Top-Down Design for Green Streets
Transcript of Top-Down Design for Green Streets
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Top-Down Design for Green Streets
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Top-Down Design for Green Streets
The American Institute of Architects
Course No. AEC1574
This program qualifies for 1.0 LU/HSW Hour
Course Expiry Date: 12/23/2023
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This course is approved by GBCI for
continuing education. Approval for this
course indicates it will be monitored by GBCI
to ensure that it upholds the quality,
relevance and rigor necessary to contribute
to ongoing learning in knowledge areas
relevant to the green building industry.
Approved for:
1.0 CE hour(s)
Course is approved for:
General
Approval date:
December 21, 2020
Course title:
Top-Down Design for Green Streets
Course ID:
920023241
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Purpose:
Trees are essential for the health of the urban environment, mitigating the heat island effect, cleaning the air, reducing stormwater runoff,
and improving residents’ health and well-being. But cities are often inhospitable to trees, where their growth may be stunted or their roots
may damage surrounding infrastructure. This course explores ways to design successful projects incorporating green infrastructure by
understanding the principles behind tree growth, proper type and amount of soil, water management, and the role of soil vault systems in
helping urban trees thrive.
Learning Objectives:
At the end of this program, participants will be able to:
• explain how trees help reduce the impact of the urban heat island effect, stormwater runoff, and air pollution while enhancing the
health of the ecosystem and its inhabitants
• describe the considerations for soil volume and specification that ensure proper growth and a healthy tree canopy and the benefits of
soil vaults in supporting pavement and providing space for tree roots and service pipes
• discuss the role and types of pavements, the benefits of porous pavements to urban trees, and the importance of pavement design
and construction that accommodates expected loads while providing large volumes of uncompacted soil for root growth below, and
• design a tree pit structure and pavement that recreates natural surroundings so the root system can access rainfall, predevelopment
flows are restored, and stormwater is cleaned.
Purpose and Learning Objectives
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Urban Environmental Challenges
Designing for Healthy Tree Canopies
Soil Specification & Soil Vault Systems
Engineering & Pavement Design
Summary & Resources
Contents
Toronto, OntarioPhoto by Matthew Henry on Unsplash
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Urban
Environmental
Challenges
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Urban Environmental Challenges
Among the many challenges
facing cities is the loss of
green space as it is replaced
with impervious surfaces.
Urban trees offer many
environmental, human health,
and economic benefits that
can mitigate the harms caused
by the urban hardscape.
Incorporating trees into green
infrastructure also contributes
to more intangible but no less
important qualities such as the
beauty of the surroundings,
social health, and positive
human physiological and
psychological responses.
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Urban Heat Island Effect
Urban heat island effect is a very real challenge in cities.
Surfaces such as building walls and roofs, roads, and parking
lots absorb and reemit the sun’s heat more than natural
landscapes such as forests and water bodies. Urban areas,
where these structures are highly concentrated and greenery is
limited, become “islands” of heat with daytime temperatures 1–
7°F higher than temperatures in outlying areas.
Heat islands contribute to a range of environmental, energy,
economic, and human health impacts:
• Increased energy consumption to keep buildings cool
• Higher levels of pollutants and greenhouse gas emissions
due to higher electricity demand
• Heat-related dangers to human health and comfort
• Diminished water quality as heated stormwater runoff
infiltrates natural water bodies and impacts the temperature
balance essential to healthy aquatic ecosystems
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Mitigating the Urban Heat Island Effect
NYC, NYPhoto by Ostap Senyuk on Unsplash
Lack of vegetation cover is a defining feature of built-up urban
areas; increasing the amount of green landscape can combat the
harms caused by the urban heat island effect.
Vegetation helps to lower urban heat through evapotranspiration
and shading and provides cooler surfaces to reduce the effect of
heat radiating from the surrounding built environment.
Suitable species selection and planting design with taller
vegetation—shrubs and trees—can also help channel cooling
breezes to where they are needed. In addition, urban vegetation
helps reduce demand for cooling energy, improve air quality by
absorbing pollutants, decrease heat-related illnesses, and
reduce thermal pollution of waterways.
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Stormwater Runoff
The growth of urban areas characterized by extensive impervious surfaces and
limited vegetative cover has altered the natural water balance, leading to a
much greater discharge of stormwater, poor-quality runoff, and damage to
aquatic habitats. Infiltration into groundwater is reduced, and water quality is
degraded as runoff gathers pollutants as it flows over hard surfaces instead of
being absorbed into the earth. Limited tree cover reduces water storage in root
systems, the release of water into the atmosphere through transpiration, and
the filtering of natural pollution through tree roots, especially.
As a consequence, increased attention is being paid to managing the water
cycle in urban development. Often referred to as low-impact design (LID),
sustainable urban design (SUDS), or water-sensitive urban design (WSUD),
this alternative approach to traditional stormwater management seeks to
minimize the extent of impervious surfaces and mitigate changes to the natural
water balance through on-site reuse of the water as well as through temporary
storage. Key principles include integrating stormwater treatment into the
landscape, protecting water quality, and reducing runoff and peak flows.
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Mitigating Stormwater Runoff
Trees and vegetation play an important role in
mitigating the damage of stormwater runoff. Permeable
surfaces, plants, and soil reduce the volume of water
runoff and slow its flow, allowing the water to infiltrate
the ground, where pollutants are absorbed.
Polluted outflows, flooding, and strain on municipal
stormwater management systems are lessened when
trees and green infrastructure reduce the amount of
urban impervious surfaces.
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The declining tree canopy is evident in common sights like the vast parking lot shown here. A 2017 study published in
Ecological Modelling looked at 10 megacities (population at least 10 million) and found that tree canopies cover about
20% of their area—but they have room for more. Theodore Endreny, one of the authors of the study, says, “By
cultivating the trees within the city, residents and visitors get direct benefits: …an immediate cleansing of the air that’s
around them….direct cooling from the trees, and even food and other products. There’s potential to increase the
coverage of urban forests in our megacities, and that would make them more sustainable, better places to live.”
Declining Tree Canopy
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Restoring the Tree Canopy
The researchers valued the contribution of urban forests at $500 million for the
average megacity—or $35 per resident. These same cities could find room for
20% more forest and would almost double the benefits by doing so. The financial
benefits come from lowered temperatures that save on energy use, moisture
absorption that reduces stormwater management costs, and air pollution removal
that improves public health.
Urban trees not only cut costs but also are proven to increase property values and
bring a range of intangible benefits to communities:
• Stronger social connections and physical activity as residents spend more time
outdoors
• Positive physiological and psychological responses to nature: reduced stress,
lower blood pressure, improved focus, and increased sense of happiness and
well-being
• Habitat and resources for other species
Pittsburgh, PAPhoto by Maria Oswalt on Unsplash
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Design for Green Streets
Among city planners and
designers, there is a shift to work
with nature to confront climate
change and to use nature’s
contributions to inform policies
and decisions that can improve
the health, well-being, and quality
of life of all who live and work in
urban areas. Green streets and
urban forests should be a part of
these policies and decisions.
This course presents solutions
that help trees thrive in urban
areas, and by extension, help
urban ecosystems and residents
thrive, as well.
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Designing
for
Healthy
Tree
Canopies
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Tree Canopies
Tree canopies can mitigate climate change, but
not only do we need more of them, we also need
them faster.
In its natural environment, a tree has a large
horizontal root structure supporting the canopy.
The analogy of a wine glass has been used to
illustrate the large, flat base beneath the stem
and the structure above. The availability of space
for a tree’s roots to develop is crucial to its ability
to grow and stay healthy. In the natural
environment, the roots of a growing tree will
extend far into the surrounding soil to more than
twice the width of the mature tree’s canopy.
Everybody has some experience of a neighbor’s
tree roots impacting a service or foundation, a
long way from the tree itself.
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Tree Canopies
There are many parts to a tree—it is a finely balanced
living organism whose complexity needs to be taken into
consideration as part of the design process. We cannot
ignore the belowground part of a tree when designing for
its long-term requirements.
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Tree Canopies
However, typically, the engineering demands of
paved structures make it impossible to grow trees
in cities. Pavements require structural support,
which historically is concrete, crushed stone, or
efficiently compacted road base, and the small
opening in the pavement for the tree is not
sufficient to support that large root plate seen in
earlier slides.
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Shortened Life Span
Trees obtain nutrients from soil via their roots, but the roots also need the
oxygen and water that occupy voids between soil particles. In
uncompacted soil, voids are abundant. For trees in hard-surfaced areas,
a fundamental conflict exists between maximizing the soil volume
available for tree rooting while providing a stable base for roads and
pavements. If soil is treated as a structural material and required to bear
the load of pedestrians, buildings, and roadways, it will be consolidated to
the point that air and water are excluded, and insufficient space is
available for roots to grow.
The average life span of a tree is very short in many of the big cities.
Some cities have an average replacement cycle of 10 to 11 years; in
other cities, it is 14 years, but that is a very, very short time considering
that the natural life span of some of these tree species is close to a
hundred years in the natural environment. This continuing replacement
cycle constitutes a massive financial cost as well as a huge lost
opportunity cost.
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Damage to Surroundings
Tree roots are opportunistic, seeking out favorable growing
conditions. To satisfy the needs of the tree, roots will explore
the space below permeable pavements where moisture is
trapped, oxygenated sand layers, moist conditions in service
trenches, and cracks in road pavements and curbs. Trees
growing in typical urban “tree boxes” are usually surrounded
by compacted soil. This often leads to the roots seeking out
the space between the compacted soil and the overlying
pavement—where air and water are present—which then
causes footpath heaving.
If the tree roots cannot expand into the surrounding soil, they
continue to grow until they have filled up the available space.
When the tree’s needs for nutrients, air, and water can no
longer be met, the health of the tree will begin to decline, and
it will eventually die. Trees grown in these conditions rarely
reach their full growth potential and cannot provide the wide
range of benefits that mature, healthy trees have to offer.
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Tree Coffins
The cause of it all is the failed practice of tree
“coffins,” where over generations, just a small box
has been provided for the tree to grow in.
This is a very short-term approach, and it does not
work; it simply yields ongoing costs and repair
work.
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Determining Target Soil Volume
The belowground area or volume needed for a tree is relative to
the mature canopy size of that tree. A larger tree obviously needs
more space for its root system, and it does not have to be round.
The shape can be varied, but we need to think in terms of soil
volume. Trees require an adequate supply of loose, well-aerated,
moist, and uncompacted soil in order to thrive. These conditions
enable the tree’s roots to obtain nutrients, oxygen, and water—all
essential for healthy tree growth.
Careful assessment needs to be made of the aboveground and
belowground space required for each tree to reach its mature
size. Various methods, described on following slides, may be
used to calculate the belowground space required for healthy root
growth and thus the desirable soil volume.
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As a general rule, feeder roots grow in the top 150 mm (6 in) to 300 mm (12 in) of the soil. This feeder zone can extend
two to seven times the diameter of the canopy drip line (area under the outer circumference of the tree branches). Major
structural roots may penetrate to greater depths. All trees must be protected from compaction in the feeder zone.
How much suitable soil do trees need to be healthy and reach maturity?
1. Mature Canopy Method: One simple method of calculating soil volume is estimating the projected area of the
mature tree canopy calculated from the area of its branch spread (usually provided by nursery catalogs) multiplied
by a depth of 0.6 m (2 ft): Mature canopy area x 0.6 m (2 ft)
2. Two-to-One Method: This method works for metric regions. For every two square meters of shade under the
mature tree canopy, allow one cubic meter of loam soil: 2 m2 shade : 1 m3 soil
These are two simple formulae that can be deployed. There are more complex ones, including online calculators that
can automate and simplify some of these calculations.
Determining Target Soil Volume
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Determining Target Soil Volume: Example
In this example, we are calculating the target soil volume
for a Quercus palustris. The nursery’s statistics will state
how large a given tree species grows; in this case, the
tree grows to a 20- to 45-foot spread.
Using the Mature Canopy Method:
We can calculate area at, say, a 35-foot spread (diameter)
using 𝐴 = 𝜋𝑟2 where r = 17.5 ft (half of the diameter).
Target Soil Volume = Area x 2 ft = 𝜋 x (17.5 ft)2 x 2 ft
= 1,924 ft3 of soil
At a 35-foot spread, Quercus palustris is going to require 1,924 cubic feet (54 m3) of loam soil by this method.
Using the Two-to-One Method:
This tree with a mature shade area of 89 m2 (962 ft2) should have 45 m3 (1,589 ft3) of uncompacted soil at planting.
Quercus palustris Pyramidal through early maturity, its form turns
more oval in older age. Fast-growing, tolerates
wet soils, likes full sun. Glossy dark green
leaves turn russet, bronze, or red. Grows to 60′
to 75′, 25′ to 45′ spread.
Hardiness Zones
The pin oak can be expected to
grow in Hardiness Zones 4 to 8.
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Bridging to Achieve Soil Volume
Soil volume targets can be achieved by
using bridging methods. We can add
soil in the primary rooting zone in the
tree pit opening, and there may be some
additional soil available in an adjacent
garden bed. If we bridge under the hard
pavement, we can connect those two
soil volumes together and provide good
growing conditions under the pavement.
This is one example of the ways in
which we can achieve target soil
volumes by being creative and thinking
outside the box.
Continuous permeable pavement tree trenchPlanting soilConcrete sidewalkSoil cellGeotextile
SubgradePlanting soilFlow spreaderCurb stopDistribution pipe
Geotextile or impermeable liner
Gravel storage
5 ft clearance
for vehicle
5 ft 7 ft 10 ft
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Designing Based on Target Canopy Cover
The designer can simply estimate soil volumes and
costs for a site based on required shade cover.
For example, to provide shade to rows of cars in a
parking lot, we can calculate what the area is. From
that, we can back-calculate the size and volume of the
tree pit and come up with some conceptual costing.
Once again, there are online calculators that can assist
with that.
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Review Question
What is the mature canopy method of determining required soil volume?
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Answer
Estimate the target volume of soil required by multiplying the projected area of the mature tree canopy
(calculated by 𝐴 = 𝜋𝑟2) by a depth of 0.6 m (2 ft).
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Soil
Specification
&
Soil Vault
Systems
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Soil Specification
Soil specification is vital for tree health and growth. Correct use
of quality soil is the “secret sauce” for tree growth and attaining
the vision you have for your project, so do not allow contractors
to cut corners. Too often, soil is overlooked to the detriment of
the tree canopy.
Soil type usually refers to the different sizes of mineral particles
in a particular sample. Each size plays a significantly different
role. For example, the largest particles, sand, determine
aeration and drainage characteristics, while the tiniest,
submicroscopic clay particles are chemically active, binding with
water and plant nutrients. The ratio of these particle sizes
determines soil type: clay, loam, clay-loam, silt-loam, and so on.
Sandy soils have very large particles, allowing water, air, and
plant roots to move freely. At the other end of the spectrum, clay
particles are so small that they pack together tightly and leave
little room for water, air, or roots.
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Soil Specification
Nutrients: Seventeen essential plant nutrients have been identified.
Carbon and oxygen are absorbed from the air while the other nutrients,
including water, are obtained from the soil and absorbed by the tree’s
roots.
Organic matter: In addition to the mineral composition of soil, humus
(organic material) also plays a crucial role in soil characteristics and fertility
for plant life. Organic matter is dead plant or animal material. It improves
sandy soil by retaining water and corrects clay soil by making it looser so
that air, water, and roots can penetrate. In all soils, it encourages beneficial
microbial activity and provides nutritional benefits.
The makeup of natural soil is constantly changing as organic material from
trees and other plants is added and eroded by wind and water. Many soils
are teeming with animal and insect life as well as bacteria and fungi.
Earthworms feed on organic matter and break it down while creating small
tunnels and tracks through the soil, helping oxygen to be transmitted.
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Soil Specification
Cation exchange: Nutrient uptake in the
soil is achieved by cation (positively charged
ion) exchange. Root hairs pump hydrogen
ions (H+) into the soil, which displace
cations attached to negatively charged soil
particles, making the cations available for
uptake by the root.
Remember, you are designing for a living
asset that will appreciate for generations if
proper principles are followed. Use a filler
soil specification by a qualified soil scientist,
and ensure the soil is sampled and tested
for compliance.
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Structural Soils
Structural soils are designed to
be “growth media” that can be
compacted to support applied
pavement design loads. In its
compacted state, structural soil
is free-draining and supports
vigorous tree root growth.
Note that the growth media itself
carries the load. Examples
include CU-Soil® (developed by
Cornell University’s Urban
Horticulture Institute), gravel-
based structural soil, and
Amsterdam tree sand (sand-
based structural soil).
Stone particle
Soil particle
Air or water
pore
Stone contact
point where load
is transferred
Loading or compaction effort
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Structural soil comprises large-gap graded gravel mixed with a horticultural soil, compacted to 95% of peak density. The
gravel compacts to provide the weightbearing capability while the soil, occupying at most 40%, provides for the needs of
the tree.
This method has been used with some success; however, there are extensive considerations to deal with including very
specific requirements for the aggregate; precise calculations of voids, tree root diameters, and compaction; consideration
of climatic factors; choice of filler soil; mixing and compaction methods; and measuring. A tendency to soil alkalinity over
time limits the choice of tree that can be grown while subsoil drainage, aeration, and feeding are further requirements.
Finally, the level of compaction required to provide pavement support seriously restricts the development of mature,
woody tree roots.
Structural Soils
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Structural Soils: Challenges
The structural component is rock, which accounts
for 70–80% by volume and leaves very little actual
growth media. To support any traffic loads,
engineers will require compaction to 96–98%
maximum dry density (MDD). This has the effect of
crushing macropores and hence leads to a loss of
drainage, aeration, and potential for root growth.
While there have been some successes over the
years, there have been many failures with this
system. Many municipalities and cities are not
permitting structural soil as an approved tree-
planting method.
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What Are Soil Vaults?
A soil vault, also known as a tree vault, is a
structure that can support all applied pavement
design loads and can be filled with a growth
media that is optimized for tree growth in the
local environment.
They can be constructed from the following:
• Block walls with suspended slabs
• Precast concrete culverts
• In-situ concrete shells
• Engineered soil cell matrices: The rest of the
course will take a detailed look at this solution.
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Soil Vaults
Soil vault systems consist of preengineered modular units, or cells, that
assemble to form a skeletal matrix, situated below pavement level, to
support the pavement load while providing a large volume of
uncompacted soil within the matrix structure for root growth. Various
designs are available, providing from 90% to over 94% of space for soil.
Different designs address the need for strength while maximizing
available space for roots, as well as for common conduits and service
pipes.
Industry professionals are increasingly insisting on the use of soil vaults.
They recognize that while soil vault technology builds upon the earlier
structural soil concept, it is clearly superior in performance. Not only is
vastly more soil made available to the tree, but also installation is
straightforward and avoids the need for the extensive calculations and
testing required for the use of structural soil.
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Available Growth Media by Volume
The largest difference between soil vaults and structural
soil is the percentage of the tree pit (excavated hole in the
ground) that is occupied by the structure.
• Structural Soil Mix: The rock is the load-bearing
structure, occupying 75–80% by volume, and typically
provides 20% volume for root growth.
• Soil Vault System: The plastic modular system is the
load-bearing structure, occupying 8–10% by volume.
By comparison then, a soil vault system has 4–4.5 times
the available space for tree root growth of a structural soil
mix. Soil vault systems provide optimal space for root
growth.
The other big difference is the available aperture size for
root growth. A good soil vault system will have 10–12″
(230–300 mm) opening sizes, whereas structural soil is
limited to the gaps between the compacted stones.
Structural Soil Soil Vault System
Void Space
Structure
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Soil Treatments Under Concrete Pavement
A four-year research project by
Bartlett Tree Research
Laboratories compared five
different planting methods
against a control tree. The
research project by Dr. Thomas
Smiley et al. was conducted in
Charlotte, NC.
The results demonstrated
irrefutably that uncompacted
loam soil systems grow the best
trees. In the case of one
variable, mean tree height, both
soil vault systems grew trees
that were taller than the control
tree.
Mean Tree Height
Compacted soil
Soil vault system 2Soil vault system 1
Gravel-based
structural soil
Sand-based
structural soil
Open control
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The study concluded that structural load-bearing modules are superior for growing large, healthy trees in the fastest
times, compared to other systems such as compacted soil and soil-based or gravel-based structural soil.
While the study was not intended to point to a “best product,” it proved that methods that support the load on a
pavement and keep that load off the growing media work better than those that don’t. This is good news for urban
planners, landscape gardeners, architects, and developers. It means that simply by choosing a structural load-bearing
soil system, they can achieve the canopy cover they require years sooner than they might with other systems.
Soil Treatments Under Concrete Pavement
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Investing in Soil Vault Systems
Both public and private entities
have asset registers recording and
tracking the value of assets. In
cities, this includes roads, parks,
street furniture, and so on.
Increasingly—and rightly so—trees
are being included as quantifiable
assets.
Let’s look at an example of the
value of trees in soil vault systems.
In an asphalt parking lot next to an
oval, five London plane trees were
planted in quite narrow islands, with
adequate space and soil volume
provided using a soil vault system
beneath the parking lot pavement.
Available soil within tree island:
45 m3 (1,589.16 ft3)
[9 m3 (317.83 ft3) per tree]
Soil cells added:
80 m3 (2,825.17 ft3)
[16 m3 (565.04 ft3) per tree]
Combined total:
125 m3 (4,414.33 ft3)
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Investing in Soil Vault Systems
The cost for the five trees (including the system) was $50,000. Four years later, as
reported by the city, the trees have grown at an unprecedented rate, from a 3″ trunk
diameter at the time of planting to 10″. Calculations (using the Burnley method) value
the trees at $17,500 each—an amazing return on investment in just four years, with so
much growth (literally and financially) still to come.
As a comparison, the city has the same species growing in a nearby parking lot using
the conventional method. The parking lot was laid, a square was cut in the pavement,
some curbing was placed around the edges, the road base was dug out, and soil was
loaded into the hole. Planted 15 years ago (versus only four), these trees are valued at
only $510 each. Of course, the initial outlay was much less ($250 per tree), but the
return on investment does not compare.
Essentially, using a soil vault system, this innovative council was able to grow trees
worth 34 times as much in one-quarter of the time! As more emphasis is placed on
generating return on investment in relation to the value of trees, adopting innovative
technology that enables trees to thrive in urban environments must be a priority.
Conventionally
planted trees
after 15 years
Soil vault
planted trees
after 4 years
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Further illustrating the differing growth rates between methods, five soil vault trees were assessed over a four-year
period against eight trees planted conventionally on the same site.
Outcomes
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Outcomes
Results recorded here show that shade from the soil vault systems
far outpaced that of the open planting methods. The trees in the
parking lot (red circle) are the same age as the ones in the grass
verge (yellow). Comparison of shadows indicates health and vigor.
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Review Question
How does organic matter contribute to healthy soil?
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Answer
Organic material, or humus, plays a crucial role in soil characteristics and fertility for plant life. Organic
matter is dead plant or animal material. It improves sandy soil by retaining water and corrects clay soil by
making it looser so that air, water, and roots can penetrate. In all soils, it encourages beneficial microbial
activity and provides nutritional benefits.
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Engineering
&
Pavement
Design
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Key Issues
Pavement has several key
purposes:
• To support loads without
excessive cracking or
deforming
• To provide a smooth surface
for vehicles to improve
comfort and efficiency
• To eliminate drainage
problems such as mud and
ponding
Load from traffic is transferred
from the pavement surface
down through the support
layers.
Applied wheel
or axle load
Concrete pavement
Pavement base course
Existing subgrade
bearing area
Narrower dispersion of
wheel load through
flexible pavement
layers
Wider dispersion of
wheel load through rigid
pavement, i.e., concrete
Soil vault system, 3 layers
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Flexible, rigid, and composite: Pavements typically consist of a number of layers, placed over the in-situ material,
which work together to withstand traffic and environmental conditions. The surface layer may be made of concrete,
asphalt, aggregate, geocells, grids, or blocks. Concrete provides a rigid pavement structure, while almost all other
pavements are flexible.
Porous and nonporous: Whether rigid or flexible, paving materials may also be porous or nonporous. Porous (or
permeable) materials have open voids between their particles or units that allow the movement of water and air around
the paving material. While some porous paving materials are almost indistinguishable from nonporous materials, their
environmental effects are quite different. Porous paving materials include the following: pervious concrete, asphalt, and
turf; single-sized aggregate; open-jointed blocks, resin-bound paving, and bound recycled glass porous paving.
The overwhelming benefit of porous paving is its contribution to growing healthy urban trees through the admission of
vital air and water to their rooting zones. Porous pavements behave almost like a healthy natural soil surface, enabling
the soil moisture to fluctuate with rapid wetting followed by drying and re-aeration. Other advantages of porous paving
include better management of urban runoff, resulting in less erosion and siltation and greater control of pollutants,
particularly heavy metals and oil, through capture and breakdown in the subgrade.
Pavement Types
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Rigid and flexible pavements distribute traffic load differently to the layers below, necessitating careful attention to
design and construction of the layers and the thickness of the surface layer. An interconnected system will share the
load. The load will be dispersed down through the matrix for minimized bearing pressure at the base of the tree pit.
Sidewalks may need to accommodate heavy point loads. When designing tree pits in sidewalks, consideration may
have to be given to emergency vehicles. While there will typically be a predominantly pedestrian or light maintenance
vehicle load, there also may be situations where emergency vehicles have to traverse the sidewalk to, for example,
rescue people from a burning building or work on the facade of a building. Design of pavements and underlying
structures needs to allow for this.
Accommodating Loads
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Types of Loads: Direct
Pavements in cities must be engineered to
withstand static and cyclic loads in
accordance with applicable standards. Fully
loaded emergency vehicles must be able to
access properties without causing
catastrophic pavement failure.
Where belowground tree pits are used, they
must be capable of supporting applied loads
while providing large volumes of
uncompacted soil for root growth below.
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Types of Loads: Lateral
In addition to direct vertical loads, pavements
are subjected to significant lateral force.
Frequent, heavy traffic may cause the road
pavement to fail adjacent to a tree pit and
unless prevented, the base course may be
displaced laterally into the tree pit void space.
It is important that engineered space for tree
root systems be capable of withstanding this
lateral force.
Even though a tree pit or a soil vault may be
in the sidewalk, consideration does have to
be given to live traffic loads that create a
lateral force bearing on the tree pit.
Engineers will need to calculate this force
and to provide for it. Your manufacturer
should have engineering staff and test data
to satisfy engineers in this regard.
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Ultimate Load Capacity
In all situations, engineering calculations must be
based on laboratory testing for ultimate load
capacity and load dispersion.
It is critical that tree vaults satisfy engineering
requirements with available ultimate load data. This
is the pressure at which the soil vault system fails
in a laboratory. From there, the other calculations
become quite simple.
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An interconnected soil vault system is preferred by engineers because it performs like a buried “space truss.” Applied
loads are shared throughout the matrix laterally and vertically. This gives confidence to engineers that they can
incorporate the system into their pavement and streetscape and into green street design.
Engineer Preferences
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Working with services is a challenge. This has been allowed for in the design
of the best soil vault systems, as the lateral members can be trimmed to
accommodate services and foundation structures without compromising the
strength of the soil vault matrix.
Working with Services
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Working with Services
There are various ways
in which services can be
integrated, and there are
many resources
available.
Your soil vault supplier
should offer a template of
the different ways to
accommodate services
within the matrix to the
satisfaction of pavement
engineers.
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Openings
Soil vault systems need to
have large-diameter openings
in all directions. This permits
both service integration within
the structure as well as the
development of large
structural root systems
without restricting the growth
of the tree.
Ensure that the system you
are designing allows
unimpeded soil volumes in all
directions laterally and
vertically without restricting
the growth of the trees.
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Soil Loading
A well-structured soil has
peds (aggregates of soil
particles) with clumps.
Soil should be loaded
from the top to preserve
the ped structure.
An open matrix will
permit soil to be loaded
into the tree pit from
above to fill all spaces
within the tree pit.
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Optimizing Compaction for Tree Stability
Soil vault systems should have an open structure that
will permit soil to be walked into the pit.
This will help bring it to a level of foot compaction that is
approximately 70 to 80% MDD, which is ideal for tree
stability.
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High Impact Strength
A robust soil vault system
will be able to cope with
installation handling and
some light excavation
equipment without breaking.
Systems made of materials
with high impact strength
help avoid delays and extra
costs on the jobsite.
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Sustainable Materials
Request and insist on using only those
tree vault systems that are made from
100% recycled material.
Such systems further the
environmental benefits of the tree
canopies they help sustain by using the
greenest materials to create green
spaces.
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Buried Structures
Permanent pavement markers can be
incorporated above the tree vault;
service crews can then access cloud-
based information about the buried
structures.
Other methods include service locator
balls, which can be installed with the
structure, and buried service registers
upon which the tree pit should be
noted and coordinated. This permits a
permanent record of the location
depth and type of structure buried
beneath a pavement for future service
integration or repairs.
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Review Question
How are services accommodated in soil vault systems?
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Answer
Lateral members can be trimmed to accommodate services and foundation structures without
compromising the strength of the soil vault matrix.
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Water
Trees in natural surroundings experience a very different
environment from trees in cities. Urban rainfall is typically
directed away from the tree—with environmental
consequences.
When we pave around the trees in cities, any rainfall is
directed away into catch basins and smooth pipes running
from the city. The results are peak velocity buildup, flash
flooding, turbulence, and erosion in the river systems while
trees do not receive the moisture that they would get in
natural surroundings.
Disconnected Environmental Cycle in Cities:
Urban Heat Island, Flash Flooding, Erosion,
Decreased Evapotranspiration
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Water
Our opportunity is to design a tree pit structure
and a pavement such that the tree performs
much like it would in its natural environment
where the root system can access rainfall. This
is also an opportunity to restore predevelopment
flows and clean the stormwater.
The rainfall that falls on the pavement is directed
into catch basins, trench drains, or rain gardens
or through permeable pavements to become
accessible by the tree root system, supporting
the continued health and growth of the tree to
cool down the neighborhood and promote
evapotranspiration.
A connected environmental cycle facilitates
rainwater harvesting, soil infiltration, on-site
detention, and filtration removal of pollutants.
Reconnecting the Environmental Cycle in Cities:
Rainwater Harvesting, Soil Infiltration,
On-site Detention, Filtration
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Rainwater
This rendering shows how we can intercept
rainwater at the surface with permeable
pavements.
This is a good option that allows oxygen exchange,
moisture to come through for the benefit of the
trees, and some on-site detention.
Please remember the test password OXYGEN. You will be
required to enter it in order to proceed with the online test.
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Rainwater
This image illustrates the way we can use catch
basins, sometimes with filtration inserts in them, or
trench drains to intercept runoff from pavement and
feed it via a pipe into the tree pit.
Ideally, water will pond over the surface of the soil
in the tree pit, which should be placed lower to
leave a space at the top of the soil vault matrix of
4″ (100 mm).
Another option is to use a large circuit of 6″
perforated pipe system to distribute the water
evenly through the tree pit for the benefit of the tree
and to offset some issues such as minor flooding in
the area.
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Deep Watering
Provision for deep watering and aeration of the root
zone is crucial, especially beneath pavements. One
highly efficient method uses a system of flexible
perforated pipe beneath the pavement. Typically,
there are two circuits. At the time a tree is planted,
one circuit is looped around the root ball within the
immediate rooting zone of the new tree, and a second
is placed in the outer rooting zone, looped throughout
the root cell matrix.
The pipe is then connected to an inlet located at the
surface that enables a hose to be attached when
water is needed. The rest of the time, the pipe allows
air to flow passively through the system and around
the roots of the tree. This arrangement enables long,
deep watering over the entire root system and the
opportunity for the soil to dry between watering, which
is better for trees than frequent light watering.
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These images depict the way an aeration pipe system is typically coordinated through the soil vault system. They show
the secondary aeration pipe and the primary aeration pipe around the root ball of the tree and how it is finished at the
surface. Aeration pipe systems maintain optimal oxygen content in tree pits.
Aeration Pipe System
Perforated feeder pipe for deep
watering and aeration system
Aeration Pipe Notes:
• Dual-action aeration pipes to
reticulate around footings/features
within tree pit as required
• Aeration inlets at surface to be
coordinated with subsurface features
such as footings to avoid clashesPerforated pipe for deep
watering and aeration system
Inlet set in tree grate to landscape
architect specifications
T-Piece
Perforated pipe for watering
in circuit around root ball
Perforated riser pipe for initial watering
and aeration of root ball zone
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Root ball anchorage is very highly recommended, particularly in cities where strong winds can develop due to the
canyon effect. Anchorage provides stability by allowing the tree’s root system to be held firmly in the ground no matter
what the underlying structure is. The trunk can flex with the wind, but the roots that are so important to the vitality,
health, and longevity of the tree can form by spreading out laterally. Without root ball anchorage, the tree root ball can
“socket” (or pivot in the ground) under wind-throw, and this damages the emerging roots, causing them to form
improperly. The tree can develop significant structural issues with time, which are very expensive to rectify. Anchors are
available that can be driven into the subgrade, as well as deadman systems if the trees are being planted on a podium.
Root Ball Anchorage
Earth anchor system with
cable and mat over root ball
Deadman system with
cable and mat over root ball
Deadman modular system with
cable and mat over root ball
Earth anchor system with
strap over root ball
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Tree Pit Details
Correct detailing and design
of a tree pit is essential to a
successful project.
This permits both accurate
estimating at bid time and
exemplary construction and
quality management.
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Shop/Construction Drawings
This image
exemplifies a good
shop drawing in
plan view. Shop or
construction
drawings should
accommodate all
known site
constraints.
The supplier or
manufacturer
should have in-
house capability to
provide accurate
construction
drawings for the
project.
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Tree Pit Details
This image shows some of the key design
elements for successful soil vault projects.
• Engineered collars support adjacent pavement
structures and prevent differential settlement.
• Any deep watering system, root ball
anchorage system, and protection of
pavements against root intrusion should be
shown.
• The dimensioning should be accurate and
clear.
• Any geotextiles or geocomposite should be
clearly noted and dimensioned.
• Root barriers, if needed to prevent root
damage to assets, and any drainage layers
should be incorporated.
Engineered
collar
Drainage
layer
Pavement
protection
Root barriers, if
needed
Geocomposite
wrap
Soil
Mound for
root ball
support
support
Accurate
dimensioning
Deep
watering
system
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Standard Template
To incorporate all required details, a
standard tree vault template may be
used as a base. Manufacturers should
offer a selection to choose from that
designers can then edit to meet the
specific project needs, or manufacturers
should be able to create bespoke shop
drawings for the project.
Manufacturers may offer online training
courses for specifiers and certification
for installers to ensure they understand
all the key aspects of a successful tree
pit installation.
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Quality Management
Management of quality for installation is vital because this is a
buried structure. And as with any constructed asset, hold points,
witness points, and photographic evidence collected throughout
the installation process allow feedback, direction, and peace of
mind that the project is being completed to the specification and
with the correct installation methods.
Manufacturers may utilize a quality assurance app that holds
documents for plans and sections and uploads hold point and
photographic information to the cloud in real time. The
information is available to all parties, including installers,
specifiers, contractors, municipalities, and the final asset owner.
This type of management system serves as proof of installation
and can be used to provide a final certification from the
manufacturer of compliance based on the completed hold points
and witness points.
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Summary &
Resources
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Summary
We need trees in urban spaces for their myriad benefits to the environment
and human health and well-being. Trees as part of green infrastructure play
a major role in maintaining sustainable urban ecosystems and must take
precedence in urban planning and design.
But trees have always battled to survive in cities, with either growth being
stunted so that trees never reach their full potential or surrounding
infrastructure being damaged by invasive root systems.
Properly designed and installed soil vaults support the pavement load while
providing a large volume of uncompacted soil for root growth, ensuring that
we—and generations to come—can reap the benefits of green streets and
healthy tree canopies.
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Akbari, Hashem, et al. Reducing Urban Heat Islands: Compendium of Strategies. EPA, 2014,
https://www.epa.gov/sites/production/files/2014-06/documents/basicscompendium.pdf. Accessed Dec. 2020.
Endreny, T. et al. “Implementing and Managing Urban Forests: A Much-Needed Conservation Strategy to Increase Ecosystem Services
and Urban Well-Being.” Ecological Modelling, vol. 360, 2017, pp. 328–335, https://doi.org/10.1016/j.ecolmodel.2017.07.016. Accessed
Dec. 2020.
Green Infrastructure Ontario Coalition. “Communicating the Benefits of the Urban Forest in a Municipal Context.” Green Infrastructure
Ontario Coalition, 2016, https://greeninfrastructureontario.org/app/uploads/2016/06/UF-Toolkit-Part-I-Communicating-Benefits-Bulletin-
Final.pdf. Accessed Dec. 2020.
McKeand, Tina and Shirley Vaughn. Stormwater to Street Trees: Engineering Urban Forests for Stormwater Management. EPA, 2013,
https://www.epa.gov/sites/production/files/2015-11/documents/stormwater2streettrees.pdf. Accessed Dec. 2020.
Smiley, E. Thomas, et al. “Comparison of Tree Responses to Different Soil Treatments Under Concrete Pavement.” Arboriculture & Urban
Forestry, vol. 45, no. 6, Nov. 2019, pp. 303–314, https://html5.dcatalog.com/?docid=55462d2c-2d0d-4aa9-a7ed-
91cc51463eb7&page=73. (Archived here: https://www.isa-arbor.com/Publications/Arboriculture-Urban-Forestry) Accessed Dec. 2020.
Resources
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