The Parking Forest:Linear Tree Wells Using Structural Soil

Sustainability for all the places between the buildings

503.334.8634www.greengirlpdx.com

greengirl@greengirlpdx.coma certified women business enterprise

Clean Water ServicesHealthy Streams Plan

Main Street Tigard

Effective Impervious Areawithin the Tualatin Basin UGB

Source: Clean Water Services Healthy Streams Plan

8%

Why do we need more trees in the urban fabric?

Water Balance BEFORE Development

25% base flow

(shallow infiltration)

50%

ev

apor

ation

0.5% runoff10

0% X

X” a

vera

gean

nual

rain

fall

25%

gro

undw

ater

(dee

p in

filtr

ation

)

Water Balance AFTER DevelopmentExample: EVERYWHERE

redu

ced

evap

orati

on

98% runoff

2%

evap

orati

on

Redu

ced

infil

trati

on

100%

ave

rage

annu

al ra

infa

llN

o infiltration

Water Balance Runoff Compared

0.5% runoff

98% runoff

Hydromodificationfrom a Watershed Perspective

• Additional volumes result in exceeding pre-developed rates that scour stream banks.

• Additional durations of flow impact habitat further.Please! Keep your

runoff volume to yourself!

What’s wrong with the trees we’ve got?

What’s wrong with the trees we’ve got?!

Photo Source: gratefulmommy.com

What about all those lovely tree-lined streets in Portland?

Trees Need Soil to Grow

Graph Source: USDA Forest Service Center for Urban Forest ResearchSlide courtesy of Todd Prager ,Todd Prager & Associates

Trees Need Soil to Grow

5 ft Crown40 cu ft Soil

20 ft Crown450 cu ft Soil

40 ft Crown2000+ cu ft Soil

Photos and observations courtesy of Todd Prager, Todd Prager & Associates

The Parking ForestAn Assembly of Common Materials

Structural Soil (dry)

The Parking ForestA Close Up

Pavement outside of

Parking Forest

Re-paved areaTree

Structural SoilAmended native

clayey soilUndisturbed native clayey soil

Crushed aggregate

Wheel stop

Two Sites

Watershed base map courtesy of Tualatin River Watershed Council

Redevelopment

Retrofit

THPRD Sunset Swim CenterRedevelopment with Pervious Concrete

THPRD Sunset Swim Center in PlanRedevelopment with Pervious Concrete

Structural SoilOur “Secret Weapon”

Source: Cornell University – CU Structural Soil - http://www.hort.cornell.edu

How do we know it works?Olympia Results 8 Years Later

Control Trees Planted in Compacted Native Soil

Trees Planted in Structural Soil

Assembly Procedure

Assembly Procedure

Assembly Procedure

Assembly Procedure

Assembly Procedure

Assembly Procedure

Assembly Procedure

Assembly Procedure

Why these materials?

• Structural soil• Amended Native Soil• Trees• Wheel stops• Crush rock (in landscape area)

Why these materials?Structural Soil

• Structural soil versus structural supports

A Second Way of Providing Tree Roots with Adequate Soil Volume Under

Pavement

. Photo courtesy of Jim Labbe

Another Way of Providing Tree Roots with Adequate Soil Volume Under Pavement

A Second Way of Providing Tree Roots with Adequate Soil Volume Under

Pavement

. Photo courtesy of Jim Labbe

A Second Way of Providing Tree Roots with Adequate Soil Volume Under

PavementJim Labbe in Nijmegen Netherlands. Photo courtesy of Jim Labbe

A Third Way of Providing Tree Roots with Adequate Soil Volume Under Pavement

Photo probably ripped off the Silva Cell website by Brian Wegener

Why these materials?

Proprietary mix versus mixed by contractor

Mixing Structural Soil

Photo Source: www.greelysand.com/cu-soil.html

Olympia Soil Recipe

4 cubic yards Crushed Rock ¾”-1¼” + 1 cubic yard Loam/organic topsoil + 1 pound Soil Binder “Stabilizer” + 46 gallons water__________Makes 4.6 cubic yards Structural Soil

Structural Soil Costs - Olympia

“The price to supply, deliver and install the structural soil material per cubic

yard ranged from $19 to $85 with an average bid of $42, the median price

was $39.50.

The city paid $24 per cubic yard for the structural soil material to the low

bidder.

This compare to a price quote we received for the supply of CU structural

soil at $65 per cubic yard plus delivery. The source was over an hour’s drive

away and delivery charges were $85 per hr with a truck and trailer hauling 22

cubic yard. The expected cost delivered was $72 per cubic yard.”

Excerpted from “Structural Soil Demonstration Project”

http://olympiawa.gov/~/media/Files/CPD/Urban%20Forestry/Forms/StructuralSoil.ash

x

Source: City of Olympia -Structural Soil Demonstration Project

Why these materials?Amended native soil

Amended native soil versus faster draining “3-way” mix

Why these materials?Trees

Trees versus only shrubs &/or groundcover:• Interception & evaporation• Evapotranspiration• Heat island effect

Why these materials?Wheel stops

Wheel stops versus curbs

Why these materials?Crushed rock

¼”x 10 crushed rock versus rounded rock or pavers

Siting Criteria

• Site is suitable for infiltration• Non-expansive soils• Soils should infiltrate at least 0.2 inches/hour*. Clay soils

are OK!

*Based on 2ft of structural soil, 25% void ratio, facility empty in 30 hours to be ready for the next Type IA storm

PCC Sylvania Lot 10Drainage Area

6133 sf

1919 sf

754 sf

615 sf

2815 sf

6103 sf

Parking Lot

Sidewalk

<50% Grass HSG C

Woods, Good HSG C

L Street

PCC Sylvania Lot 10in Plan

The Parking Forest Retrofitat PCC Sylvania Lot 10

Ignore the piles of dirt. They’re

gone.

Design Differences at PCC Sylvania Lot 10

Left most of the parking lot

unmodified/ impervious

Installed an impervious liner adjacent to impervious

asphalt

Installed an impervious liner adjacent to impervious

asphalt

Stakeholders chose a native

conifer

Water quality and quantity monitoring

Design Differences at PCC Sylvania Lot 10

Straight head-in parking

Donated “old school” solid bottom wheel

stops = not ideal

Just trees

Erosive forcesare an issue at PCC

Non-attentive forcesare an issue at PCC

Thank You! Partners Who Donated In-Kind

Services/Products

Alder Geotechnical Services

Siting Criteria at PCC

• Site is suitable for infiltration• Non-expansive soils• Soils should infiltrate at least 0.2 inches/hour*. Clay soils

are OK!

*Based on 2ft of structural soil, 25% void ratio, facility empty in 30 hours to be ready for the next Type IA storm

More Informationwww.parkingforest.org

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