CDOT closed I-70 at 9 a.m. on Tuesday and reopened it at 4:30 p.m. I-70 was closed in both directions at Georgetown to the east and Silverthorne to the west.
I-70 is expected to be closed for 20-minute stretches on Wednesday while crews finish their work.
CDOT says crews are breaking larger boulders into smaller pieces, but it is a slow and tedious process. They plan to remove about 40 boulders from the hillside.
CDOT took down two large boulders on Tuesday morning, the largest of which was 8 feet in diameter.
No boulders have reached the highway. Engineers have draped huge steel mesh curtains to rein in boulders on some slopes, and built retaining walls to keep falling rocks off I-70.
In addition to the highway closure, five homes and one business in the immediate area were evacuated as a precaution.
Alternate routes take motorists up through Kremmling to the north or Fairplay to the south. The northern route takes westbound drivers off I-70 at Empire and winds them over Berthoud Pass and into Kremmling. From Kremmling, they'll take State Highway 9 south to I-70 in Silverthorne.
The southern route takes eastbound drivers south on State Highway 9 from Frisco, through Breckenridge, over Hoosier Pass and into Fairplay. From there they will take US 285 into Denver.
CDOT says traffic is light on I-70 and the detour route over Berthoud Pass is moving well. The southern detour route over Hoosier Pass was blocked by two semi trucks, but CDOT was working to clear them out of the way as quickly as possible.
"You have mountain goats up on the hill and they get to kicking those rocks, and you have them come down," said Archie Marrone, who works at MountainBuzz Cafe & Pizzeria in Georgetown, across the highway from where the work was taking place.
"Plus you have the winter spring thaw that loosens them up. I keep an eye out for them," Marrone said.
Falling boulders killed two people on I-70 near Georgetown in 1999.
The atmosphere was almost festive in Georgetown, a community with deep roots in Colorado's mining and narrow-gauge railroad history. Residents gathered in small groups on street corners Tuesday to gaze up at the work about a quarter- to a half-mile away.
"I wanted to watch the rocks fall down," Georgetown resident Tom Elliott said.
Elliott said a rock hit a Volkswagen he was riding in several years ago, bouncing on the trunk lid and over the top but missing the windshield.
"I've been aware of these rocks for years, so I try to avoid them," he said.
Rockslides are a natural hazard of mountain travel, CDOT spokesperson Stacy Stegman said. The state transportation department has a rating system to monitor potential rock falls at more than 750 sites across Colorado, including 18 around Georgetown alone.
"We're never going to be able to prevent all of those. What we do by way of mitigation is try to reduce the probability of a major slide or try to slow the rocks so they don't come into contact with vehicles," she said.
Some of the boulders threatening I-70 were as large as pickups, but none that size had come down by mid-day.
An average of 30,000 vehicles travel I-70 near Georgetown every 24 hours, and closing it down cuts off traffic between Denver and some of Colorado's biggest ski areas.
The Loveland Ski Area offered free breakfast to skiers and snowboarders who came up early to beat the closure.
"The parking lot is not even half full," said Randal Nichols, 31, a snowboarder from Springville, Calif., who made it to Loveland from the Denver area before the closure. "There was actually longer lines for breakfast than the lifts."
Loveland marketing assistant Carly Knutson said skier numbers were down slightly Tuesday but still better than expected.
Vail Resorts offered lodging packages for customers who wanted to ski during the highway closures. Keystone Ski Resort extended its operating hours Tuesday and Wednesday.
(KUSA-TV © 2011 Multimedia Holdings Corporation with The Associated Press)