High Altitude Rehabilitation

Historic Colorado Tunnel Presents Unique Repair Challenges in the Rockies

For the past few years, construction crews and engineers have been hard at work repairing the Carlton Tunnel, high in the Rocky Mountains near Leadville, Colorado. The tunnel’s remote location – miles from the nearest paved road and an hour-and-a-half drive from there to the nearest town – adds to the challenges of working at nearly 11,000 ft elevation.

Background

The Carlton Tunnel has a long and varied background. Completed by the Colorado Midland Railway company in 1893, the Busk Ivanhoe Tunnel (as it was originally called) replaced the nearby Hagerman Tunnel to provide rail access across the mountains. The new tunnel was nearly 1,500 ft lower in elevation than its predecessor and eliminated 7 miles of winding track – many on trestles – leading up to the old Hagerman Tunnel. The new, lower alignment provided a much more economically feasible route across the Continental Divide.

Construction of the tunnel was achieved using the drill-and-blast, top heading-and-bench method. Timber supports were used as needed in the varied geology. According to an 1894 article in Engineering News, in some areas the rock was self-supporting while in other areas it disintegrated upon exposure. About 78 percent of the tunnel was supported by timbers. 

Construction of the 9,394-ft long Busk Ivanhoe Tunnel took over three years to excavate (an average daily advance of 4.2 ft per heading) at a cost of approximately $1 million, with 20 workers losing their lives. The tunnel rose at a grade of 1.41 percent from east to west, reaching an elevation of 10,953 ft. The inner dimensions were 15 ft wide by 21 ft high. The completed tunnel then carried freight and passengers by rail between Leadville and Aspen. 

The railroad tunnel operated for more than two decades before it experienced financial difficulties and was sold to the State of Colorado in 1922. At this point, the tunnel was renamed the Carlton Tunnel and it was converted to a single-lane vehicular tunnel carrying SR 104, with the internal dimensions reconfigured to about 14 ft wide by 14 ft high. Traffic along the route alternated directions on half-hour intervals. 

The construction of a dam at the western end of Ivanhoe Lake near the Carlton Tunnel’s western portal allowed for the tunnel to fulfill a new role. As the water level rose following completion of the dam, water was diverted through the tunnel via an open ditch, carrying the precious water across the Continental Divide and into local waterways before ending up at Turquoise Lake reservoir. The reservoir feeds into the Arkansas River watershed that supplies water to Eastern Slope communities. Eventually, a 36-in. concrete pipe was placed with the tunnel to convey water. 

The facility operated as a vehicular tunnel into the 1940s. It was then purchased by the Highline Canal Company in 1945 and has continued in service for water conveyance to the present day. Today, the valuable water rights are shared by Pueblo Water and the City of Aurora, and operated by Busk Ivanhoe Water System Authority.

Need for Rehabilitation

In the 1950s and 60s, repairs were completed in the tunnel that included the placement of new timber sets in about 35 percent of the alignment. Additionally, wood stave pipe was replaced with reinforced concrete pipe for water conveyance. 

Until recently, crews were only able to access about 1,000 ft of the tunnel due to large collapses. Despite the collapses, the Busk Ivanhoe Water System Authority was still able to maintain about 50 cfs of flow, which allowed it to achieve its annual water yield of 7,000 acre-ft. However, that number started dropping, and, following a major collapse in 2018, that number eventually reached 12 cfs in 2019 as collapses within the tunnel had damaged the concrete pipe carrying the water. The need for major renovations was apparent. 

To lead the repair plan, the authority issued an RFP to engineering companies specializing in tunnels, eventually selecting Mott MacDonald as its engineer. At the same time, the owner wanted to incorporate Early Contractor Involvement to help develop a collaborative pathway forward, and selected Drill Tech Drilling & Shoring Inc. as its contractor on a time and materials basis. The collaborative approach, it was felt, was needed due to the complex nature and unknowns facing the job.

The Drill Tech/Mott MacDonald team began the repair project in 2021 with work on the western portal, with repair work within the tunnel beginning in 2022, and continuing in 2023 and 2024. The tunnel is used to transport water when snowmelt from the mountains feeds Ivanhoe Lake from about March through June. The construction window typically begins in July when the roads are passable and runs into October when snow starts falling in the high mountains again.

“The biggest challenge we face is the remote access of the site,” said Ryan Carroll of Drill Tech. “The site is located at 10,000 ft elevation and the nearest paved road is 14 miles away, so materials have to be delivered to the end of the pavement, where we have to ferry it all up to the jobsite. In fact, our crewmembers camp at the site during the season because it is so remote. They work 12-hour shifts, six days a week and take Sundays off to go to town and resupply. It just makes everything challenging from a logistics standpoint.”

“Because of the remoteness, you can’t do everything by the textbook – you can only work with the tools that are available,” said Dave Jurich of Mott MacDonald. “For example, there may be some areas where traditional support isn’t feasible because of access limitations and the fact that you need to pre-order and store everything onsite.”

Adding to the difficulties is a lack of records from the original construction, according to Shawna Von Stockhausen of Mott MacDonald. “While we don’t have the original records, we do have records from repairs that were completed in the 1950s. With those, we are at least able to infer what ground conditions we might expect,” she said. In fact, the last time an inspection of the full tunnel was completed was in 1988. 

Construction

Drill Tech is using bags of pre-mixed Quikrete concrete that it batches with water into a Cemen Tech plant set up on site. Crews are placing steel sets and lagging inside the existing timbers and backfilling and a 5-in. lining with fiber reinforced shotcrete, creating an approximately 12 ft by 12 ft opening. When completed, it is the hope of the Busk Ivanhoe Water System Authority to be able to drive vehicles through the tunnel as they had been able to do in the past, in addition to increasing the water capacity.

Depending on the ground encountered and the condition of the timbers, different approaches are used based on collaboration with Mott MacDonald, Drill Tech and the owner. In areas where the timbers have collapsed, crews may spend several days to weeks mucking out before they are able to install the steel sets. In some areas the collapsed portions of the tunnel reach more than 30 ft high, Carroll said. 

During the first two years of repair, Drill Tech used muckers to transport material from the tunnel through the 11 ft by 11 ft portal. This past year, Drill Tech was able to deploy two Atlas Copco MT 2010 haul trucks with ejector beds, which crews loaded using a Cat 306 excavator. This combination allowed Drill Tech to quadruple its production. 

Moving Forward

With three years of repair work completed, the tunnel is already seeing an increase in flow to 25 cfs, which is close to the point where the authority can achieve its annual water yield.

“With three years of construction behind us we have made good headway,” said Ed Perko of the Busk Ivanhoe Water Authority. “If not for Mott MacDonald and early contractor involvement with Drill Tech, we would not have been able to get our water yield this year and would have lost income.”

Von Stockhausen concurs that teamwork is paramount in this non-traditional project. “Having a good working relationship across all parties and being clear on the goals are keys to success,” she said. “We are all doing whatever it takes and being creative to get the job done safely and efficiently.”

About 1,000 ft of the nearly 10,000-ft alignment has been repaired to date, and plans are for crews to return in 2025. Perko is optimistic that crews will be able to repair significantly more footage as they expect conditions within the tunnel to improve. However, because full capacity has not been restored, it is anticipated that collapses farther into the tunnel have occurred, resulting in damage to the pipe.

“From what we are able to see, there appears to be a clear stretch of tunnel ahead of us with no cave-ins, so we hope to make a lot of headway next year,” Perko said. “We also plan on addressing the east portal in the future, which would make it easier to get staff, materials and equipment there.

“With the help of Mott MacDonald and Drill Tech, we have made a lot of progress. It takes a good team to be able to overcome the challenges of working in unknown surroundings and in a remote site with limited access to tools and supplies.”

Jim Rush is editor of TBM.