Comparison of different tree techniques
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 | Surface Area of Soil Volume Needed Assuming 34” deep**2 soil per 30’ diameter canopy tree*1 | Structural Capacity (Traffic load supported) |
|---|---|---|---|---|---|
| Open tree pit (no pavement or foot traffic above rooting space) | 1 ft3 | 500 2 | 1413 ft3 | 500 ft2 | Not even foot traffic should be allowed on open tree pits |
| Structural cells | 0.92 ft3 | 5 ft by 5 ft | 1536 ft3 | 543 ft2 | 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 soils | 0.2 ft3 | 5 ft by 5 ft | 7065 ft3 | 2493.5 ft2 | 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 | 500 ft2 | 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 | Not large enough to grow 30 ft diameter tree |
1Based 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%.
234 inches is the depth of 2 stacks of Silva Cells and was selected to obtain equal depths among all treatments.