The SourceCivil Engineering MagazineThe last piece of the Interstate Highway System may have been the toughest
History Lesson

The last piece of the Interstate Highway System may have been the toughest

By T.R. Witcher

When it finally opened to traffic on Oct. 14, 1992, the 12.5 mi segment of Interstate 70 through Glenwood Canyon, on the Western Slope of Colorado, completed what may be America’s greatest public works project: the federal Interstate Highway System. These last miles may very well have been the hardest.

Plans for a national network of roads had been under consideration since just after World War I, but America’s modern interstate system began with the passage of the 1956 Federal Aid Highway Act, which authorized construction of a 40,000 mi network — now officially known as the Dwight D. Eisenhower National System of Interstate and Defense Highways — that would link the United States and be paid for almost entirely by the federal government. (Read “Special Report: The Interstate Highway System at 50,” Civil Engineering, June 2006, pages 36-43, 78.)

Initial plans for I-70 called for it to run west from Baltimore to Denver but not to extend into the Rocky Mountains. Colorado leaders quickly pressed for a route through the mountains, however, and military officials wanted a route that would connect the central United States with Southern California. Eventually I-70 was aligned to connect with I-15 in central Utah rather than in northern Utah at Salt Lake City, terminating at Ft. Cove.This meant builders would be faced with plotting a course directly through one of the most beautiful and challenging stretches of the Colorado Rockies. 

According to CDOT, the finished project cost $490 million in 1992 dollars; adjusted for inflation, that would be $925.5 million today.

The route turned out to be less of a single road than a “network of viaducts, bridges, and tunnels constructed through an extraordinarily narrow, environmentally sensitive gorge” (“Glenwood Canyon 12 Years Later,” by Karen Stufflebeam Row, Eva LaDow, and Steve Moler, Public Roads, Federal Highway Administration, March/April 2004).

Carved by the mighty Colorado River over the course of 70 million years, the rock walls of the narrow Glenwood Canyon reach as high as 2,000 ft. It had long been seen as an impassable route, even by the area’s indigenous Utes. But during Colorado’s 19th-century silver boom, speculators and developers recognized the canyon was the most direct link between the Western Slope and Denver. So the Denver and Rio Grande Railroad built a railroad through the canyon in 1887. A “primitive dirt road” followed in the early 1900s, according to the Public Roads article. That road was “upgraded to a two-lane paved highway, designated U.S. 6, in the 1930s.” In 1940, U.S. 6 was extended across Vail Pass and through the canyon.

The two-lane highway worked well for the next several decades, but its narrow shoulders and lack of auxiliary infrastructure meant any kind of recreation — walking, cycling, or accessing the river itself — was extremely difficult. “Motorists turned off the highway at the many informal pullouts to fish, picnic, or camp, leaving garbage, ash-filled fire rings, and other debris within just a few feet of the river,” according to the Public Roads article. 

Meanwhile, the interstate system was expanding through Denver and the Rockies. Planning for I-70 through the mountains began in 1960, and some early work began in the canyon. In 1965 two tunnels were driven through Horseshoe Bend, roughly 1 mi east of Glenwood Springs, just beyond the canyon’s western end, to prevent drivers from having to negotiate the sharp and hazardous curve.

The construction of the road through the canyon might have progressed faster but for the passage of the monumental 1969 National Environmental Policy Act. NEPA, according to a 2019 Historic Context report prepared for the Colorado Department of Transportation by the engineering and design firm Mead & Hunt, required a multidisciplinary approach to federal construction projects that drew on the “natural and social sciences and the environmental design arts” to deal with environmental impacts. NEPA also required public hearings. The result, wrote engineer John Haley in his history of the project, was that projects that might have gotten started within, say, one to three years could now take a decade or longer, with a commensurate rise in costs (Wooing a Harsh Mistress: Glenwood Canyon’s Highway Odyssey,Greeley, Colorado: Canyon Communications, 1994).

Prior to the 1960s, the Mead & Hunt report noted, the federal Bureau of Public Roads (a precursor to the FHWA) and state transportation departments “gave little consideration to the environmental and scenic impact of highway locations and designs.” Engineers developed their plans, calculated their costs, and expected to be met by a grateful public. Moving forward, transportation planning would no longer solely be the work of engineers and bureaucrats but ordinary citizens as well. As Haley archly noted, “Engineers became acquainted with environmentalists. Many were shocked by the experience.”

For years opinion was divided on whether to build through Glenwood Canyon at all — but among the many who supported the project, few argued against protecting the sublime landscapes. According to Haley, a 1971 film produced by the Glenwood Springs Chamber of Commerce (now the Glenwood Springs Chamber Resort Association) called I-70: Where and How? argued that standard cut-and-fill construction methods “could not adequately preserve and protect the rugged beauty of the canyon.” In the film, Haley wrote, “Various shots of unsightly cuts created by I-70 construction in the Glenwood Springs area seemed to prove this point.”

Instead the film argued for a technique used in Italy, where mountain roads were built on concrete spans well off the canyon floor to create an ecologically sustainable zone for vegetation, wildlife, and recreation seekers. 

The film was well received and the method garnered public support, but there were still questions. Alternate routes were proposed. For years stakeholders debated whether the canyon road should be two lanes and 48 ft wide (the preferred option of most environmentalists), four lanes and 56 ft wide (the initial recommendation), or four lanes and 68 ft wide (the option ultimately selected by the planners).

State officials had requested federal approval of the route through Glenwood Canyon after dismissing two alternate bypasses as too rugged or expensive. But the federal government was not ready to sign off. The FHWA instructed the state to consider alternate routes more thoroughly and to create a citizens’ advisory committee to help guide the planning of the highway.

One member of that committee, local businessman Mark A. Skrotzki, emerged as a longstanding critic of the road. “It is impossible to put such a highway through this canyon without permanently scarring the several million year old unique geologic formations and further disrupting the Colorado River headwaters,” he wrote in a petition to Richard Lamm, governor of Colorado from 1975 to 1987.

Skrotzki would eventually enlist celebrity John ­Denver and former Secretary of the Interior Stewart Udall in a fierce and long-lasting public bid to curtail the project. According to the Mead & Hunt report, Denver once staged a media event pressing for a two-lane roadway “where he threw a rock across the canyon waters to demonstrate its narrowness.” But it took him six throws to hit the other side. 

Most residents of the Western Slope concluded that the project was going to happen one way or another, and it was best to work with planners to ensure that the highway preserved the beauty of the canyon. 

The U.S. DOT granted final approval in 1979; construction began in 1980. To that point, the key figure on the project had been Colorado Department of Highways district engineer Richard A. “Dick” Prosence, P.E., who had helped survey the route in the 1960s and had been involved in the planning of the highway. CDOH project manager Ralph J. Trapani, P.E., M.ASCE, oversaw construction of the roadway. (The CDOH became the CDOT in 1991.) 

aerial shot of Glenwood Canyon and parts of Interstate 70
Work on the Glenwood Canyon stretch ushered in an era of lengthy but critical environmental reviews; planning and construction of the roadway took decades. (Photograph courtesy of Joseph J. Kracum, Kracum Resources LLC)

Even before construction began, engineers realized the I-70 project would require a host of new approaches. The very act of surveying would be difficult in the tight, twisty canyon. As Haley wrote, “the party chief could not see the rodmen unless he set up the survey instrument on the south side of the river to sight targets on the north side of the canyon wall.” Conventional survey equipment was too slow and inefficient, he continued, so engineers turned to a new technology: Hewlett Packard’s Total Stations. These “highly sophisticated, battery-powered electronic instruments with no reflective targets” used “light beams to measure horizontal and vertical angles” and displayed the results digitally.

Haley’s book recounted many other challenges, starting with the geology of the canyon itself. Planners assumed bedrock was much closer to the surface of the canyon floor than it turned out to be — in some places it was 125 ft below ground. “At the east end of the canyon, deep, 60-foot-thick layers of highly compressible clays were found under the proposed roadway,” Haley wrote. Unstable talus slopes “ruled out normal piling foundations. The number of boulders embedded in talus, and the damage that heavy pile-drivers could do to the environment, also foreclosed this option.”

In the Mead & Hunt report, Trapani likens the underlying layer of earth at the east end of Glenwood Canyon to a tube of toothpaste — material began to squeeze out as soon as weight was added. 

“Spread footings offered the only reasonable foundation alternative in most talus deposits,” Haley wrote, “but they required stabilizing and strengthening the loose, porous … unreliable material below the footings.” Workers tried to pump thousands of gallons of cement grout into one “void-­riddled talus (deposit),” but the grout just disappeared. 

The solution was an emerging technique offered by foundation specialist Hayward Baker called compaction grouting, by which, as Haley described it, workers bored holes “around the perimeter of the proposed footing. Then they pumped them full of a stiff (cement) grout at low pressure, which formed a cemented curtain wall. Inside this wall they drilled another series of holes on four-foot centers, into which they pumped a zero-slump (cement) grout at pressures so high the ground surface would heave, confirming that they had filled the voids within the perimeter wall.”

The interstate project, Haley wrote, was also one of the first projects to use posttensioned concrete pavement slabs — common in buildings and bridges — on a roadway.The slabs were to be placed on “variable foundations of compacted earth, rock, gravel, Styrofoam, tunnel muck, construction debris, and sometimes even bedrock,” he explained, and many of them would be cantilevered, with only one edge connected to retaining walls. “Normal concrete-paving designs with conventional reinforcement were just not adequate under these foundation conditions,” he wrote. “Posttensioned concrete slabs support the loads irrespective of the underlying foundation materials.” 

Before placing each 200 ft long section, crews laid the cables “in conduits crisscrossed at 45-degree angles, to be tensioned after the concrete had attained its required strength,” Haley wrote. “The cables were tensioned to specified loads, then grouted with epoxy resin. This placed the entire slab in uniform compression and distributed the loads evenly without cracks.” 

The unprecedented use of posttensioned tendons in the retaining walls and roadway slabs gave the composite structure an operational life expected to extend for decades — or even centuries, according to the FHWA at the time.

To put the slabs in place, engineers used a 105 ft tall, 350 ft long erection gantry brought over from France. The self-propelled gantry was able to “move from pier to pier, pivot(ing) around curving canyon walls and tall trees while assembling the segmental deck from the top of the piers,” wrote Haley.

At the same time the CDOH developed 26 fences with flexible posts to protect workers and drivers from rockfalls. “By grouting bundles of wire strands into steel casings, fence posts are created that are rigid enough to support the fence mesh,” Haley wrote, “but will flex as a system under repeated rockfall impacts and then rebound to their original position.”

The project employed two chief design teams. One was led by Gruen Associates (in partnership with Nelson, Haley, Patterson & Quirk, the engineering firm at which Haley was a partner). Gruen was tasked with surveying and creating the preliminary designs for the east end of the canyon; its project lead was the talented ­Italian-born architect-engineer Edgardo Contini. The second design team, which handled the canyon’s western half, was led by Daniel, Mann, Johnson & Mendenhall; DMJM’s project lead, Joseph Passonneau, P.E., was also an architect and engineer.

The unprecedented use of posttensioned tendons in the retaining walls and roadway slabs gave the composite structure an operational life expected to extend for decades — or even centuries, according to the FHWA at the time.

Both men were instrumental in marrying the roadway’s technical bravura with careful attention to aesthetics. The Mead & Hunt report noted that Contini devised the plan to use elevated roadways to protect vegetation (and presumably wildlife). Meanwhile, “Passonneau created the terraced alignment that enabled the highway to fit into the narrow canyon and the overhang design atop retaining walls that softened the appearance of the concrete wall with shadows and created the illusion of a shorter wall.”

Additionally, the report continued, designers used tall, single-column piers, mostly 10 ft wide, that “minimized visual effects to the natural landscape and enabled wildlife to cross underneath the highway. Piers were rusticated with deep grooves spaced in random patterns to reflect the patterns on rock joints within the canyon and had a warm tan or light brown color.”

The final centerpiece of the work was a 4,000 ft long pair of tunnels at the trailhead to Hanging Lake, which preserved the ambiance of one of the most scenic stretches of the canyon (another Contini idea). Each of the parallel tunnels was drilled from four faces — one face at each end and two faces from an open cut in the center. The open cut was then backfilled and landscaped, essentially causing it to vanish into the landscape. 

Workers drilled holes and packed them with dynamite in a staggered pattern along prominent joints to create “portals that appeared to be naturally occurring cuts in the rock,” according to the Mead & Hunt report. Additionally, the tunnel portals were designed with parabolic curves “that created distinct shadow lines to highlight and complement the jagged geologic formations within the canyon.”

Because the rock near the site at Cinnamon Creek was good — free from geological faults and groundwater, Haley wrote — the tunnels there could be designed with a lightweight system of “grouted rock dowels, limited applications of (shotcrete), and a thin, cast-in-place concrete liner. This first-of-its-kind design reduced heavy construction costs by 20 percent.”

Between the eastbound and westbound tunnels, is the traffic control center. The facility, which also houses a plant with equipment to ventilate the tunnels, was designed to monitor vehicles and respond to crashes with a fleet of emergency vehicles and tow trucks. Since the highway was upgraded to meet interstate standards, incidents have decreased by 40 percent, according to the Public Roads article.

According to CDOT, the finished project cost $490 million in 1992 dollars; adjusted for inflation, that would be $925.5 million today. It included the three tunnels, 15 mi of retaining walls, and 40 bridges and viaducts and required 30 million lb of structural steel, 30 million lb of reinforcing steel, and 400,000 cu yd of concrete. 

The planners and engineers never forgot their commitment to the natural environment along the way. Haley wrote, “Horticulturists created the state’s largest nursery just to provide the 150,000 trees, shrubs, bushes, flowers and other perennial plants needed in the canyon for post-­construction landscaping.” (Landscaping was overseen by De Leuw, Cather & Company.)

According to the FHWA, the Glenwood Canyon project won more than 30 awards. Among those were ASCE’s 1993 Outstanding Civil Engineering Achievement Award.

Haley boldly claimed that the Interstate Highway System was the “largest, most ambitious and most successful public works project of all time.” If that’s the case, then Glenwood Canyon proves its ultimate merit: a project that could meet the nation’s demands for economic and technological progress while protecting its rich natural landscape. 

This article first appeared in the March/April 2021 issue of Civil Engineering as “The Last Miles Were the Hardest: The Completion of Interstate 70.”

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  1. Although not facing the same environmental and esthetic issues that it did in Glenwood Canyon, I-70 also experienced an environmental and construction challenge through Spotted Dog Canyon and the San Rafael Swell in Central Utah.

  2. The Glenwood Canyon segment may have been a great engineering work, but it did NOT complete Interstate 70. There is still a gap between Baltimore and Denver. At Breezewood PA, just before I 70 merges with I 76, I 70 is interrupted with a short stretch of travel on US 30 where traffic lights have to be negotiated. It may be a small gap, but it is still a gap in the Interstate Highway system.

  3. Enjoyed this article and understanding the multiple challenges recognized, understood and ultimately addressed when designing and constructing this last link of the interstate highway system.


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