To Enhance Landscape Environments

The University of North Carolina campus in Chapel Hill used a soil cell system to expand the root volume for a large plaza of trees. The designers wanted the new trees to grow to similar sizes. The consistent amounts of shared soil volumes in this soil cell system allowed this to be possible. Over 700 modular frames were used throughout the entire plaza area with twelve trees sharing the space. This provided over 700 cubic feet of soil per tree. Bald cypress trees were used in the new plaza area because they are an underutilized native species that meets the new campus landscape standards.

Sand set pavers were used for the plaza pavement thus allowing the capture of some storm water and surface water at tree openings. These trees will provide a shady gathering space to enhance the campus landscape. These trees are also projected to have a longer life span and the project will be monitored by the university to determine the effectiveness of the soil cell system and it possible future use as a campus design standard.

At the University of North Carolina, Chapel Hill, a soil cell system was used to provide ample soil volumes for native bald cypress trees at a new campus plaza. Photo courtesy of DeepRoot Green Infrastructure

Meeting Sustainability Requirements

Compliance with new sustainability regulations and criteria can be met through the use of a soil cell systems. In 2011, landscape architecture firm CMG designed a courtyard for a large technology company based in Menlo Park, California. The landscape architects proposed a design that caused the impervious surface area to increase from 40 percent to 60 percent. This increase necessitated that the design meet California's C3 requirements, which mandate that 85 percent of the storm water runoff over the lifetime of the treatment facility be captured and treated.

CMG landscape architects decided to use a soil cell system to provide a planting environment that was lightly compacted, provided ample amounts of soil for the trees, and provided underground bio-retention areas under the pedestrian pavements.

The primary considerations by the designers for these bio-retention areas were as follows:

1. Treatment of major pollutants in surface and rain water

2. Seasonal groundwater levels

3. Geotechnical concerns

4. Distribution of treatment flows to bio-retention areas

5. Overflow requirements

6. Pollutant removal rates

Geotechnical issues and seasonal groundwater levels were addressed through the engineering and design of a soil cell system that was developed based on a review of the site's existing soils and existing seasonal groundwater levels. The engineering evaluation was critical to ensuring that the paving section and adjacent buildings remained stable. This soil cell system included an under-drain to address groundwater levels and heavy soils with limited infiltration capacity. Treatment flows were directed to the bio-retention soils through slot drains and perforated distribution pipes. The distribution pipes ran the entire length of the treatment area and distributed treatment flows to the underlying bio-retention soil. The pollutant removals were centered around removing Total Suspended Solids (TSS) from the bio-retention areas.

A soil cell system was used to meet new storm water runoff requirements at a courtyard project in Menlo Park, California. Photo courtesy of DeepRoot Green Infrastructure

Soil Cells Provide a Multi-Directional Approach to Urban Challenges

Toronto, Canada has been at the forefront for urban forestry initiatives. They have set minimum soil volumes for street trees of 30 cubic meters (1,059 cubic feet thus exceeding the 1,000 cubic feet recommended by James Urban) per tree, and they have also set a goal of increasing their overall tree canopy from 17 percent to 40 percent. A recent (2007) project for a six-block stretch of Bloor Street in the Bloor-Yorkville Business District involved the planting of 138 new London Plane trees. The trees were planted in the large volume (30 cubic meters) of soil to ensure their growth and health.

The project required the installation of trees within the existing utility framework. The new street design also included wider sidewalks and seasonal flowerbeds within a curbed planting island. Slot drains are used to capture rainwater on the sidewalk and direct it to small catch basins that remove debris and floatable materials. This captured rainwater is used to irrigate the soil volume underneath the sidewalks via a perforated pipe that extends throughout the Soil Cell system.