GLENWOOD SPRINGS, Colo. -- Highway workers began breaking up a huge boulder perched precariously on a cliff 900 feet above Interstate 70 in western Colorado on Wednesday, days after a rock slide that closed the highway indefinitely.

State officials said they blasted the boulder just before 6 p.m., and maintenance crews immediately began work to clear the debris on the highway. They worked throughout Wednesday to make the repairs needed to open a lane of traffic in each direction when rockfall mitigation is complete, said CDOT spokeswoman Mindy Crane.

The highway remained closed Wednesday night because crews lost daylight. They will hike to the site Thursday morning to conduct follow-up rock scaling and assess the stability. Crane said CDOT hoped to have some portion of the highway open on Thursday.

Workers used compressor-powered drills to break up a boulder 20 feet in diameter that was sitting above the highway in Glenwood Canyon. A helicopter was used to haul the drilling equipment to the site.

A CDOT geologist felt the rock was "unstable," said CDOT spokeswoman Stacey Stegman. "He doesn’t know when it would come down on its own, maybe five years. Maybe not. But it’s enough so that we have concerns that we don’t want to leave live traffic under this rock knowing that it does have the potential to come down in the meantime.”

“Ideally we would love it to come down in small pieces. And that’s, I think, the goal but it’s pretty unpredictable. We’re not exactly sure what’s going to happen. We don’t want to see more damage to the highway because that’s just going to delay being able to open it,” Stegman said.

A 17-mile stretch of I-70 has been closed since Monday after the slide rained boulders onto the road and punched gaping holes in an elevated section of the highway.

A woman commuting to work in northwest Colorado was killed Wednesday when a basketball-size boulder fell on a car on U.S. 40, about 60 miles north of the I-70 incident, the State Patrol said.

That part of U.S. 40 is one of the detours around the I-70 rock slide, but there was no indication the woman had gone that way because of the slide.

The coroner identified her as Karen Lynn Evanoff, 55, of Craig, a passenger in the car. The unidentified driver was unhurt.

“It was one stray rock. And in fact we even saw elk tracks up above and so we think that maybe it was just an animal that kicked one down," Stegman said. "It’s one of those freak occurrences. But it’s horrible.”

Victor Domenico, owner of Domenico Transportation in Denver, said the 200-mile detour around the slide was doubling the cost of some truck deliveries between Denver and western Colorado. The longer trip increases fuel and payroll costs, and some drivers have to lay over for a night to stay within federal limits on their hours behind the wheel, he said.

The slide blocked the main driving route from the Denver airport to the Aspen Skiing Co.'s four Aspen-area mountains, but the company said more than half its winter visitors fly into the Aspen airport.

Bill Tomcich, president of Stay Aspen Snowmass, a reservations agency, said he had heard no reports of cancellations.

The slide did not affect access to most of the state's other ski areas.