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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===
You can also review Urban's presentation he gave at the University of Washington in 2014 about some of the lessons learned in his book here: [https://botanicgardens.uw.edu/wp-content/uploads/sites/7/2014/10/Urban_Soils_Jim_Urban.pdf Urban Soil and Site Assessment Presentation] <ref>Urban, J. 2014. Urban Soil and Site Assessment [Presentation]. University of Washington Botanic Gardens. Seattle, WA https://botanicgardens.uw.edu/wp-content/uploads/sites/7/2014/10/Urban_Soils_Jim_Urban.pdf.</ref>
You can also review Urban's presentation he gave at the University of Washington in 2014 about some of the lessons learned in his book here: [https://botanicgardens.uw.edu/wp-content/uploads/sites/7/2014/10/Urban_Soils_Jim_Urban.pdf Urban Soil and Site Assessment Presentation] <ref>Urban, J. 2014. Urban Soil and Site Assessment [Presentation]. University of Washington Botanic Gardens. Seattle, WA https://botanicgardens.uw.edu/wp-content/uploads/sites/7/2014/10/Urban_Soils_Jim_Urban.pdf.</ref>
==Inspection and maintenance==
==Inspection and Maintenance==
Tree trenches have fewer maintenance requirements than bioretention cells or bioswales, but maintenance is still critical to their success. The most critical maintenance task is the removal of trash, sediment and debris accumulated in inlet structure sumps, gravel diaphragms and tree openings at curb cuts. This should be done at least once per year, however the frequency will depend on pavement uses, traffic volumes and tree canopy size. Inspect new trenches closely during the first two years of operation to measure the rate of accumulation and set an optimal maintenance frequency.
Tree trenches have fewer maintenance requirements than bioretention cells or bioswales, but maintenance is still critical to their success. The most critical maintenance task is the removal of trash, sediment and debris accumulated in inlet structure sumps, gravel diaphragms and tree openings at curb cuts. This should be done at least once per year, however the frequency will depend on pavement uses, traffic volumes and tree canopy size. Inspect new trenches closely during the first two years of operation to measure the rate of accumulation and set an optimal maintenance frequency.
See further details here: [[Stormwater Tree Trenches: Maintenance]]
See further details here: [[Stormwater Tree Trenches: Maintenance]]
<br>
Also take a look at the [[Inspection and Maintenance: Bioretention & Bioswales]] page by clicking below for further details about proper inspection and maintenance practices:
{{Clickable button|[[File:Cover Photo.PNG|150 px|link=https://wiki.sustainabletechnologies.ca/wiki/Inspection_and_Maintenance:_Bioretention_%26_Bioswales]]}}
==Performance==
==Performance==
For other recent research on the water management benefits of urban trees, and modelling approaches see the following articles and projects.
For other recent research on the water management benefits of urban trees, and modelling approaches see the following articles and projects.
* '''[https://www.sciencedirect.com/science/article/abs/pii/S0048969721063749?via%3Dihub Stormwater runoff volume reduction benefits of urban street tree canopy (Selbig et al., 2022)]''' <ref> Selbig, W.R., Loheide II, S.P., Shuster, W., Scharenbroch, B.C., Coville, R.C., Kruegler, J., Avery, W., Haefner, R., Nowak, D. Quantifying stormwater runoff volume reduction benefits of urban street tree canopy. Science of the Total Environment. v.806 (2022) 151296. https://www.sciencedirect.com/science/article/abs/pii/S0048969721063749?via%3Dihub </ref>
* '''[https://www.sciencedirect.com/science/article/abs/pii/S0048969721063749?via%3Dihub Stormwater runoff volume reduction benefits of urban street tree canopy (Selbig et al., 2022)]''' <ref> Selbig, W.R., Loheide II, S.P., Shuster, W., Scharenbroch, B.C., Coville, R.C., Kruegler, J., Avery, W., Haefner, R., Nowak, D. Quantifying stormwater runoff volume reduction benefit of urban street tree canopy. Science of the Total Environment. v.806 (2022) 151296. https://www.sciencedirect.com/science/article/abs/pii/S0048969721063749?via%3Dihub </ref>
** In a paired-catchment study design involving medium density residential areas in Wisconsin, with removal of 29 mature green ash and Norway maple street trees as the treatment, tree removal resulted in a 4% increase in runoff volume over the evaluation period, while peak discharge was generally not affected. Runoff volume reduction benefit of the mature street trees was estimated at 6376 L per tree, which is similar to values reported in previous studies based largely on simulation.
** In a paired-catchment study design involving medium density residential areas in Wisconsin, with removal of 29 mature green ash and Norway maple street trees as the treatment, tree removal resulted in a 4% increase in runoff volume over the evaluation period, while peak discharge was generally not affected. Runoff volume reduction benefit of the street tree canopy was estimated at 6376 L per tree, which is similar to values reported in previous studies based largely on simulation.
* '''[https://open.library.ubc.ca/soa/cIRcle/collections/ubctheses/24/items/1.0378388 Stormwater tree trench and bioswale performance in Vancouver, BC (Vega 2019)]''' <ref> Vega, O.M. Green infrastructure in the City of Vancouver: performance monitoring of stormwater tree trenches and bioswales. UBC Theses and Dissertations. https://open.library.ubc.ca/soa/cIRcle/collections/ubctheses/24/items/1.0378388 </ref>
* '''[https://open.library.ubc.ca/soa/cIRcle/collections/ubctheses/24/items/1.0378388 Stormwater tree trench and bioswale performance in Vancouver, BC (Vega 2019)]''' <ref> Vega, O.M. Green infrastructure in the City of Vancouver: performance monitoring of stormwater tree trenches and bioswales. UBC Theses and Dissertations. https://open.library.ubc.ca/soa/cIRcle/collections/ubctheses/24/items/1.0378388 </ref>
** A study of a stormwater tree trench featuring structural soil medium and two bioswales in Vancouver, British Columbia found that these practices are effective in treating heavy metals, suspended solids and other typical stormwater pollutants, and are effective tools for reducing runoff volume by promoting infiltration to native soils.
** A study of a stormwater tree trench featuring structural soil medium and two bioswales in Vancouver, British Columbia found that these practices are effective in treating heavy metals, suspended solids and other typical stormwater pollutants, and are effective tools for reducing runoff volume by promoting infiltration to native soils.