The original tunnels, completed in 1961, were narrow and created a “black hole” effect, causing motorists to dramatically slow their speeds, creating bottlenecks. Courtesy of Mike Keleman, Colorado Department of Transportation
The CDOT is at the midpoint of a project to widen key twin tunnels on I-70 that have created a weekend bottleneck as motorists flock to the mountains for skiing and summer recreation.
February 25, 2014—The Colorado Department of Transportation (CDOT) is at the midpoint of a project to widen its twin tunnels, which create a persistent bottleneck on I-70. On weekends and holidays, motorists jam the highway—the main artery linking Denver with mountain skiing and recreation centers. Traffic jams from the tunnel can extend for as far as 30 mi.
The bottleneck has been exacerbated by what the CDOT calls the “black hole effect.” The tunnels, built in 1961, have flat, gray openings and weak lighting inside. Additionally, the tunnels are just 28 ft wide, with narrow shoulders. “The speed limit is 55 miles per hour, [but] people drop down to 30 to 40 miles per hour,” says Benjamin Acimovic, P.E., M.ASCE, an engineer for the CDOT. “They really hit their brakes hard going into that tunnel, even though it’s a short tunnel and you can see the light on the other side. The darkness engulfs you and you step on your brakes out of reflex. You feel like you are driving into a wall.”
So the CDOT is widening the Twin Tunnels from 28 ft to 53 ft. This will enable the addition of a third lane, a 10 ft wide outside shoulder, a 4 ft inside shoulder, and a concrete guardrail. The project also includes visual enhancements to the tunnel entrances to ease the transition for motorists. The eastbound tunnel, which was completed in December, is approximately 790 ft long. The westbound tunnel is approximately 823 ft long. Construction will begin on the westbound tunnel in April.
The improved tunnels are just the first phase of a larger vision that covers 144 mi of I-70 between Glenwood Springs and Golden, says David Singer, the I-70 Mountain Corridor Environmental Program Manager for the CDOT. Singer notes that the CDOT identified a long-term multimodal solution for the I-70 Mountain Corridor in 2011 following a decade of analysis and coordination. The department employed a context-sensitive solution approach that involved many stakeholders in the development of the project.
The Twin Tunnels are in a sensitive area. The land bridge over the tunnels provides a crucial crossing for local wildlife. Nearby Clear Creek is a source of recreation and drinking water for local communities, as well as Denver. And there are important viewsheds and historical communities in the area. Early in the planning process, these factors precluded a mountain cut that would have eliminated the tunnels altogether.
Rock conditions varied dramatically from one end of the
eastbound tunnel to the other. Courtesy of Mike Keleman,
Colorado Department of Transportation
“CDOT looked at a number of different alternatives, but maintaining the current alignment allowed CDOT to maximize its existing infrastructure,” Singer says. “One of the benefits of doing all this work in the past is [that] we had a lot of data to refer to.”
The CDOT’s key goals for the project were to increase mobility and increase safety. A dangerous curve immediately past the eastbound tunnel bore was of particular concern.
“People start speeding up after they go through the tunnel and immediately encounter a sharp curve,” Singer says. “That was a big area for truck spills and accidents. So the project straightened out the curve in addition to adding a third lane of capacity.”
The tunnels are being enlarged via drilling and blasting from either end toward the center. The widening has been complicated by rock conditions that varied greatly from the east end to the west. Some areas accommodated drilling as deep as 15 ft; other areas as little as 5 ft. Additionally, although the CDOT had plans from the original construction, there were no “as-built” documents.
“Inside the tunnels, we had unknown conditions behind the liners,” Acimovic says. “What we found early from our geophysics scans was [that] we might have voids.” The state created an emergency project to fill the voids before demolition began.
Managing risk was one of the greatest challenges the team faced. “We weren’t going to understand how much poor rock we had until we pulled the liner off and started blasting and exposing it,” Acimovic says. “We designed a system of dowels, bolts, channels, and shotcrete to support the good rock.”
As the team moved into the poorer rock, it deployed a system of steel bracing in addition to the dowels, bolts, and shotcrete to support the tunnel until the final supports, the geomembrane drainage system, and the liner could be installed.
To manage risk in the project, the state created risk pools and determined that the construction manager/general contractor delivery method was the best approach to the project. The project design team was led by Atkins, of Surrey, United Kingdom, and included Parsons Brinckerhoff, of New York City, as the tunnel lead. The state selected a joint venture of Edward Kraemer & Sons, of Plain, Wisconsin, and Obayashi Corporation, based in Tokyo, as the contractor. The risk management strategy was very successful, Acimovic says.
With the eastbound tunnel complete, CDOT officials note that the bottleneck has been relieved and motorists are offering positive comments about the project. When westbound tunnel construction begins in the spring, traffic will be diverted onto a nearby county owned frontage road, as it was for the eastbound tunnel. The use of this road as a detour is a part of a greater partnership between CDOT and Clear Creek County. As a part of this agreement, the project will also include stream enhancements and the creation of a trailhead in close proximity to the tunnels.
The second phase of the $160-million project is scheduled to open to traffic in December. The CDOT has plans to convert the new third lane of eastbound I-70 in this segment to a toll lane, Singer says. “This is consistent with CDOT’s policy to better manage congestion on our most heavily traveled corridors,” he explains. “This managed lane will provide a more reliable option for the public.
“During peak periods on Sundays and holidays, that third lane is going to be managed. The existing two general-purpose lanes will remain free. There are a lot of people trying to get to the airport, trying to get their families home after a long day of skiing or playing in the mountains. Many times they are willing to pay for additional travel-time savings,” Singer says.