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{{:Comparison of pros and cons of bioretention soil mixes}} | {{:Comparison of pros and cons of bioretention soil mixes}} | ||
− | === | + | ===General guidelines=== |
The following guidelines are written in a format similar to a specification, and can serve as a basis for a specification, however, they are NOT a finished specification. Any specification for construction must be developed specifically for that project by a person skilled in writing specifications and construction documents. Additional items are needed particularly in Part One – General. 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 growing media D. | The following guidelines are written in a format similar to a specification, and can serve as a basis for a specification, however, they are NOT a finished specification. Any specification for construction must be developed specifically for that project by a person skilled in writing specifications and construction documents. Additional items are needed particularly in Part One – General. 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 growing media D. | ||
− | + | ====Scheduling==== | |
− | |||
− | |||
Schedule all utility installations prior to beginning work in this section. | Schedule all utility installations prior to beginning work in this section. | ||
− | + | ====Submittals==== | |
− | + | =====Soil test analysis===== | |
Submit soil testing results from an approved soil-testing laboratory for each soil mix for approval. Soil suppliers that regularly prepare Bioretention Soil Mix D that meets these requirements may submit past testing of current production runs to certify that the mix to be supplied meets the requirements provided the testing results are less than 12 months prior to the submission date. | Submit soil testing results from an approved soil-testing laboratory for each soil mix for approval. Soil suppliers that regularly prepare Bioretention Soil Mix D that meets these requirements may submit past testing of current production runs to certify that the mix to be supplied meets the requirements provided the testing results are less than 12 months prior to the submission date. | ||
*The testing laboratory shall be a member of the Soil Science Society of America's, North American Proficiency Testing Program (NAPT). The testing lab shall specialize in agricultural soil testing. Geotechnical engineering soil testing labs are not acceptable. | *The testing laboratory shall be a member of the Soil Science Society of America's, North American Proficiency Testing Program (NAPT). The testing lab shall specialize in agricultural soil testing. Geotechnical engineering soil testing labs are not acceptable. | ||
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***Cation Exchange Capacity (CEC)Chemical analysis shall be interpreted by the Owner based on plant material specified and testingrecommendations. | ***Cation Exchange Capacity (CEC)Chemical analysis shall be interpreted by the Owner based on plant material specified and testingrecommendations. | ||
− | + | =====Product Data===== | |
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 that the product meets the requirements of the specification. | 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 that the product meets the requirements of the specification. | ||
− | + | =====Samples for Verification===== | |
Submit one gallon minimum samples for the Coarse Sand, Compost, Topsoil, Bioretention Soil Mix A 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 one gallon minimum samples for the Coarse Sand, Compost, Topsoil, Bioretention Soil Mix A 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. | ||
− | + | =====Cone Penetrometer reading certification===== | |
Submit for approval, a written certification that the Bioretention Soil Mix A was sufficiently compacted to fall within the required resistance ranges. The Owner may verify the certification prior to approval. | Submit for approval, a written certification that the Bioretention Soil Mix A was sufficiently compacted to fall within the required resistance ranges. The Owner may verify the certification prior to approval. | ||
− | + | ====Bioretention Soil Mix D compaction testing==== | |
*Bioretention Soil Mix A shall be tested in-situ with a cone penetrometer, to the full depth of the installed soil profile or 30 inches deep, whichever is less. This is a field test to confirm correct compaction. One test shall be performed approximately once every 300 square feet of Bioretention Soil Mix A surface area. The cone penetrometer shall be available at the project site at all time when the contractor is working. | *Bioretention Soil Mix A shall be tested in-situ with a cone penetrometer, to the full depth of the installed soil profile or 30 inches deep, whichever is less. This is a field test to confirm correct compaction. One test shall be performed approximately once every 300 square feet of Bioretention Soil Mix A surface area. The cone penetrometer shall be available at the project site at all time when the contractor is working. | ||
*Maintain a volumetric moisture meter on site to verify that moisture readings were within the required ranges during the installation and testing. | *Maintain a volumetric moisture meter on site to verify that moisture readings were within the required ranges during the installation and testing. | ||
*The cone penetrometer shall be “Dickey-John Soil Compaction Tester”, or “AgraTronix Soil Compaction Meter,” both distributed by Ben Meadows www.benmeadows.com*The contractor shall certify that penetration resistance readings meet the requirements. The contractor’s penetrometer shall be made available to the owner, at all times, to confirm resistance readings. | *The cone penetrometer shall be “Dickey-John Soil Compaction Tester”, or “AgraTronix Soil Compaction Meter,” both distributed by Ben Meadows www.benmeadows.com*The contractor shall certify that penetration resistance readings meet the requirements. The contractor’s penetrometer shall be made available to the owner, at all times, to confirm resistance readings. | ||
− | + | ====Delivery, storage and handling==== | |
When warranties are required, verify with Owner's counsel that special warranties stated in this article are not less thanremedies available to Owner under prevailing local laws. | When warranties are required, verify with Owner's counsel that special warranties stated in this article are not less thanremedies available to Owner under prevailing local laws. | ||
*Bulk Materials: Do not deliver or place backfill, soils and soil amendments in frozen, or when the material is overly wet defined as the material sticks to the hand when squeezed.Retain subparagraph below for bare-root stock ifrequired; this is not an ANSI Z60.1 requirement. | *Bulk Materials: Do not deliver or place backfill, soils and soil amendments in frozen, or when the material is overly wet defined as the material sticks to the hand when squeezed.Retain subparagraph below for bare-root stock ifrequired; this is not an ANSI Z60.1 requirement. | ||
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*Provide erosion-control measures to prevent erosion or displacement of bulk materials and discharge ofsoil-bearing water runoff or airborne dust to adjacent properties, water conveyance systems, and walkways. Provide sediment control to retain excavated material, backfill, soil amendments and planting mix within the project limits as needed. | *Provide erosion-control measures to prevent erosion or displacement of bulk materials and discharge ofsoil-bearing water runoff or airborne dust to adjacent properties, water conveyance systems, and walkways. Provide sediment control to retain excavated material, backfill, soil amendments and planting mix within the project limits as needed. | ||
− | + | ====Project conditions==== | |
THIS SECTION NOT NEEDED THIS SECTION NOT NEEDED THIS SECTION NOTNEEDED THIS SECTION NOT NEEDED 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. | THIS SECTION NOT NEEDED THIS SECTION NOT NEEDED THIS SECTION NOTNEEDED THIS SECTION NOT NEEDED 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. | ||
− | + | ====Excavation around utilities==== | |
*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. | *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. | ||
− | === | + | ===Product guidelines=== |
− | + | ====Coarse sand==== | |
*Coarse sand, ASTM C-33 Fine Aggregate, with a Fines Modulus Index of 2.8 and 3.2. | *Coarse sand, ASTM C-33 Fine Aggregate, with a Fines Modulus Index of 2.8 and 3.2. | ||
*Sands shall be clean, sharp, natural sands free of limestone, shale and slate particles. | *Sands shall be clean, sharp, natural sands free of limestone, shale and slate particles. | ||
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*Complete [[Design guidelines for soil characteristics - tree trenches and tree boxes#Submittals|submittals]]. | *Complete [[Design guidelines for soil characteristics - tree trenches and tree boxes#Submittals|submittals]]. | ||
− | + | ====Compost==== | |
Compost shall meet the requirements of the US Composting Council “Landscape Architecture/Design Specifications for Compost Use”, section “Compost as a Landscape Backfill Mix Component”, with the following additional requirements: | Compost shall meet the requirements of the US Composting Council “Landscape 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. | ||
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**Complete [[Design guidelines for soil characteristics - tree trenches and tree boxes#Submittals|submittals]]. | **Complete [[Design guidelines for soil characteristics - tree trenches and tree boxes#Submittals|submittals]]. | ||
− | + | ====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 may contain up to 5 percent by volume stones, roots or masonry debris. Topsoil shall not contain metal debris, glass 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 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 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. | ||
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**Complete [[Design guidelines for soil characteristics - tree trenches and tree boxes#Submittals|submittals]]. | **Complete [[Design guidelines for soil characteristics - tree trenches and tree boxes#Submittals|submittals]]. | ||
− | + | ====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 sand (combined) – 25 to 40 percent, by dry weight | *silt plus sand (combined) – 25 to 40 percent, by dry weight | ||
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*Submittals shall be completed per Section 1.4 and shall be interpreted by Owner based on plant material specified and testing recommendations. | *Submittals shall be completed per Section 1.4 and shall be interpreted by Owner based on plant material specified and testing recommendations. | ||
− | + | ====Structural soil==== | |
*A mixture of stone and soil formulated to be compacted to 95% of maximum dry density, Standard Proctor and support tree roots. | *A mixture of stone and soil formulated to be compacted to 95% of maximum dry density, Standard Proctor and support tree roots. | ||
*Structural Soil shall be “CU Soil” as manufactured by Amereq, Inc New York City, NY, or approved equal. | *Structural Soil shall be “CU Soil” as manufactured by Amereq, Inc New York City, NY, or approved equal. | ||
**Complete [[Design guidelines for soil characteristics - tree trenches and tree boxes#Submittals|submittals]]. | **Complete [[Design guidelines for soil characteristics - tree trenches and tree boxes#Submittals|submittals]]. | ||
− | + | ====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. Soil Cells shall be capable of supporting loads up to and including AASHTO H-20, when used in conjunction with approved pavement profiles. | 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. Soil Cells shall be capable of supporting loads up to and including AASHTO H-20, when used in conjunction with approved pavement profiles. | ||
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*Complete [[Design guidelines for soil characteristics - tree trenches and tree boxes#Submittals|submittals]]. | *Complete [[Design guidelines for soil characteristics - tree trenches and tree boxes#Submittals|submittals]]. | ||
− | + | ===Execution guidelines=== | |
− | + | ====Site examination==== | |
Examine the surface grades and soil conditions for any circumstances that might bedetrimental to soil drainage. | Examine the surface grades and soil conditions for any circumstances that might bedetrimental to soil drainage. | ||
− | + | ====Soil preparation==== | |
*Excavate to the proposed sub grade. Maintain all required angles of repose of the adjacent materials as shown on the drawings or as required to support adjacent materials or structures. Do not over excavate into compacted sub grades of adjacent pavement or structures. Remove all construction debris and material. | *Excavate to the proposed sub grade. Maintain all required angles of repose of the adjacent materials as shown on the drawings or as required to support adjacent materials or structures. Do not over excavate into compacted sub grades of adjacent pavement or structures. Remove all construction debris and material. | ||
*Confirm that the sub grade is at the proper elevation and compacted as required. Sub grade elevations shall slope parallel to the finished grade and/or toward the subsurface drain lines as shown on the drawings. | *Confirm that the sub grade is at the proper elevation and compacted as required. Sub grade elevations shall slope parallel to the finished grade and/or toward the subsurface drain lines as shown on the drawings. | ||
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*Clean up any soil or dirt spilled on any paved surface, including at the end of each working day. | *Clean up any soil or dirt spilled on any paved surface, including at the end of each working day. | ||
− | + | ====Soil cell installation==== | |
Install Soil Cells in accordance with the manufacturers requirements including all accessories, geotextile, geogrid, and aggregate layers. | Install Soil Cells in accordance with the manufacturers requirements including all accessories, geotextile, geogrid, and aggregate layers. | ||
− | + | ====Bioretention Mix D installation==== | |
*Loosen or till the subsoil of the sub grade to a depth of 2 to 3 inches, or as required, with a backhoe or other suitable device. Where required on the drawings or other requirements, loosen the subsoil to a depth of 18 to 24 inches below the required subgrade elevation to improve infiltration into the subgrade soil. Loosen the soilusing a backhoe by digging into the subgrade soil and lifting then dropping the soil in place. Do not over work the soil or break up the large clumps in the soil created by the process. Do not allow the loosened soil to become re-compacted by other work. | *Loosen or till the subsoil of the sub grade to a depth of 2 to 3 inches, or as required, with a backhoe or other suitable device. Where required on the drawings or other requirements, loosen the subsoil to a depth of 18 to 24 inches below the required subgrade elevation to improve infiltration into the subgrade soil. Loosen the soilusing a backhoe by digging into the subgrade soil and lifting then dropping the soil in place. Do not over work the soil or break up the large clumps in the soil created by the process. Do not allow the loosened soil to become re-compacted by other work. | ||
*Install all required drainage aggregate and drain lines. | *Install all required drainage aggregate and drain lines. | ||
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**The tolerance for finished grades shall be plus or minus ½ inch in 10 feet. | **The tolerance for finished grades shall be plus or minus ½ inch in 10 feet. | ||
− | + | ====Bioretention Soil Mix A compaction==== | |
Compact the Bioretention Soil Mix A so that the pressure reading of a cone penetrometer is between 75 and 200 psi with the volumetric soil moisture between 5 and 15 percent. | Compact the Bioretention Soil Mix A so that the pressure reading of a cone penetrometer is between 75 and 200 psi with the volumetric soil moisture between 5 and 15 percent. | ||
− | + | ====Structural soil installation==== | |
Install and compact Structural Soil in accordance with the manufactures requirements. | Install and compact Structural Soil in accordance with the manufactures requirements. | ||
− | + | ====Cleanup==== | |
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. | 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. | ||
− | + | ====Protection==== | |
Protect work and materials from damage including: compaction, contamination, and erosion including due to operations by other Contractors or trespassers. Repair all damage and loosen compaction prior to acceptance. | Protect work and materials from damage including: compaction, contamination, and erosion including due to operations by other Contractors or trespassers. Repair all damage and loosen compaction prior to acceptance. | ||
− | + | ====Repair of settled Bioretention Soil Mix D==== | |
At the end of twelve months after the date of substantial completion of the Bioretention Soil Mix A 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. | At the end of twelve months after the date of substantial completion of the Bioretention Soil Mix A 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. | ||
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 is 2 cubic feet of rootable soil volume per square foot of mature tree canopy size. This soil is considered essential to healthy growth of trees. (See canopy size for several common tree species).
Stormwater volume credits for tree trenches and tree boxes include
If only rock-based structural soil is used, the volume credit is limited to water storage in the media. The rock component of the structural soil should not be included in the volume calculation for evapotranspiration. The decrease in stormwater volume retained is assumed to be linear with decreases in soil volume below the minimum recommended volume. For example, if a rock-based structural soil is used that contains 80 percent rock and 20 percent soil, only the 20 percent soil component counts toward the evapotranspiration volume credit. This approach is utilized within the Minimal Impact Design Standards (MIDS) calculator.
An extensive literature review was completed prior to developing the minimum soil volume recommendation discussed above.
Minimum soil volume needed to grow healthy trees has been studies 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.
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 that compares 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 alone, it would appear that approximately 1.5 times the amount of Cornell University (CU) structural soil is needed as needed to grow the same size tree growing in sandy loam (Bassuk 2010).
However, a pot study that compared growth of trees in CU structural soil to trees growing in loam indicates that tree growth in CU structural soil vs. loam soil is likely limited by more than just plant available water holding capacity, as the pot study found that 5 parts of structural soil were needed to provide the soil value of 1 part of loam soil (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 in the following soil types and volumes:
There were no significant differences in above ground growth between trees in the small loam soil pots and the ones in the large structural soil, indicating that 5 times the amount of structural soil was needed to produce the same size tree grown in the small loam soil. As only 1/5 of structural soil is composed of soil, it appears that perhaps only the soil component of the structural soil is useable to the tree as growing medium. Longer term studies are recommended to confirm the exact proportion ofstructural soil needed to provide the value of 1 cubic foot of loam soil.
A study by Bartlett Tree Research Laboratories is finding that trees growing in loam soils in suspended pavement are growing better than trees grown in rock based structural soil (Smiley et al 2006, Smiley 2013). Since 2004, they have been comparing tree growth in natural soil under suspended pavement compared to growth of trees grown using other ways 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. Throughout the 9 years since the trees were planted, Elm growth (trunk diameter and tree height) has been best in the suspendedpavement with natural soil.
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 has a separate section on Urban trees (Hinman and Wulkan 2012) and provides some guidance on soil quality and volume for trees, but does not include a soil specificationfor 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.
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 volume provided conforms to tree soil volume requirements.
Bioretention Mix D consists of sand, unscreened topsoil, and compost per the guidelines 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 spacesbetween 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 lack permeability. Using an unscreened topsoil and preserving ped structure as much as possible allows for a higher clay content in Bioretention Soil Mix A 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 (25-40 percent by dry weight) is beneficial because it provides higher cation exchange capacity (for increased nutrientretention 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.
The following guidelines are written in a format similar to a specification, and can serve as a basis for a specification, however, they are NOT a finished specification. Any specification for construction must be developed specifically for that project by a person skilled in writing specifications and construction documents. Additional items are needed particularly in Part One – General. 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 growing media D.
Schedule all utility installations prior to beginning work in this section.
Submit soil testing results from an approved soil-testing laboratory for each soil mix for approval. Soil suppliers that regularly prepare Bioretention Soil Mix D that meets these requirements may submit past testing of current production runs to certify that the mix to be supplied meets the requirements provided the testing results are 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 that the product meets the requirements of the specification.
Submit one gallon minimum samples for the Coarse Sand, Compost, Topsoil, Bioretention Soil Mix A 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 A 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 thanremedies available to Owner under prevailing local laws.
THIS SECTION NOT NEEDED THIS SECTION NOT NEEDED THIS SECTION NOTNEEDED THIS SECTION NOT NEEDED 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.
Sieve size | % passing |
---|---|
3/8 inch | Example |
#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 “Landscape 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. *Topsoil may contain up to 5 percent by volume stones, roots or masonry debris. Topsoil shall not contain metal debris, glass other sharp objects. Topsoil shall not contain any chemicals at levels that are harmful to plants, fish or exceed EPA limitations for human contact.
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 are
It is understood that 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.
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. Soil Cells 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 bedetrimental to soil drainage.
Install Soil Cells in accordance with the manufacturers requirements including all accessories, geotextile, geogrid, and aggregate layers.
Compact the Bioretention Soil Mix A so that the pressure reading of a cone penetrometer is between 75 and 200 psi with the volumetric soil moisture between 5 and 15 percent.
Install and compact Structural Soil in accordance with the manufactures 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 including due to operations by other Contractors or trespassers. Repair all damage and loosen compaction prior to acceptance.
At the end of twelve months after the date of substantial completion of the Bioretention Soil Mix A 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.