A massive rock slide on Interstate 70 in Glenwood Canyon forced thousands of motorists onto a circuitous, 146-mile detour and could prompt delays on the heavily traveled route for up to a month.

Officials say Monday night’s slide underscores the challenges of managing hundreds of miles of Colorado roads winding through rockfall-prone slopes that cycle through repeated freezes and thaws.

The closed interstate will reopen partially Thursday afternoon at the earliest, with traffic alternating each direction in the eastbound lanes. Workers must repair the damaged roadway and shore up unstable rocks towering above before traffic can start moving again.

A solid timetable for full reopening has not been announced, although officials estimate it could be up to a week before vehicles can simultaneously head in both directions. It will be about a month before fixes are complete.

Until then, motorists are encouraged to divert several hours on U.S. 40 to the north. An average of about 300 vehicles ordinarily travel the canyon every hour.

The slide, which occurred about 9 p.m. just west of the Hanging Lake Tunnel, sent boulders tumbling into at least three vehicles. No injuries were reported, but the rocks gouged holes in the asphalt and damaged guardrails.

“Smart-car-size boulders fell down canyon walls,” said Amy Ford, a spokeswoman for the Colorado Department of Transportation.

Mitigation work

Mitigation work was done in Glenwood Canyon during the summer, and at least three major rock slides have closed the same stretch of I-70 in the past 12 years.

While routine checks, sensors and proactive rock removal aim to mitigate the danger on mountain roads, Mother Nature sometimes holds surprises.

A slide just south of Durango early Sunday left roughly 700 tons of rocks on U.S. 550.

CDOT has a $9 million rockfall program aimed at keeping tabs on the more than 750 Colorado locations — including Glenwood Canyon — that are recognized as having chronic rockfall hazards. Those funds pay for netting, visual monitoring and rock removal, among other remediation.

“It’s a combination of all those efforts,” Ford said. “It’s an incredibly difficult task. As much as we do, we still have to deal with mountain environments.”

Monday’s slide wasn’t even the first of the day. About 19 hours earlier in the same area, car-sized boulders fell onto the interstate and prompted lane closures.

CDOT engineers analyzed the canyon’s walls after the 2 a.m. slide and, although they ordered further review, found that the area appeared to be stable.

“They felt like it was safe to reopen the road based on that first review,” Ford said.

Then about 9 p.m., more large boulders tumbled down onto two semi-trailers and at least one car. One of the semi-trailers was severely damaged.

Ray Hatch said he was behind the wheel of one of the trucks on a trip from Denver to Flagstaff, Ariz., when he was hit by the slide.

“All I saw was this big cloud of snow coming down,” said Hatch, who lives in Las Vegas. “Then it hit my windshield and I couldn’t see anything.”

Hatch’s truck came to a stop after running into a large boulder that lodged in the road before him. He called the experience a “big surprise” and said he didn’t know at first what hit him.

The 24-mile stretch of I-70 between Gypsum and Glenwood Springs has been closed during cleanup, cutting off the state’s main route from the Front Range to the Western Slope, including the popular resort destination of Aspen. Semi-trailers traveling cross-country also rely heavily on the passage.

CDOT used a helicopter Tuesday to view the damaged area and determine the stability of the canyon walls. Officials also were examining the road for structural issues.

In June, I-70 through the canyon was closed for about eight hoursafter 1-ton boulders fell onto the highway.

In March 2010, another major slide near the Hanging Lake Tunnel closed the interstate for about four days. That incident prompted a disaster declaration by then-Gov. Bill Ritter.

Another slide there in 2004 cost $700,000 to repair and closed the interstate for a day.

During the I-70 closure in 2010, a 55-year-old Craig woman taking a detour was killed when a rock smashed through the roof of her car.

Early thawing

CDOT says rock slides are fairly common in Glenwood Canyon during spring thawing.

Recent warmer temperatures have sped the process and probably prompted Monday’s slides.

“There have been problems in there even when it was (U.S.) 6 before the interstate,” said Jonathan White, an emeritus geologist with Colorado Geological Survey who specializes in rockfall. “It’s just the nature of the terrain.”

White explained that the canyon is lined with granite and that tiny separations in the rock face can be widened by melting ice and snow, which causes chunks to break off.

“When you drive along the canyon, you can see all these steep slopes are nothing but broken rock,” said White, who worked on surveying the rockfall risk when the interstate through the canyon was built in the late 1980s and early 1990s.

Some of the boulders that landed on I-70 Monday are so large that they must first be blasted with explosives so that pieces can be loaded onto trucks and hauled away.

The cost of repairing the interstate is being calculated. Bustang service will stop at Eagle as long as the road remains closed.

Jonathan Godt, program coordinator for the U.S. Geological Survey’s landslide hazards initiative, pointed to CDOT as a national leader in mitigating rockfall danger. Still, he said, there is no magic bullet.

“Although, transportation departments and other facilities managers take actions to reduce rockfall and rock slide potential, the only foolproof way to mitigate their impacts is to avoid cliffs, canyons and steep slopes altogether,” he said. “(That’s) hard to do in the mountainous parts of Colorado.”

Jesse Paul: 303-954-1733 or jpaul@denverpost.com

Staff writer Kirk Mitchell contributed to this report.

Glenwood Canyon Detour

CDOT officials advise motorists to take U.S. 40 as an alternate route. The detour, which adds 146 miles to a normal trip between Wolcott and Rifle, could take up to four hours.

Westbound motorists should get off I-70 at Colorado 131 at Wolcott to Steamboat Springs then head west on U.S. 40 to Craig. In Craig, drivers should head south on Colorado 13 to Rifle, where they can get back on the interstate.

Eastbound motorists are encouraged to take the same route in reverse.