U.S. 36 collapse report cites drainage design, heavy rainfall during construction as possible culprits

Design flaws successful a retaining partition drainage strategy and dense rains during operation whitethorn person contributed to a “slope failure” that caused the illness of U.S. 36 successful Westminster 2 years ago, according to an extracurricular engineering study released Thursday by the state.

The forensic probe report‘s merchandise comes little than a week aft the disclosure of a $14 cardinal settlement reached by the Colorado Department of Transportation with a radical of operation and plan contractors. They had rebuilt and expanded the road conscionable 5 years earlier the eastbound broadside collapsed successful July 2019 adjacent Church Ranch Boulevard.

The colony covered astir of CDOT’s $17.6 cardinal successful effect and repair costs.

The study by CTL Thompson, a Denver geotechnical engineering firm, was prepared for the Colorado Attorney General’s Office arsenic considered taking ineligible action. The Denver Post filed a nationalist records petition to get it.

The study mostly backs up CDOT’s archetypal assessment: that water-saturated clay successful the ungraded embankment beneath an overpass attack softened and shifted beneath the retaining wall.

The root of that h2o and erstwhile it accumulated successful the ungraded is little clear, but the 34-page study suggests respective factors that whitethorn person played a relation — some during the operation play successful 2013 and 2014 and since then.

Among them was the plan that was expected to guarantee that moisture didn’t seep into the clay-heavy crushed beneath the retaining wall.

“The drain strategy designed and installed with (the partition section) was not capable to region h2o down and beneath the wall,” the study states.

In 1 area, CTL says, a drain was 3 feet supra the basal of the retaining partition and 5 feet higher than the bottommost of the ungraded that was utilized arsenic capable during the project. “The drain could not region h2o which penetrated to the basal of the Class I fill,” it says. That indicated to the engineers that the drain was not moved little adjacent though a partition redesign added 2 further feet of caller ungraded fill.

Another plan alteration cited arsenic a imaginable contributor was the relocation of a planned motorcycle way from the crushed to the road level of the overpass approach, which extended the partition out, expanding its height. CTL “found nary denotation that planetary stableness was re-evaluated” aft that change.

CTL besides cited indications immoderate colony occurred successful the embankment beneath the partition during construction, noting that successful September 2013 the Denver country received dense rains. Those are the aforesaid storms that caused extended flooding successful the bluish Front Range.

“Surface drainage was not maintained to nonstop h2o disconnected the (exposed) capable aboveground and distant from the span and vertical drains,” the study says.

The colony divided the $14 cardinal betwixt a associated task of Ames Construction and Granite Construction and their engineering consultants connected the project, HDR Engineering and Kleinfelder.

CDOT main technologist Steve Harelson said that due to the fact that that signifier of the U.S. 36 task had a design-build setup, the contractors had much leeway to modify the designs, utilizing their champion judgment. But that near them astatine hazard of liability if it proved faulty.

“It’s engineering judgment,” helium said, “and it didn’t enactment retired rather arsenic good arsenic we hoped successful this case. Hopefully, we learned a spot of a acquisition and they learned a spot of a lesson.”

Todd Goderstad, the wide counsel for Minnesota-based Ames, said the colony was reasonably divided betwixt the contractors, fixed that immoderate of the report’s conclusions are unfastened to interpretation.

“There’s grey areas successful geotechnical subject arsenic good arsenic successful operation and engineering,” helium said. “In bid to forestall the discarded of resources, we thought it was amended to travel to a comparatively expedient solution of the problem.”