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Design flaws in a retaining wall drainage system and heavy rains during construction may have contributed to a “slope failure” that caused the collapse of U.S. 36 in Westminster two years ago, according to an outside engineering report released Thursday by the state.
The forensic investigation report‘s release comes less than a week after the disclosure of a $14 million settlement reached by the Colorado Department of Transportation with a group of construction and design contractors. They had rebuilt and expanded the highway just five years before the eastbound side collapsed in July 2019 near Church Ranch Boulevard.
The settlement covered most of CDOT’s $17.6 million in response and repair costs.
The report by CTL Thompson, a Denver geotechnical engineering firm, was prepared for the Colorado Attorney General’s Office as considered taking legal action. The Denver Post filed a public records request to obtain it.
The report largely backs up CDOT’s initial assessment: that water-saturated clay in the soil embankment beneath an overpass approach softened and shifted beneath the retaining wall.
The source of that water and when it accumulated in the soil is less clear, but the 34-page report suggests several factors that may have played a role — both during the construction period in 2013 and 2014 and since then.
Among them was the design that was supposed to ensure that moisture didn’t seep into the clay-heavy ground beneath the retaining wall.
“The drain system designed and installed with (the wall section) was not sufficient to remove water behind and below the wall,” the report states.
In one area, CTL says, a drain was three feet above the base of the retaining wall and five feet higher than the bottom of the soil that was used as fill during the project. “The drain could not remove water which penetrated to the base of the Class I fill,” it says. That indicated to the engineers that the drain was not moved lower even though a wall redesign added two additional feet of new soil fill.
Another design change cited as a possible contributor was the relocation of a planned bike path from the ground to the highway level of the overpass approach, which extended the wall out, increasing its height. CTL “found no indication that global stability was re-evaluated” after that change.
CTL also cited indications some settlement occurred in the embankment below the wall during construction, noting that in September 2013 the Denver area received heavy rains. Those are the same storms that caused extensive flooding in the northern Front Range.
“Surface drainage was not maintained to direct water off the (exposed) fill surface and away from the bridge and vertical drains,” the report says.
The settlement divided the $14 million between a joint venture of Ames Construction and Granite Construction and their engineering consultants on the project, HDR Engineering and Kleinfelder.
CDOT chief engineer Steve Harelson said that because that phase of the U.S. 36 project had a design-build setup, the contractors had more leeway to modify the designs, using their best judgment. But that left them at risk of liability if it proved faulty.
“It’s engineering judgment,” he said, “and it didn’t work out quite as well as we hoped in this case. Hopefully, we learned a bit of a lesson and they learned a bit of a lesson.”
Todd Goderstad, the general counsel for Minnesota-based Ames, said the settlement was reasonably split between the contractors, given that some of the report’s conclusions are open to interpretation.
“There’s gray areas in geotechnical science as well as in construction and engineering,” he said. “In order to prevent the waste of resources, we thought it was better to come to a relatively expedient resolution of the problem.”