Caution: Because tree trenches and boxes are similar to bioretention systems, it is Highly Recommended that designers be familiar with Design criteria for bioretention.
Green Infrastructure: 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.
Information: James Urban recently published 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

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 research)

Recommended minimum soil volume requirements for urban trees

Caution: the recommended minimum soil volume is 2 cubic feet of rootable soil volume per square foot of mature tree canopy size. See canopy size for several common tree species.

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).

Effect of soil volume on stormwater volume credits

schematic for infiltration from biofiltration with an underdrain
Schematic showing fate of water in a tree trench with a raised underdrain. Soil volume (porosity) has a direct effect on the volume of water lost below the underdrain since water stored there eventually drains into the underlying soil. Soil volume also affects stormwater volume lost to evapotranspiration, since water stored in the media above the underdrain can be captured by a tree.

Soil volume affects the stormwater volume credits for tree trenches and tree boxes. Stormwater volume credits for tree trenches and tree boxes include

  • storage of water below an underdrain,
  • interception of rain water by the tree canopy, and
  • evapotranspiration from the tree(s).

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.

Summary of literature review on soil volume

An extensive literature review was completed prior to developing the minimum soil volume recommendation discussed above. The results of this review are presented below.

Summary of research on minimum soils volumes needed

Minimum soil volume needed to grow healthy trees has been studied several ways, including

  • field surveys investigating minimum soil volumes that grew healthy trees;
  • calculation of minimum soil volume needed based on tree water requirements; and
  • calculation of minimum soil volume needed based on tree nitrogen requirements (Kopinga 1991).

Each of these techniques indicates similar ranges of minimum soil volume needed:

  • 1 to 3 cubic feet of soil per square foot of canopy; and
  • 1000 to 2100 cubic feet of soil for a large tree (median of 1500 cubic feet and mean of 1506 cubic feet).

To put these numbers in perspective in relation to tree size and typical street tree spacing:

  • Using the above numbers, a tree with 2 cubic feet of soil per square foot of canopy would need 1413 cubic feet of soil to grow 30 feet wide.
  • Assuming 2 cubic feet of soil per square foot of canopy, 1,500 cubic feet of soil would be able to support a 31 foot wide tree.

Precedents for minimum soil volume standards

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
  • Large stature trees (≥24” diameter at maturity): 1589 cubic feet (c.f) for single trees; 1059 c.f. for trees sharing soil volume; 530 c.f. allowable shared soil volume
  • Medium Stature trees: (≥16” diameter at maturity): 989 c.f for single trees; 653 c.f. for trees sharing soil volume; 336 c.f. allowable shared soil volume
  • Small stature trees: (≥8” diameter at maturity): 600 c.f for single trees; 389 c.f. for trees sharing soil volume; 212 c.f. allowable shared soil volume
  • For all boulevards where trees are planted, the minimal soil depth will be 450 mm (17.7 inches), and all other soil habitat zones (public/private front lawn, cul de sac, active parkland) will be 900 mm.
  • Where soil habitat zones (e.g. boulevard and front lawn) must be connected to achieve the required soil volumes, root pathways or Silva Cells will be used to provide a functional connection between the two areas.
Emeryville, CA
  • 600 cubic feet (17 cubic meters) for a small tree
  • 900 cubic feet (25 cubic meters) for a medium tree
  • 1200 cubic feet (34 cubic meters) for a large tree
  • 50 percent credit for planting areas under adjacent paving using 100% planting soil with Silva Cell or similar products.
Toronto, Ontario, Canada
  • 30 cubic meters (1059 cubic feet) of soil per tree
  • 20 cubic meters(706 cubic feet) per tree for trees with shared volume
  • Minimum 0.9 m (3’) and maximum 1.2 m (4’) depth
Markham and Oakville, Ontario, Canada; Burnaby MetroTown Development Area, British Columbia, Canada
  • 30 cubic meters (1059 cubic feet) of soil per tree
  • 15 cubic meters (530 cubic feet) per tree for trees with shared volume
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
  • Zoning Bylaw requires 10 cubic meters (353 cubic feet) of growing medium per tree planted in hard surfaced parking lots on private developments.
  • Subdivision and Development Servicing Bylaw requires 10 cubic meters (353 cubic feet) of growing medium per tree (generally street trees)
  • growing medium defined as screened, weed free, composted soil mixed according to BC Landscape Standards for the intended use and confirmed with a soil analysis report.
  • In hardscape environments, street trees are expected to be planted using structured supports such as Silva Cell to achieve the expected growing volume
Winnipeg, Manitoba, Canada, Tree Planting Details and Specifications, Downtown Area and Regional Streets
  • 8.5 cubic meters
  • (300 c.f.) to 12.75 cubic meters (450 c.f.) of soil per tree
  • 17.0 cu.m. (600 c.f.) to 25.5 cubic meters (900 c.f.) for 2 trees with shared volume
  • Optimal planting medium depth 900 mm (36in.). Minimum planting medium depth of 760 mm (30in.) will be accepted where 900 mm is not feasible.
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)
  • Small trees (shorter than 30’) = 10’x10’x3’ = 300 c.f.
  • Medium trees (Less than 50’ height or spread) = 1,200 c.f.
  • Large trees (Greater than 50’ height or spread) = 2,700 c.f.
Minnesota B3 Guidelines
  • Small trees (e.g. serviceberry) = 400 c.f.
  • Medium trees (e.g. ironwood) = 800 c.f.
  • Large trees (e.g. hackberry) = 1,200 c.f.
If using structural soils, total soil volumes above need to be multiplied by 5 to obtain equivalent volume of soil useable by the tree.

Comparison of rock based structural soil and traditional tree soils

photo showing comparison of trees grown using different planting techniques
Overview of Bartlett’s study comparing trees grown using various planting techniques (Smiley 2013).
photo comparing trees grown with suspended pavement and in gravel/soil
Side by side comparison of suspended pavement and structural soil trees (Smiley 2013).

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:

  • 0.4 cubic feet of loam soil (small loam soil)
  • 0.4 cubic feet of structural soil (small structural soil)
  • 2 cubic feet of loam soil (large loam soil)
  • 2 cubic feet of structural soil (large structural soil).

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.

graph comparing trunk diameter over time for four planting techniques
Comparison of mean trunk diameter, in inches (y-axis), for four tree planting techniques. The studies were conducted using Bosque Elm (Smiley, 2013).
graph comparing height over time for four planting techniques
Comparison of mean tree height, in inches (y-axis), for four tree planting techniques. The studies were conducted using Bosque Elm (Smiley, 2013).

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).

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 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.

Recommended soil for trees for stormwater

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
  • 55-65% construction sand
  • 10-20% top soil
  • 25-35% organic matter2
  • 60-70% construction sand
  • 15-25% top soil
  • 15-25% organic matter2
  • to receive P credit for water captured by underdrain the P content must be less than 30 mg/kg (ppm) per Mehlich III (or equivalent) test; NOTE a minimum P concentration of 12 mg/kg is recommended for plant growth.
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
  • 50-70% construction sand
  • 30-50% organic matter
  • 70-85% construction sand
  • 15-30% organic matter
  • to receive P credit for water captured by underdrain the P content must be less than 30 mg/kg per Mehlich III (or equivalent) test; NOTE a minimum P concentration of 12 mg/kg is recommended for plant growth.
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
  • 85-88 percent by volume sand and
  • 8 to 12 percent fines by volume,
  • 3 to 5 percent organic matter by volume
  • recommended P content between 12 and 30 mg/kg per Mehlich III (or equivalent) test
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
  • All components below by dry weight:
  • 60-75% sand
  • Min. 55% total coarse and medium sand as a % of total sand
  • Less than 12% fine gravel less than 5 mm (Calculated separately from sand/silt/ clay total)
  • 2 to 5 % organic matter
  • recommended P content between 12 and 30 mg/kg per Mehlich III (or equivalent) test
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
  • 60-80% sand meeting gradation requirements of MnDOT 3126, ―Fine Aggregate for Portland Cement Concrete
  • 20-40% MnDOT 3890 Grade 2 Compost
  • 30% organic leaf compost
High infiltration rates, relatively inexpensive As compost breaks down, nutrients available for plants decreases
F Not in original manual
  • 75% loamy sand by volume:
    • Upper Limit: 85-90% sand with %Silt + 1.5 times %Clay > 15%.
    • Lower Limit: 70-85% sand with %Silt + 2 times %Clay < 30%.
    • Maximum particle size < 1-inch
  • 25% MnDOT 3890 Grade 2 Compost
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.

General guidelines

Caution: 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. 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.

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.

  • The testing laboratory shall be a member of the Soil Science Society of America's 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 Methods of Soil Analysis Part 1 and 3, published by the Soil Science Society of America, or the ASTM testing required.
  • Testing of topsoil and Bioretention Soil Mix D shall be required as defined below:
    • Physical analysis.
      • USDA particle size analysis shall include gravel, clay, silt, and coarse, medium and fine sand fractions.
      • Hydraulic Conductivity testing (Bioretention Soil Mix D only) using ASTM F1815 at 80 percent and 85 percent compaction at proctor density (ASTM D 698-91). This is a LABORATORY TEST to determine water flow at specified compaction rates.
    • Chemical analysis. Note that nutrient levels and chemical analysis shall include recommendations from the testing laboratory for ranges of each element appropriate for the types of plants to be grown in the soil mix.
      • Nutrient levels by parts per million including phosphorus, potassium, calcium, magnesium, manganese, iron, copper, zinc and calcium
      • Percent organic content
      • pH
      • Soluble salt by electrical conductivity
      • Cation Exchange Capacity (CEC). Chemical analysis shall be interpreted by the Owner based on specified plant material and testing recommendations.
Product data submittal

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.

Samples for verification submittal

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.

Cone penetrometer reading certification submittal

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.

Bioretention Soil Mix D compaction testing

  • Bioretention Soil Mix D 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 every 300 square feet of Bioretention Soil Mix D surface area. The cone penetrometer shall be available at the project site at all times when the contractor is working.
  • Maintain a volumetric moisture meter on site to verify that moisture readings are 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
  • 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 than remedies available to Owner under prevailing local laws.

  • Bulk Materials: Do not deliver or place backfill, soils and soil amendments in frozen conditions, or when the material is overly wet (defined as the material sticks to the hand when squeezed). Retain subparagraph below for bare-root stock if required; this is not an ANSI Z60.1 requirement.
  • Provide protection including tarps, plastic and or matting between all bulk materials and any finished surfaces sufficient to protect the finish material.
  • Provide erosion-control measures to prevent erosion or displacement of bulk materials and discharge of soil-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

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.

Product guidelines

Specific guidelines for Mix D are discussed below.

Coarse sand

  • 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.
  • Sand pH shall be lower than 7.5
  • Provide the following particle size distribution:
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 CouncilArchitecture/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 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 odor: compost shall have a strong, sweet, aerobic odor indicating active biological activity. Compost with a sour anaerobic odor (indicating composting in excessive water) or an odor similar to denatured alcohol (indicating incomplete composting) shall be rejected
  • Compost testing and analysis: compost analysis shall be provided by the compost supplier. Before delivery of the compost, the supplier must provide the following documentation:


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 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.
  • 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
    • Gravel: less than 10 percent by volume
    • Sand: 30 to 70 percent by volume
    • Silt: 10 to 50 percent by volume
    • Clay: 10 to 25 percent percent by volume
  • Chemical Parameters
    • Organic Matter: 2 to 8 percent by dry weight
    • pH: 5.0 to 7.3
    • Phosphorus: Sufficient to meet the maximum requirements in the Plant/Bio-retention mix once the other products are added to the mix
    • Complete 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:

  • silt plus clay (combined): 25 to 40 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
  • fine gravel less than 5 millimeters: up to 12 percent by dry weight (calculated separately from sand/silt/ clay total)
  • organic matter content: 2 to 5 percent, percent loss on ignition by dry weight
  • saturated hydraulic conductivity: 1 to 4 inches per hour
  • ASTM F1815 at 85 percent compaction, Standard Proctor ASTM D698
  • phosphorus between 12 and 30 parts per million (ppm)
  • cation exchange capacity greater than 10 meq/g

Suggested mix ratio ranges by volume are

  • Coarse sand: 50 to 65 percent
  • Topsoil: 25 to 35 percent
  • Compost: 10 to 15 percent

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.

Structural soil

  • A mixture of stone and soil formulated to be compacted to 95 percent of maximum dry density, Standard Proctor and to support tree roots.
  • Structural soil shall be “CU Soil” as manufactured by Amereq, Inc. New York City, NY, or approved equal.
  • Complete 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, 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:

  • The structure design shall permit an uninterrupted mass of soil throughout the structure. All openings between different cell units shall have a minimum area of 36 square inches
    • to permit a continuous mass of soil that allows for capillary transfer of water;
    • to foster the growth of large tree roots; and
    • to permit the structure to be built around, over, under and through existing and proposed utilities.
  • The structure shall permit damp soils with large soil peds to be installed and for the soil to be installed without vibrating the structure. The structural openings must allow all of the soil to be checked for compaction and complete filling of all cell areas.
  • The Soil cell top surface shall be perforated to allow the free flow of water through the deck. Soil cell installation shall include all accessories, geotextiles, geogrids, and aggregate layers required by the Soil cell manufacture.
  • Complete submittals.

Execution guidelines

Site examination

Examine the surface grades and soil conditions for any circumstances that might be detrimental 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.
  • 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.
  • Protect adjacent walls, walks and utilities from damage or staining by the soil. Use ½ inch plywood and/or plastic sheeting as directed to cover existing concrete, metal and masonry work and other items as directed during the progress of the work.
  • 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 manufacturer's 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 soil using 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 Bioretention Soil Mix D in approximately 12 inch lifts to the required depths. Lightly compact each lift with a maximum of two passes with a 5 horsepower vibrating plate tamper to achieve the required penetration resistance. Maintain volumetric soil moisture between 5 and 15 percent during installation and compaction.
  • Scarify the surface of each lift with the teeth of a back hoe or similar equipment prior to installing additional lifts.
  • Do not drive over delivered soil to spread or grade. Install soil in narrow bands, working out from the installed soil such that soil delivery and spreading equipment does not have to pass over previously installed soil. The width of each band of installed soil shall not exceed the reach of the delivery equipment.
  • Coordinate the installation of water harvesting distribution and drain lines within the Bioretention Soil Mix D.
  • Install soil in Soil Cells according to the cell manufacturer’s requirements.
  • Grade the finished surface of the Bioretention Soil Mix D to the grades indicated on the drawings plus extra soil for settlement as noted below.
    • The Bioretention Soil Mix D, when properly installed at the compaction levels indicated, will continue to settle. It is not the intent of these requirements to construct a soil profile that is immediately stable. Add an additional 1 inch of soil for each 10 inches of installed soil depth. The grades shown on the drawings are the grades after the anticipated settlement.
    • The tolerance for finished grades shall be plus or minus ½ inch in 10 feet.

Bioretention Soil Mix D compaction

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.

Structural soil installation

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.

Information: 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 The Most Important Factor for Growing Healthy Trees. The manual page on Alleviating compaction from construction activities, while written for active construction sites, provides some guidance that can be applied to alleviating compaction

Repair of settled Bioretention Soil Mix D

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.

Literature review of soils optimized for tree growth

Examples of soil guidelines and specifications for optimized tree growth are discussed below.

  • Toronto Street Trees Guide to Standard Planting Options. 2010. City of Toronto Urban Design Streetscape Manual. In collaboration with Parks, Forestry & Recreation.
    • High-quality soil shall consist of a minimum 0.9 meter and maximum 1.2 meter depth, over and above any required drainage system and/or granular material, be uncompacted, and be sandy loam with the following composition:
      • Sand: 50 to 60 percent
      • Silt: 20 to 40 percent
      • Clay: 6 to 10 percent
      • Organic: 2 to 5 percent
      • pH = 7.5 or less
  • DTAH et al 2013 tree manual for Toronto recommends:
    • 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.
    • Soil organic matter should be between 3 and 5 percent dry weight in the upper layer of the profile at the time the soil is installed. Organic matter in the lower soil level should be between 1.5 and 3 percent dry weight. This difference can be accomplished by making two different soils or tilling additional compost into the surface of the soil.
    • See pages 56 to 62 of the manual for detailed conceptual explanation of other tree soil requirements, including the importance of soil structure and obtaining unscreened soil with soil ped structure intact, recommendations for soil mixes and proportion of coarse to medium sands; difference between natural soil organic matter and compost.
  • Urban (Up by Roots: Healthy Soils and Trees in the built environment, 2008) contains extensive explanations of tree soil needs. Some relevant conclusions and recommendations include the following.
    • Develop a soil that has good drainage capability, while providing adequate moisture and nutrient holding capacity for the plants.
    • Ensure the final coarse to medium size sand content exceeds 50 percent, because at lower amounts of sand, drainage rates are not increased.
    • As the sand and aggregate content increases above 55 percent, the water and nutrient holding capacity goes down and the drainage rate goes up.
    • High sand content soils are reliably drained, are more tolerant of a wide range of compaction rates, and are are less likely to become over compacted, but require a continuous supply of water and nutrients, which increases maintenance. Note that directing stormwater to these soils lessens this problem because it increases water and nutrient supply, though the trees will likely still be exposed to periodic droughts.
    • As the sand is reduced in the mix, close to the 55 percent threshold, compaction rates become critical. With too much compaction in finer-grained soil, drainage rates become unacceptable. With too little compaction in finer grained soil soil settlement can become unacceptable. The window between too much and too little compaction is hard to manage during construction.
    • Soil mix design is a multi-step process and does not end with the publication of specifications. The specifications should set performance standards and the types of mix components to be used. At the time of writing the specifications, the designer cannot know the actual source of the soil or organic amendment. Minor variations in these materials will change the mix proportions.
    • Testing of the soil to be used in the mix must include the measuring of the difference sizes of sand particles, known as sand fractions.
    • A good measure to evaluate the performance of a soil mix is its infiltration rate when compacted to a known level. A developing standard is to test infiltration at 80 and 85 percent of maximum dry density as measured by the Standard Proctor test.

Literature review of soils optimized for bioretention

Using trees as stormwater BMP’s adds the following soil requirements to those of traditional street trees that are not planted for stormwater management.

  • As more stormwater is directed to the trees, there is more danger of soils clogging, so limiting the fines content of the soil becomes more crucial. However, research shows that while the hydraulic conductivity of a bioretention practice typically decreases initially, hydraulic conductivity goes back up as plants and microbes improve soil structure over time and thus the infiltration rate does not decrease significantly long term (Hatt et al., 2009; Jenkins et al., 2010; Li & Davis, 2008b; Barrett et al., 2011).
  • Where nutrient reduction in stormwater runoff is a goal, limiting the nutrient content of the soil is required to minimize nutrient leaching from the soil.

A representative sampling of recent comprehensive literature on bioretention media guidelines is summarized below.

  • 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).
    • 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.
    • Although there is some concern among tree specialists that the North Carolina media mix (Mix C) may have too little organic matter to grow healthy trees, this soil has been used extensively to grow trees with success in North Carolina (Winston and Hunt, 2013, personal communication). North Carolina does receive more annual precipitation than Minnesota, so moisture retention is more crucial in Minnesota than in North Carolina and trees may not perform as well in Minnesota as in North Carolina in a soil with low moisture content. Research by Fassman et al. (2013), however, indicates that sand based bioretention media can have as much plant available water as horticultural soils with less sand and more organic matter.
  • Australian research on using trees for bioretention: Breen 2004; Denman 2006; Denman 2011 (all describing same experiment)
    • Trees grew equally well in soils of a wide range of saturated hydraulic conductivities, and grew better when irrigated with stormwater versus tapwater. “Trees were grown outdoors in experimental biofiltration systems, constructed with 240 millimeter (9.45 inch) diameter columns, cut into 600 millimeter (23.6 inch) lengths. The constructed soil profiles were 500 millimeters (20 inches) deep with 10 percent (v:v) composted green waste added to the surface 200 millimeters (8 inches). The three soils used were sands with saturated hydraulic conductivities (SHC) of 4, 95 and 170 millimeters per hour (0.16, 3.75 and 6.7 inches per hour) and the soils are referred to as low, medium and high SHC soil, respectively."
    • Tree growth was similar in the three soils studied (Denman 2006).
    • Trees grew taller and had greater root density when irrigated with stormwater compared to tapwater.
    • 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).
  • Fassman et al., 2013, (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:
      • incorrect media specification, where the media has incorrect physical/chemical properties for removing targeted pollutants;
      • incorrect media specification, where the media may have high clay content or extremely fine particles, and vulnerability to compaction which cause inadequate drainage and over-extended ponding;
      • incorrect compaction, often resulting from poor compaction specifications or using media vulnerable to compaction. The media is either under-compacted and too loose resulting in low contact time, or over-compacted and too dense (resulting in inadequate drainage and over-extended ponding); and
      • clogging, where excessive sediment loads restrict the pores of the media, hindering infiltration and causing inadequate drainage and over-extended ponding. Clogging most commonly occurs at the surface as crusting, capping, or sealing. Sediment from unstable catchments or from catchments with active construction, or fine particles within the filter media may contribute to clogging.
    • Cites that Warynski and Hunt (2011) “performed an inspection of 20 bioretention cells throughout the state. They found 82 percent of bioretention cell filter media having incorrect particle size distributions (PSD), and 44 percent of bioretention cells having incorrect permeabilities. Furthermore, 50 percent of bioretention cells were undersized.”
    • Summarizes recommended typical bioretention media composition specifications from worldwide sources (see table 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.

    • While hydraulic conductivity initially declines as the filter media is compacted, FAWB (2009a) and Barret et al. (2011) found it often recovers back to the design value over time as increased plant root growth counters the effects of compaction and clogging.
    • Summarizes worldwide sampling of typical bioretention media hydraulic conductivity specifications (see table below).

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

    • Particle size distribution (PSD) is used as a gauge of the hydraulic performance of a filter media in several international guidelines.
    • PSD may be a useful gauge of the potential hydraulic performance of a filter media, but it should not be used to replace hydraulic conductivity testing.
    • In addition to meeting gradation limits, media should be well-graded over the entire range to avoid structural collapse due to particle migration (FAWB, 2009a).
    • Summarized a world wide sampling of specifications for compaction during construction (see table below).

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

    • If specific compaction details (such as water content) are not designed for, filter media may easily be over or under compacted, leading to an undesirable hydraulic conductivity and potential failure of the bioretention cell. An advantage of materials that are relatively insensitive to compaction is a greater certainty of achieving design conductivity range.
    • The density of predominantly sand based media are less susceptible to the effects of compaction and water content.
    • Mixes that met the author’s desired hydraulic conductivity range did not meet international PSD guidelines. The three mixes that best fit their desired hydraulic conductivity range had higher fines than the recommended guidelines, while two out of three are poorly graded. The authors conclude that “With this result, it is clear the PSD-based guidelines should not be used as a substitute to hydraulic conductivity testing.”
    • A coarse sand (all passing 2 millimeters, with U 3) or high silt/clay component (considered to be greater than 20 percent, as might be found in natural soils) will be susceptible to the field installation compaction method, and hence so will be the hydraulic conductivity. In practice, if these materials are used, careful installation procedures and post-installation testing of infiltration and/or bulk density would be strongly recommended.
    • Mixes that satisfied aggregate PSDs tended to produce extremely high hydraulic conductivities, even when mixed with relatively high proportions of compost (which also violates many international guidelines).
    • Because the sand based bioretention media developed in this project had low organic matter content (less than 10 percent v/v compost, equating to 1 to 3 percent (w/w) total carbon), and also had very low clay and silt components, it was expected that plants might not grow well in them because they were thought to have little ability to store and supply water and nutrients to the plants. Therefore the authors tested two of their sand based mixes for plant growth against a rich horticultural soil mix, containing 30 percent compost by volume, known to grow vigorous plants. They grew 2 bioretention plant species for 6 months in pots of each of the three soils, with a low rate of 9 month slow release fertilizer added to the sand-based mixes only. No differences in plant growth or plant available water were observed between the plants growing in sand based mixes vs. the horticultural mix. They concluded that: “the volume of stored water that plants can access for growth is similar in all substrates, being 21 to 24 percent of the total soil volume. This is because the two commercial mixes have a large amount of water that is held very tightly (to the organic matter) and therefore inaccessible to plants… It is unlikely the three sand-based mixes developed will be any more drought prone than existing commercial mixes with high organic contents, as all store similar volumes of plant-available water per unit depth...” If these results also hold true in longer term field experiments, plants should be able to grow just as well in bioretention media with low organic matter content as in horticultural mixes designed for vigorous plant growth. Minimizing soil organic matter content is crucial to maximize nutrient reduction by bioretention systems and to minimize nutrient leaching from the soil.
  • Australian guidelines for bioretention media (FAWB 2009)
    • Based on extensive research, a bioretention BMP requires three layers of media:
      • the filter media itself (400 to 600 millimeters (15.75 to 23.62 inches) deep, or as specified in the engineering design);
      • a transition layer (100 millimeters (4 inches) deep)); and
      • a drainage layer (50 millimeters (2 inches) minimum cover over underdrainage pipe).
    • The biofiltration system will operate so that water will infiltrate into the filter media and move vertically down through the profile.
    • The biofiltration system is required to support a range of vegetation types (from ground covers to trees) that are adapted to freely draining soils with occasional wetting.
    • The material should be based on natural or amended natural soils or it can be entirely engineered… In the case of natural or amended natural soils, the media should be a loamy sand.
    • In general, the media should have an appropriately high permeability under compaction and should be free of rubbish, deleterious material, toxicants, declared plants and local weeds (as listed in local guidelines/Acts), and should not be hydrophobic.
    • The filter media should contain some organic matter for increased water holding capacity but be low in nutrient content.
    • Requires minimum 3 percent organic matter by weight; does NOT specify maximum organic content, just maximum nutrient content. NOTE: Research in North Carolina indicates that using trees with suspended pavement and soils higher in organic matter content can still attain good nutrient reduction if soil nutrient content is below North Carolina standards (Jonathan Page, NC State, 2013, personal communication).
    • Maintaining an adequate infiltration capacity is crucial in ensuring the long-term treatment efficiency of the system. The ability of a biofiltration system to detain and infiltrate incoming stormwater is a function of the filter surface area, extended detention (ponding) depth, and the hydraulic conductivity of the filter media. Most importantly, design of a biofiltration system should optimize the combination of these three design elements.
    • For a biofiltration system in a temperate climate with an extended detention depth of 100 to 300 millimeters (4 to 12 inches) and whose surface area is approximately 2 percent of the connected impervious area of the contributing catchment, the prescribed hydraulic conductivity will generally be between 100 to 300 millimeters per hour (4 to 12 inches per hour) in order to meet best practice targets. This configuration supports plant growth without requiring too much land space.
    • The infiltration capacity of the biofiltration system will initially decline during the establishment phase as the filter media settles and compacts, but this will level out and then start to increase as the plant community establishes itself and the rooting depth increases(FAWB, 2009) based on research by Hatt et al. (2009).
    • The hydraulic conductivity of potential filter media should be measured using the ASTM F1815-06 method. This test method uses a compaction method that best represents field conditions and so provides a more realistic assessment of hydraulic conductivity than other test methods. Note: if a hydraulic conductivity lower than 100 millimeters per hour (4 inches per hour) is prescribed, the level of compaction associated with this test method may be too severe and so underestimate the actual hydraulic conductivity of the filter media under field conditions. However, FAWB considers this to be an appropriately conservative test, and recommends its use even for low conductivity media.
    • PSD is of secondary importance compared to hydraulic conductivity. A material whose PSD falls within the following recommended range does not preclude the need for hydraulic conductivity testing; i.e., it does not guarantee that the material will have a suitable hydraulic conductivity. However, the following composition range (percentage w/w) provides a useful guide for selecting an appropriate material:
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
    • Clay and silt are important for water retention and sorption of dissolved pollutants; however they substantially reduce the hydraulic conductivity of the filter media. This size fraction also influences the structural stability of the material (through migration of particles to block small pores and/or slump). It is essential that the total clay and silt mix is less than 3 percent (w/w) to reduce the likelihood of structural collapse of such soils. The filter media should be well-graded; i.e., it should have all particle size ranges present from the 0.075 millimeter to the 4.75 millimeter sieve (as defined by AS1289.3.6.1 - 1995). There should be no gap in the particle size grading, and the composition should not be dominated by a small particle size range. This is important for preventing structural collapse due to particle migration.
    • Total Nitrogen (TN) Content – less than 1000 milligrams per kilogram.
    • Orthophosphate (PO43-) Content – less than 80 milligrams per kilogram. Soils with total phosphorus concentrations greater 100 miligrams per kilograms should be tested for potential leaching. Where plants with moderate phosphorus sensitivity are to be used, total phosphorus concentrations should be less than 20 milligrams per kilogram.
    • For engineered media, they recommend
      • a washed, well-graded sand with an appropriate hydraulic conductivity should be used as the filter medium.
      • the top 100 millimeters (4 inches) of the filter medium should then be ameliorated with appropriate organic matter, fertilizer and trace elements. This amelioration is required to aid plant establishment and is designed to last four weeks, the rationale being that beyond this point the plants receive adequate nutrients via incoming stormwater.


  • Clay, silt and sand soil particles: Per USDA size designations. It is critical to NOT use testing laboratories that report results in engineering size designations such as the Unified or AASHTO systems.
  • Coarse Sand: Process washed and graded sand from regional sand suppliers.
  • Compost: Decomposed plant based biomass.
  • Bioretention Soil Mix A: A mixture of topsoil, coarse sand and compost intended to fill bio- retention planters to support the treatment ofstorm water and the growth of trees and other plants.
  • Screened Soil: Any soil run through any type of screen with a mesh size 3 square inches or smaller.
  • Soil Cells: also called structural cells; pre-engineered structural system to hold up the sidewalk and be filled with soil to support tree roots and treat stormwater.
  • Soil Peds: Clumps of soil that naturally aggregate during the soil building process.
  • Structural Soil: A mixture of stone and soil formulated to be compacted to 95 percent of maximum dry density, Standard Proctor, and support tree roots.
  • Topsoil: Fertile, friable, loamy soil, harvested from natural topsoil sources.


Related pages

The following pages address incorporation of trees into stormwater management under paved surfaces

This page was last modified on 7 May 2018, at 08:31.

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