The state of Colorado is still fighting a year later to hold road builders accountable for the unusual highway collapse on U.S. 36 that shut down toll lanes, snarled northwest metro traffic for months and required breakneck repair work.

The Colorado Department of Transportation hasn’t released the findings of a forensic investigation into the July 2019 incident, in which a “slope failure” destroyed an eastbound overpass approach near Church Ranch Boulevard in Westminster. The Colorado Attorney General’s Office denied The Denver Post’s most recent request for the report last week, saying it was keeping the findings confidential until the state’s probe ends.

What began as cracking concrete on the approach — which had been rebuilt as part of a large highway project about five years earlier — quickly turned into something far worse: Pavement shifted and the retaining wall buckled as water-saturated clay in the embankment’s soil sank and receded.

As the scope of the problem became clear on the 300-foot stretch next to a railroad overpass, CDOT fully closed the eastbound side late on July 12, 2019 — about 36 hours after drivers began reporting the initial cracks. In the following days, the retaining wall bulged and sagged even more.

For nearly four months, both directions of traffic were shoehorned into the westbound lanes of the highway, causing bottlenecks for commuters while crews raced to repair and rebuild the eastbound side’s embankment and retaining wall.

It reopened to traffic Oct. 3. Last week, CDOT spokesperson Matt Inzeo told The Post that the final repair tab was $17.6 million, shy of the $20 million set aside for the emergency work.

Bruce Finley, The Denver Post

A large crack along eastbound U.S. 36 between Wadsworth and Church Ranch boulevards is pictured on July 14, 2019.

In the days after the collapse, state Rep. Matt Gray and state Sen. Faith Winter, who chair the legislature’s transportation committees, issued a joint statement demanding accountability for the roadway’s failure.

“Something definitely went wrong,” Gray, a Democrat whose district is just north of the failure site, said in an interview. “It’s not something that should have happened, and whoever is responsible does need to answer to the folks who spent hours of additional time in traffic away from their families because of a preventable mistake.

“So I understand the legal process can take some time, but those people still deserve answers — as do taxpayers.”

Lawrence Pacheco, the spokesperson for Attorney General Phil Weiser, declined to address questions about the state’s approach to seeking damages or reimbursement. The office also is holding tight to the findings by CTL Thompson, a Denver geotechnical engineering firm paid about $125,000 by CDOT to investigate the causes of the collapse and help assign blame.

“This is an ongoing investigation and materials related to it are confidential and privileged attorney-client communications” under the Colorado Open Records Act, Pacheco wrote in an email.

U.S. 36 from Denver to Boulder was rebuilt from 2012 to 2016 by Ames Construction and Granite Construction in two phases that had different contract setups. The section that failed was part of the straightforward first phase, when the companies acted as a joint venture.

A consortium called Plenary Roads Denver won a 50-year public-private partnership deal that included the second phase of construction — also handled by Ames and Granite — as well as operation of new tolled express lanes and maintenance of the entire stretch for decades to come.

After the failure, an expert told The Post that construction was a more likely culprit than maintenance, but there’s plenty of complexity in the matter.

Joe Amon, The Denver Post

CDOT and its contractor Kraemer North America worked on rebuilding eastbound U.S. 36 by placing geofoam blocks along the section of the collapsed highway in Westminster on Sept. 5, 2019. About 6,000 foam blocks were placed before installation of new highway pavement on top.

The state’s discussions with Ames and Granite have been underway for months. It’s not clear whether the AG’s office has pursued a claim or is engaging in less formal talks.

Ames and Granite are both large companies based out of state. Reached this week, Ames general counsel Todd Goderstad said it was premature to discuss the issue. Granite spokesperson Erin Kuhlman also declined comment.