Denver airport weighs bridges, tunnels, gondolas as train backup


The next time Denver International Airport’s increasingly crowded underground train breaks down, imagine that stranded passengers have an easy backup option: At last, an underground corridor with moving walkways to ease what would be a long hike to DIA’s far-out island concourses.

Or think futuristic: Small, on-demand pods that zip through new tunnels and up concourse ramps, depositing passengers right inside gate areas. Or maybe just think Colorado, with a gondola that rides along swooping cables high in the air, delivering riders from the terminal — with its tented roof resembling snow-covered peaks — to stations atop each concourse.

For now, the only thing that happens on the rare occasions when DIA’s concourse trains break down is pandemonium, as occurred for half the day in August 2021. But does it have to be?

Those three ideas are among more than a dozen that emerged from engineering firms, consultants and startups in early 2022 after airport officials asked for the private sector’s input on what it would take to solve DIA’s greatest vulnerability: the train, which roughly 70% of non-connecting passengers must ride to get to and from their gates.

Most of the pitches, reported by The Denver Post for the first time, are nowhere near ready for construction, standing more as broad concepts than firm proposals. Only some came with rough cost estimates, but they suggest that any robust plan for an alternate way to move people would cost at least in the hundreds of millions of dollars.

For all the fanciful ideas, it’s the most straightforward — at least in theory — that has captured DIA executives’ initial interest: Two soaring bridges that would connect from Concourse A, which already has a bridge link to the terminal, across yawning gaps to concourses B and C.

DIA officials said Jacobs Engineering Group, which made a compelling case for bridges in one of its submissions, will perform a more detailed feasibility study under one of its on-call contracts with the airport. The firm also will look more deeply with tunneling experts at challenges for underground options, including a backup walkway.

As outlined by Jacobs, bridges linking the concourses might be glass-encased for Colorado’s climate, with moving walkways. Jacobs also raised the idea of a system for automated pay-per-use pods that would run along the top.

“It was interesting to see a bridge concept that might be realistic,” said Bill Poole, DIA’s senior vice president of planning and design, who served on an evaluation team that sifted through the concept submissions.

Basic math long had doomed that idea. While the A-Bridge spans 365 feet, the distance from Concourse A to Concourse B is nearly 1,400 feet. And it’s nearly 1,100 feet from B to Concourse C. Neither could have supports blocking the taxiway, and DIA says each would have to be at least 70 feet off the ground — higher than the A-Bridge’s 45 feet — to allow jumbo jets to pass underneath.

A bridge rendering shows the interior of a glass-enclosed bridge spanning between concourses as proposed by Jacobs Engineering Group
A bridge rendering shows the interior of a glass-enclosed bridge spanning between concourses as proposed by Jacobs Engineering Group to DIA in 2022. It would contain moving walkways, concessions and, potentially, suspended pay-per-use automated pods. (Image from Jacobs Engineering Group transportation alternatives proposal to DIA)

Both new bridges would be longer than a 780-foot bridge that opened in 2022 at Seattle-Tacoma International Airport and took the crown as the world’s longest span crossing over a taxiway.

But Jacobs, and some other submitters to DIA that considered bridge-related ideas, argued bridge engineering has advanced enough to do it. They also caught airport officials’ attention by pointing out the potential for new retail and food concessions, built-in solar panels, and other revenue-generating or cost-saving opportunities.

“We want to understand it,” said Jim Starling, the airport’s chief construction and infrastructure officer. “I mean, No. 1, that’s a humongous span between A and B. So that would be a substantial structure. We wanted to get a little more information (on) the feasibility of actually doing that — what type of structure that would be … and getting some ideas on cost.”

Jacobs didn’t hazard a guess, other than estimating a bridge would cost slightly less than boring a new passenger walkway tunnel. Another proposal that included the possibility of bridges was submitted by Parsons, another large engineering firm. It estimated a rough cost of $500 million for two bridges between the concourses.

Travelers take train to main terminal ...

Hyoung Chang, The Denver Post

Travelers prepare to board a train to the main terminal from the Concourse B station at Denver International Airport on Friday, Nov. 20, 2020.

Reacting to recent breakdown — and reality of growing air traffic

When airport CEO Phil Washington issued the solicitation for ideas on alternatives to the train in October 2021, he was responding to its most recent breakdown.

Two months earlier, a wheel assembly broke on a train car, causing damage to the concrete tracks. The incident resulted in a partial train shutdown that wreaked havoc for hours, backing up passenger flows as trains ran at reduced capacity. DIA didn’t activate its full backup shuttle-busing plan for concourses B and C, which brings its own logistical hassles and can’t nearly match the train’s current maximum capacity of roughly 6,600 riders per hour in each direction.

But Washington also was acting in recognition of a problem on DIA’s horizon — one that eventually will make it necessary to consider more than simply a backup people-moving system.

DIA’s leaders project that passenger traffic will grow to 100 million a year within the next decade, up from a recent projection of nearly 70 million for 2022. Even with plans to expand the concourse train’s capacity in coming years, the cramped system will be pushed to its limits at some point in the 2030s, requiring new ways to get between the concourses.

So DIA’s solicitation for concept ideas asked submitters to think through a range of possibilities, including ones that could supplement the train on a daily basis. Eighteen wildly varying submissions came in by the January 2022 deadline. (See details on each.)

Glydways Inc. proposed that Denver International Airport build an automated people-moving system
Glydways Inc. proposed that Denver International Airport build an automated people-moving system in new tunnels. On-demand pods would drop off and pick up passengers at the ends of each concourse, near gate areas. (Image from Glydways Inc. transportation alternatives proposal to DIA)

For most of the year, DIA held them tight as the internal team evaluated the concept proposals. After the review was done, The Post obtained the documents recently through a public-records request.

Some firms spitball through a range of ideas, while others fix on just one or two as the most promising. Together, they explore the limits of three realms: subterranean, ground level and, when it comes to bridges and gondolas, in the sky.

Old ideas animate some proposals, while others would make use of relatively untested emerging technology. The Boring Co., billionaire Elon Musk’s tunneling firm, promises fast-and-cheap boring of 12-foot-diameter tunnels that might loop DIA’s terminal and three concourses, to be filled with Teslas — on a grander scale than its proof-of-concept project beneath the Las Vegas Convention Center.

Among the gondola-related proposals are ones by Doppelmayr and Leitner-Poma, both storied ski-lift makers that have branched out to manufacture other cable-based systems, including trams that can run above- or below-ground. Each floated several concepts, including aerial systems.

Denver International Airport's existing network of tunnels
Denver International Airport’s existing network of tunnels beneath its terminal and concourses, as shown in an image in WSP USA’s concept proposal to DIA in 2022, include passageways for its train system (center tunnels), utilities, baggage transport and service vehicles. The blue-shaded spaces show areas reserved for a future automated baggage system. (Image from WSP USA transportation alternatives proposal to DIA)

Lack of backup system is seen by some as design fault

Several proposals acknowledged that solving what many see as DIA’s original sin — airport designers’ decision to leave out a backup pedestrian tunnel along the train’s 1.25-mile length — would be tricky. And some declined to endorse specific ideas, instead laying out a range of possibilities for DIA officials to consider, with their pros and cons.

“We know that at this stage DEN is looking for ‘big ideas’ to solve redundancy and capacity challenges,” HDR Engineering says in its submission. “We also know that the current information available will not allow any team to propose a credible ‘silver bullet’ solution that will meet DEN’s needs.”

DIA and Alstom Transportation, the operator of the Automated Guideway Transportation System, or AGTS for short, frequently point out that its uptime rate is 99.8%. That makes it highly reliable — but with sometimes-catastrophic results on the rare occasions since DIA’s 1995 opening that the system has failed, whether for less than an hour or a full day. Thousands of passengers were stranded each time, raising calls for a fix.

Alstom Transportation, which operates Denver International Airport's underground train, called the Automated Guideway Transportation System, submitted a concept proposal
Alstom Transportation’s concept proposal would expand DIA’s current out-and-back train system to a “dual racetrack” configuration, in which trains would run on two independent loops. (Image from Alstom Transportation’s alternatives proposal to DIA)

For Alstom, which submitted a proposal, an expansion of that AGTS system might be in order. Two other submitters agreed. Each suggested building new train tunnels that would pass through outer areas of each concourse on one or both sides, in a variety of configurations — expanding capacity greatly while providing redundancy when part of the system breaks down. But such projects could be disruptive to ground operations. And they’d be costly: While Alstom didn’t pin down the cost, Denver-based consultant Logplan and Parsons separately estimated costs at $1.3 billion and $1.5 billion for differing plans.

Starling said any expansion of the AGTS is unlikely until it’s time to discuss the building of a fourth concourse, which probably won’t be needed at least until the 2030s. And there are scenarios in DIA’s older master plans in which new gates are built in an expansion off the terminal — rather than out past Concourse C.

The need for new people-moving systems — even cheaper ones — faces plenty of skepticism both inside and outside DIA. When the review of the concept proposals came up during a wider airport presentation this fall to a Denver City Council committee, Councilman Kevin Flynn cast doubt on the wisdom of spending big money to solve such a rare breakdown problem with the AGTS.

“I don’t disagree that we need to do a cost-benefit analysis on this,” Washington replied.

Renderings in AECOM's proposal to Denver International Airport for an underground walkway
Renderings in AECOM’s proposal to Denver International Airport for an underground walkway would use an existing baggage tunnel, as submitted in its response to a DIA request for concepts in 2022. These images show views from inside the tunnel and of “nodes” where passengers would take escalators or elevators to enter or exit each concourse — where they could choose the walkway or the existing train. (Image from AECOM transportation alternatives proposal to DIA)

Taking a closer look at underground walkways

Among the submissions, several proposed repurposing DIA’s existing network of seven main tunnels that carry the trains, utility lines, service vehicles and baggage tugs beneath the airport, though not all run fully out to Concourse C. Even with many constraints on their use, some firms included ideas for squeezing passenger walkways into those tunnels — while others would dig or bore new ones for walkways or other people-moving systems.

More than 20 years ago, concerns about the train’s reliability spurred proposals for a shorter underground walkway between concourses A and B, especially as B-based United Airlines began using A gates. But a $60 million project discussed in 2000 and 2001 — just over $100 million in today’s dollars — never moved forward.

The Denver office of WSP USA, a multinational engineering and design company, was among a handful of the 2022 submitters that made a full-throated case for an underground walkway all the way from the terminal to Concourse C, or done in phases. Its submission suggests DIA’s east baggage tunnel could handle all tug traffic, freeing up the west baggage tunnel for modifications.

A five-year project would cost roughly $180 million to $240 million, according to its estimates — a fraction of some other proposers’ price tags for systems requiring newly bored tunnels.

“Our solution isn’t a major program that will create nearly a decade of planning and construction,” WSP says. “Rather, it is right-sized to meet the current need, freeing up funds for larger growth objectives and asset management of aging facilities.”

WSP USA's proposal to Denver International Airport, as shown in its concept submittal to DIA in 2022
WSP USA’s proposal to Denver International Airport, as shown in its concept submittal to DIA in 2022, shows how walkways for passengers might fit into an existing tunnel. (Image from WSP USA transportation alternatives proposal to DIA)

But DIA’s Poole and Starling say such use of the existing tunnels needs careful consideration. They’re also skeptical such lengthy pedestrian tunnels — requiring treks of roughly a third of a mile between each concourse — would get much daily use, even with moving walkways.

Hartsfield-Jackson Atlanta International Airport, which has a similar design of parallel terminals and concourses, has made a dual underground train and walkway setup work. Its train is slightly longer than DIA’s, at 1.5 miles, though it stops more often, connecting seven buildings with walkways that pass artwork and visual effects.

“It is often perceived that a long tunnel might diminish the customer experience; however, we challenge this perception,” AECOM says in a similar walkway proposal to DIA. “We are well-versed in delivering amazing architecture and experiences in a wide variety of situations, including tunnels.”


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