Content uploaded by Susan D Day
Author content
All content in this area was uploaded by Susan D Day on Feb 25, 2016
Content may be subject to copyright.
Managing Stormwater
for Urban Sustainability
Using Trees and
Structural Soils
Susan Downing Day and
Sarah B. Dickinson, Editors
This manual was made possible in part by a grant from the United States
Department of Agriculture Forest Service Urban & Community Forestry
Program on the recommendation of the National Urban & Community
Forestry Advisory Council (NUCFAC).
Project title: “Development of a Green Infrastructure Technology that Links
Trees and Engineered Soil to Minimize Runoff from Pavement”.
Editors: Susan Downing Day, and Sarah Beth Dickinson
Contributing Authors: Nina Bassuk, Julia Bartens, Laurence Costello, Joseph
E. Dove, Jason Grabosky, Ted Haffner, J. Roger Harris, E. Gregory McPherson,
Peter Trowbridge, Theresa Wynn, and Qingfu Xiao
Design & Production: Sarah Beth Dickinson
How to cite this manual:
Day, S.D, and S.B. Dickinson (Eds.) 2008. Managing Stormwater for Urban
Sustainability using Trees and Structural Soils. Virginia Polytechnic Institute
and State University, Blacksburg, VA.
Copyright © 2008, Susan Downing Day and Sarah Beth Dickinson
Acknowledgements
This manual is the culmination of a four-year project that has relied on the hard
work and insight of many people. We appreciate the work of John O. James, Stephanie
Worthington, Mona Dollins, Liz Crawley, Velva Groover, Félix Rubén Arguedas, Andy
Hillman, and many others in bringing this project to completion.
Contributing Authors
Nina Bassuk, Ph.D., Professor and Program Leader of the Urban Horticulture Institute,
Cornell University
Julia Bartens, Graduate Research Assistant, Department of Horticulture, Virginia Tech
(current position: Ph.D. student, Department of Forestry, Virginia Tech)
Laurence Costello, Ph.D., Extension Specialist, University of California at Davis
Susan Downing Day, Ph.D., Assistant Professor, Departments of Forestry and
Horticulture, Virginia Tech
Sarah B. Dickinson, Research Associate, Department of Horticulture, Virginia Tech
Joseph E. Dove, Ph.D., P.E., Research Assistant Professor, Department of Civil and
Environmental Engineering, Virginia Tech
Jason Grabosky, Ph.D., Associate Professor, Department of Ecology, Evolution and
Natural Resources, Rutgers University
Ted Haffner, Graduate Research Assistant, Department of Horticulture, Cornell
University (current position: Associate Landscape Architect, Terry Guen Design
Associates, Chicago, IL)
J. Roger Harris, Ph.D., Professor and Head, Department of Horticulture, Virginia Tech
E. Gregory McPherson, Ph.D., Director, Center for Urban Forestry Research PSW, USDA
Forest Service
Peter Trowbridge, MLA, Professor and Chair, Landscape Architecture, Cornell
University
Theresa Wynn, Ph.D., Assistant Professor, Biological Systems Engineering, Virginia
Tech
Qingfu Xiao, Ph.D., Research Water Scientist, Department of Land, Air, and Water
Resources, University of California at Davis
Contents
Introduction 1
Chapter 1— Trees and Structural Soils- A System Overview 5
Trees— Mimicking the Hydrologic Benefits of a Forest in
the City 6
Structural Soils— Supporting Tree Growth and Pavement 7
Subsoils 10
Limitations concerning subsoil infiltration 11
Chapter 2— System Design to Meet Site Requirements 13
Specifications 13
Surface Treatments 13
Reservoir Sizing and Overflow Pipe Design 14
Geotextiles 18
By Joseph E. Dove
Trees and Other Plants 20
Chapter 3— Surface Treatments 25
Structural Soils and Turf 25
By Nina Bassuk, Ted Haffner, Jason Grabosky, and Peter
Trow br id ge
Using Porous Pavement on Structural Soils 30
By Ted Haffner, Nina Bassuk, Jason Grabosky, and Peter
Trow br id ge
Chapter 4— Research and Recommendations 33
Tree Root Penetration into Compacted Soils Increases
Infiltration 33
Based on Research by Julia Bartens, Susan Day, Joseph E.
Dove, J. Roger Harris, and Theresa Wynn, Virginia Tech
Tree Development in Structural Soils at Different Drainage
Rates 34
Based on Research by Julia Bartens, Susan Day, J. Roger
Harris, Joseph E. Dove, and Theresa Wynn, Virginia Tech
Drainage Rate at the Mini Parking Lot Demonstration Site
in Blacksburg, VA 35
Based on Research by Mona Dollins, Virginia Tech
System Effects on Water Quality 36
Based on Research by Qingfu Xiao, University of California
at Davis
Helpful Resources 39
Appendices 43
CU-Soil Specification and Mixing Procedure 44
Carolina Stalite Structural Soil Specification 51
Carolina Stalite Mixing Specification 54
Figures
Figure 1. Typical runoff from a parking lot going into a
storm sewer. 1
Figure 2. This system both serves as a parking lot and as a
stormwater management facility. 2
Figure 3. An example of a retention/detention pond
adjacent to a conference center on the Virginia Tech
campus in Blacksburg, Virginia. 5
Figure 4. This photograph shows the effect of soil volume
on tree growth. 7
Figure 5. Compacted soil from a typical construction site.
Lack of structure prohibits root penetration and growth. 8
Figure 6. CU-Soil, the structural soil developed at Cornell
University in the 1990s. 8
Figure 7. Conceptual diagram of structural soil including
stone-on-stone compaction and soil in interstitial spaces. 10
Figure 8. The top illustration shows a diversion mound
system as used on a roadway. The photo to the left shows
the installation of diversion mounds and the right photo
is the same divestion mounds with structural soil being
installed. 16
Figure 9. Enlarged view of woven and nonwoven
geotextiles. 19
Figure 10. Visual comparison of a healthy pin oak leaf
(left) and a chlorotic leaf (right). 20
Figure 11. Davis Soil, a non-loadbearing soil (i.e. not
a structural soil) with high infiltration rate and high
potential for water storage. 21
Figure 12. Area of park used for a weekly farmers market
in Chicago. 25
Figure 13. Photo simulation of turf-covered perimeter
parking at a big box lot in Ithaca, NY. 25
Figure 14. Aerial view of structural soil and turf
experimental plots at Cornell University in Ithaca, NY. 26
Figure 15. Construction detail for turfgrass and structural
soil profile. 27
Figure 16. In winter when the sod is dormant, the median
serves as additional storage and display space for the
dealership inventory. 29
Figure 17. The left figure shows rain on a traditional
asphalt parking lot. The right figure shows rain on a
porous asphalt parking lot. 30
Figure 18. A comparison of traditional asphalt (left) and
porous asphalt (right) when wet. 31
Figure 19. Ash roots penetrating geotextile after
compacted subsoil has been washed away. 33
Tables
Table 1 . Comparison of physical properties of CU-Soil,
Carolina Stalite and a silt-loam soil. 9
Table 2. Reservoir depths and the corresponding levels of
mitigated rain events based on the 30% void space within
the structural soil mix (assuming an empty reservoir). 14
Table 3. Pollutant removal of single storm event. 37
Table 4. Pollutant removal of multiple storm events. 37
Introduction 1
Introduction
U
and decreases vegetative cover. These disruptions increase stormwater
runoff at the expense of groundwater recharge, degrading water quality
and impairing aquatic habitats. The repercussions of this non point source
pollution are being felt worldwide. Creative Best Management Practices
(BMPs) that harness the ability of vegetation and soils to mitigate urban runoff
are needed. This material is a culmination of four years of research at Virginia
Tech, Cornell University and the University of California at Davis investigating
how a novel stormwater BMP that relies on shade trees and structural soils
can be designed and how it will function. We do not have the answer to every
question but the approach presented here works and is in place now at our
demonstration sites around the country. We developed this guide to assist
others in implementing this BMP. We hope it will expand your toolbox and
create new approaches for harnessing the power of trees in urban settings.
Challenges for Stormwater Management in Urban Areas
Urban areas are challenged by extensive impervious surfaces, damaged soils,
and little room for greenspace or for stormwater management facilities. The
remove pollutants. The system described in this manual addresses all three of
these goals by utilizing trees and structural soils to aid in water interception,
Figure 1. Typical runo from a parking lot going into a storm sewer. Noce
that traces of oil are visible to the naked eye. There are many other
pollutants in parking lot runo such as various metals, sediment, salts, and
lier.
Photo by Susan Day.
2Day, S.D. and S.B. Dickinson (eds.) 2008. Managing Stormwater for Urban
Sustainability Using Trees and Structural Soils.
Figure 2. This system both serves as a parking lot and as a stormwater management
facility. In addion to this double use of space, the structural soils also provide vastly
greater soil volumes for tree root growth than tradional parking lot construcon.
Note: Gravel base course is oponal, since the structural soil is designed to be as strong
as a base.
Figure by Sarah Dickinson.
Introduction 3
How Does This System Work?
The system guides water to a structural soil retention area beneath the
pavement where it is then temporarily stored. Water leaves the reservoir via
creates a large rooting volume, trees have the potential to develop full
canopies, allowing increased interception of precipitation. Tree roots take up
into the subsoil. Together, trees and structural soils can create a zero runoff
installations of this system. This is attributed to the distributed nature of the
system: because the reservoir is beneath the pavement, there is a one-to-one
ratio of land area receiving rainfall and land area treating stormwater.
Before deciding on any BMP, site constraints should be evaluated. This system is
designed to be installed beneath pavement and therefore stormwater management
is distributed throughout the site and not conned to unpaved porons of the site.
The system has not been evaluated for treang large amounts of collected runo from
adjacent areas. Inltraon BMP’s are not appropriate for sites that need to handle
highly polluted or contaminated water due to risk of groundwater contaminaon.
There are also some topographical and geological features that could limit the use of an
inltraon BMP (see the limitaons secon in Chapter 2).
Distributed Stormwater Management in Urban Settings
Distributed stormwater management techniques, such as bioswales, are
used to retain stormwater at many sites throughout the urban landscape
as opposed to collecting runoff at a more centralized facility, such as a
detention pond, or relying on a storm sewer system. But some sites do not
impervious surfaces in a dispersed fashion. In addition, sites that are largely
The system described in this manual can make it possible to use distributed
stormwater management that takes advantage of the stormwater mitigation
for stormwater management and vegetation are very limited. This system may
system also provides an alternative to detention ponds where lack of space is
not yet the primary concern.
4Day, S.D. and S.B. Dickinson (eds.) 2008. Managing Stormwater for Urban
Sustainability Using Trees and Structural Soils.
Project Background and Resources
This manual is the result of a series of research studies carried out at Virginia
Tech, Cornell University, and the University of California at Davis. This
research evaluated multiple aspects of the novel stormwater BMP described
here. Work at Virginia Tech focused on tree health and root development
in the system, as well as the ability of tree roots to enhance subsurface
physical characteristics of the structural soil mixes as they pertain to storing
stormwater, and the feasibility of a wide variety of surface treatments—
everything from porous asphalt to turf. Research at Davis in the Department
of Land and Water Resources produced baseline evaluations of the ability of
several structural soil mixes to remove typical urban runoff contaminants.
Each university partnered with private groups or municipalities and installed
one or more demonstration sites to evaluate the system as a whole. Overall,
the system presented here has been successful. We have prepared this manual
to help stormwater engineers, public works departments, and others to put
this new approach—or elements of it—into practice.
How this Manual is Organized
The manual is designed to guide you through the features of the system,
including its limitations, and how to design a system to suit the site’s needs.
Original research papers are referenced and are available from university
libraries or by contacting the authors. Brief summaries of this research appear
in the manual.
Chapter 1 introduces the stormwater management system, its attributes and
limitations.
Chapter 2 provides information on designing a system with structural soils
and trees based on the needs of individual sites.
Chapter 3 describes surface treatments that can be used in conjunction with
this stormwater management BMP, namely turf and porous pavement. All the
information in this section is based on a series of publications from Cornell
University’s Urban Horticulture Institute.
Chapter 4 summarizes several original research projects related to the
development and evaluation of this system which were conducted by the
contributors of this manual. The research in this section was made possible in
part through a grant from the United States Department of Agriculture Forest
Service Urban & Community Forestry Grants Program on the recommendation
of the National Urban & Community Forestry Advisory Council (NUCFAC).
Chapter 1— Trees and Structural Soils- A System Overview 5
Chapter 1— Trees and Structural
Soils- A System Overview
Stormwater management in urbanized settings faces special challenges:
paved surfaces and buildings generate high amounts of runoff while at
the same time leaving little space for constructed stormwater management
facilities or for the soil and vegetation combination that could reduce the need
for these facilities.
The system described in this manual seeks to address these limitations by
using structural soils to simultaneously allow healthy tree growth, water
the structural soil that supports them combine to form a shallow but extensive
reservoir for capturing and storing stormwater. Structural soils are engineered
soil mixes with a high porosity that allow tree roots to penetrate freely,
into the soil beneath. Tree canopies effectively intercept rainfall, reducing
throughfall to the ground and lengthening the time of runoff concentration
into stormwater systems. Trees also actively transpire, taking up water and
pollutants and contaminants can be removed from the stormwater via
This double use of land surface area (e.g. parking lot and stormwater
a large area, which more closely mimics natural hydrology than stormwater
Figure 3. An example of a retenon/detenon pond
adjacent to a conference center on the Virginia Tech
campus in Blacksburg, Virginia. This treatment uses
space that could be otherwise directed towards other
uses.
Photo by Susan Day.
6Day, S.D. and S.B. Dickinson (eds.) 2008. Managing Stormwater for Urban
Sustainability Using Trees and Structural Soils.
canopy increases opportunities for returning rainfall to the atmosphere
via evapotranspiration and through canopy interception and storage of
components of this system, namely trees and structural soils.
Addional benets of trees
• Shading, reducing
ambient temperature
• Removing pollutants
from the air
• Improve aesthecs
Seehp://www.fs.fed.
us/psw/programs/cufr/
formoreinformaon
Trees— Mimicking the Hydrologic
Benets of a Forest in the City
Natural forests with their complete canopy
cover, large leaf areas, and permeable
soils handle rainwater effectively through
water to groundwater and the atmosphere
and protecting water quality in surface
waterways. Replicating elements of this
hydrologic cycle in urban settings, however, is
people, and other urban denizens compete
for land and soil resources.
Urban forests are also widely recognized as
an effective means of handling stormwater.
Like their forestland counterparts, urban
trees intercept rainfall, direct precipitation
take up stormwater through their roots.
In addition, urban tree roots penetrating
through typically impermeable urban soil
layers into more permeable zones have the
rates. However, urban canopy cover (and thus
rain interception) is greatly limited by urban
soil conditions such as compaction, reduced
rooting volume, and elevated pH. Even open
ground in urbanized areas is commonly
disturbed or compacted, limiting normal soil
hydrologic functions. This system directly
addresses the limitations of urban soils to
support vegetation and handle water. The
system provides a highly permeable rooting
environment that can support large trees,
the city.
Chapter 1— Trees and Structural Soils- A System Overview 7
Structural Soils— Supporting Tree Growth and Pavement
Why were structural soils
designed?
Typically, soils beneath
pavement are compacted to
meet engineering requirements
to support the loads from
vehicles, pavement and
structures. Unfortunately,
most plant life cannot survive
in soils compacted for these
purposes. Roots cannot
penetrate extremely strong
soils. In addition, compacting
soil destroys soil structure,
collapsing the large pore spaces
needed to provide the balance of
air and water that roots require.
The result is soil that can support
pavement but cannot support
trees. Structural soils were
designed to meet requirements
for pavement support while
still allowing adequate pore
space to support tree roots.
Structural soils must be carefully
constructed and tested according
to meet these requirements.
Figure 4. This photograph shows
the eect of soil volume on tree
growth. Both rows of willow oaks
were planted at the same me on
Pennsylvania Avenue, Washington,
D.C. The trees on the le are in tree
pits, and those on the right are in an
open grassed area.
Photo by Nina Bassuk.
A good structural soil will have known
water-holding, drainage, structural
and load-bearing characteriscs. It
should be able to be compacted to
95% of standard Proctor density and
sll support plant growth. It will also
have a research-based track record of
success and body of best pracces.
Just any mix of a stone and soil is
not a structural soil. Some so-called
structural soils have failed miserably
when praconers thought they were
purchasing a good soil but were just
purchasing an untested mix with no
research vericaon. The two discussed
here have been thoroughly tested yet
each product should sll be required
to undergo tesng aer installaon to
ensure that the nal product meets
the standards of the specicaon. In
the case of CU-Structural Soil it must
be purchased from licensed producers
who are required to test their materials
to adhere to a research-based
specicaon.
8Day, S.D. and S.B. Dickinson (eds.) 2008. Managing Stormwater for Urban
Sustainability Using Trees and Structural Soils.
How do structural soils work ?
Structural soils are engineered to meet
compaction requirements for parking
lots, roads and other paved surfaces
and, at the same time, allow tree root
penetration under the pavement. Excavated
root systems from structural soils have
illustrated that deep rooting of trees in
these soils appears to prevent heaving of
sidewalks, curbs and gutters by tree roots.
Structural soil can therefore expand the
soil volume available for the roots of trees
in plazas and parking lots and other paved
areas.
There are many types of
structural soils, but they are
based on the same principal:
large “structural” particles,
typically an angular stone,
form a matrix that distributes
the load from pavement and
structures through stone-
to-stone contact ultimately
spreading the load across
the supporting subsoil. The
gaps between the structural
high quality mineral soil with
good water-holding capacity
and tilth. Hydrogel is often
used in addition to the mineral
segregation of the soil during
mixing and installation. When
structural soils are compacted, they form a rigid matrix while suspending
soil as a rooting medium within the interconnected voids of the stone matrix.
Roots are able to easily penetrate this uncompacted mineral soil within the
compacted stone matrix. As roots expand in the structural soil, they appear
to encapsulate, rather than displace the stone matrix or deform temporarily
to move between the smallest pores. Because stone is the load-bearing
component of the structural soil, the aggregates used should meet regional or
state department of transportation standards for pavement base courses.
-Adapted from Bassuk, et al. 2005
Figure 5. Compacted soil from
a typical construcon site.
Lack of structure prohibits root
penetraon and growth.
PhotobyJohnW.Layman.
Figure 6. CU-Soil, the structural soil developed
at Cornell University in the 1990s. Soil parcles
within the media are clearly visible and allow soil
nutrients and water holding capacity for healthy
root growth.
PhotobyTedHaner.
Chapter 1— Trees and Structural Soils- A System Overview 9
Table 1 . Comparison of physical properes of CU-Soil, Carolina Stalite and a silt-loam
soil. Note: The Stalite specicaons usually call for sandy loam but plant available
moisture with Stalite was tested using the same intersal silty clay loam as was used
with the CU-Soil.
TablebasedoninformaonfromHaner,E.C.2008.
The history of structural soil
This manual examines stormwater management techniques that detain
soils, CU-Soil (Amereq Inc., New York, NY) was developed at Cornell University
for tree root development. This new type of soil mix resulted from research
exploring a means to create a substrate that would both allow adequate tree
root growth and support pavement for sidewalks, streets, and parking lots. It
from other types of tree soils. Since then, other structural soils have been
developed that use other components (e.g. Carolina Stalite, a heat expanded
shale (Carolina Stalite Company, Salisbury, NC). The structural component of
Carolina Stalite is porous and lightweight in comparison to the gravel used
required to prevent segregation during mixing.
10 Day, S.D. and S.B. Dickinson (eds.) 2008. Managing Stormwater for Urban
Sustainability Using Trees and Structural Soils.
Figure 7. Conceptual diagram of structural soil including stone-on-stone compacon
and soil in intersal spaces.
FigurebySarahDickinson,adaptedfromNinaBassuk.
Contact points where
load is transferred
Soil aggregate
Stone particle
Air or water pore spaces
around the soil aggregates
Compactive force
Subsoils
structural soil reservoir will be constructed. For optimum functioning of the
system, including healthy root development, the stormwater reservoir should
drain within two days. If the subsoil is permeable, or has some permeable
through structural soils is extremely rapid. If soils are impermeable but have
permeable layers beneath them, root penetration into the subsoil base may
page 15). Although a separation geotextile is not normally required below
structural soil sections, when the structural soil is being used as a reservoir
for stormwater, subsoil may be saturated at times, resulting in lower soil
strength. Therefore, a geotechnical engineer should always be consulted to
determine if a separation geotextile is advisable between the subsoil and
structural soil components (see Geotextiles section).
Chapter 1— Trees and Structural Soils- A System Overview 11
Limitations concerning subsoil infiltration
Some conditions only require minor adjustments to the design to use
Some of these limitations include:
to the risk of groundwater contamination. Always refer to local
regulations.
feet from the surface, limited drainage, and extreme slopes are not
the ground water without any contaminants or pollutants being
regions of the country. Carolina Stalite is produced in the eastern
United States and high transportation costs make its use in western
states impractical.
Citations
Bassuk, N.L., J. Grabosky, and P. Trowbridge. 2005. Using CU-Structural Soil
in the Urban Environment. Urban Horticulture Institute, Cornell University,
Ithaca, NY.
Haffner, E.C. 2008. Porous asphalt and turf: exploring new applications
through hydrological characterization of CU Structural Soil® and Carolina
Stalite Structural Soil. Master’s Thesis. Department of Horticulture, Cornell
University.
12 Day, S.D. and S.B. Dickinson (eds.) 2008. Managing Stormwater for Urban
Sustainability Using Trees and Structural Soils.
Chapter 2— System Design to Meet Site Requirements 13
Chapter 2— System Design to
Meet Site Requirements
Specications
Surface Treatments
The intent of this BMP is to manage stormwater from the immediate
vicinity— it is not meant to handle large amounts of stormwater concentrated
from surrounding land areas. Regardless, the system requires that water be
directed into a structural soil reservoir beneath the soil surface. There are two
options for this that can be used alone or in combination:
• Local rainfall data and runo calculaons will determine the minimum depth
for the structural soil reservoir. The reservoir can be designed to store the
desired rain event (e.g. a 25-year storm).
• For opmal growth of trees, designs must provide adequate depth and
extent of structural soil (see Reservoir Sizing).
• Determine the type of soil and the seasonal water table levels underneath
the reservoir. Clay soils will drain much more slowly than sandy soils and will
inuence how much water the reservoir can take and will also determine
inltraon and groundwater recharge rates from the reservoir into the
subsoil below the reservoir.
• Inltrometer measurements may not accurately reect drainage rates of the
reservoir as a whole. This is because water moves laterally very quickly in
structural soils and zones of rapid inltraon can have a disproporonately
large eect.
Sustainable site design requires coordination and consultation with diverse
professions. For instance, a geotechnical engineer can determine if this
topography.
A stormwater engineer may determine the quantity of water that the system
will need to be able to handle. In addition to water quantity, they should be
familiar with the contaminants and pollutants that will be present in the
stormwater and local regulation and permit requirements.
consulted during the design process for choosing tree species and other
plantings that will perform well for a given system design and climate.
14 Day, S.D. and S.B. Dickinson (eds.) 2008. Managing Stormwater for Urban
Sustainability Using Trees and Structural Soils.
Option 1: Pervious Pavement
through the wearing surface and into the structural soil reservoir below.
rates. There are many types of pervious pavement and the choices continue
to expand. For more information on alternatives to traditional impervious
pavement, see Chapter 3.
Option 2: Traditional, Impervious Pavement
Water can easily be directed beneath traditional pavement as well. Structural
soils allow rapid lateral water movement, so water entering at one point in a
structural soil system will seek its own level, spreading out in the reservoir
in accordance with the subsoil topography. Gravel swales on the edges of
impervious areas allow water to enter the system. This design also can be
used as a “backup” system for pervious pavement if there are concerns of
clogging.
Reservoir Sizing and
Overflow Pipe Design
In order to properly
mitigate any storm,
exact rainfall data must
be obtained from local
meteorological stations.
To help design the proper
reservoir depth to
accommodate any rain
event, the adjacent table
(Table 2) can be used
as a general aid. This
information is based on a
conservative estimation
of the total porosity
of any structural soil
of 30%. If actual total
porosity is calculated
for your particular structural soil mix, the chart can be adjusted accordingly.
It is important to note that while depths less than 24” will both support
and mitigate a storm event up to 5.4” in 24 hours, for larger tree species, a
reservoir depth of 24” to 36” is optimum.
Table 2. Reservoir depths and the corresponding levels
of migated rain events based on the 30% void space
within the structural soil mix (assuming an empty
reservoir). Numbers in the gray box illustrates the
depths necessary to accommodate opmum healthy
tree root development.
TablebyTedHaner.
Chapter 2— System Design to Meet Site Requirements 15
Although a structural soil
reservoir is a great way
to collect rainwater and
runoff as regulated by the
National Pollution Discharge
Elimination System (NPDES)
guidelines and decrease
demands on existing
municipal storm water
systems, there may be rain
events that generate more
runoff than the reservoir
below can handle. Installing
design stormwater retention
level of the reservoir can
prevent system failure during
extreme weather events.
Placement of the overflow pipe should be determined based on the
to remove water from the rooting zone (the top 18 to 24 inches of structural
soil) within 48 hours, the depth of the structural soil reservoir should be
level of the rooting zone it will be removed by the pipe.
Two systems combined insure against
system failure.
1. The structural soil reservoirs at a
predetermined depth allow water storage
and inltraon to recharge groundwater, if
soil condions below the reservoir permit.
2. Tradional piping infrastructure located
at a level high enough that water will not
backup under the pavement if the reservoir
is overlled by mulple storm events. The
combinaon of the two ensures the system
will work during storm events that are larger
than the design capacity of the system.
Helpful Hints
• Design to capture all the runo from the desired storm event. The system
can easily be designed to capture all of the runo from a 100— year storm in
most cases. At a minimum, design the reservoir to handle the “water quality
storm” for your region. This is the threshold which encompasses 90% of the
yearly runo producon.
• Inltraon expectaons: water should not stay in the upper 18 to 24 inches
of the reservoir for more than 48 hours. Longer residencies in the tree roong
zone may interfere with tree establishment, growth, health, and stability of the
roong system.
16 Day, S.D. and S.B. Dickinson (eds.) 2008. Managing Stormwater for Urban
Sustainability Using Trees and Structural Soils.
Figure 8. The top illustraon shows a diversion mound system as used on a
roadway. The photo to the le shows the installaon of diversion mounds for an
access road. The diversion mounds are circled in red. In the photo to the right you
can see the mounds during the structural soil installaon process.
Figure by Joe Dove. Photos by Susan Day.
Use addional drainage as necessary to decrease ooding and inundaon from
extreme storm events. Although structural soil is highly porous, ooding will
occur if the rate of water leaving via inltraon is slower than the rate that
water enters the system via rain and runo (see Reservoir Sizing above).
CU-Soil specicaons require that the mineral soil component of the mix be
heavy clay loam or loam with a minimum of 20% clay, because of its greater
water- and nutrient-holding capacity. Carolina Stalite structural soil mixes
specify a sandy loam since the porous structural parcles also hold water,
but soils with a ner texture (i.e. more clay) can also be used. Structural soil
should also have organic maer content ranging from 2-5% to ensure nutrient
and water holding while encouraging benecial microbial acvity.
Chapter 2— System Design to Meet Site Requirements 17
Level and Unlevel Sites
Does the reservoir need to be level? A level or nearly level reservoir will
can be designed in two ways. First the subsoil can be excavated in a series of
terraces. This is appropriate for a slightly sloped parking area, for example.
Alternately, diversion mounds (Figure 8) can be used to direct water under
pavement on a slope. This technique was employed at an access road
installation in Blacksburg, Virginia. Runoff collected in roadside swales and
was then directed under the road pavement with diversion mounds that
intersected the swales. In such cases, hydrostatic buildup under the pavement
must be prevented by appropriate drainage. Because the reservoir will allow
be minimal.
A good, well drained topsoil may be used around the newly installed tree if the
pavement opening allows. If this is not practical, structural soil can be used
right up to the tree root ball. In drier climates, establishing some tree species
directly in structural soil may require frequent irrigation because of the high
porosity of the soil. Tree roots need to establish good root-soil contact before
sensitive to drought during establishment (e.g. swamp white oak (Quercus
bicolor
after planting. Because structural soil gives tree roots a larger volume of soil,
irrigation may not be necessary after establishment. Again, this is climate
dependent and the expertise of a plant professional with local knowledge
should be sought.
While structural soils may have less total moisture on a per volume basis than
in conventional soil (around 16% versus a normal 25% in a agricultural soil),
the plant available moisture within the structural soil matrix is actually quite
comparable to a normal landscape soil (in the range of 8-11% by volume).
Traditional planting designs in paved areas surround the planting hole with
materials which restrict root penetration and growth. Because the use of
structural soils expands total rooting volume, trees have access to greater
water resources and can usually be managed very similarly to trees planted in
landscape soils. Similar to trees in the landscape, supplemental water should
be provided until the tree is established and then irrigation practices should
follow local climatic requirements.
18 Day, S.D. and S.B. Dickinson (eds.) 2008. Managing Stormwater for Urban
Sustainability Using Trees and Structural Soils.
Geotextiles
By Joseph E. Dove
Geotextiles are part of the broad class of materials called Geosynthetics, which
are synthetic polymer materials that are used in a wide range of geotechnical
engineering applications such as reinforcement, erosion control, separation,
geosynthetics are available from the International Geosynthetics Society
(http://www.geosyntheticssociety.org/guideance.htm).
the general appearance of a cloth fabric. They are typically manufactured
from polypropylene or polyester and are categorized as either woven or
nonwoven. Woven geotextiles are produced by interweaving two orthogonal
sets of yarns. They typically have high tensile strength and resistance to
elongation. Non-woven geotextiles are manufactured by extruding individual
are then interlocked through needle punching or heat bonding processes.
Needlepunched geotextiles typically have high permeability; whereas heat
bonded non-woven geotextiles have higher tensile strength characteristics.
In the structural soil system, possible locations for a geotextile include
(Figure 8): 1) between the top of the natural (subgrade) soil and the base of
the structural soil, and/or 2) below the aggregate base soil supporting the
pavement or other surface treatment and the top of the structural soil. In
separation functions. However in the second case, the geotextile provides a
separation function only. The reinforcing function arises when the subgrade
resulting in rutting at the ground surface. This function typically requires
geotextiles with high tensile strength. A civil engineer can determine if
a reinforcing geotextile is required and recommend tensile strengths for
selecting candidate materials, if needed. The separation function in the
second case arises to prevent the aggregate base from commingling with
the structural soil below. This downward migration can result in decreased
pavement performance and a separation geotextile may be warranted as a
mitigation measure. A check can be made to assess if the aggregate base soil
soil to fall into the voids between the underlying structural soil particles.
Fortunately, migration of aggregate base soil has not proved to be a problem in
other installations. Geotextiles are not be required if the above consequences
Chapter 2— System Design to Meet Site Requirements 19
Figure 9. Enlarged view of woven and nonwoven geotexles.
Photosfrom“IGSGeosynthecsinDrainageandFiltraonbyJ.P.
GourcandE.M.Palmeira.”
Selection of a geotextile is made after the required material properties are
estimated from design computations performed by a civil engineer (for
geotextile for this application is the reduction in mechanical performance
Applications” provides guidance geotextile selection. This standard is
intended for geotextiles used in subsurface drainage, separation, stabilization
each function. Selection of the minimum geotextile material properties
selection of locally available candidate geotextile products with the required
engineering properties is made from information published by manufacturers.
Most manufacturers of geotextiles provide the M288-00 survivability class for
each of their products.
It has been found that the woven geotextiles tested in the structural soil
system do not prevent tree root penetration, a summary of this research is in
Citation
Upper Saddle River, NJ.
20 Day, S.D. and S.B. Dickinson (eds.) 2008. Managing Stormwater for Urban
Sustainability Using Trees and Structural Soils.
Trees and Other Plants
Trees are an integral component of this stormwater system and must grow
well in order to realize maximum stormwater mitigation. By enlarging the
rooting volume typically available to trees in paved areas, canopy size has
the potential to increase faster and trees may ultimately reach a greater size.
Rainfall interception, storage, and ultimately evapotranspiration from leaf
surfaces, are directly related to canopy size. In addition, rainfall captured by
tree canopies is often directed down limbs and trunks into the soil at the base
of the tree—effectively bypassing the pavement.
Trees are living organisms and have certain requirements in order to grow
stormwater system. However, tree selection should never be undertaken
arborist, or related professional). Pest resistance, urban forest diversity,
regional climate factors, growth form, invasive potential and numerous other
Soil Chemistry
Structural soils can have very different pHs than local mineral soils. Structural
soils with a limestone base will typically have high pH. A structural soil with a
granite base may have lower pH. The soil pH determines nutrient availability
among other things. A pH of 7 is neutral, with lower pH being acid and a
higher pH, basic or alkaline. The ideal pH for most trees is about 5 to 6.5, but
urban soils are typically very basic (pH 7.5 to 8.5) because of disturbance,
including concrete and limestone debris mixed into the soil. A typical
yellowing, of the leaves (Figure 10). If the structural soil used in the system
has a high pH, then a “pH tolerant” tree species should be used. These include
many elms and ashes and certain maples and oaks as well as a variety of other
species (see the tree guide sources at the end of this chapter). The key is to
test the structural soil pH and select trees that tolerate it.
Figure 10. Visual comparison of a healthy
pin oak leaf (le) and a chloroc leaf
(right). This chlorosis ulmately interferes
with carbohydrate producon in the plant
and is a result of nutrient deciencies
stemming from elevated soil pH.
Photo by Susan Day.
Chapter 2— System Design to Meet Site Requirements 21
Soil Volume
Trees need enough room to grow—for their roots as well as their canopy.
Tree pits (a.k.a. cutouts, planters) should be as large as possible—but how
large is that? The key to designing sites that support large trees is to have
essentially unlimited rooting space. A typical 4 × 4 ft. cutout with no access
to surrounding soil limits tree growth almost immediately. A 25× 25 ft. cutout
limits growth very little until the tree is quite large. The usable rooting space
provided by any cutout can be expanded by a continuous structural soil bed
under pavement. Some species are more adept at exploiting weakness in
pavement, penetrating compacted soils, or reaching nearby open spaces.
However, the system should be designed to support the tree fully without
infrastructure damage. Structural soils have been shown to support
should supply rooting space without compromising structural integrity.
Again, species selection and site conditions must be compatible so a plant
professional should be consulted. Always consider local regulations and
permitting requirements.
Innovave Soluon: High Shipping Costs of Structural Soils for Western
States
High shipping costs can make using
Carolina Stalite, produced in North
Carolina, prohibively expensive
in Western states. The University
of California at Davis designed an
engineered soil from local, inexpensive
volcanic rock and gave it the name
of Davis Soil. This soil has been
successfully used to increase drainage
in open areas adjacent to parking lots
and in certain turf applicaons. Davis
Soil is not considered a structural
soil because it cannot support
the weight of pavement, cars and
other structures. It can maintain
perviousness under foot trac and
supports healthy tree growth. It is very porous (40 % porosity), and so it is
able to store stormwater which can be then be used by trees. In addion,
its large surface area with many nooks and crannies act to trap common
stormwater pollutants. Contact Qingfu Xiao at qxiao@ucdavis.edu for
more informaon on obtaining Davis Soil.
Figure 11. Davis Soil, a non-
loadbearing soil (i.e. not a
structural soil) with high
inltraon rate and high potenal
for water storage.
PhotobyQingfuXiao.
22 Day, S.D. and S.B. Dickinson (eds.) 2008. Managing Stormwater for Urban
Sustainability Using Trees and Structural Soils.
(see Chapter 4, Tree Development in Structural Soils at Different Drainage
Rates), root systems developed best when water was retained in the rooting
white oak (Quercus bicolor) or American elm (Ulmus americana) can survive
turf or groundcovers can be used if climate permits. See Chapter 3 for more
information on surface treatments.
Although high water tables may limit tree rooting depth, when species
selection and site design allow trees to root into lower soil regions and
penetrate through impervious zones, they may be an effective tool to increase
in highly restrictive soils. To ease establishment, trees should ideally be
established in mineral topsoil, with the structural soil components being
reserved for under the pavement. However, establishing trees directly in
structural soil can simplify installation. If trees will be irrigated regularly
during establishment and climatic conditions are appropriate, this approach
can be used.
Tree root systems are wide spreading. For maximum tree growth, provide
rooting area about twice the diameter of the ultimate canopy for which you
are designing.
Chapter 2— System Design to Meet Site Requirements 23
General tree guide sources:
Dirr, Michael. Woody Landscape Plants.
PLANTS Database, hp://www.plants.usda.gov/
Northern Trees, hp://orb.at.u.edu/TREES/index.html
Tree guide sources for the Eastern United States:
Appleton, B. 2001. New York / Mid Atlanc Gardener’s Book of Lists. Taylor
Publishing Company, Dallas.
Bassuk, N.L. Cornell Department of Horculture Woody Plant Database, hp://
hosts.cce.cornell.edu/woody_plants/
Bassuk, N.L., J. Grabosky, and P. Trowbridge, 2005. Using CU-Structural Soil in
the Urban Environment, hp://www.hort.cornell.edu/uhi/outreach/csc/index.
html
Day, S.D. Virginia Urban Tree Selector, hp://www.cnr.vt.edu/dendro/
treeselector/
Trowbridge, P.J. and N.L. Bassuk. 2004. Trees in the Urban Landscape: Site
Assessment, Design, and Installaon. Wiley and Sons, New York.
Tree guide sources for the Western United States:
McPherson, E.G., J.R. Simpson, P.J. Peper, Q. Xiao, D.R. Pienger and D.R.
Hodel. 2001. Tree Guidelines for Inland Empire Communies. Sacramento, CA:
Local Government Commission
McPherson, E.G., J.R. Simpson, P.J. Peper, K.I. Sco and Q. Xiao. 2000. Tree
Guidelines for Coastal Southern California Communies. Sacramento, CA: Local
Government Commission
McPherson, E.G., J.R. Simpson, P.J. Peper and Q. Xiao. 1999. Tree Guidelines
for San Joaquin Valley Communies. Sacramento, CA: Local Government
Commission
24 Day, S.D. and S.B. Dickinson (eds.) 2008. Managing Stormwater for Urban
Sustainability Using Trees and Structural Soils.
Chapter 3— Surface Treatments 25
Special Concerns
Soil Migraon
The excavaon of a seven-year-old tradional installaon of a London plane
(Platanus x acerifolia) tree in CU-Soil with a pervious surface did not show
any aggregate migraon. The pores between stones in the structural soils are
mostly lled with soil so there are few empty spaces for soil to migrate to.
Frost Heave
By design, structural soils are gap-graded to provide rapid drainage, and limits
the silt fracon to be consistent with very low frost heave suscepbility as
dened by the US Corp of Engineers Cold Weather Research Laboratories.
However, two important issues are related to this queson. First, if the design
system is installed as a trench under the pavement, there needs to be an
awareness of the depths of layers in each pavement layer prole, and their
dierent frost heave potenals. The designer needs to be sure there is not a
major dierence in frost heave potenal at the interface of the two systems
or else the pavement surface will move and crack as the total layered systems
will behave dierently. Secondarily, frost concerns also suggest snow removal
concerns, so the placement of trees in the system and the needs of snow
removal and storage on site need to be addressed with the maintenance
authority to prevent the loss of the trees or damage to the system.
Observaon of structural soil throughout the US and Canada shows that the
depth of the reservoir negates any heaving due to consequent freezing and
thawing. Addionally, there have been no observed instances of freeze/thaw
damage in any structural soil installaons in the een plus years since its
incepon.
Chapter 3— Surface Treatments 25
Chapter 3— Surface Treatments
This section describes two surface treatments that can be used with this
system: turf and porous pavement. The sections in this chapter are
summaries from manuals published by the Urban Horticulture Institute
(Cornell University). A citation to the complete manual is provided at the end
of each section.
Structural Soils and Turf
By Nina Bassuk, Ted Haffner,
Jason Grabosky, and Peter
Trowbridge
Introduction
Turf is primarily used as a
ground cover in residential
lawns, parks, playgrounds and
providing a sense of open space
and as a protective surface for
recreation. If turf is properly
installed, it can have additional
lanes, and parking lots. In these
instances, turf can contribute to
a sense of open green space and
reduce temperatures in urban
settings that may otherwise be
paved.
When turf is used for these
applications, however, it is
will compact the soil. These
situations also limit drainage,
healthy root growth, and the
ability of turf to grow at all.
Cornell Developments in Turf
Use
Cornell University has
combined turf with structural
soil to create a healthy growing
medium for the grass that
Figure 12. Area of park used for a weekly farmers
market in Chicago. Compacon from foot and
vehicle trac has denuded the grass in this
secon of the park.
PhotobyTedHaner.
Figure 13. Photo simulaon of turf-covered
perimeter parking at a big box lot in Ithaca, NY.
For best results, turf should be only placed in
parking stalls and not in driving lanes of the
parking lot.
PhotosimulaonbyTedHaner.
Chapter 3— Surface Treatments 2626 Day, S.D. and S.B. Dickinson (eds.) 2008. Managing Stormwater for Urban
Sustainability Using Trees and Structural Soils.
Chapter 3— Surface Treatments 27
to be virtually maintenance free, and can be used in areas that receive high
both people, cars and temporary structures to safely use a turf covered surface
the turf surface and hold it in a reservoir underneath the grass. Increased
water and air within the structural soil media not only allows for healthier
root and shoot growth for the grass, but also allows rainwater and runoff to
be collected and held within the reservoir in large amounts until it can slowly
system infrastructure and also recharges the groundwater levels over time.
This combination, then, not only serves the environment from a water quality
standpoint, but also adds a “sustainably green” component to highly urbanized
areas.
Figure 14. Aerial view of structural soil and turf experimental plots at Cornell University
in Ithaca, NY. Surface Treatments: PA= Porous Asphalt, Z= Zoysia Grass, F= Tall Fescus,
C= Tradional Asphalt.
GraphicsbyTedHaner.UnderlyingphotobyGoogleEarth.
Chapter 3— Surface Treatments 26
Chapter 3— Surface Treatments 27
Figure 15. Construcon detail for turfgrass
and structural soil prole. Note that the 24”
reservoir depth was based on local rainfall data
and will vary by region according to the local
rainfall data and/or ancipated runo amounts.
FigurebyTedHaner.
• Minimize vehicular wear on the turf as much as possible. To do this, place
turf only in parking stalls and not the driving lanes of the lot.
• Angle parking stalls to minimize turning from automobile wheels. Excessive
turning causes the turf grass leaf blades to tear and can create bare patches in
the turf. Research indicates that turf can recover from this damage but it takes
extra me.
• Use turf only in overow parking areas on the outskirts of large parking lots.
• Use inset stonework between stalls, or posts to demark parking stalls. This
design maneuver may cost more upfront to install, but will save me and
money during post-installaon maintenance.
• Specify proper post-installaon maintenance regimes. Mowing every 10
days is necessary, as is the applicaon of annual fall ferlizaon with proper
applicaon rates.
• Never snow plow the turf poron of the parking lot. The blades from the
plow will damage the turf surface, removing the turf and necessitang costly
replacement.
Chapter 3— Surface Treatments 2828 Day, S.D. and S.B. Dickinson (eds.) 2008. Managing Stormwater for Urban
Sustainability Using Trees and Structural Soils.
Chapter 3— Surface Treatments 29
Designing and Working with Turf and Structural Soil
achieve. With many different factors involved in the process, it is not as simple
as spreading seed or unfurling a roll of sod. Proper decision making at every
step of the planning, design, installation, and post-installation process are
absolutely necessary.
Working with turf and structural soil requires a change in the way that
designers and contractors go about their work. Rather than just installing
sod or seeding grass directly onto existing soil, entire areas will need to be
excavated to a depth of at least 18” to 24” (to accomodate stormwater- see
soil. Once the structural soil mix is in place it must be compacted with a
vibratory or rolling compactor. Once compacted, the sod should be installed
directly onto the structural soil and then irrigated for a number of weeks
until established. Once established, research indicates that maintenance
requirements are minimal, other than regular mowing and periodic
fertilization.
With the previous guidelines, a few simple construction details will provide
the bulk of information needed for bidding and installation of a construction
project. While a few simple drawings are helpful, keep in mind that every
design is different and will necessitate the level of detail appropriate for
each different design scenario. Additional details will be needed for, ADA
compliance curbing, tree planting and staking, hydrant water supply, signage
FAQs
What type of maintenance is needed for a turfgrass and structural soil system?
Our research was performed with the idea of the most basic maintenance
regime in mind. Test plots on the Cornell campus received no maintenance
other than roune mowing once every 7 to 10 days during the growing season.
Addional annual ferlizaon in the fall is recommended with the proper
applicaon rates.
What happens when neighboring tree roots expand in structural soil?
There will come a me when the roots will likely displace the stone because
there are no pavement layers above the structural soil, but if the roots are,
as we have observed, deep down in the prole, the pressure they generate
during expansion would be spread over a larger surface area. We have seen
roots move around the stone and actually surround some stones in older
installaons, rather than displace the stones.
Chapter 3— Surface Treatments 28
Chapter 3— Surface Treatments 29
Case Study
Turf on CU-Soil has been successfully used at a Mercedes dealership (Crown
Automobile) in Alabama. At this installaon, the soil in an enre median
was excavated and replaced with CU-Soil and then sod was placed on top.
The median can now properly withstand the compacon from the weight
of the cars and serves as a exible open space for the dealership, providing
impromptu space to display inventory, or as overow parking for the
dealership. Aer three years, this installaon is maintenance free and as
healthy as the day it was installed.
Figure 16. In winter
when the sod is
dormant, the median
serves as addional
storage and display
space for the
dealership inventory.
This exibility is
invaluable to the
dealership.
PhotobyBillIsaacs.
Citation:
Haffner, E.C. 2008. Porous asphalt and turf: exploring new applications
through hydrological characterization of CU Structural Soil and Carolina
Stalite Structural Soil. Master’s Thesis. Department of Horticulture, Cornell
University.
Chapter 3— Surface Treatments 3030 Day, S.D. and S.B. Dickinson (eds.) 2008. Managing Stormwater for Urban
Sustainability Using Trees and Structural Soils.
Chapter 3— Surface Treatments 31
Using Porous Pavement on Structural Soils
By Ted Haffner, Nina Bassuk, Jason Grabosky, and Peter Trowbridge
into the subgrade below, naturally recharging groundwater levels.
Porous asphalt is similar to traditional asphalt in every way but the mix
porous pavements should be limited to 1-6%.
Figure 17. The le gure shows rain on a tradional asphalt parking lot- aer it hits the
surface it typically runs o into a storm sewer system. The right gure shows rain on
a porous asphalt parking lot- aer it its the surface it inltrates through the pavement
into the structural soil reservoir below. Water then inltrates into the ground,
recharging the groundwater over me.
BothguresbyTedHaner.
Structural soil and porous asphalt are a new combination of 15- and 30-year-
in Ithaca, NY and was installed in 2005. Porous asphalt parking lots are
numerous and the oldest include the Walden Pond Reservation in Concord,
MA, the Morris Arboretum in Philadelphia, PA, as well as an ever expanding
list of corporations and universities across the United States. Structural
soil has been used extensively without porous asphalt pavement and the
(Gleditsia
triacanthos) planting at the Staten Island Esplanade Project in New York City,
the second is a London planetree (Platanus acerifolia) planting on Ho Plaza
on the Cornell campus, Ithaca, NY. There are now hundreds of installations of
various sizes across the United States and Canada.
Chapter 3— Surface Treatments 30
Chapter 3— Surface Treatments 31
Figure 18. A comparison of tradional asphalt (le) and
porous asphalt (right) when wet. The gaps created by
leaving out the ner parcles in porous asphalt allow
water to inltrate pavement and into the structural soil
reservoir below. As a result, porous asphalt appears
dull when wet, because water runs through and does
not pond, which creates a high fricon surface.
PhotobyTedHaner.
Concerns of Clogging
The best maintenance for any type of porous pavement is a vacuum treatment
pavement, although the oldest installations have never been vacuumed and
show little effects of clogging. Porous asphalt systems should not be pressure
washed since this treatment further embeds sediment within the surface.
Additionally, porous asphalt systems should never be sealed. Once a sealant is
applied, the system will not work ever again.
Porous Bituminous Asphalt Specification
Ithaca, NY Porous Asphalt Medium Duty Parking Lot
1. Bituminous surface course for porous paving shall be two and one-half (2.5)
inches thick with a bituminous mix of 5.5% to 6% by weight dry aggregate.
In accordance with ASTM D6390, draindown of the binder shall be no greater
than 0.3%. If more absorptive aggregates, such as limestone, are used in the
mix then the amount of bitumen is to be based on the testing procedures
outlined in the National Asphalt Pavement Association’s Information Series
131 – “Porous Asphalt Pavements” (2003) or NYSDOT equivalent.
Chapter 3— Surface Treatments 3232 Day, S.D. and S.B. Dickinson (eds.) 2008. Managing Stormwater for Urban
Sustainability Using Trees and Structural Soils.
binder meeting the requirements of PG 76-22. The elastomeric polymer shall
be styrene-butadiene-styrene (SBS), or approved equal, applied at a rate of
3% by total weight of the binder. The composite materials shall be thoroughly
storage stable.
3. Aggregate in the asphalt mix shall be minimum 90% crushed material and
have a gradation of:
U.S. Standard
Sieve Size Percent Passing
½” (12.5mm) 100
3/8” (9.5mm) 92-98
4 (4.75mm) 32-38
8 (2.36mm) 12-18
16 (1.18mm) 7-13
30 (600 mm) 0-5
200 (75 mm) 0-3
4. Add hydrated lime at a dosage rate of 1.0% by weight of the total dry
aggregate to mixes containing granite. Hydrated lime shall meet the
requirements of ASTM C 977. The additive must be able to prevent the
separation of the asphalt binder from the aggregate and achieve a required
tensile strength ratio (TSR) of at least 80% of the asphalt mix.
The asphaltic mix shall be tested for its resistance to stripping by water in
accordance with ASTM D-3625. If the estimated coating area is not above 95
percent, anti-stripping agents shall be added to the asphalt.
Citation:
Haffner, T., Bassuk, N.L., Grabosky, J., and P. Trowbridge. 2007. Using Porous
Asphalt and CU-Structural Soil. http://www.hort.cornell.edu/uhi/outreach/
csc/index.html Urban Horticulture Institute, Cornell University, Ithaca, NY.
Chapter 3— Surface Treatments 32Chapter 4— Research and Recommendations 33
Chapter 4— Research and
Recommendations
Tree Root Penetration into Compacted Soils Increases
Inltration
Based on Research by Julia Bartens, Susan Day, Joseph E. Dove, J. Roger
Harris, and Theresa Wynn, Virginia Tech
Research Summary
A container experiment with
recently transplanted black
oak (Quercus velutina) and red
maple (Acer rubrum) tested
whether roots can penetrate into
compacted soil and once they
penetrate, if they can increase
species were grown in pine bark
and surrounded on all sides
and the bottom with compacted
soils. Within 12 weeks, both tree
species were able to penetrate
into compacted soil and increase
by 153%. There was no
difference in performance
between black oak (coarse roots)
In a second container experiment, green ash (Fraxinus pennsylvanica)
were grown in CU-Soil and were separated from the compacted subsoil by
geotextile. Roots were able to penetrate into compacted subsoil and increase
Next Steps/Research Needs
applies to larger scale trees in the ground needs to be done. Tree species with
different requirements should also be observed.
Citation
Bartens, J., S. D. Day, J. R. Harris, J. E. Dove, and T. M. Wynn. 2008. Can urban
management? Journal of Environmental Quality, 37 (6):2048-2057.
Figure 19. Ash roots penetrang geotexle aer
compacted subsoil has been washed away.
Roots increased inltraon by a factor of 27.
Photo by Susan Day.
34 Day, S.D. and S.B. Dickinson (eds.) 2008. Managing Stormwater for Urban
Sustainability Using Trees and Structural Soils.
Tree Development in Structural Soils at Different Drainage
Rates
Based on Research by Julia Bartens, Susan Day, J. Roger Harris, Joseph E.
Dove, and Theresa Wynn, Virginia Tech
Research Summary
A container experiment involving 2 tree species (swamp white oak (Quercus
bicolor), and green ash (Fraxinus pennsylvanica), 3 drainage rates (slow,
medium, rapid), and 2 structural soils (CU-Soil and Carolina Stalite)
evaluated the optimal reservoir detention times for tree root development
and water uptake from the reservoir. Structural soils had an impact on root
distribution— tree roots grew wider in Carolina Stalite than with CU-Soil.
Drainage rate also had an impact on tree growth; Root:shoot ratios for swamp
white oak were much higher for the slow drainage treatment and trees were
and no difference in Root:shoot ratios for the different drainage rates was
observed but roots did grow deepest in the rapidly draining treatment.
Recommendations based on this research
In general, water should drain from the parking lot within 2 days so adequate
root systems can develop. For water uptake from the reservoir it is clearly
inundation can prevent this deeper root exploration, depending upon species.
Transpiration rates were varied but similar to trees grown in traditional
landscapes. Of course, size of tree canopy is important in determining
amount of water that can be removed. In general, the largest trees with the
best developed root systems removed the greatest amount of water from the
stormwater reservoirs.
Next Steps/Research Needs
Temperatures of the structural soils could be compared in future experiments
because this could also be affecting the root growth and maybe of interest if
information about lateral root growth (which was limited in this experiment
tolerances can be expected to respond similarly, more species trials would be
useful.
Chapter 4— Research and Recommendations 35
Citation
Bartens, J., J. R. Harris, S. D. Day, J. E. Dove, and T. M. Wynn. 2008 Ecologically
integrated stormwater distribution using urban trees and structural soils. (in
review)
Drainage Rate at the Mini Parking Lot Demonstration Site in
Blacksburg, VA
Based on Research by Mona Dollins, Virginia Tech
Research Summary
A Mini Parking Lot demonstration site which had a Carolina Stalite structural
allowed to naturally drain into the clay textured subsoil beneath. The water
levels were checked from 15 observation wells every 5 minutes (during the
determine the speed of drainage and lateral water movement through the
system.
Within 2.5 hours, the water had completely drained from the reservoir. Lateral
water movement within the reservoir was very rapid through the structural
soil media traveling over 18 feet in a matter of minutes.
Next Steps/Research Needs
Drainage data from larger systems, at varying depths, and different types of
subsoils should be tested to gain better understanding of the systems behavior
in different conditions.
recommended (see the blue box on page 15).
36 Day, S.D. and S.B. Dickinson (eds.) 2008. Managing Stormwater for Urban
Sustainability Using Trees and Structural Soils.
System Effects on Water Quality
Based on Research by Qingfu Xiao, University of California at Davis
Research Summary
Research shows that 97.9-99% of the hydrocarbons found in pollutants
suspension, microorganisms biodegrade the hydrocarbons into their
constituent parts of simple chemical components which cease to exist as
pollutants and render them harmless to the environment.
Surface runoff from four types of parking lots was collected (commercial,
older institutional (>10 years), newer institutional (<3 years), and
residential). Pollutant removal (nutrients, heavy metals, soil column tests) by
3 types of substrates (CU-Soil, Davis Soil, and Carolina Stalite) were compared.
Tests: single event test, multiple events test and synthetic runoff test.
All three engineered soils were effective at removing nutrients and materials
in polluted surface runoff. Pollutant removal rates were strongly related to the
type and size of the rainfall event.
Next Steps/Research Needs
Research that determines the pollutant saturation point for these soils should
Once tree roots explore the reservoir it is expected that they would enhance
pollutant removal— but research is needed to accurately evaluate these
effects.
How effective the system is at removing/degrading nutrients and pollutants
with trees in the system.
other BMPs need to be used for pre-treating the surface runoff.
Chapter 4— Research and Recommendations 37
Table 3. Pollutant removal of single storm event. CU= CU Soil, CS= Carolina Stalite, and
DS= Davis Soil.
TablebyQingfuXiao.
Table 4. Pollutant removal of mulple storm events. CU= CU Soil, CS= Carolina Stalite,
and DS= Davis Soil.
TablebyQingfuXiao.
38 Day, S.D. and S.B. Dickinson (eds.) 2008. Managing Stormwater for Urban
Sustainability Using Trees and Structural Soils.
Resources 39
Helpful Resources
Balages, J.D., L. Legret, and H. Madiec, 1995. Permeable pavements: pollution
management tools. Water Science and Technology, 32 (1): 49-56.
175-180.
Bartens, J. 2006. Trees and structural soil as a stormwater management
system in urban setting. Master’s Thesis. Department of Horticulture, Virginia
Tech.
Bartens, J., S. D. Day, J. R. Harris, J. E. Dove, and T. M. Wynn. 2008. Can urban
management? Journal of Environmental Quality, 37 (6):2048-2057.
Bassuk, N.L., J. Grabosky, and P. Trowbridge. 2005. Using CU-Structural Soil
in the Urban Environment. Urban Horticulture Institute, Cornell University,
Ithaca, NY. http://www.hort.cornell.edu/uhi/outreach/csc/index.html
in Copenhagen with emphasis on the effect of different establishment.
Arboriculture & Urban Forestry, 33(5): 330-337.
Cahill, T., 1993. Porous pavement with underground recharge beds,
engineering design manual. Cahill Design Associates, West Chester, PA.
Cahill, T., 2008. A second look at porous pavement/underground recharge.
Watershed Protection Techniques, US Environmental Protection Agency, 1:
76-78.
Cahill, T., M. Adams, and C. Marm, 2003.Porous asphalt: the right choice for
porous pavements, Hot Mix Asphalt Technology, September/October.
com.
Colandini, V., M. Legret, Y. Brosseaud, and J.D. Balades, 1995. Metallic
pollution in clogging materials of urban porous pavements. Water Science and
Technology, 32(1): 57-62.
the process and the evidence. Australian Journal of Soil Restoration, 33: 221-
239.
40 Day, S.D. and S.B. Dickinson (eds.) 2008. Managing Stormwater for Urban
Sustainability Using Trees and Structural Soils.
Day, S. and N. Bassuk, 1994. A review of the effects of soil compaction and
amelioration treatments on landscape trees. Journal of Arboriculture, 20: 9-
17.
Day, S.D., J.R. Seiler, and N. Persaud, 2000. A comparison of root growth
differing soil water contents. Tree Physiology, 20: 257-263.
Evans, M., N.L. Bassuk, and P.J. Trowbridge, 1990. Street trees and sidewalk
construction. Landscape Architecture. 80(3): 102-103.
Resources Journal, Winter 1995-1996: 14-18.
Raton, London, New York, Singapore.
Goldstein, J., N.L. Bassuk, P. Lindsey, and J. Urban, 1991. From the ground
down. Landscape Architecture, 81(1): 66-68.
Grabosky, J. 1996. Developing a structural soil material with high bearing
strength and increased rooting volumes for street trees under sidewalks.
Master’s Thesis. Department of Horticulture, Cornell University.
Doctoral Dissertation. Department of Horticulture, Cornell University.
Grabosky, J. and N. Bassuk, 1995. A new urban tree soil to safely increase
rooting volumes under sidewalks. Journal of Arboriculture, 21: 187-200.
Grabosky, J. and N. Bassuk, 1996. Testing of structural urban tree soil materials
for use under pavement to increase street tree rooting volumes. Journal of
Arboriculture, 22: 255-263.
Grabosky, J. and N. Bassuk, 1998. Urban tree soil to safely increase rooting
Grabosky, J. and N. Bassuk, 2008. Sixth- and tenth- year growth measurements
for three tree species in a load-bearing stone-soil blend under pavement and
a tree lawn in Brooklyn, NY, U.S. Arboriculture & Urban Forestry, 34(4): 265-
266.
Grabosky, J., N.L. Bassuk, L. Irwin, and H. Van Es, 2001. Shoot and root growth
of three tree species in sidewalks. Journal of Environmental Horticulture,
19(4):206-211.
Resources 41
measuring street tree shoot growth in two skeletal soil installations compared
to tree lawn plantings. Journal of Arboriculture, 28(2):106-108.
Grabosky J., N. Bassuk, and P. Trowbridge, 1999. Structural Soils: A new
medium to allow urban trees to grow in pavement. Landscape Architecture
Technical Information Series (LATIS).
Grabosky, J. and E.F. Gilman, 2004. Measurement and prediction of tree growth
reduction from tree planting space design in established parking lots. Journal
of Arboriculture, 30:154-159.
Gregory, J.H., M.D. Dukes, P.H. Jones and G.L. Miller, 2006. Effect of urban soil
117-124.
Haffner, E.C. 2008. Porous asphalt and turf: exploring new applications
through hydrological characterization of CU Structural Soil and Carolina
Stalite Structural Soil. Master’s Thesis. Department of Horticulture, Cornell
University.
Haffner, T., Bassuk, N.L., Grabosky, J., and P. Trowbridge. 2007. Using Porous
Asphalt and CU-Structural Soil. Urban Horticulture Institute, Cornell
University, Ithaca, NY. http://www.hort.cornell.edu/uhi/outreach/csc/index.
html
soil. Forestry Science, 27: 660-666.
Upper Saddle River, NJ.
Lindsey, P. and N.L. Bassuk, 1991. Specifying soil volume to meet the water
needs of mature urban street trees and trees in containers. Journal of
Arboriculture, 17: 141-149.
Lindsey, P. and N.L. Bassuk, 1992. Redesigning the urban forest from the
ground below: a new approach to specifying adequate soil volumes for street
trees. Arboricultural Journal. 16(1): 25-39.
Loh, F.C.W., J.C. Grabosky, and N.L. Bassuk, 2003. Growth Response of Ficus
benjamina to limited soil volume and soil dilution in a skeletal soil container
study. Urban Forestry & Urban Greening, 2(1):53-62.
42 Day, S.D. and S.B. Dickinson (eds.) 2008. Managing Stormwater for Urban
Sustainability Using Trees and Structural Soils.
McPherson G., J.R. Simpson, P.J. Peper, S.E. Maco, and Q. Xiao, 2005. Municipal
Smiley, T. E., L. Calfee, B. Fraedrich and E.J. Smiley, 2006. Compaction of
structural soil and noncompacted soils for trees surrounded by pavement.
Arboriculture and Urban Forestry, 32(4): 164-169.
Thelen, E., W.C. Grover, A.J. Hoiberg, and T.I. Haigh, 1972. Investigation of
Porous Pavements for Urban Runoff Control. Environmental Protection
Agency.
Trowbridge, P. and N.L. Bassuk, 2004. Trees in the urban landscape: site
assessment, design and installation. Chapter 3:61-81. Wiley and Sons, Inc.
viable urban forest. ASLA Proceedings Annual Conference, pp. 350-351. 13(2):
64-71.
Xiao Q., and E. McPherson, 2003. Rainfall interception by Santa Monica’s
municipal urban forest. Urban Ecosystems, 6:291-302.
Appendices 43
Appendices
Note:
to the reader and are presented “as is” from resources provided by the
an endorsement of or warranty of these products by Virginia Tech, Cornell
University or University of California at Davis or any of their employees.
CU-Soil is a patented material and must be purchased from a licensed supplier.
Amereq (http://www.amereq.com/) licenses the manufacturing of CU-Soil to
ensure quality control of installations.
Carolina Stalite is composed primarily of a manufactured component available
from Carolina Stalite Company (Salisbury, NC). It is available through the
horticultural division of Carolina Stalite (www.permatill.com).
44 Day, S.D. and S.B. Dickinson (eds.) 2008. Managing Stormwater for Urban
Sustainability Using Trees and Structural Soils.
CU-Soil Specication and Mixing Procedure
CU-Soil is a patented material and must be purchased from a licensed
supplier. Amereq (http://www.amereq.com/) licenses the manufacturing
of CU-Soil to ensure quality control of installations.
Appendices 45
46 Day, S.D. and S.B. Dickinson (eds.) 2008. Managing Stormwater for Urban
Sustainability Using Trees and Structural Soils.
Appendices 47
48 Day, S.D. and S.B. Dickinson (eds.) 2008. Managing Stormwater for Urban
Sustainability Using Trees and Structural Soils.
Appendices 49
50 Day, S.D. and S.B. Dickinson (eds.) 2008. Managing Stormwater for Urban
Sustainability Using Trees and Structural Soils.
Appendices 51
Carolina Stalite Structural Soil Specication
52 Day, S.D. and S.B. Dickinson (eds.) 2008. Managing Stormwater for Urban
Sustainability Using Trees and Structural Soils.
Appendices 53
54 Day, S.D. and S.B. Dickinson (eds.) 2008. Managing Stormwater for Urban
Sustainability Using Trees and Structural Soils.
Carolina Stalite Mixing Specication
Appendices 55
