on “limited drilling” from one 1999 study and a 2004 seismic study, Lovekin wrote.
Four more borings, each between 100 feet and 150 feet deep, have been drilled into bedrock at Filing 10 to provide more information, Hoffmann said.
Data from those holes, along with other work, “does not change the basic conclusions” that CTL made in a 2004 report, Hoffmann said in a statement.
The developer is working with "a variety of engineers and technical consultants" to ensure that the Geological Survey’s concerns about drainage and erosion will be addressed, too, said Stephanie Edwards, executive vice president for the developer.
Gold Hill Mesa sits on the former site of the Golden Cycle Mill, which refined ores mined in Cripple Creek for the first half of the 20th Century. Parts of the development are atop an old decant pond, up to 130 feet deep in places, full of tailings left over from that refining process.
Mine tailings are known for their instability — a source of concern since the development’s inception.
Construction began around 2007, guided by a state-approved plan to mitigate contaminants in the soil. That plan required developers and home builders to top the tailings with at least two feet of mixed soil and two feet of clean soil. They were also required to install a plastic barrier to prevent residents and others from digging into the tailings.
Sinking soils and consequent damage to homes have been recorded at Gold Hill Mesa, the Colorado Geological Survey reported last year.
But the developer’s representatives and many residents flatly deny that there has been an inordinate amount of sinking or subsidence; they’ve instead blamed cracked foundations and other damage on construction defects.
More testing to examine the potential for settlement and liquefaction at Filing 11 has been underway since last fall.
“I don’t think anybody can argue with wanting to do more state-of-the-art testing,” said Edwards.
The developer still has not agreed to fulfill one of Lovekin’s requests: that the entire site be tested for its potential for liquefaction.
Edwards said the developer “wouldn’t be opposed” to a global liquefaction study, but land rights are a complicating factor. The developer would have to get permission to conduct testing on lots that were sold to builders and then to current homeowners, she said.
“If there ever were to be more testing on the already existing property, that’s kind of tricky because of property ownership,” she said.
Lovekin has asserted that liquefaction anywhere in Gold Hill Mesa could damage Filings 10 and 11.
But Fischer said that liquefaction’s effects are limited by lateral distance.
“If you want to understand what’s happening way over there, you need to do testing way over there,” Fischer said.
“We feel pretty good that the thoroughness of this investigation will address whatever concerns that are making them think that testing outside would be necessary,” he later added.
Filing 10 was originally approved in 2018 before it was resubmitted to the Colorado Geological Survey for review, city spokeswoman Jamie Fabos has said.
Most filings at Gold Hill Mesa won approval from city staff without input from state geologists on each individual phase. The state agency reviewed a broad proposal in 2004, which served as a basis for the approval of subsequent phases.
The Colorado Geological Survey evaluated Filings 10 and 11 at Gold Hill Mesa because of an ordinance, passed unanimously by the Colorado Springs City Council in 2017, that requires the agency review site plans and make recommendations in advance of development across the city.
The decision of whether to approve construction at both plots ultimately rests with city planners and engineers.
While construction has been put on hold on the two filings, work can continue at already approved plots.