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+ | [[file:check it out.png|150px|thumb|alt=image|<font size=3>[https://www.deeproot.com/blog/blog-entries/writing-bullet-proof-resistant-soil-specifications?utm_medium=email&utm_campaign=DeepRoot%20Newsletter%20July%202019&utm_content=DeepRoot%20Newsletter%20July%202019+CID_930020a63aca598a8c6d4fdb513662ab&utm_source=Email&utm_term=Link Read this blog] called Writing Bullet Proof Resistant Soil Specifications, published July 24, 2019</font size>]] | ||
+ | |||
+ | {{alert|Because tree trenches and boxes are similar to bioretention systems, it is Highly Recommended that designers be familiar with [[Design criteria for bioretention]]. |alert-warning}} | ||
+ | |||
+ | {{alert|Trees can be an important tool for retention and detention of stormwater runoff. Trees provide additional benefits, including cleaner air, reduction of heat island effects, carbon sequestration, reduced noise pollution, reduced pavement maintenance needs, and cooler cars in shaded parking lots.|alert-success}} | ||
+ | |||
+ | {{alert|James Urban recently published [http://www.deeproot.com/blog/blog-entries/why-trees-may-fail-to-establish-or-thrive-and-what-to-do-about-it a blog] discussing why trees fail to establish. Many of the points raised in the article are related to the following design guidelines for tree quality and planting|alert-info}} | ||
+ | |||
+ | [[File:Technical information page image.png|100px|left|alt=image]] | ||
+ | |||
+ | ==Design phase maintenance considerations== | ||
+ | {{alert|Maintenance considerations are an important component of design|alert-warning}} | ||
+ | |||
+ | Implicit in the design guidance is the fact that many design elements of infiltration and filtration systems can minimize the maintenance burden and maintain pollutant removal efficiency. Key examples include | ||
+ | *limiting drainage area; | ||
+ | *providing easy site access (''REQUIRED''); | ||
+ | *providing [[Glossary#P|pretreatment]] (''REQUIRED''); and | ||
+ | *utilizing native plantings (see [https://stormwater.pca.state.mn.us/index.php?title=Plant_and_vegetation_information_for_stormwater_management Plants for Stormwater Design]). | ||
+ | |||
+ | For more information on design information for individual infiltration and filtration practices, [http://stormwater.pca.state.mn.us/index.php/Category:Design_criteria link here]. | ||
+ | |||
==Soil volume guidelines== | ==Soil volume guidelines== | ||
Providing adequate rootable soil volume is crucial to growing healthy trees. Several researchers have investigated minimum soil volumes needed to grow healthy trees. Results from these studies have been used to develop the following guidelines. (See summary of [[Studies analyzing minimum soil volume needed by trees|research]]) | Providing adequate rootable soil volume is crucial to growing healthy trees. Several researchers have investigated minimum soil volumes needed to grow healthy trees. Results from these studies have been used to develop the following guidelines. (See summary of [[Studies analyzing minimum soil volume needed by trees|research]]) | ||
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==Soil quality guidelines== | ==Soil quality guidelines== | ||
− | At this time, most Low Impact Development (LID) manuals only address trees in traditional bioretention practices (i.e. they do not provide separate soil specifications for systems that use only urban trees without herbaceous vegetation). In nature, trees typically grow with herbaceous vegetation in the same soils, but generally drier climates have fewer trees (i.e. more areas of herbaceous vegetation without trees) than wetter climates. In general tree soils need more moisture holding capacity than soils that just support herbaceous plants (depending on the tree and herbaceous species). Manufacturers of proprietary tree stormwater BMP’s such as, for example, Silva Cells tree soil systems and Cornell University (CU) structural soils, provide recommendations for soils to be used in their systems. The [ | + | At this time, most Low Impact Development (LID) manuals only address trees in traditional bioretention practices (i.e. they do not provide separate soil specifications for systems that use only urban trees without herbaceous vegetation). In nature, trees typically grow with herbaceous vegetation in the same soils, but generally drier climates have fewer trees (i.e. more areas of herbaceous vegetation without trees) than wetter climates. In general tree soils need more moisture holding capacity than soils that just support herbaceous plants (depending on the tree and herbaceous species). Manufacturers of proprietary tree stormwater BMP’s such as, for example, Silva Cells tree soil systems and Cornell University (CU) structural soils, provide recommendations for soils to be used in their systems. The [https://www.ezview.wa.gov/Portals/_1965/Documents/Background/2012_LIDmanual_PSP.pdf Puget Sound manual] (Hinman and Wulkan, 2012; see Section 6.4, page 203) has a separate section on urban trees and provides some guidance on soil quality and volume for trees, but does not include a soil specification for urban trees. Many jurisdictions have their own soil specifications for urban tree planting that are not specifically targeted towards stormwater management but provide excellent guidance regarding tree needs. Books, such as [[Design guidelines for soil characteristics - tree trenches and tree boxes#References|Urban (2008)]], also address soil needs for trees. |
This section provides a discussion of existing literature and research on soils designed to optimize tree growth and soils designed for optimized bioretention function. A summary of [[Design guidelines for soil characteristics - tree trenches and tree boxes#Literature review of soils optimized for tree growth|examples of soil guidelines and specifications]] for optimized tree growth from the literature is included at the end of this section. | This section provides a discussion of existing literature and research on soils designed to optimize tree growth and soils designed for optimized bioretention function. A summary of [[Design guidelines for soil characteristics - tree trenches and tree boxes#Literature review of soils optimized for tree growth|examples of soil guidelines and specifications]] for optimized tree growth from the literature is included at the end of this section. | ||
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=====Soil test analysis submittal===== | =====Soil test analysis submittal===== | ||
Submit soil testing results from an approved soil-testing laboratory for each soil mix. Soil suppliers that regularly prepare Bioretention Soil Mix D may submit past test results of current production runs to certify that the mix to be supplied meets the requirements, provided the test is less than 12 months prior to the submission date. | Submit soil testing results from an approved soil-testing laboratory for each soil mix. Soil suppliers that regularly prepare Bioretention Soil Mix D may submit past test results of current production runs to certify that the mix to be supplied meets the requirements, provided the test is less than 12 months prior to the submission date. | ||
− | *The testing laboratory shall be a member of the Soil Science Society of America's [http://www.naptprogram.org/ North American Proficiency Testing Program] (NAPT), specializing in agricultural soil testing. Geotechnical engineering soil testing labs are not | + | *The testing laboratory shall be a member of the Soil Science Society of America's [http://www.naptprogram.org/ North American Proficiency Testing Program] (NAPT), specializing in agricultural soil testing. Geotechnical engineering soil testing labs are not recommended. |
− | *Testing shall comply with the requirements of the | + | *Testing shall comply with the requirements of the Methods of Soil Analysis Part 1 and 3, published by the [https://www.soils.org/ Soil Science Society of America], or the [http://www.astm.org/ ASTM] testing required. |
*Testing of topsoil and Bioretention Soil Mix D shall be required as defined below: | *Testing of topsoil and Bioretention Soil Mix D shall be required as defined below: | ||
**Physical analysis. | **Physical analysis. | ||
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===Product guidelines=== | ===Product guidelines=== | ||
+ | Specific guidelines for Mix D are discussed below. | ||
====Coarse sand==== | ====Coarse sand==== | ||
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====Compost==== | ====Compost==== | ||
− | [[Turf#Compost|Compost]] shall meet the requirements of the [http://compostingcouncil.org/ US Composting Council] “[ | + | [[Turf#Compost|Compost]] shall meet the requirements of the [http://compostingcouncil.org/ US Composting Council] “[https://archive.epa.gov/wastes/conserve/tools/greenscapes/web/pdf/la-specs.pdf Architecture/Design Specifications for Compost Use”], section ''Compost as a Landscape Backfill Mix Component'', with the following additional requirements: |
*Compost feedstock shall be yard waste trimmings and/or source-separated municipal solid waste to produce fungi-dominated compost. Compost shall not be derived from biosolids or industrial residuals. | *Compost feedstock shall be yard waste trimmings and/or source-separated municipal solid waste to produce fungi-dominated compost. Compost shall not be derived from biosolids or industrial residuals. | ||
*Compost physical appearance: compost shall be dark brown approximately the color of a 70 percent dark chocolate bar or darker. Particles of compost when broken shall be the same color inside as outside. | *Compost physical appearance: compost shall be dark brown approximately the color of a 70 percent dark chocolate bar or darker. Particles of compost when broken shall be the same color inside as outside. | ||
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====Topsoil==== | ====Topsoil==== | ||
Topsoil texture shall be a naturally produced soil of loam, sandy loam to sandy clay loam, within the following parameters, and suitable for the germination of seeds and the support of vegetative growth. | Topsoil texture shall be a naturally produced soil of loam, sandy loam to sandy clay loam, within the following parameters, and suitable for the germination of seeds and the support of vegetative growth. | ||
− | *Topsoil may contain up to 5 percent by volume stones, roots or masonry debris. Topsoil shall not contain metal debris, glass or other sharp objects. Topsoil shall not contain any chemicals at levels that are harmful to plants, fish or exceed | + | *Topsoil may contain up to 5 percent by volume stones, roots or masonry debris. Topsoil shall not contain metal debris, glass or other sharp objects. Topsoil shall not contain any chemicals at levels that are harmful to plants, fish or exceed EPA limitations for human contact. |
*Topsoil should not be screened or processed in a manner that breaks down soil peds. Soil peds of 2 inches in diameter or greater should be visible throughout the source pile. | *Topsoil should not be screened or processed in a manner that breaks down soil peds. Soil peds of 2 inches in diameter or greater should be visible throughout the source pile. | ||
− | *Manufactured topsoil where sand or compost has been added to a soil material to meet the specification shall be rejected. Clumps or peds of soil within the sample shall be the same color and texture on the inside as the outside of the clump or ped. | + | *Manufactured topsoil where sand or compost has been added to a soil material to meet the specification shall be rejected. Manufactured soil is rejected because it is screened and does not have adequate structure. Clumps or peds of soil within the sample shall be the same color and texture on the inside as the outside of the clump or ped. |
*Physical Parameters | *Physical Parameters | ||
**Gravel: less than 10 percent by volume | **Gravel: less than 10 percent by volume | ||
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====Bioretention Soil Mix D==== | ====Bioretention Soil Mix D==== | ||
Bioretention Soil Mix D soil shall be a mixture of coarse sand, compost and topsoil in proportions which meet the following: | Bioretention Soil Mix D soil shall be a mixture of coarse sand, compost and topsoil in proportions which meet the following: | ||
− | *silt plus | + | *silt plus clay (combined): 25 to 40 percent, by dry weight |
*total sand: 60 to 75 percent, by dry weight | *total sand: 60 to 75 percent, by dry weight | ||
*total coarse and medium sand: minimum of 55 percent of total sand, by dry weight | *total coarse and medium sand: minimum of 55 percent of total sand, by dry weight | ||
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*saturated hydraulic conductivity: 1 to 4 inches per hour | *saturated hydraulic conductivity: 1 to 4 inches per hour | ||
*ASTM F1815 at 85 percent compaction, Standard Proctor [http://www.astm.org/ ASTM] D698 | *ASTM F1815 at 85 percent compaction, Standard Proctor [http://www.astm.org/ ASTM] D698 | ||
− | *phosphorus between 12 and | + | *phosphorus between 12 and 30 parts per million (ppm) |
*[[Glossary#C|cation exchange capacity]] greater than 10 meq/g | *[[Glossary#C|cation exchange capacity]] greater than 10 meq/g | ||
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====Soil cells==== | ====Soil cells==== | ||
− | Pre-engineered modular structures designed to hold up pavement and to be filled with soil to support tree roots and treat storm water, with the goal of protecting soil within the cells from compaction from the loads on the overlying pavement, shall be capable of supporting loads up to and including [ | + | Pre-engineered modular structures designed to hold up pavement and to be filled with soil to support tree roots and treat storm water, with the goal of protecting soil within the cells from compaction from the loads on the overlying pavement, shall be capable of supporting loads up to and including [https://lockesolutions.com/translating-hs-20-traffic-lingo/ AASHTO] H-20, when used in conjunction with approved pavement profiles. |
Soil Cells shall meet the following requirements: | Soil Cells shall meet the following requirements: | ||
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====Protection==== | ====Protection==== | ||
Protect work and materials from damage including compaction, contamination, and erosion due to operations by other contractors or by trespassers. Repair all damage and loosen compaction prior to acceptance. | Protect work and materials from damage including compaction, contamination, and erosion due to operations by other contractors or by trespassers. Repair all damage and loosen compaction prior to acceptance. | ||
+ | |||
+ | {{alert|Excessive soil compaction is considered the most important factor affecting tree health since it affects soil water availability, soil aeration, and soil drainage. Shanstrom (2014) provides a discussion of compaction in [http://www.deeproot.com/blog/blog-entries/the-most-important-factor-for-growing-healthy-trees-2 The Most Important Factor for Growing Healthy Trees]. The manual page on [http://stormwater.pca.state.mn.us/index.php/Alleviating_compaction_from_construction_activities Alleviating compaction from construction activities], while written for active construction sites, provides some guidance that can be applied to alleviating compaction|alert-info}} | ||
====Repair of settled Bioretention Soil Mix D==== | ====Repair of settled Bioretention Soil Mix D==== | ||
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***Organic: 2 to 5 percent | ***Organic: 2 to 5 percent | ||
***pH = 7.5 or less | ***pH = 7.5 or less | ||
− | *DTAH et al 2013 [http:// | + | *DTAH et al 2013 [http://treecanada.ca/wp-content/uploads/2017/10/CUFC-2014-19-James-Urban-Toronto%E2%80%99s-Pioneering-Standards-for-Trees-in-Hard-Boulevards.pdf tree manual for Toronto] recommends: |
**pH 6.0 to 7.8 | **pH 6.0 to 7.8 | ||
**Drainage rates of between 12 millimeter and 75 millimeter per hour. Drainage rates must be measured in the field; however - soil can be tested in the lab if it is compacted to a prescribed density. | **Drainage rates of between 12 millimeter and 75 millimeter per hour. Drainage rates must be measured in the field; however - soil can be tested in the lab if it is compacted to a prescribed density. | ||
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A representative sampling of recent comprehensive literature on bioretention media guidelines is summarized below. | A representative sampling of recent comprehensive literature on bioretention media guidelines is summarized below. | ||
− | *[ | + | *[https://files.nc.gov/ncdeq/Energy+Mineral+and+Land+Resources/Stormwater/BMP+Manual/C-2%20%20Bioretention%201-19-2018%20FINAL.pdf North Carolina Bioretention Soil Specification] (referenced e.g. in North Carolina Department of Environment and Natural Resources. 2009. Stormwater BMP Manual Chapter 12: Bioretention. Revised 07-24-09.). This is one of the most widely used bioretention media specifications in the US (i.e. several other states have also adopted it). Some important conclusions and recommendations include the following. |
**Bioretention soils may clog more easily in northern climates where de-icers are used (Bannerman 2013, personal communication). Additional research is needed to determine if flushing salts out of the soil in spring with water eliminates SAR (sodium adsorption ratio) effects). | **Bioretention soils may clog more easily in northern climates where de-icers are used (Bannerman 2013, personal communication). Additional research is needed to determine if flushing salts out of the soil in spring with water eliminates SAR (sodium adsorption ratio) effects). | ||
**A minimum of one tree, three shrubs, and three herbaceous species should be incorporated in the bioretention planting plan unless it is a grassed cell. | **A minimum of one tree, three shrubs, and three herbaceous species should be incorporated in the bioretention planting plan unless it is a grassed cell. | ||
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**The low SHC soil was more effective in reducing nitrogen losses, particularly the inorganic forms(Denman 2006). | **The low SHC soil was more effective in reducing nitrogen losses, particularly the inorganic forms(Denman 2006). | ||
**Averaged across all species, planting resulted in an increase in infiltration rate compared to the unplanted control (Breen et al 2004). | **Averaged across all species, planting resulted in an increase in infiltration rate compared to the unplanted control (Breen et al 2004). | ||
− | *Fassman et al., 2013, ([ | + | *Fassman et al., 2013, ([https://knowledgeauckland.org.nz/media/1616/tr2013-011-media-specification-for-stormwater-bioretention-devices.pdf Media Specification for Stormwater Bioretention Devices]) produced a 115 page report that includes a literature review of bioretention media, a summary of trends in how bioretention media specifications are changing over time based on recent research and experience, and research to determine best bioretention media for New Zealand. The following conclusions were reported. |
**Common reasons for hydraulic failure of bioretention media include: | **Common reasons for hydraulic failure of bioretention media include: | ||
***incorrect media specification, where the media has incorrect physical/chemical properties for removing targeted pollutants; | ***incorrect media specification, where the media has incorrect physical/chemical properties for removing targeted pollutants; | ||
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==References== | ==References== | ||
*Bakker. J.W. 1983. ''Groeiplaats en watervoorziening van straatbomen''. Groen, 39(6)205-207; OBIS, 1988. Bomen in straatprofielen – Voorbeelden – Groeiplaatsberekening. Uitgeverij van de Vereniging van de Nederlandse gemeenten, ‘s-Gravenhage 1988. 63 p. Cited in Kopinga 1991. | *Bakker. J.W. 1983. ''Groeiplaats en watervoorziening van straatbomen''. Groen, 39(6)205-207; OBIS, 1988. Bomen in straatprofielen – Voorbeelden – Groeiplaatsberekening. Uitgeverij van de Vereniging van de Nederlandse gemeenten, ‘s-Gravenhage 1988. 63 p. Cited in Kopinga 1991. | ||
− | *Barret, M., M. Limouzin, and D. Lawler. 2011. [ | + | *Barret, M., M. Limouzin, and D. Lawler. 2011. [https://ascelibrary.org/doi/10.1061/41173%28414%2943 Performance Comparison of Biofiltration Designs]. World Environmental and Water Resources Congress 2011. pp. 395-404. |
− | *Bassuk, Nina. 2010. [ | + | *Bassuk, Nina. 2010. [https://www.ecolandscaping.org/01/developing-healthy-landscapes/soil/using-cu-structural-soil-to-grow-trees-surrounded-by-pavement/ Using CU-Structural Soil to Grow Trees Surrounded by Pavement]. In The Great Soil Debate Part II: Structural soils under pavement. ASLA Annual Meeting Handout. |
− | *DTAH, Lead Consultant, ARUP, Engineering, and James Urban, Urban Trees + Soils Urban Forest Innovations, Arborist. 2013. [ | + | *DTAH, Lead Consultant, ARUP, Engineering, and James Urban, Urban Trees + Soils Urban Forest Innovations, Arborist. 2013. [https://issuu.com/dtah/docs/iii-iv_best-practices-manual_append Tree Planting Solutions in Hard Boulevard Surfaces Best Practices Manual]. Project # A21065. Prepared for: City of Toronto. |
*Facility for Advancing Water Biofiltration (FAWB). 2009. [http://graie.org/SOCOMA/IMG/pdf/FAWB_Filter_media_guidelines_v3_June_2009-2.pdf Biofiltration Filter Media Guidelines (Version 3.01)]. | *Facility for Advancing Water Biofiltration (FAWB). 2009. [http://graie.org/SOCOMA/IMG/pdf/FAWB_Filter_media_guidelines_v3_June_2009-2.pdf Biofiltration Filter Media Guidelines (Version 3.01)]. | ||
− | *Fassman, EA, R. Simcock, and S. Wang. 2013. [ | + | *Fassman, EA, R. Simcock, and S. Wang. 2013. [https://knowledgeauckland.org.nz/media/1616/tr2013-011-media-specification-for-stormwater-bioretention-devices.pdf Media specification for stormwater bioretention devices, Prepared by Auckland UniServices for Auckland Council. Auckland Council technical report, TR2013/011]. |
*Helliwell, D.R. 1986. ''The Extent of Tree Roots''. Arboriculture Journal 10:341-347. Updated in Letter to the Editor. Arboricultural Journal: The International Journal of Urban Forestry, Volume 16, Issue 2, 1992. | *Helliwell, D.R. 1986. ''The Extent of Tree Roots''. Arboriculture Journal 10:341-347. Updated in Letter to the Editor. Arboricultural Journal: The International Journal of Urban Forestry, Volume 16, Issue 2, 1992. | ||
− | *Hinman, C., and B. Wulkan. 2012. [ | + | *Hinman, C., and B. Wulkan. 2012. [https://ecology.wa.gov/Regulations-Permits/Guidance-technical-assistance/Stormwater-permittee-guidance-resources/Low-Impact-Development-guidance Low Impact Development: Technical Guidance Manual for Puget Sound]. Publication No. PSP 2012-3. |
*Jenkins, J. K. G., Wadzuk, B. M., & Welker, A. L. 2010. ''Fines Accumulation and Distribution in a Storm-Water Rain Garden Nine Years Postconstruction''. Journal of Irrigation and Drainage Engineering, 136(12):862-869. | *Jenkins, J. K. G., Wadzuk, B. M., & Welker, A. L. 2010. ''Fines Accumulation and Distribution in a Storm-Water Rain Garden Nine Years Postconstruction''. Journal of Irrigation and Drainage Engineering, 136(12):862-869. | ||
*Kent, D., S. Shultz, T. Wyatt, and D. Halcrow. 2006. ''Soil Volume and Tree Condition in Walt Disney World Parking Lots''. Landscape Journal 25:1–06 | *Kent, D., S. Shultz, T. Wyatt, and D. Halcrow. 2006. ''Soil Volume and Tree Condition in Walt Disney World Parking Lots''. Landscape Journal 25:1–06 | ||
− | *Kopinga, J. 1991. [ | + | *Kopinga, J. 1991. [https://joa.isa-arbor.com/article_detail.asp?JournalID=1&VolumeID=17&IssueID=3&ArticleID=2412 The Effect of Restricted Volumes of Soil on the Growth and development of Street Trees]. Journal of Arboriculture 17(3):57-63 |
*Li, H., & Davis, A. P. (2008b). ''Urban Particle Capture in Bioretention Media. I: Laboratory and Field Studies''. Journal of Environmental Engineering. 134(6):409-418. | *Li, H., & Davis, A. P. (2008b). ''Urban Particle Capture in Bioretention Media. I: Laboratory and Field Studies''. Journal of Environmental Engineering. 134(6):409-418. | ||
*Lindsey, P. and N. Bassuk. 1991. [http://www.hort.cornell.edu/uhi/research/articles/JArb17(6).pdf Specifying Soil Volumes to Meet the Water Needs of Mature Urban Street Trees and Trees in Containers]. Journal of Arboriculture 17(6):141-149. | *Lindsey, P. and N. Bassuk. 1991. [http://www.hort.cornell.edu/uhi/research/articles/JArb17(6).pdf Specifying Soil Volumes to Meet the Water Needs of Mature Urban Street Trees and Trees in Containers]. Journal of Arboriculture 17(6):141-149. | ||
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*Schoenfeld, P.H., and J. van den Burg. 1984. ''Voortijdige bladval en groeiafname bij ‘Heidemij’populier in beplantingen langs autowegen''. Nederlands Bosbouw Tijdschrift 56:12-21. Cited in Kopinga 1991. | *Schoenfeld, P.H., and J. van den Burg. 1984. ''Voortijdige bladval en groeiafname bij ‘Heidemij’populier in beplantingen langs autowegen''. Nederlands Bosbouw Tijdschrift 56:12-21. Cited in Kopinga 1991. | ||
*Smiley, E. T. 2013. Bartlett Tree Research Lab, Charlotte North Carolina, Adjunct Professor Clemson Univ., unpublished data. | *Smiley, E. T. 2013. Bartlett Tree Research Lab, Charlotte North Carolina, Adjunct Professor Clemson Univ., unpublished data. | ||
− | *Smiley, E. Thomas, Lisa Calfee, Bruce R. Fraedrich, and Emma J. Smiley. 2006. [ | + | *Smiley, E. Thomas, Lisa Calfee, Bruce R. Fraedrich, and Emma J. Smiley. 2006. [https://joa.isa-arbor.com/article_detail.asp?JournalID=1&VolumeID=32&IssueID=4&ArticleID=2952 Comparison of Structural and Noncompacted Soils for Trees Surrounded by Pavement]. Arboriculture & Urban Forestry 32(4):164-169. |
− | *Urban, J. 1992. [ | + | *Urban, J. 1992. [https://static1.squarespace.com/static/52ec31b2e4b04eb0bbd9c075/t/5339ee00e4b029d638529446/1396305408895/Bringing+Order+to+the+Technical+Dysfunction+within+the+Urban+Forest.pdf Bringing Order to the Technical Dysfunction Within the Urban Forest]. Journal of Arboriculture 18(2): 85-90. |
<noinclude> | <noinclude> | ||
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*[[Requirements, recommendations and information for using trees with an underdrain as a BMP in the MIDS calculator]] | *[[Requirements, recommendations and information for using trees with an underdrain as a BMP in the MIDS calculator]] | ||
− | [[Category:Design criteria]] | + | [[Category:Level 3 - Best management practices/Specifications and details/Design criteria]] |
− | [[Category: | + | [[Category:Level 3 - Best management practices/Structural practices/Tree trench and box]] |
− | |||
</noinclude> | </noinclude> |
Implicit in the design guidance is the fact that many design elements of infiltration and filtration systems can minimize the maintenance burden and maintain pollutant removal efficiency. Key examples include
For more information on design information for individual infiltration and filtration practices, link here.
Providing adequate rootable soil volume is crucial to growing healthy trees. Several researchers have investigated minimum soil volumes needed to grow healthy trees. Results from these studies have been used to develop the following guidelines. (See summary of research)
Based on research, the recommended minimum soil volume of 2 cubic feet of rootable soil volume per square foot of mature tree canopy size is considered essential to healthy growth of trees. (See canopy size for several common tree species).
Soil volume affects the stormwater volume credits for tree trenches and tree boxes. Stormwater volume credits for tree trenches and tree boxes include
The first of these credits, storage of water below an underdrain, is a direct function of soil volume. The interception credit is a function of tree species and leaf area.
The evapotranspiration (ET) credit is a function of plant available water and is indirectly related to soil volume (e.g. available pore space). The decrease in stormwater volume available for ET is assumed to be linear with decreases in soil volume below the minimum recommended volume of 2 cubic feet of rootable soil per square foot of mature tree canopy. This approach is utilized within the Minimal Impact Design Standards (MIDS) calculator. However, in the case of rock-based structural soils, the relationship between soil volume and plant available water is not solely a function of the ratio of rock to soil. This is because the soil added to a structural soil tends to adhere to and coat the rock, thereby increasing the effective soil area in the structural soil. For a typical structural soil consisting of 80 percent rock and 20 percent loam, we recommend that 2 parts of structural soil are needed to equal 1 part of loam. This ratio can be dropped to 1.5 if the loam soil is used around the root ball.
For specific information on stormwater volume and pollutant credits for tree trenches and tree boxes, click here.
An extensive literature review was completed prior to developing the minimum soil volume recommendation discussed above. The results of this review are presented below.
Minimum soil volume needed to grow healthy trees has been studied several ways, including
Each of these techniques indicates similar ranges of minimum soil volume needed:
To put these numbers in perspective in relation to tree size and typical street tree spacing:
Because of the importance of providing adequate rootable soil volume to grow healthy trees, several jurisdictions have enacted minimum soil volume policies. These are summarized in the following table. Other examples can be found at this link.
Examples of jurisdictions with minimum tree soil volume requirements
Link to this table.
Jurisdiction | Minimum tree soil volume |
---|---|
Kitchener, Ontario, Canada |
|
Emeryville, CA |
|
Toronto, Ontario, Canada |
|
Markham and Oakville, Ontario, Canada; Burnaby MetroTown Development Area, British Columbia, Canada |
|
North Vancouver Lonsdale Street Guideline, British Columbia, Canada | 15 cubic meters (530 cubic feet) per tree for trees with shared volume |
Calgary, Alberta, Canada | Provide for a volume of soil suitable for a 25 year tree life span, this is approximately 14 cubic meters [494 c.f.) for a single tree. An additional 7 cubic meters [247 c.f.] of soil is required for each additional tree in interconnected plantings |
Langley, British Columbia, Canada |
|
Winnipeg, Manitoba, Canada, Tree Planting Details and Specifications, Downtown Area and Regional Streets |
|
Denver, CO | Internal (city) standard: 750 c.f. of soil volume per tree |
University of Florida Extension Recommendations (Urban Design for a Wind Resistant Forest) |
|
Minnesota B3 Guidelines |
|
Research comparing rock based structural soil to loam soil indicates that significantly larger soil volumes are needed to produce the same size tree using structural soil vs. loam soils. Based on plant available water holding capacity, it appears that approximately 50 percent more Cornell University (CU) structural soil compared to a sandy loam soil to grow the same size tree (Bassuk 2010).
A pot study comparing growth of trees in CU structural soil to trees growing in loam indicates that tree growth in CU structural soil is likely limited by more than just plant available water holding capacity (Loh et al., 2003). The pot study compared the growth of fig trees in structural soil and loam soil in pots over 500 days. Tree growth was compared for the following soil types and volumes:
There were no statistically significant differences in above ground growth between trees grown in small loam soil pots and trees grown in large pots containing structural soil. This suggests greater amounts of structural soil were needed to produce the same size tree grown in the small loam soil. Only 1/5 of structural soil is composed of soil, suggesting the soil component of the structural soil is most important to the tree as a growing medium. Longer term studies are needed to confirm the exact proportion of structural soil needed to provide the value of 1 cubic foot of loam soil.
Since 2004, Bartlett Tree Research Laboratories have conducted a study comparing tree growth in natural soil under suspended pavement to growth in other media designed to prevent rooting volume compaction under pavements (stalite soil, and gravel soil (i.e. structural soil)), as well as to trees grown in compacted soil. Each tree was provided 5.7 cubic meters (200 cubic feet) of rooting space. Results to date show trees growing in loam soils in suspended pavement have greater trunk diameter and height than trees grown in rock based structural soil (Smiley et al., 2006; Smiley, 2013).
At this time, most Low Impact Development (LID) manuals only address trees in traditional bioretention practices (i.e. they do not provide separate soil specifications for systems that use only urban trees without herbaceous vegetation). In nature, trees typically grow with herbaceous vegetation in the same soils, but generally drier climates have fewer trees (i.e. more areas of herbaceous vegetation without trees) than wetter climates. In general tree soils need more moisture holding capacity than soils that just support herbaceous plants (depending on the tree and herbaceous species). Manufacturers of proprietary tree stormwater BMP’s such as, for example, Silva Cells tree soil systems and Cornell University (CU) structural soils, provide recommendations for soils to be used in their systems. The Puget Sound manual (Hinman and Wulkan, 2012; see Section 6.4, page 203) has a separate section on urban trees and provides some guidance on soil quality and volume for trees, but does not include a soil specification for urban trees. Many jurisdictions have their own soil specifications for urban tree planting that are not specifically targeted towards stormwater management but provide excellent guidance regarding tree needs. Books, such as Urban (2008), also address soil needs for trees.
This section provides a discussion of existing literature and research on soils designed to optimize tree growth and soils designed for optimized bioretention function. A summary of examples of soil guidelines and specifications for optimized tree growth from the literature is included at the end of this section.
Bioretention Soil Mix D is recommended for bioretention with trees. Mixes B or C could also be used for bioretention with trees, but if mix B or C is used, limit the saturated hydraulic conductivity to a maximum of 4 inches per hour. Rock based structural soil may be used if the volume provided conforms to tree soil volume requirements.
Bioretention Mix D consists of sand, unscreened topsoil, and compost per the guidelines discussed below. It uses unscreened topsoil and recommends blending using a front end loader to preserve topsoil peds as much as possible. In an undisturbed soil, soil particles are clumped together into large units called peds. Peds range in size from the size of a large sand grain to several inches, so ped structure significantly increases pore space in the soil compared to a screened soil without peds. Pore spaces between soil peds improve air and water movement, water holding capacity, as well as root growth. Preserving ped structure is especially important in finer soils, because finer soils without ped structure have only very small pores and therefore have low permeability. Using an unscreened topsoil and preserving ped structure as much as possible allows for a higher clay content in Bioretention Soil Mix D compared to typical bioretention mixes because the ped structure maintains pore space and infiltration rates despite the higher clay content. The higher clay and silt content (25 to 40 percent by dry weight) is beneficial because it provides higher cation exchange capacity (for increased nutrient retention beneficial for plant growth and for increased pollutant removal) and higher water holding capacity (beneficial for plant growth).
Comparison of pros and cons of bioretention soil mixes
Link to this table.
Mix | Composition in original Manual | Proposed updated composition | Pros | Cons |
---|---|---|---|---|
A |
|
|
Likely to sorb more dissolved P and metals than mix B because it contains some fines; best for growth of most plants | Likely to leach P; if topsoil exceeds maximum allowed clay content, higher fines content could result in poor hydraulic performance and long drawdown times |
B |
|
|
Easy to mix; least likely to clog | Likely to leach P, lack of fines in mix results in less dissolved pollutant removal; harder on most plants than mix A because it dries out very quickly |
C | Not in original MN Stormwater Manual |
|
Likely to sorb more dissolved P and metals than mix B because it contains some fines; less likely to leach P than mix B because of low P content | Harder on most plants than mix A because it dries out very quickly. Research in Wisconsin indicates that in cold climates, excess of Na ions can promote displacement of Mg and Ca in the soil, which breaks down soil structure and decreases infiltration rate, and can also cause nutrient imbalances1 |
D | Not in original MN Stormwater Manual |
|
Best for pollutant removal, moisture retention, and growth of most plants; less likely to leach P than mix B because of low P content | Harder to find. Research in Wisconsin indicates that in cold climates, excess of Na ions can promote displacement of Mg and Ca in the soil, which breaks down soil structure and decreases infiltration rate, and can also cause nutrient imbalances |
E | Not in original manual |
|
High infiltration rates, relatively inexpensive | As compost breaks down, nutrients available for plants decreases |
F | Not in original manual |
|
Finer particles in loamy sand holds moisture for better plant growth | Lower infiltration rates, requires careful soil placement to avoid compaction, requires custom mixing |
1This problem can be avoided by minimizing salt use. Sodium absorption ratio (SAR) can be tested; if the SAR becomes too high, additions of gypsum (calcium sulfate) can be added to the soil to free the Na and allow it to be leached from the soil (Pitt et al in press).
2MnDOT Grade 2 compost is recommended.
Any specification for construction must be developed specifically for that project by a person skilled in writing specifications and construction documents. Terminology and requirements in the final specifications must be consistent with the terminology in other parts of the construction documents including plans and detail nomenclature. Unless otherwise stated, the guidelines apply to Bioretention Soil Mix D.
Schedule all utility installations prior to beginning work.
The following submittals are Highly Recommended.
Submit soil testing results from an approved soil-testing laboratory for each soil mix. Soil suppliers that regularly prepare Bioretention Soil Mix D may submit past test results of current production runs to certify that the mix to be supplied meets the requirements, provided the test is less than 12 months prior to the submission date.
For each type of product, including soil cells, structural soil, coarse sand and compost, submit the manufacturer's product literature with technical data sufficient to demonstrate the product meets the requirements of the specification.
Submit one gallon minimum samples for the coarse sand, compost, topsoil, Bioretention Soil Mix D and structural soil. Label samples to indicate product name, source and contractor. Samples will be reviewed for appearance only. Delivered materials shall closely match the samples.
Submit for approval, a written certification that the Bioretention Soil Mix D was sufficiently compacted to fall within the required resistance ranges. The Owner may verify the certification prior to approval.
When warranties are required, verify with Owner's counsel that special warranties stated in this article are not less than remedies available to Owner under prevailing local laws.
Do not proceed with work when sub grade soil is frozen or is overly wet (defined as the sub grade material sticks to the hand when squeezed).
Contractor shall carefully examine the civil, record, and survey drawings to become familiar with the existing underground conditions before digging. Notification of Local Utility Locator Service is required prior to all excavation.
Specific guidelines for Mix D are discussed below.
Sieve size | % passing |
---|---|
3/8 inch | 100 |
#4 | 95 to 100 |
#8 | 80 to 100 |
#16 | 50 to 85 |
#30 | 25 to 60 |
#50 | 5 to 30 |
#100 | 4 to 10 |
3200 | 2 to 4 |
Compost shall meet the requirements of the US Composting Council “Architecture/Design Specifications for Compost Use”, section Compost as a Landscape Backfill Mix Component, with the following additional requirements:
Topsoil texture shall be a naturally produced soil of loam, sandy loam to sandy clay loam, within the following parameters, and suitable for the germination of seeds and the support of vegetative growth.
Bioretention Soil Mix D soil shall be a mixture of coarse sand, compost and topsoil in proportions which meet the following:
Suggested mix ratio ranges by volume are
Compost quality and particle size, coarse sand shape and variations in particle distribution, topsoil component, and silt / clay amounts within the tolerance will cause the soil blend within these suggested mix ratio ranges to drain too fast or too slow. The contractor shall adjust the final mix proportions to achieve the required drainage rate and percent organic matter. Any variation of required products, above or below the percentages listed below, needed to attain the required drainage rate or percent organic matter, shall not be grounds to change the agreed upon price of the installed material.
Lightly mix the Bioretention Soil Mix D using a front end loader to preserve topsoil peds as much as possible. Topsoil peds 2 inches in diameter or larger should be visible in the finished stockpile. Do not over mix or screen the material.
Submittals shall be completed and interpreted by the Owner based on specified plant material and testing recommendations.
Pre-engineered modular structures designed to hold up pavement and to be filled with soil to support tree roots and treat storm water, with the goal of protecting soil within the cells from compaction from the loads on the overlying pavement, shall be capable of supporting loads up to and including AASHTO H-20, when used in conjunction with approved pavement profiles.
Soil Cells shall meet the following requirements:
Examine the surface grades and soil conditions for any circumstances that might be detrimental to soil drainage.
Install soil cells in accordance with the manufacturer's requirements including all accessories, geotextile, geogrid, and aggregate layers.
Compact the Bioretention Soil Mix D so that the pressure reading of a cone penetrometer is between 75 and 200 pounds per square inch (psi) with the volumetric soil moisture between 5 and 15 percent.
Install and compact Structural Soil in accordance with the manufacturer's requirements.
Once installation is complete, remove any excess soil and trash from pavements, structures or fixtures. Remove any spilled oil or other stains on surfaces caused by the work.
Protect work and materials from damage including compaction, contamination, and erosion due to operations by other contractors or by trespassers. Repair all damage and loosen compaction prior to acceptance.
Twelve months after the date of substantial completion of the Bioretention Soil Mix D installation work, inspect the site and restore any areas where the grades have settled beyond the elevations shown on the drawings by an amount greater than 5 percent of the soil depth.
Examples of soil guidelines and specifications for optimized tree growth are discussed below.
Using trees as stormwater BMP’s adds the following soil requirements to those of traditional street trees that are not planted for stormwater management.
A representative sampling of recent comprehensive literature on bioretention media guidelines is summarized below.
Summary of recommended bioretention filter media mixes from worldwide sources
Link to this table.
Guideline | Aggregate | Organic | Note |
---|---|---|---|
Auckland Regional Council (2003), Waitakere City Council (2004) | Sandy loam, loamy sand, loam, loam/sand mix (35 - 60% v/v sand) | Not specified | Clay content < 25% v/v1 |
Prince George’s County, Maryland (2007) | 50 - 60% v/v sand | 20 - 30% v/v well aged leaf compost, 20 - 30% v/v topsoil2 | Clay content < 5% v/v |
The SUDS manual (Woods-Ballard et al. 2007) | 35 - 60% v/v sand, 30 - 50% v/v silt | 0 - 4% v/v organic matter | 10 - 25% v/v clay content |
Facility for Advanced Water Biofiltration (FAWB, 2009a) | Washed, well graded sand with specified PSD band | 3% w/w organic material | Clay content < 3% w/w, top 100 mm to be ameliorated with organic matter and fertilizer |
Seattle Public Utilities (2008) | 60 - 65% v/v mineral aggregate, PSD limit (“clean sand” with 2 - 5% passing #200 sieve), U3 ≥ 4 | 35 - 40% v/v fine compost which has > 40% w/w organic matter content | |
Puget Sound Partnership (2009) | 40% v/v compost, or 8 - 10% w/w organic matter | ||
North Carolina Cooperative Extension Service (Hunt & Lord 2006) | 85 - 88% v/v washed medium sand4 | 3 - 5% v/v organic matter | 8 - 12% v/v silt and clay |
City of Austin (2011) | 70 - 80% v/v concrete sand5 | 20 - 30% v/v screened bulk topsoil2 | 70 - 90% sand content, 3 - 10% clay content, silt and clay content < 27% w/w. Warning not to use sandy loam (“red death”).6 |
1 % v/v is percent by volume; % w/w is percent by weight (mass)
2“Topsoil” is a non-technical term for the upper or outmost layer of soil, however there is no technical standard for topsoil.
3U, Coefficient of Uniformity = D60/D10, where D60 is particle diameter at 60% passing and D10 is particle diameter at 10% passing.
4A specific definition for “medium sand” was not identified. ASTM D2487-10 classifies coarse-grained sandsas those with > 50% retained on the (USA) No. 200 sieve (75 m) and > 50% of coarse fraction passing the No. 4 sieve (4.76 mm). Clean sands contain < 5% fines. Fine-grained soils are silts and clays whereby > 50% passes the No. 200 sieve.
5Concrete sand is described by ASTMD2487-10 as coarse sand that is retained by a (USA) No. 10 sieve (2.00mm)
6“Red death” is commercially available fill material in Austin marketed as sandy loam.
Summary of recommended hydraulic conductivities of bioretention filter media
Link to this table.
Publication | Hydraulic conductivity |
---|---|
Auckland Council Rain Garden Construction Guide (2011) | 12.5 mm hr-1 (minimum) |
California Bioretention TC-32 (CASQA, 2003) | 12.5 mm hr-1 (minimum) |
City of Austin (2011) | 50.8 mm hr-1 (minimum) |
USEPA (2004) | 12.7 mm hr-1 (minimum) |
FAWB (2009b) | 100 - 300 mm hr-1 (temperate climates) 100 - 500 mm hr-1 (tropical climates) |
Prince George’s County, Maryland (2007) | 12.7 mm hr-1 (minimum) |
The SUDS manual (Woods-Ballard et al. 2007) | 12.6 mm hr-1 |
North Carolina Cooperative Extension Service (Hunt and Lord 2006) | 25.4 mm hr-1 (for nitrogen removal) 50.8 mm hr-1 (for phosphorus, metal and other pollutant removal) |
Puget Sound Partnership (2009) Seattle Public Utilities (2011) | 25.4 - 305 mm hr-1 |
Summary of recommended installation methods
Link to this table.
Jurisdiction | Guideline on lifts | Guideline on compaction |
---|---|---|
Prince George’s County, (2007) | 200 to 300 mm lifts | Natural compaction with light watering |
ARC TP10 (2003) | 300 to 400 mm lifts | Loose compaction by light tamping with backhoe bucket |
North Shore City Council (2009) | 300 mm lifts | Natural compaction with wetting of soil |
Melbourne (FAWB 2009b) | Two lifts if depth is over 500 mm | Light compaction; single pass with vibrating plate for small systems; single pass with roller for large systems |
Seattle Public Utilities (2008) | Loose lifts | Compact to 85 to 90% of modified maximum dry density |
California Stormwater (CASQA 2003) | 460 mm or greater lifts | Light compaction |
Texture | Percent | Size |
---|---|---|
Clay and silt | < 3% | < 0.05 mm |
Very fine sand | 5 to 30% | 0.05 to 0.15 mm |
Fine sand | 10 to 30% | 0.15 to 0.25 mm |
Medium to coarse sand | 40 to 60% | 0.25 to 1.0 mm |
Coarse sand | 7 to 10% | 1.0 to 2.0 mm |
Fine gravel | < 3% | 2.0 to 3.4 mm |
The following pages address incorporation of trees into stormwater management under paved surfaces
This page was last edited on 14 February 2023, at 12:30.