Two years after the long-awaited renovation of the 16th Street Mall in the Mile High City, Denverites are seeing signs of life at the end of the long, not-so-winding road designed by IM Pei.
The main driver of the 12-block makeover is to address deteriorating infrastructure, primarily the granite paving system built in 1982 that was failing due to inadequate subsurface drainage, says Travis Bogan, the department’s director of special projects. of Transportation and Infrastructure of Denver.
City planners hope the makeover will also breathe new life into the 40-year-old landmark, which serves as the unofficial backbone of downtown Denver. The project will increase public safety and mobility along the kilometer-long stretch with a cobblestone design that incorporates orientation and safety features for the visually impaired.
The first four blocks opened for business from September and the entire project is scheduled to be completed by 2025.
Landmark status requires the pedestrian mall to stay true to the modernist design envisioned by IM Pei and Olin in the 1970s: a diamond-patterned granite paving surface inspired by Navajo rug imagery from the southwest of a diamond rattlesnake skin.
Photo courtesy Office of the City Forester – City and County of Denver
Solving the unknowns
Voters approved the $149 million restoration project in 2017, part of a $937 million package to elevate infrastructure throughout the city, and construction began in April 2022. Since then the price has increased to $172.5 million. And the timeline has been extended by a year due to global supply chain issues and the discovery of complex underground utilities and storm systems that were determined to be historically significant, resulting in utility work that the first four building blocks took longer than expected.
These “unknown unknowns” have been the project’s biggest challenge, says Tim Springer, civil infrastructure manager for PCL Construction, the project’s builder. “After 2,000 potholes, you’d think we’d know where everything is, but there are utilities from the 1890s … many that nobody knew anything about.”
In the summer of 2023, to work more efficiently, the project team re-sequenced work to minimize lag time between construction activities and implemented a “two-pronged” approach to work simultaneously on more areas of Calle 16.
“This approach was made possible by the identification of a divide in the Denver water line halfway along the project corridor on California Street,” says Springer. Although the project team resolved utility conflicts in the first blocks of the project in the west, this split allowed utilities to advance to California and continue moving east without disrupting the service to 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 work below grade for the mall. original 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 below-grade service infrastructure or somehow improve those existing facilities, he explains. “Also, we installed conduits that could provide an element of future proofing so that future technologies could be added to the mall as technology continues to advance,” says Schlageter.
Businesses in 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 eliminate some common obstacles businesses face during major construction projects, Bogan says.
So far, “the city has awarded $1.3 million to 124 businesses on or very near 16th Street in the form of stabilization grants” up to $15,000 in size.
Elements of whimsy, like this beehive-shaped climbing structure, and areas where pedestrians can interact are incorporated into the new service areas along the 12-block promenade.
Photo courtesy of PCL Construction
ADA paving system
PCL is widening the sidewalks, improving the drainage system and adding trees to create a shaded promenade. Historic lighting has been preserved and restored, updated with the latest LED technology. Crews are also removing the pedestrian walkway between the two bus lanes and moving the bus lane to the middle of the street, as well as creating amenity zones on each side with planters, patios and other meeting spaces, giving the shopping center a European touch.
To keep the historic elements alive, crews are also recreating the kilometer-long grid of red, black and gray granite cobblestones, key to the mall’s Art Nouveau design.
“There are a lot of elements of this mall that have never been done before — this cobblestone component is one of them,” says Schlageter. That’s because the ADA didn’t exist when the original mall was built.
“After 2,000 potholes, you’d think we’d know where everything is, but there are utilities from the 1890s … many that nobody knew anything about.”
—Tim Springer, manager of Civil Infrastructure, PCL
“We have two federal requirements that conflict with each other: the ADA and the historic mall designation, so the project had to go through many iterations to build a [paver] system that could work and still comply with the spirit of the ADA,” says Schlageter.
The new mall also features a ropeless groove drainage design, and the project team paid close attention to the texture of the paving stones to prevent visually impaired people from walking in front of the traffic lane.
There isn’t a clear solution for this setup in this environment for the blind and visually impaired, says Schlageter, so members of that community were invited to “road test” potential designs, and eventually decide on cobblestone and non-slip. minnesota pavers. Mock-ups were created three years ago to make sure people could feel the difference between the paving stones on the walkway and the stones approaching the carriageway, and the final design changed as a result of the your opinion
The extensive soil cell system supports vertical loading without compacting soils or adversely affecting tree root health.
Rendering courtesy of Dig Studio
Hidden in Plain Site
Denver’s frequent freeze-thaw cycles, coupled with the lack of release of penetrating water, resulted in frequent paver failure at the original mall. The rehabbed mall’s cordless drainage system is designed to “hide in plain sight,” explains Schlageter. “The system provides efficient access to maintenance, but for many it will be relatively unnoticeable as an infrastructure element.”
The design was strategic, as it fulfilled a very important double purpose: the collection of surface water and the capture and routing of the water that would infiltrate through the cobblestones.
“If any water makes its way through the pavers, the sub-slab underneath has grooves designed to allow water that enters under the pavers to flow into the slot drainage system,” adds Springer.
Water quality elements were designed in accordance with Denver’s Ultra-Urban Green Infrastructure Guide, which encourages the construction of systems that are aesthetically pleasing and functionally focused to enhance the public experience and showcase drainage concepts progressive, explains Schlageter.
Crews are installing the traffic lane, which was moved to the middle of the street for safety and to create wider sidewalks with amenity zones.
Photo courtesy of 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 less than ideal as the trees grew. “The city felt that the tree canopy was so important that they completely changed the design of the project,” says Schlageter.
It also became another innovative feature of the mall, built with two systems on top of each other. “One is a complete infrastructure system, a kind of bridge deck, to accommodate all the loads at the top, keeping the root zone uncompacted to maximize the full root growth of the trees below,” says Schlageter.
“There are a lot of elements of this mall that have never been done before — that [ADA compliant] the paver component is a”.
—Matt Schlageter, Director of Civil Engineering, Martin/Martin
Ten-year-old off-site trees weighing 20,000 pounds each are being planted in 8-foot by 8-foot boxes using a soil cell system of 1,000 cubic feet of soil for each tree. The system is behind a prefabricated shear wall, and unlike the previous mall, the tree roots are not confined and can grow and flourish underground. “These trees have as much root area as they’ll need to grow substantially, and in 30 years I guess you’ll hear complaints from building occupants that it’s too green,” jokes Schlageter.
“Not only have we increased the tree cover from about 150 to 220 trees, but we have also increased biodiversity. [to 10 species]which should promote a healthier urban tree canopy,” notes Bogan.
The day-to-day challenges faced by the crews, for example, environmental changes that lead to changing cobblestones, have required a level of precision and number of form checks “beyond anything I’ve done before, up to and including everything we do in bridges,” says Springer.
The project’s mantra has been “go slow to go fast,” adds Schlageter. “Although it was a bold and difficult move to make, it was the right decision and it will pay off in the end; everyone will appreciate this level of high quality.”
