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→Structural Soil Comparisons
'''Stormwater tree trench installations include:'''
'''Stormwater tree trench installations include:'''
* Overlying impermeable or [[permeable pavements]]
* Overlying impermeable or [[permeable pavements]]
* Trees (tolerant to northern. urban conditions)
* Trees (tolerant to northern urban conditions)
* Planting soil
* Planting soil
* Modular soil support or "soil cell" structures (optional)
* Modular soil support or "soil cell" structures (optional)
===Drainage Area===
===Drainage Area===
Typical contributing drainage areas are between 150-300 m2 per tree, with a maximum of 450 m2 per tree.
Typical contributing drainage areas are between 150 to 300 m<sup>2</sup> per tree, with a recommended maximum of 450 m<sup>2</sup> per tree. For optimal performance recommended ratios of impervious drainage area to pervious facility footprint area (I:P area ratio) range from 5:1 on low permeability soils (HSG C and D) to 15:1 on high permeability soils (HSG A and B).
===Setback from Buildings===
===Setback from Buildings===
===Soil Volume===
===Soil Volume===
Each tree planted should have access to a minimum 30 m<sup>3</sup> of soil volume, including the growing medium within the tree pit and growing or structural soil medium below adjacent supported pavement. If more than one tree shares the same trench a minimum 20 m<sup>3</sup> of soil per tree may be acceptable.
Each tree planted should have access to a minimum 30 m<sup>3</sup> of soil volume, including the growing medium within the tree pit and growing or structural soil medium below adjacent supported pavement. If more than one tree shares the same trench a minimum 20 m3 of soil per tree may be acceptable. It should be noted that structural soils are mostly filled with rock and will therefore have much lower soil volumes. However, trees have been found to grow reasonably well in these soils because roots only occupy a portion of the total soil medium.
Structural soils consist of 3 components, mixed in the following proportions by weight: a load bearing stone lattice, soil, and a tackifier. Soils may be clay loam or coarser textured soil if drainage is a priority. Common tackifiers include ‘hydrogel’, a coated potassium propenoate-propenamide copolymer) or ‘stabilizer’, a plant based organic product sourced from the US.
*Crushed [[stone]] (granite or limestone) should be narrowly graded, highly angular with no fines. Stone sizes may vary between 20 to 75 mm. In British Columbia, a larger 75 mm stone (range between 60 and 80 mm) is used because it was found to allow for larger soil volumes (up to 33% of the total soil medium volume).
*[[Geotextiles]] are used with structural soils to prevent migration of fines from the road or sidewalk base into the structural soils. In BC, a Nilex 4545 fabric is used for this purpose, but other fabrics may also be suitable.
*Compaction to 95% SPD is achieved in 1 m lifts to 95% SPD. Testing of compaction of levels is accomplished with a trolled nuclear densometer for larger rock size mixes.
====Structural Soil Comparisons====
{|class="wikitable"
|+Specifications for Stormwater Tree Trenches using Structural Soils
|-
!Structural Soil Type
!Median Stone size/range
!Soil Texture
!Tackifiying Agent
!Approximate Porosity
|-
|'''[https://gailmaterials.net/wp-content/uploads/2019/08/cu-structural_soil_specifications.pdf CU-Soil™]'''
|30mm (20-40mm)*
|
Gravel: <5%<br>
Sand: 20-45%<br>
Silt: 20-50%<br>
Clay: 20-40%<br>
Cation Exchange Capacity (CEC) >10<br>
pH: 5.5 – 6.5<br>
Organic Content: 2 – 5% by dry weight
|[http://www.amereq.com/pages/12/index.htm Hydrogel (coated potassium propenoate-propenamide copolymer)]
|26%
|-
|'''B.C Soil'''
|75mm/60 – 80mm)
|
Sand: 45-55%<br>
Silt: 25-35%<br>
Clay: 0 – 10%<br>
Silt + Clay: 25 – 45%<br>
pH: 6.0 – 7.0<br>
Organic Content: 15-20%**
|[http://www.stabilizersolutions.com/products/stabilizer/ Stabilizer]
|33%
|-
| colspan="5" style="text-align: left;" |<small>'''Note:'''<br>
"*" = Larger or smaller stone sizes are accepted as long as they do not comprise more than 10% above or 10% below the indicated range.<br>
"**" = Soil texture is the City of Vancouver specification for structural soils</small>
|}
===Modular Soil Support Systems===
===Modular Soil Support Systems===
===Structural Soil Medium===
===Structural Soil Medium===
Structural soil is an engineered soil medium that can be compacted to support sidewalk or roadway pavement installation requirements while also permitting tree root growth. Structural soil medium filled trenches are installed adjacent to tree pits to provide room for tree roots to spread out under the supported pavement portion of the tree trench.
Structural soil is an engineered soil medium that can be compacted to support sidewalk or roadway pavement installation requirements while also permitting tree root growth. Structural soil medium filled trenches are installed adjacent to tree pits to provide room for tree roots to spread out under the supported pavement portion of the tree trench. The available soil for root growth ranges from 25 to 33% depending on the stone size. Larger stone sizes will typically allow for greater soil volume.
===Structural Concrete Panels===
===Structural Concrete Panels===