Dark, damp and claustrophobic confines

Maintaining a 58-year-old water tunnel under the Continental Divide is no easy fix.

October 12, 2020 | By: Todd Hartman

Some maintenance jobs are harder than others, and Denver Water just finished a doozy.

The task: patch some of the leaky spots in a 23-mile water tunnel that cuts under the Continental Divide, and do it in damp, dark and claustrophobic confines — and do it fast.

The repairs recently completed at the Roberts Tunnel were among the first needed inside the nearly 60-year-old conveyance that can move up to 480 million gallons of water per day from Dillon Reservoir on the West Slope into the South Platte River Basin on the Front Range.

a man stands in a tunnel, his back to the camera, wearing a hard hat, and other gear and water up above his knees.
Working inside the Roberts Tunnel required having to walk through water that was up to 3-feet deep in places due to water seeping inside. For the project, crews built a small dam to create a dry work area which can be see in the back of the photo. Photo credit: Denver Water.


The utility has inspected the tunnel — one of Denver Water’s most critical pieces of infrastructure — for decades and found few problems.

But time has taken its toll. Over the last several weeks, engineers set out to plug several small cracks and holes in the tunnel’s concrete lining that have most likely developed due to the hydraulic pressure of Dillon Reservoir pushing on its west side. Crews also patched up areas where the concrete was deteriorating.

One positive: the repairs at issue are within the first 2 miles of the tunnel’s west side, limiting the distance workers had to travel inside the tunnel’s confines. That’s where the pressure from the reservoir is likely having the greatest impact, by forcing water through small cracks and crevices in the tunnel’s concrete.

“The repairs are very challenging,” said Brad Piede, a Denver Water engineer overseeing the project.

Construction workers inject urethane, a grout-like material into cracks and holes in the tunnel’s concrete lining. Photo credit: Denver Water.


The process started in August, when Denver Water closed the gate under Dillon Reservoir and shut off flow through the tunnel. That step, in and of itself, required Denver Water to take numerous additional steps to ensure consistent water flows from other facilities for customers.

Turning off that flow allows work inside the tunnel to begin. Even so, some water still flows along the bottom of the 10-foot-wide tunnel, so workers had to divert that water around work areas to allow them to perform repairs.

Most of the fixes involved injecting grout into the holes as well as behind the concrete liner.

But that’s not all.

A man in safety gear, sprays the wall of a tunnel.
A worker injects urethane into a crack. You can also see an area where the concrete has deteriorated. Photo credit: Denver Water.


“The grout has to set fast enough so water intruding into the area doesn’t wash it away,” Piede said.

The holes and cracks in the tunnel concrete don’t pose a major threat to the tunnel itself.

When water is flowing through the tunnel at full capacity, the pressure evens out and probably reduces the penetration of water into — and out of — the tunnel lining, Piede said.

But, he added, it’s important to reduce any deterioration that comes with time.

“You don’t want the tunnel to go into disrepair, and let these things fester, either,” he said. “This is preventative maintenance.”

Small holes and cracks, left unattended, could get worse over time and potentially create more serious problems.

On top of the technical challenges posed by the repairs, workers were racing the clock. Denver Water can’t keep the Roberts Tunnel turned off very long.

Water from Dillon Reservoir is diverted through the Harold D. Roberts Tunnel under the Continental Divide into the South Platte River before being sent to one of Denver Water's treatment plants.
Water from Dillon Reservoir in Summit County flows through the Roberts Tunnel to the Front Range. Dillon is Denver Water’s largest reservoir. Photo credit: Denver Water.


The project schedule was only six weeks. Keeping the tunnel off also affects hydropower production, flows in the North Fork of the South Platte River and operations across the system.

The work wrapped up Oct. 6. Piede said the project required a few changes “on the fly,” but that repairs “went well and on schedule. We’d call it successful.”

So, as you flip on your shower in the morning, you might want to give a quick thought, once again, to all the work and infrastructure built over decades that delivers that water to your bathroom. And maybe say a quiet thank you to the laborers who worked deep inside the earth in a wet, dark tube, ensuring the flow will continue long into the future.

Watch a video of some of the work with commentary from Gary Metz, senior project manger from Restruction Corporation:


3 thoughts on “Dark, damp and claustrophobic confines”

  1. Thank you so much for the videos on Roberts Tunnel. My father worked on the project to build the tunnel. Every time you post a video, I take him on a walk down memory lane and he entertains me with construction stories.

  2. I always enjoy the consistently excellent pieces in TAP. I have learned so much and I must say, I was just “blown away” (to use on old, old phrase) by the photos and descriptions in the piece about repairing the Roberts Tunnel. Having lived in Denver and lucky enough to have had a second home near Dillon for many years I have always been interested in Denver Water’s management of the lake and the tunnel.
    Thank you Mr. Hartman. – Great job !!

  3. I don’t understand why bringing water back through the tunnel means the S Platte goes from 400+ cfs flows to an ankle deep trickle in a matter of days. Seems like an unnecessarily drastic change that could have been spread out over a week or two rather than done so quickly resulting in a shock to the fishery. This after leaving the north fork running so low fishing in the ranches was outright cancelled for the season.

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