Hardscapes suspended over soil cells have been shown to be more suitable for urban trees. These StrataCells from GreenBlue interlock and can be stacked as high as required. A compactor is recommended to help fill their voids with dirt.

The interlocking feature of the soil cell system provides correct dispersion of wheel loads throughout the matrix for high compressive strength.

After excavating the tree pit, the natural ground needs to be compacted a minimum of three times with a vibrating machine. If a granular sub-base is required to provide improved bearing strength, three compactions are also specified. Once the matrix is installed and filled, a filter grid fabric is placed on top.

Urban areas can often be completely covered by one hardscape surface or another – which when not considered properly can mean a nonexistent presence of trees in cityscapes. Given the enormous social and environmental benefits of trees, urban landscape professionals should consider methods suitable for planting trees in urban areas. Suspended hardscapes over noncompacted soil is a construction technique that has been recommended worldwide for urban development projects.

Suspended hardscapes provide the best available tree growth according to Dr. Thomas Smiley, an arboriculture researcher at Bartlett Tree Experts. Research at the urban plaza experimental plot at Bartlett Laboratories in Charlotte, N.C., compared soil treatments under hardscapes to determine which one provided a suitable rooting environment for urban tree plantings. Suspended hardscapes is a treatment that is sometimes overlooked by landscape architects when designing urban tree sites.

"Trees in the suspended hardscape treatment were larger and faster-growing than most of the other treatments, and had better color and more root growth," Dr. Smiley said in his findings.

The suspended hardscape is supported at the surface by structural support modules commonly known as soil cells. GreenBlue's StrataCell® system is one such solution for tree pit use in urban development projects. The structural integrity of these cells supports the concrete above, while preventing tree roots from getting out of the cell zone and interfering with utilities and other services. The cells allow for an increased volume of appropriate soil blend rather than structural soil, which is unsuitable for tree planting because of the containment of rocks and debris and the fact that it is compacted. If suspended hardscapes are used for urban development projects, they will need to be engineered to take expected loads without breaking. Laboratory tests in North America have demonstrated that this system can sustain the weight of an ASV PT-50 multi-terrain loader. The system's interlocking feature provides correct dispersion of wheel loads throughout the matrix, therefore featuring extremely high compressive strength. Due to this structural integrity, the modules can even be used under trafficked areas.