LEADVILLE —

It may be a month before Colorado Department of Transportation engineers have the still-growing hole near the summit of Tennessee Pass filled and U.S. 24 fixed.

"The roof of the tunnel is collapsing, and it's trying to take our road with it," CDOT engineer Joe Elsen said Friday.

The hole appeared Monday above a century-old railroad tunnel. Its timber braces had folded, revealing a 100-foot-deep section of the tunnel.

Pieces of the hole's edge break off and bounce into the darkness below every day. The pit at the highway's shoulder, about halfway between Redcliff and Leadville, now measures about 30 feet by 30 feet.

Elsen said the next step in the highway-recovery process is to put the repair contract out to bid. Bids must be submitted by 2 p.m. Monday to be eligible. CDOT hopes to have a contractor picked late Monday or Tuesday.

"We want this done quickly," Elsen said.

CDOT has closed off less than a mile of U.S. 24 about 9 miles north of Leadville, forcing traffic to Colorado 91 and Interstate 70.

The roadblock isn't a problem for those traveling from Denver to Leadville but could add up to an hour of travel time for drivers coming from Redcliff and an estimated 15 minutes from Minturn.

The hole also poses problems for the three-day Children's Hospital Courage Classic charity bike ride, which is scheduled to pedal over Tennessee Pass on July 24.

"At this time, I don't see how they'll go there," CDOT spokeswoman Ashley Mohr said.

About 2,400 cyclists are signed up for the ride.

The Children's Hospital Colorado Foundation expects to announce a route change on Monday, spokeswoman Tina Garbin said.

"We had a fire plan in case there were fires, and now there's a sinkhole," Garbin said.

Mohr said the hole should be filled and U.S. 24 reopened by Aug. 23, when Stage 4 of the USA Pro Cycling Challenge is scheduled to race over Tennessee Pass.

A look inside of a large sinkhole on highway 24 at mile marker 165 between Red Cliff and Leadville Friday morning, July 13th, 2012. Crew from the Colorado
A look inside of a large sinkhole on highway 24 at mile marker 165 between Red Cliff and Leadville Friday morning, July 13th, 2012. Crew from the Colorado Department of Transportation were on hand surveying the sinkhole which opened up Monday. The sinkhole, developed above a century-old Rio Grand Railway tunnel that measures 25x20 feet and approximately 50-60 feet deep. CDOT plans to put out the construction contract right away to begin repairs ASAP. (Andy Cross, The Denver Post)

To mend the sinkhole, CDOT engineer Matt Figgs said the tunnel — which stretches under the roadway — will have to be plugged on each end with a foam material injected into the void.

"It's not as simple as back the trucks up and start filling it," Figgs said.

The repair is estimated to cost $1 million to $2 million, CDOT said Friday evening.

The 2,500-foot long tunnel crosses U.S. 24 in two other places south of Tennessee Pass. Crews will continue to survey those sections of the highway, but the tunnel runs under the area at 180 and 200 feet — posing less of a threat.

"Folks shouldn't worry about those other ones yet," Mohr said. "But we definitely have our eye on them."

On Friday afternoon, Elsen and Figgs crouched downhill from the edge of the hole, using a mirror to reflect light onto the timber braces buried 40 feet below the pavement.

This isn't the first time the braces have broken on the tunnel, owned by Denver and Rio Grande Railroad.

A Summit County Journal article from May 29, 1909, reports that the tunnel — which cost $1 million to construct — caved in "and will have to be abandoned."

The article does not report which portion of the tunnel collapsed, adding to the murky history of the road and railway.

"Some points of the tunnel have had problems," Mohr said. "It's just unclear; they didn't keep records."

Despite the tunnel's collapse, CDOT obtained a license from the railroad company in 1925 to begin construction of the road above it.

Union Pacific spokesman Mark Davis said Rio Grande moved the track from the collapsed tunnel around the mountain area. In 1944, the railroad company received the right to construct a new tunnel about 50 feet to the west of the collapsed structure.

This time, the tunnel was built with sturdier, concrete trusses. Railway cars traveled into Camp Hale — established for training during World War II in Eagle Park.

The 10th Mountain Division Memorial stands not far from Camp Hale and near the sinkhole-caused roadblock.

Shane O'Donnell, 42, hiked to the spot Thursday with Lauren Jerd, 26, and Marlowe Kent, 32, catching a ride to town and back again Friday.

The three started their journey on the Continental Divide Trial in New Mexico in May and hope to reach the Canadian border by October.

Carrying a 55-pound backpack, O'Donnell stopped to peer into the sinkhole, protected by temporary barriers.

"It was so odd to me," he said before continuing on to Silverthorne. "You know, to hear about a sinkhole on the road."

Tegan Hanlon: 303-954-1729, thanlon@denverpost.com, or twitter.com/teganhanlon