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[http://greenthisway.com/Green_This_Way/GTW_New_System_files/Managing%20Stormwater%20Using%20Trees%20and%20Structural%20Soil.pdf Day and Dickinson (2008)] provide information on use of trees in structural soils, including design specifications. The following considerations should be made in using structural soils. | [http://greenthisway.com/Green_This_Way/GTW_New_System_files/Managing%20Stormwater%20Using%20Trees%20and%20Structural%20Soil.pdf Day and Dickinson (2008)] provide information on use of trees in structural soils, including design specifications. The following considerations should be made in using structural soils. | ||
− | + | *Soil pH | |
Care must be taken to select species tolerant of structural soil pH. For example, if limestone basedstructural soil is used, trees tolerant of alkaline pH must be selected, as limestone can raise the pH of soil to 8.0 or higher (Bassuk 2010 soil debate, Urban 2008). | Care must be taken to select species tolerant of structural soil pH. For example, if limestone basedstructural soil is used, trees tolerant of alkaline pH must be selected, as limestone can raise the pH of soil to 8.0 or higher (Bassuk 2010 soil debate, Urban 2008). | ||
− | + | *Drainage Rate | |
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Because rock based structural soils drain quickly (greater than 24 inches per hour), designers should select tree species tolerant of extremely well drained soils (Bassuk 2010). | Because rock based structural soils drain quickly (greater than 24 inches per hour), designers should select tree species tolerant of extremely well drained soils (Bassuk 2010). | ||
− | + | *Volume of rock based structural soil needed for healthy tree growth | |
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Because only 20 percent of the volume of a rock based structural soil is actually soil, a greater total volume of rock based structural soil is needed compared to growing the same size tree in a sandy loam soil. A pot study by Loh et al (2003) found that 5 parts of structural soil were needed to provide the soil value of 1 part of loam soil (Loh et al, 2003). Similarly, an on-going study at Bartlett Tree labs is finding that over the past 9 years, trees growing in loam soil in suspended pavement have been consistently outgrowing trees growing in equal volumes of rock based structural soils, stalite soil, and compacted soil. | Because only 20 percent of the volume of a rock based structural soil is actually soil, a greater total volume of rock based structural soil is needed compared to growing the same size tree in a sandy loam soil. A pot study by Loh et al (2003) found that 5 parts of structural soil were needed to provide the soil value of 1 part of loam soil (Loh et al, 2003). Similarly, an on-going study at Bartlett Tree labs is finding that over the past 9 years, trees growing in loam soil in suspended pavement have been consistently outgrowing trees growing in equal volumes of rock based structural soils, stalite soil, and compacted soil. | ||
Standard keys to success in bioretention construction apply to trees for bioretention, including the following.
See the bioretention section for other construction guidelines and specifications.
In addition to general bioretention guidelines and specifications, the following guidelines and specifications apply specifically to trees for stormwater design.
Each of the above techniques is described and compared below. Links to construction guidelines are also provided.
In areas that do not have enough open space to grow large trees, techniques like suspended pavement can be used to extend tree rooting volume under HS-20 load bearing surfaces and create favorable conditions to grow large trees in urban areas. This rooting volume can also be used for bioretention. While suspended pavement has been built in several different ways, all suspended pavement is held slightly above the soil by a structure that “suspends” the pavement above the soil so that the soil is protected from the weight of the pavement and the compaction generated from its traffic.
One of the earliest examples of trees grown in suspended pavement is in Charlotte, North Carolina, where a reinforced concrete sidewalk was installed over the top of poured in place concrete columns. While this is an effective way to grow large trees, it is labor intensive and requires intensive surveying to ensure that column heights are precise.
A more recently developed, and less labor intensive technique to build suspended pavement is through the use of soil cells. An example is the Silva Cell, a modular proprietary pre-engineered structural cell manufactured by Deeproot Green Infrastructure (NOTE: Reference herein to any specific commercial products, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the Minnesota Pollution Control Agency). The modular design allows flexibility to size the rooting/bioretention volume as needed for each site. Underground utilities can be accommodated within Silva Cells systems. The Silva Cell system consists of silva cell frames that are 48 inches (1200 millimeters) long by 24 inches (600 millimeters) wide by 16 inches (400 millimeters) high. Frames can be stacked up to three high, and a deck is installed above the top frame. An air space between the top of the soil and the deck prevents tree roots in the upper soil layers from lifting the overlying pavement.
Because soil in a suspended pavement system is protected from compaction from loads on the pavement, a wide range of soils can be used in these systems, so soil can be tailored to desired functions (e.g. tree growth and stormwater management). The Silva Cell system, for example, can support vehicle loading up to AASHTO H-20 rating of 32,000 pounds (14,500 kilograms) per axle. According to Deeproot’s website “This rating refers to the ability of a roadway to safely accommodate 3 to 4 axle vehicles, such as a large semi-truck and trailer.” The cells are made of “an ultra high-strength compound of glass and polypropylene, with galvanized steel tubes to add support and prevent plastic creep.”
Construction documents and specifications for a wide range of Silva Cell applications can be found on Deeproot Green Infrastructure’s website. Many other resources are available on their website including, for example
Rock based structural soils are engineered to be able to be compacted to 95% Proctor density [1] without impeding root growth. Rock based structural soils are typically gap graded engineered soils with the following components:
Desired characteristics for the stone base used in rock based structural soils include the following.
According to University of California-Davis (1994), 2 inch stones would be able to support most tree roots. Because limestone has been found to crush on some projects, granite stone is recommended.
Soil needs to have adequate nutrient and water holding capacity to provide for the tree’s needs.
The tackifier, if used, should be non-toxic and non-phototoxic.
Construction drawings are available on Cornell University’s website, including
Several types of rock based structural soils have been developed, including
Day and Dickinson (2008) provide information on use of trees in structural soils, including design specifications. The following considerations should be made in using structural soils.
Care must be taken to select species tolerant of structural soil pH. For example, if limestone basedstructural soil is used, trees tolerant of alkaline pH must be selected, as limestone can raise the pH of soil to 8.0 or higher (Bassuk 2010 soil debate, Urban 2008).
Because rock based structural soils drain quickly (greater than 24 inches per hour), designers should select tree species tolerant of extremely well drained soils (Bassuk 2010).
Because only 20 percent of the volume of a rock based structural soil is actually soil, a greater total volume of rock based structural soil is needed compared to growing the same size tree in a sandy loam soil. A pot study by Loh et al (2003) found that 5 parts of structural soil were needed to provide the soil value of 1 part of loam soil (Loh et al, 2003). Similarly, an on-going study at Bartlett Tree labs is finding that over the past 9 years, trees growing in loam soil in suspended pavement have been consistently outgrowing trees growing in equal volumes of rock based structural soils, stalite soil, and compacted soil.
Based on the above studies, Urban (2008) recommends: “Given the extreme inefficiency of the ratio of excavated volume to soil usable by the tree, strips of structural soil less than 20 feet wide might be better constructed as soil trenches or structural cells, where more soil can be included for less cost. A 5-foot wide soil trench set of structural cells…will provide more soil usable by the tree than a 20 foot wide trench of soil/aggregate structural soil. Soil/aggregate structural soils may have applications as a transition to other options, and to add soil in places where other options may not be practical. These might include tight, contorted spaces and fills around utility lines and against foundations where full compaction is required (p. 306)”
More information about rock based structural soils is available online, for example, at
Sand based structural soils were first developed in Amsterdam when some trees were in poor condition because of an “unfavorable rooting environment” (Couenberg 1993). Because the natural soils in Amsterdam, bog-peat, was non-load bearing, the top 2 meters of soil had been replaced with a medium coarse sand, which had insufficient nutritional value. Amsterdam soils were developed in an effort to grow better trees but still provide adequate bearing capacity for pavement bearing light loads, suchas sidewalks. The Dutch studied various mixes for tree growth, soil settlement, and several other parameters. The resulting Amsterdam Tree Soil contains medium coarse sand with 4 to 5 percent organic matter and 2 to 4 percent clay by weight and also meets other criteria, including, for example, (1) the medium coarse sand must meet specific gradation requirements, (2) soil mix must be free of salt, (3) mix must contain less than 2 percent particles below 2 micrometers, and (4) amount of particles below 2 micrometers must be considerably less than the amount of organic materials (Couenberg 1993).
All medium coarse sand (the layer above the Amsterdam Tree Soil) is compacted to 95 percent to 100 percent Proctor density. Amsterdam tree soil is not compacted to 100 percent density, but “is compacted until the soil has a penetration resistance between 1.5 and 2 MegaPascal (187 to 250 pounds per square inch (PSI)) ... Comparison of soil density values after filling with soil density at 100 percent Proctor Density has shown that soil density of Amsterdam Tree Soil after filling amounts to 70 to 80 percent Proctor Density” (Couenberg 1993).
Amsterdam Tree Soil was found to settle 19 millimeters [0.75 inches] in 3 years compared to the surrounding pavement, which was acceptable according to Dutch standards (Couenberg 1993), but may not be acceptable to many communities in the US to minimize risk of litigation related to trip and fall hazards.
The standard design in Amsterdam includes the following from bottom to top:
While Amsterdam sand based soils work well in Amsterdam, trees grown in Amsterdam soils in Minnesota would likely need significant irrigation, as they have low water holding capacity. Amsterdam receives an average of 36 inches of rain per year, which is higher than the range of average yearly rainfall in Minnesota. Rainfall events are also more frequent and lower intensity in Amsterdam than in Minnesota. Summer temperatures are also higher in Minnesota than in Amsterdam, resulting in higher water needs by trees. According to Couenberg (1993), who helped develop Amsterdam soils “the Amsterdam tree soil has been developed in an area where there is sufficient rainfall. If this soil is used in other climatic areas, adaptations to the tree pit will have to be made after the local situation has been monitored.”
Most of Amsterdam also has a high water table which is controlled to have almost no seasonal fluctuation, and groundwater wicks up into tree rooting zones by capillary action, so trees always have access to water from the ground water table. According to Urban (2008), “tests of Amsterdam planting soil in other locations in Europe without high water tables showed less promising results with overly dry soil, which required significant supplemental water.”So, in summary, sand based structural soils may be a viable in Minnesota if (1) some settlement is acceptable with light structural loads, and (2) trees are irrigated.
Soil boxes are concrete boxes designed for bioretention. They are typically proprietary products, such as, for example, the boxes made by Filterra and Contech. Rooting volume capacity of these boxes is typically limited to large shrubs. Soil volumes provided by these boxes are typically not sufficient to grow healthy large trees. A standard 6 feet by 6 feet Filterra box, for example, provides 72 cubic feet of soil, assuming a 2 foot depth of soil. Given that the recommended soil volume for trees is 2 cubic feet of soil per 1 square foot of tree canopy, this is only enough to support a tree with a 5 foot radius canopy.