by SYNKD—Landscape design, build and maintain all on the same page

Denver employs an advanced below-ground infrastructure to handle stormwater and promote urban tree growth.

16th Street in Denver, CO, has long been a cultural and commercial landmark, stretching across 12 city blocks with a pedestrian-friendly design envisioned by world-renowned architect I.M. Pei and landscape architect Laurie Olin in 1982. However, after four decades, the corridor faced critical infrastructure challenges, including outdated stormwater management systems and failing tree-planting methods.

Today, through a $172.5 million revitalization project, the city has embraced cutting-edge green infrastructure solutions—including advanced stormwater management systems and modular soil cell technology—to create a more sustainable and resilient urban environment. The transformation not only preserves 16th Street’s aesthetic but also introduces modern engineering innovations that address climate-related challenges and infrastructure demands.

At the heart of 16th Street’s design is its iconic diamond-patterned promenade, composed of granite pavers originally inspired by both the patterns of Navajo blankets and western diamondback rattlesnake skin. While visually stunning, the original 1982 drainage and subsurface infrastructure had significantly deteriorated, leading to pavement and tree failures, and unsuitable conditions for a progressive city like Denver. The redesign had to balance design preservation with the need for modern functionality, all while integrating green infrastructure seamlessly into the space.

What Lies Beneath

Addressing these subterranean challenges required a fusion of engineering innovation and environmental sensitivity. Traditional stormwater solutions had succumbed to Denver’s unpredictable freeze-thaw cycles, necessitating a paradigm shift. The answer lay in a bespoke, curb-less slot drain system, artfully concealed beneath the historic pavers. Unlike conventional drainage designs, this system is hidden in plain sight beneath the pavers, ensuring the aesthetic integrity of the promenade while preventing water from pooling and undermining structural stability.

Revolutionizing Urban Forestry

Pei’s original vision included a lush tree canopy along the pedestrian mall, but the 1982 planting system relied on precast concrete planters that restricted root growth and an outdated soil cell system. As a result, many trees struggled to survive.

To rectify this, the city introduced an advanced engineered deep soil zone (EDSZ) system, Stratavault™ from Citygreen. Historically referred to as a ‘soil cell solution,’ Stratavault goes further, delivering trafficable load support for pavements and roads as well as large volumes of uncompacted soil, allowing deep structural roots to develop, and stormwater to be retained, clearly benefitting both tree longevity and drainage.

The reconstructed mall now has 220 new mature trees comprised of 10 different species. Each block contains continuous soil volumes providing a minimum of 1,000 cubic feet of soil per tree. This sustainable tree canopy solution will become the largest installation of trees in a suspended paving system in the state of Colorado.

Engineered for Everything

A key challenge in the redevelopment of 16th Street was designing a space that accommodated both pedestrians and transit while preserving the trees and underground systems, in the shared zones.

To meet the challenge, two layers of Stratavault were used: one supporting essential services such as drainage and utilities, and a second serving as a reinforced bridge-like structure to support the tree roots without compacting the soil beneath.

Additionally, a custom-engineered tree grate system, Invisigrate™, was developed by Citygreen to integrate seamlessly with the pavers. Unlike traditional grates, Invisigrate is installed beneath the pavement, maintaining the mall’s design aesthetic while providing essential root protection and aeration. These tree grates were engineered to withstand loads of up to 16,000 pounds per wheel, ensuring they could endure both transit vehicles and occasional heavy delivery trucks without compromising tree health.

Citygreen conducted a detailed structural engineering analysis of their custom-designed tree grate to ensure performance, strength, and safety. The engineers employed Finite Element Analysis (FEA), a computer modelling technique, to precisely predict how the structure would respond to real-world stresses like pedestrian and vehicle loads.

Using stringent industry standards (AASHTO and AISC), the analysis assessed critical factors such as stress distribution, maximum deformation, and potential plastic strain to validate design durability. Extensive evaluations were performed at multiple loading points, confirming the structure met or exceeded all requirements for strength and stiffness, with maximum stresses occurring at expected connection points. This rigorous approach was instrumental to compliance sign off and exemplified Citygreen’s unique capability to design and deliver a tailored solution for Denver’s landmark project that also met Buy America compliance.

Redesigned for Resilience

Sustainability played a central role in the mall’s transformation. Denver has long been a leader in environmental initiatives, and aligning with the city’s climate resilience goals was essential. The expanded tree canopy not only enhances the urban landscape but also contributes to carbon sequestration, improves air quality, and mitigates the urban heat island effect.

Invisigrate tree grate passes load testing of 16,000 lbs. per wheel

It sets a new benchmark for sustainable urban renewal projects. By integrating advanced stormwater management techniques and engineered deep soil zone systems, Denver has not only restored an iconic public space but also future-proofed it for generations to come.

With completion expected later this year, the transformed 16th Street will stand as a testament to the power of innovation, sustainability, and collaboration in shaping vibrant, green, and future-ready urban environments.

As cities worldwide face increasing climate challenges, this project serves as a model for how urban infrastructure can be designed with resilience in mind. By prioritizing green solutions, ensuring compatibility with existing utilities, and maintaining historical integrity, Denver’s approach provides a scalable template for reimagining city landscapes in the face of climate change.

The success of the 16th Street redevelopment was made possible through a collaborative effort between the city and county of Denver, RTD, Downtown Denver Partnership, Jacobs, Stantec, Olin, Dig Studio Landscape Architects, Brightview Landscaping, Bowman Construction Supply, Atkinson-Nolan & Associates, Martin & Martin Consulting Engineers, and PCL Construction.