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Protecting Tree Growth and
Fertility in the Streetscape:

Chicago's Navy Pier, Delta City Centre,
Ottawa and Hatcher Road, Phoenix

The Navy Pier South Dock has evolved into a tree grove promenade. The trees are mature 'Marmo' maples, a resilient variety of the Freeman's maple originally bred at the nearby Morton Arboretum, and 'WOW' American sycamores, one of the largest hardwood trees of the Eastern U.S. forests, which have been locally-sourced from within 50 miles and are native to the Chicago region. The benches are made from reclaimed wood and recycled steel.

Tree growth and fertility are strongly influenced by soil composition and structure, as it affects the movement of air, water and other nutrients required for trees to flourish. Effectively the "architecture of the soil" is usually the most critical element in the success or failure of urban trees, according to the people at GreenBlue Infrastructure Solutions Founded in 1992 in Ontario, the company, as the name suggests, was established to provide solutions for green infrastructure challenges, particularly focusing on assisting trees in their battle to establish in urban areas; more specifically, a goal of "drastically improving urban planting success and increasing leaf canopy in urban areas." This has involved analyzing the causes of failure and reasons for premature mortality in urban trees, then systematically researching solutions and designing practical product solutions.

A well-structured soil functions like a reservoir. It enables a tree to accept, store and transmit water, nutrients and energy. Good soil allows the space required for life and the necessary biochemical exchanges for tree growth. Of critical importance is room for roots to propagate. Too often trees are planted in cramped planting pits with poor subsoil, resulting in retarded growth, with roots tending to colonize the area immediately underneath a paved surface and causing structural pavement damage.

As paved surfaces require solid, compacted ground for pedestrian movement and vehicular traffic, how can street trees be adequately nurtured in an urban setting without compromising or damaging the structural integrity of paved surfaces? One solution is the GreenBlue ArborSystem, an engineered tree pit system that offers soil structure conducive to root growth, while also providing adequate support for pavements. It keeps roots in the designated tree pit area and away from pavement, utilities, and other infrastructure. It also incorporates irrigation and aeration, all fully integrated into the tree grate/tree guard.

This system has engineering specifications to support greater vertical loads, while bringing tree roots close to the sidewalk. Engineers have calculated that with only 12 inches of granular sidewalk depth, the system matrix can support maximum pedestrian and sidewalk traffic loads. With vertical and lateral forces also considered in the engineering make-up of tree pits, a series of soil cell modules ('StrataCell') lock together to form a strong monolithic framework. Highly secure connectors allow the soil modules to click together fast and simply. This open, skeletal structure, void space in excess of 94 percent, provides an enormous growth zone for the roots and for stormwater harvesting. These high soil-volume tree pits offer optimum conditions for trees to survive surrounded by hardscaping.

Trees for Navy Pier Waterfront
Chicago's Navy Pier on Lake Michigan is undergoing an extensive transformation to revitalize 50 waterfront acres. Construction crews have been working "round the clock" to renovate the pier in time for its centennial celebration in 2016. Navy Pier is a destination enjoyed by millions of visitors each year. With Phase I now complete, this revitalization scheme will be a model of sustainability that will improve the health and vitality of the local community. Designed by award-winning landscape architecture firm James Corner Field Operations, the plan incorporates the latest in ecological design principles and environmental best practices. This waterfront renewal provides vibrant public spaces for recreation and social life.

With the keen emphasis on environmental sustainability, the project contractor and landscape architect turned to GreenBlue and its ArborSystem to ensure longevity of the newly planted trees. StrataCell modules support the load bearing requirements of the paved high-traffic pedestrian area, while providing uncompacted soil volume for healthy root growth. The tree grove promenade of mature 'Marmo' maples and 'WOW' American sycamores soften the hard edges of the downtown waterfront, offering a more natural foreground for the broad waters of Lake Michigan. Through this sustainable design, the renovation of Navy Pier is making a positive environmental impact.

Stormwater runoff at Navy Pier is captured through native landscape plantings, the trees wells and the herringbone patterned permeable pavements made of recycled content and locally-sourced aggregate. The stormwater goes to bioinfiltration basins, then to underground storage cisterns for reuse as irrigation. The tree grates are stainless steel with built in LEDs.

Project Team
Owner: Navy Pier, Inc.
Landscape Architect: James Corner Field Operations
Contractor: Christy Webber Landscapes
GreenBlue Consultant: Jeremy Bailey

Construction at the South Dock of Chicago's Navy Pier includes 59 large precast concrete tree pits in eight sizes, the largest being 28' long. The tree root balls measured more than 7' in diameter. The city specified 9.2 cubic meters of uncompacted, rootable soil for each tree pit, which is not ideal, given that mature tree roots frequently occupy 30 cubic meters of soil. The minimum soil volume specified by the city of Toronto, for instance, is 30 cubic meters per tree. The soil cells, however, allow the tree roots to extend beyond the uncompacted soil. The soil cells create a forest floor environment for the trees, and are always installed with adequate drainage and root ventilation (two inlets) to allow the soil to breathe.


Delta City Centre, Ottawa, Ontario
The reconstruction of the streetscapes along Queen Street and Albert Street on either side of the Delta Hotel City Centre in Ottawa included the installation of new concrete sidewalks, architectural pavers, trees, tree grates, tree guards and StrataCells.

Streetscape reconstruction paralleling the Delta Hotel City Centre in Ottawa included new concrete sidewalks, architectural pavers and trees: Hackberries (Celtis occidentalis), a hardy native of Eastern North America, along Albert Street (pictured); and 'Morten Accolade' elms, a hardy, hybridized tree resistant to the Dutch elm disease, installed along Queen Street. The installation included StrataCells, tree grates and tree guards. Buried hydroducts within the excavation area was a concern, but the soil cells were easily cut to fit around the ducts.

The Delta Ottawa City Centre is ideally located in a prime downtown location, within walking distance to the Canadian Parliament buildings, national museums and the Rideau Canal, a waterway that connects Ottawa to Kingston, Ontario. Bryan Jones, lead landscape architect from The HOK Planning Group, commented the project was a "complete refresh" for the Delta Hotel chain and sets a precedent for further renovations for the hotel chain across Canada. The hotel, built in the early 1970s, had not experienced a significant update since the early 1990s. The landscape architects decided to use soil cells to help ease the stress experience by new urban trees during Ottawa's harsh winters and the prolonged dry spells of the summers.

According to the manufacturer, the soil support modules (StrataCells) are 100 percent recycled polymer. The cells form a "skeletal matrix" that supports sidewalk and traffic loads, allowing "almost five times more available growing media compared to the traditional rock/soil method, therefore facilitating healthy root growth." Their "high structural integrity" allow these matrices to be used beneath vehicular areas.

Like with any renovation, there were unforeseen obstacles hidden from the contractors during the project.

"When excavation began for the StrataCells, the original building piles were uncovered that impeded the intended footprint of the cells by approximately 20 percent," explained Bryan Jones. "Since the system is flexible, the contractor was able to cut the cells to fit over the pilings, making use of all the space and allowing the trees to be planted as intended."

"The system was extremely easy to install once the upfront planning and layout was done," said. Enzo Di Chiara of Prestige Design & Construction. Enzo notes the soil cells connect "like building blocks," but are also lightweight and easy to handle. "It simply comes down to the ease of placement for the particular application. Overall, these types of systems are very comparable in pricing, so ease of installation reduced the overall cost."

Project Team
Owner: City of Ottawa / Delta Hotels & Resorts
Lead Landscape Architect: Bryan Jones of HOK Planning Group
Contractor: Prestige Design & Construction
GreenBlue Consultant: Jeremy Bailey


The soil cells (StrataCells) are being assembled under the sidewalk. The modules interlock horizontally and vertically. The cells are normally installed at a depth of 1' below finished levels, loaded with a screened sandy loam soil mix, then lightly compacted either by treading or using a small plate compactor. The plate compactor simply rides across the modules. The vibration allows the soil to settle and eliminate voids. The weight of granular subbase material above prevents surface heaving.

Phoenix Community Development
Tree installation systems that maximize root growth are particularly critical in hot climates like Arizona. Here, the challenges are retaining water for supporting tree growth, and ameliorating compacted subbase areas that deter root growth. Retarded root growth means that during the wet season between June and September, trees can easily be uprooted. Phoenix implemented a Community Development Improvement project that included installation on a quarter mile stretch of Hatcher Road using StrataCells, ReRoot root management and RootRain irrigation and aeration to support a permeable concrete surface and the adjacent roadway. Linked tree pits shared soil volume, therefore maximizing rooting space. The RootRain System allows for the exchange of organic gases and oxygen throughout the soil spaces below the pedestrian surfaces, and direct irrigation into the root zone.

This is Hatcher Road in Phoenix before and after installation of soil cells. The 100 percent recycled polymer cell modules keep the weight of the hardscape from compressing the soil, rather like a soil skeleton. The void space is large enough to allow roots to develop and thicken for long-term stability and transport of moisture, nutrients and movement of plant sugars around the tree. Palo Verde and Mulga (Acacia aneura) trees were installed.

Project Team
Owner: City of Phoenix
Lead Landscape Architect: Aaron Jensen of City of Phoenix
Contractor: AJP Electric
GreenBlue Consultant: Jeremy Bailey

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January 24, 2019, 1:38 am PST

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