Design guidelines for soil characteristics - tree trenches and tree boxes
Contents
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.
Recommended minimum soil volume requirements for urban trees
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 oftrees. Insert a table of species versus mature canopy size.
Stormwater volume credits for tree trenches and tree boxes include*storage of water in the planting media;*interception of rain water by the tree canopy; and*evapotranspiration from the tree(s). 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.
Summary of literature review on soil volume
An extensive literature review was completed prior to developing the minimum soil volume recommendation discussed above.====Summary of research on minimum soils volumes needed====Minimum soil volume needed to grow healthy trees has been studies 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.
Comparison of rock based structural soil and traditional tree soils
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 treegrowing 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:*0.4 c.f. of loam soil (small loam soil)*0.4 c.f. of structural soil(small structural soil)*2 c.f. of loam soil (large loam soil)*2 c.f. of structural soil (large structural soil) 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 (ie structural soil), as well as to trees grown in compacted soil. Each tree was provided 5.7 m3 (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 (see Figures 4.1-4.4).
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 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. ===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 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). ===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.
General=========Scheduling=
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.
- 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.
- 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 be 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 85percent compaction at proctor density (ASTM D 698-91). This is a LABORATORY TEST to determine water flow at specified compaction rates. Laboratories that provide thistesting include: ****Hummel Soil Labs, www.turfdoctor.com; (607) 387-5694,35 King Street, PO Box 606, Trumansburg, NY, 14886.****Turf Diagnostics & Design, www.turfdiag.com; (913)-723-3700, 613 E. 1st Street, Linwood, KS, 66052.**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 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.
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.
======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 priorto 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.*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.
=====Delivery, storage and handling=====When warranties are required, verify with Owner'scounsel 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.*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 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 withOwner's counsel that special warranties stated in this article are not lessthan 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 theexisting underground conditions before digging. Notification of Local Utility Locator Service is required prior to all excavation.
Products
=====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” 100
- 4 95-100
- 8 80-100
- 16 50-85
- 30 25-60
- 50 5-30
- 100 4-10
#200 2-4
- Submittals shall be completed per Part 1 - Section – Submittals for approval.
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 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: **A statement that the Compost meets federal and state health and safety regulations.**Compost testing methodologies and sampling procedures shall be as provided in Test methods for the Examination of Composting and Compost (TMECC), as published by the US Composting Council.*Submittals shall be completed per Part 1 - Section – Submittals for approval.
=====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 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 specificationshall be rejected. Clumps or peds of soil within the sample shall be the same color and texture on the inside as the outside of the clump or ped.*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 (standard pH units)
- Phosphorous: Sufficient to meet the maximum requirements in the Plant/Bio-retention mix once the other products are added to the mix
- Submittals shall be completed per Part 1 - Section – Submittals for approval.
=====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 sand (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 D968*phosphorus between 12 and 36 parts per million (ppm)*cation exchange capacity greater than 10 meq/g
The following are suggested mix ratio ranges. 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 % organic matter. Any variation of required products, above or below the percentages listed below, needed to attain the required drainage rate or %organic matter, shall not be grounds to change the agreed upon price of the installed material. Approximate ranges byvolume Coarse Sand 50-65% Topsoil 25-35% Compost 10-15%
- Lightly mix the Bioretention Soil Mix A using a front end loader to preserve topsoil peds as much as possible. Topsoil peds 2” in diameter or larger should be visible in the finished stockpile. Do not over mix or screen the material.*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. *Structural Soil shall be “CU Soil” as manufactured by Amereq, Inc New York City, NY, or approved equal.*Submittals shall be completed per Section 1.4.
=====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.
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 140 square inches to permit:*a continuous mass of soil that allows for capillary transfer of water;
- fosters the growth of large tree roots including the zone of rapid taper roots that can become up to 12 inches in diameter at tree maturity; and permits the structure to be built around, over; under and thru existing and proposed utilities.*Each cell stack shall be structurally independent of all adjacent stacks such that the cell layout can achieve maximum layout flexibility.*The structure shall permit damp soils with large soil peds to be installed and the soil installed without vibrating the structure. The structural openings must allow all the soil to be checked for compaction and complete filling of all cell areas.*The Soil Cell deck shall be perforated to allow the free flow of water thru the deck. Soil cell installation shall include all accessories, geotextiles, geogrids, and aggregate layers required by the Soil Cell manufacture.
Submittals shall be completed per Section 1.4.
Execution
Site examination
A. Examinethe 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.*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 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 2-3 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 18-24 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 Bioretention Soil Mix A in approximately 12 inch lifts to the required depths. Lightly compact each lift with a maximum of two passes with a 5 HP vibrating plate tamper to achieve the required penetration resistance. Maintain volumetric soil moisture during installation and compaction at between 5 and 15 percent.
- 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 A.*Install soil in Soil Cells according to the cell manufacturer’s requirements.*Grade the finished surface of the Bioretention Soil Mix A to the grades indicated on the drawings plus extra soil for settlement as noted below. **The Bioretention Soil Mix A 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 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%.
=====Structural soil installation=====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.
=====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.
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% of the soil depth.
Definitions
- Clay, silt and sand soil particles: Per USDA size designations. It is critical NOT to 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. Coarse Sand will be further defined in Part 2 - Products.*Compost: Decomposed plant based biomass. Compost will be further defined in Part 2 - Products.*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. Bioretention Soil Mix A will be further defined in Part 2 - Products.*Screened Soil: Any soil run through any type of screen with a mesh size 3 inches square 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 rootsand treat storm water. Soil Cells will be further defined in Part 2 - Products.*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. Structural Soil will be further defined in Part 2 - Products.*Topsoil: Fertile, friable, loamy soil, harvested from natural topsoil sources, Topsoil will be further defined in Part 2 - Products
References
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- Lindsey, P. and N. Bassuk. 1991. Specifying Soil Volumes to Meet the Water Needs of Mature Urban Street Trees and Trees in Containers. Journal of Arboriculture 17(6):141-149. *Loh, F. C. W.; Grabosky, J. C.; Bassuk, N. L. (2003) Growth Response of Ficus Benjamina to Limited Soil Volume and Soil Dilution in a Skeletal Soil Container Study. In Urban For. Urban Gree. 2 (1)53-62.*Pitt, R., S. Clark, P. Johnson, and J. Voorhees. In Press. Evapotranspiration and Related Calculations for Bioretention Devices. CHI Monograph 14 Available October 2013 from http://rpitt.eng.ua.edu/Class/StormWaterManagement/Fall%202009/Pitt_Evapo_final__copy_changes_accepted.pdf*Schoenfeld, P.H. 1975. De groei van Hollandse iep in the kustprovincies van Nederalnd. Nederlands Bosbouw Tijdschrift 47:87-95. Cited in Kopinga 1991.
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- Smiley, E. T. 2013. Bartlett Tree Research Lab, Charlotte North Carolina, Adjunct Professor Clemson Univ., unpublished data.
- Smiley, E. Thomas, Lisa Calfee, Bruce R. Fraedrich, and Emma J. Smiley. 2006. Comparison of Structural and Noncompacted Soils for Trees Surrounded by Pavement. Arboriculture & Urban Forestry 32(4): 164-169.
- Urban, J. 1992. Bringing Order to the Technical Dysfunction Within the Urban Forest. Journal of Arboriculture 18(2): 85-90.