Two years into the Mile High City’s much-anticipated renovation of the 16th Street Mall, Denverites are seeing signs of life at the end of the long and not-so-winding I.M. Pei-designed road.

The primary driver of the 12-block makeover is to address deteriorating infrastructure, primarily the 1982-constructed granite paver system that was failing because of inadequate subsurface drainage, says Travis Bogan, director of special projects for Denver’s Dept. of Transportation and Infrastructure.

City planners hope that the makeover will also breathe new life into the 40-year-old historic landmark, which serves as the unofficial backbone of downtown Denver. The project will increase public safety and mobility along the mile-long stretch with a paver design that incorporates wayfinding and safety elements for individuals with visual impairments.

The first four blocks have opened for business as of September, and the entire project is scheduled to wrap up in 2025.

Landmark status requires that the pedestrian mall stay true to the modernist design envisioned by I.M. Pei and Olin in the 1970s—a diamond-patterned granite paver surface inspired by the Southwestern Navajo rug imagery of a diamondback rattlesnake skin.
Photo courtesy Office of the City Forester – City and County of Denver

Solving for the Unknowns

Voters approved the $149-million restoration project in 2017, part of a $937-million package to elevate infrastructure across the city, and construction broke ground in April 2022. Since then, the price tag has increased to $172.5 million. And the timeline has been waylaid by a year due to global supply chain issues and the discovery of complex underground utilities and storm systems that were determined to have historic significance, resulting in utility work that took longer than planned in the first four blocks of construction.

These “unknown unknowns” have been the project’s greatest challenge, says Tim Springer, manager of civil infrastructure with PCL Construction, the project’s design builder. “After 2,000 potholes, you’d think we would know where everything is, but there are utilities from the 1890s ... many no one knew anything about.”

In the summer of 2023, to work more efficiently, the project team resequenced work to minimize lag time between construction activities and implemented a “two front” approach to get work done in more areas of 16th Street simultaneously.

“This approach was possible due to identifying a divide in the Denver Water line about halfway along the project corridor at California Street,” says Springer. While the project team resolved utility conflicts in the first few blocks of the project to the west, this divide allowed utilities to progress forward at California and continue to move east without disrupting service in other areas.

“It’s a bowl of spaghetti below grade,” adds Matt Schlageter, director of civil engineering for Martin/Martin, which is providing engineering services for the project and provided utilities and below grade work on the original mall in 1982.

The existing mall acts as a gathering place on the surface but functions as an infrastructure distribution system below grade. A key piece of the project was to replace and upgrade much of the utility infrastructure below grade or to somehow enhance these existing facilities, he explains. “In addition, we installed conduits that could offer an element of future proofing so that future technologies could be added to the mall as technology continues to advance,” Schlageter says.

Businesses along the mall have struggled to stay open during construction, and the project’s delay created more stress. The city and county of Denver and the Downtown Denver Partnership developed a variety of tactics to help remove some common roadblocks that businesses face during large construction projects, Bogan says.

To date, “the city has granted $1.3 million to 124 businesses on or in close proximity to 16th Street in the form of stabilization grants” ranging in size up to $15,000.

Elements of whimsy, like this hive-shaped climbing structure, and areas where pedestrians can interact are incorporated into the new amenity zones along the 12-block walk.
Photo courtesy PCL Construction

ADA Paver System

PCL is widening the sidewalks, improving the drainage system and adding trees to create a shady walk. Historic lighting has been retained and restored, updated with the latest LED technology. Crews are also removing the pedestrian walkway between the two bus lanes and moving the bus transitway to the middle of the street as well as creating amenity zones along each side with planters, patios and other gathering spaces, giving the mall a European feel.

To keep the historic elements alive, crews are also recreating the mile-long grid of red, black and gray granite pavers, key to the mall’s modernist design.

“There are a lot of elements of this mall that have never been done before—this paver component is one,” Schlageter says. That’s because ADA didn’t exist when the original mall was built.

“We have two federal requirements conflicting with each other—ADA and the historic designation of the mall—so the project had to go through many iterations to build a [paver] system that could function and still meet the spirit of ADA,” Schlageter says.

The new mall also features a curbless slot drain design, and the project team paid close attention to the texture of the pavers to help keep those with visual impairments from walking in front of the transit way.

There is not a cut-and-dried clear solution for this configuration in this setting for the blind and visually impaired, says Schlageter, so members of that community were invited to “road test” potential designs, eventually deciding on slip resistant and cobbled stone pavers from Minnesota. Mock-ups were created three years ago to make sure people could feel the difference between the pavers in the walkway and stones approaching the transitway, and the final design changed as a result of their feedback.

Extensive soil cell system supports vertical loading without compacting the soils or adversely impacting tree root health.
Rendering courtesy Dig Studio

Hidden in Plain Site

Denver’s frequent freeze and thaw cycles, coupled with a lack of release for penetrating water, resulted in a frequent paver failure on the original mall. The curbless drainage system on the rehabilitated mall is designed to be “hidden in plain sight,” Schlageter explains. “The system offers efficient access to maintenance, but for many will be relatively imperceptible as an infrastructure element.”

The design was strategic as it served a very important dual purpose: collection of surface water and the capture and routing of water that would percolate through the pavers.

“If any water does work its way through the pavers, the subslab underneath has grooves designed to allow the water that gets under the pavers to flow to the slot drain system,” Springer adds.

At-grade water quality elements were designed in accordance with the Denver Ultra Urban Green Infrastructure Guide, which encourages the construction of systems that are both aesthetically pleasing and functionally focused to enhance the public experience and showcase progressive drainage concepts, Schlageter explains.

Crews install the transitway, which was moved to the middle of the street for safety and to create wider sidewalks with amenity zones.
Photo courtesy PCL Construction

Tree Root Network

Pei envisioned a tree canopy along the mile-long pedestrian mall, but the original design consisted of precast concrete planters, which were not ideal as the trees grew. “The city deemed the tree canopy to be so significant that it totally modified the design of the project,” Schlageter says.

It also became another innovative feature of the mall—constructed with two systems on top of each other. “One is a complete infrastructure system, a bridge deck of sorts, to accommodate all of the loads on top while keeping the root zone uncompacted to maximize the full root growth of the trees underneath,” Schlageter says.

Ten-year-old trees grown off site and weighing in at 20,000 lb each are being planted in 8-ft by 8-ft boxes using a soil cell system of 1,000 cu ft of soil for each tree. The system is located behind a precast cutoff wall and, unlike the previous mall, the tree roots are not confined and can grow and flourish underground. “Those trees have as much root zone as they will ever need to grow substantially, and in 30 years my guess is you’re going to hear complaints from occupants in the buildings that it’s too green,” Schlageter jokes.

“Not only did we increase the tree cover from about 150 to 220 trees, but we also increased the biodiversity [to 10 species], which should promote a healthier urban tree canopy,” Bogan notes.

The day-to-day challenges the crews are facing—for example, the environmental changes that lead to shifting pavers—has required a level of precision and number of form checks “beyond anything I’ve done before—beyond even what we do on bridges,” Springer says.

The project’s mantra has been “go slow to go fast,” Schlageter adds. “While this was a bold, difficult move to make, it was the right decision, and at the end of the day, it will be worth it; everyone will appreciate this level of high quality.”