Top-Down Design for Green Streets

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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






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






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.






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






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.






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.






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






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.






Designing

for

Healthy

Tree

Canopies






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.






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.






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.






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.






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.






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.






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.






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






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.






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






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.






Review Question

What is the mature canopy method of determining required soil volume?






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).






Soil

Specification

&

Soil Vault

Systems






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.






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.






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.






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






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






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.






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.






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.






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






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






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






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)






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






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






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.






Review Question

How does organic matter contribute to healthy soil?






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.






Engineering

&

Pavement

Design






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






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






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






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.






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.






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.






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






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






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.






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.






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.






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.






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.






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.






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.






Review Question

How are services accommodated in soil vault systems?






Answer

Lateral members can be trimmed to accommodate services and foundation structures without

compromising the strength of the soil vault matrix.






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






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






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.






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.






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.






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






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


Deadman modular system with


Earth anchor system with

strap over root ball






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.






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.






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






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.






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.






Summary &

Resources






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.






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




https://www.epa.gov/sites/production/files/2014-06/documents/basicscompendium.pdf

https://doi.org/10.1016/j.ecolmodel.2017.07.016

https://greeninfrastructureontario.org/app/uploads/2016/06/UF-Toolkit-Part-I-Communicating-Benefits-Bulletin-Final.pdf

https://www.epa.gov/sites/production/files/2015-11/documents/stormwater2streettrees.pdf

https://html5.dcatalog.com/?docid=55462d2c-2d0d-4aa9-a7ed-91cc51463eb7&page=73

https://www.isa-arbor.com/Publications/Arboriculture-Urban-Forestry



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