Comparison of soil volumes, open space, and underground space needed for open grown tree vs. tree in suspended pavement, rock based structural soil, sand based structural soil, and soil boxes.
Link to this table

Technique Soil volume provided per cubic foot rooting zone (not including pavement profile where applicable) Open space needed to grow a 30’ diameter canopy tree assuming 34” soil depth1 Total volume recommended per 30’ diameter canopy tree1 Structural Capacity (Traffic load supported)
Open tree pit (no pavement or foot traffic above rooting space) 1 ft3 500 2 1413 ft3 Not even foot traffic should be allowed on open tree pits
Structural cells 0.92 ft3 5 ft by 5 ft 1536 ft3 Can support vehicle loading up to AASHTO H-20 rating of 32,000 lbs. per axle (U.S. Federal Highway Bridge Standard). This rating refers to the ability of a roadway to safely accommodate 3-4 axle vehicles, such as a large semi-truck and trailer (Deeproot website)
Rock based structural soils3 0.5 to 0.75 ft2 5 ft by 5 ft 2826 to 2120 ft3 Can be used under pedestrian mall paving, sidewalks, parking lots, and low-use access roads
Sand based structural soils 1 ft3 5 ft by 5 ft 1413 ft3 No standard test data available; Amsterdam sand settled 19 mm in 3 years compared to the surrounding pavement (Couenberg 1993), which is generally not acceptable in the U.S.
Soil boxes 1 ft3 Not large enough to grow 30 ft diameter tree Not large enough to grow 30 ft diameter tree

1 Based on 2 ft3 of soil volume per 1 ft3 of canopy area, assumes Silva Cells are used for structural cells, assumes 92% void space in Silva Cells; assumes CU Structural Soil is used for rock based structural soil; assumes soil component of rock based structural soil is 20%.
2 Although a typical rock-based soil includes 80% rock and 20% soil, the effective volume of the soil is greater than 20%. The recommended ratios in the table reflect information provided in Grabowsky et al., 2009, and by Dr. Nina Bassuk, Cornell University (personal communication).


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