1. The Parking Forest in Gresham
Linear Tree Wells Using Structural Soil
Sustainability for all the places between the buildings
503.334.8634
www.greengirlpdx.com
greengirl@greengirlpdx.com
a certified women business enterprise
4. Why was I invited here today?
Tualatin Riverkeepers’ Vision: the Parking Forest
• Increase canopy in parking lots
• Manage stormwater
• No loss of parking spaces
13. Trees Need to Access Soil to a
Depth of 3 Feet Minimum
Photo Source: gratefulmommy.com
14. Trees Need Adequate Soil Volume to Grow
Graph Source: USDA Forest Service Center for Urban Forest Research
Slide courtesy of Todd Prager ,Todd Prager & Associates
33326720013367
Soil Area Needed Assuming a 3ft min depth (ft2)
15. Structural Soil Composition
Source: Cornell University – CU Structural Soil - http://www.hort.cornell.edu
Structural stability (similar to
porous pavement aggregate):
Tree health:
16. Question
• What is different about structural soils than regular
construction practices and soils in sidewalk/planter strip
applications?
17. If you have lots of money…
• Structural soils are designed to be on compacted
subgrade and compacted to the standard 95% modified
proctor density.
• Requires a depth of structural soil = 3 feet, for tree
health.
18. If you want to save lots of money
on structural soil…
Careful compaction to hit the “Goldilocks” zone:
• Not so much that tree roots can’t grow into it
• Not so little that vehicular or pedestrian traffic cannot be
supported
• = 85% to 90% Modified Proctor Density
19. Additional Benefit for Tree Health
• Shallow depths of structural soil = little or (more
likely) no fertilization
33. Question
• What are the implementation issues for using structural
soils? Design, availability of materials, construction,
inspection, etc. How are each of these addressed in a
project?
34. Materials are Commonly Available
• Rock
• Clay
• Optional amendments (Our mix included mycorrhizae,
biota, biochar, Stabilizer = hydrogel and moisture holding
element)
• Hydrogel
35. Design: You must answer,
“Where will the structural soil be used?”
Pedestrian areas only:
• Sand Based Structural Soil (SBSS, aka Amsterdam Tree
Soil, Tree Sand)
Pedestrian and vehicular areas:
• CU Structural Soil (proprietary from NY)
• Carolina Stalite Structural Soil
• Olympia Soil Mix
• Sunmark Environmental Mix (proprietary from Troutdale)
36. Choosing Adequate Depth for Structural Stability
• Our geotechnical engineer determined an appropriate
pavement depth based on:
37. Geotechnical engineering report excerpt
• For structural soil placed on a wet, uncompacted (clayey)
subgrade! (Saved us 1 foot of depth of structural soil. Over
the 125 lf of trench, cost savings = $3600.)
38. Consider using more than just structural soil
Amended native soil in places
where structure isn’t needed (see
next slide) gets the trees off to a
good start
45. 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.as
hx
Source: City of Olympia -Structural Soil Demonstration Project
46. Vendors & Costs
• Olympia DIY soil mix, $42/cy delivered
• Sunmark Environmental, $105/cy delivered
• CU Structural Soil (coming soon), national average
$58.25/cy (not delivered)
47. Question
• How can the stability of the adjacent road subgrade and
curb be protected during the installation of structural
soils?
51. A Second Way of Providing Tree Roots with
Adequate Soil Volume Under Pavement
. Photo courtesy of Jim Labbe
52. 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
53. A Second Way of Providing Tree Roots with
Adequate Soil Volume Under Pavement
Jim Labbe in Nijmegen Netherlands.
Photo courtesy of Jim Labbe
54. A Third Way of Providing Tree Roots with
Adequate Soil Volume Under Pavement
Photo probably ripped off the Silva Cell website by Brian Wegener
55. Question
• How much more does it cost to construct a planter strip
and sidewalk using structural soils vs. traditional
methods? Consider design, excavation, materials,
installation, and other issues for a 100’ section of sidewalk
(replacing existing curbtight sidewalk with 4’ planter strip
and new sidewalk). Are there economies of scale we can
expect for larger projects?
59. Economies of Scale
• Economies of scale are available in:
• Engineering design
• Geotechnical investigation
• Mobilization
• Structural soil testing
60. According to Brian Kalter
of Amereq, Inc (Licenses CU Structural Soil)
• Email dated 4/4/14:
“As far as installation costs, CU-Soil™ should not cost more
to install on a per cubic yard basis than a typical base
material since it is installed in 6” lifts and compacted to
95% Proctor Density.”
• …except it’s usually required to be installed to a depth of
3 feet!
61. Question
• Are there standard or typical details, cross-sections, and
specs available for constructing a planter strip and
sidewalk using structural soils that we could review?
62. Specs & Designs to review
• I’ve consolidated everything I could find, plus my own
details and specs, here:
http://www.parkingforest.org/specs.htm
63. Thank You!
Sustainability for all the places between the buildings
503.334.8634
www.greengirlpdx.com
greengirl@greengirlpdx.com
a certified women business enterprise
Notas del editor
Structural soils are a mix of materials that provide structural stability (large, angular aggregate), health support to the trees. Nutrition comes from clay and compost; establishment support comes from the mycorrhizae and biota.
Structural soils must be carefully placed with a “Goldilocks” approach to compaction: just enough to provide structural stability but not so much that roots won’t be able to grow into it (around 85% modified proctor density or in our case).
To reduce the volume Structural soils must be carefully placed with a “Goldilocks” approach to compaction: just enough to provide structural stability but not so much that roots won’t be able to grow into it (around 85% modified proctor density or in our case).
To reduce the volume Structural soils must be carefully placed with a “Goldilocks” approach to compaction: just enough to provide structural stability but not so much that roots won’t be able to grow into it (around 85% modified proctor density or in our case).
Hi Maria. I wasn't familiar with that term before looking at the link you sent (thank you!), but we've created that same blend for similar purposes on past commercial properties. We worked with a civil engineer to determine cobble size for bridging under an asphalt parking lot, with the objective if maintaining air flow, water infiltration, and biotic communities to act as a biological filter. The engineer specified the substrate material, and we added the organic matter, bacterial inoculants,mycorrhizal fungi. All storm water was directed through this mixture and into tree wells around existing Ponderosa. It worked amazingly well. After about 10 years, we've only lost 2% of the trees we were trying to preserve. Sunmark is a good source, and I"m not sure who else would be a good resource for this type of practice. You could check with Cody (works for SunMark); he probably knows who else may be supplying these materials. Good luck! Rick Rick Martinson WinterCreek Restoration PO Box 1543 Bend, OR 97709 541-948-0661 541-382-1227 fax wntrcrkrest@bendbroadband.com OSU Department of Horticulture 4017 Agriculture and Life Sciences Building Oregon State University Corvallis, OR 97331 Office: (541) 737-3695 martinsr@onid.oregonstate.edu
I think it wouldn't have to be even that deep with the approach we used at the Parking Forest. The three foot deep requirement is for structural soils that are compacted to 95% proctor density (a very high level of compaction similar to conventional pavements). The subgrade at 3 feet is compacted, too, for extra protection and this gives the trees the three foot depth they need to form their structural roots. In the parking forest BMP, though, we approached the structural soil, not as if it were a conventional subgrade bearing material, but instead, as if it were a porous pavement material. It acts and is as much like this, anyway with it large open-graded (all the same size) rocks. We didn't compact the subgrade so the tree roots could go into it and we only used the minimum amount of structural soil (in our case, 2 feet) needed to support vehicular loads on a wet, uncompacted condition, taking the same approach as if we were building a porous pavement (even though in one case, we paved over the structural soil with conventional asphalt). If we limit ourselves to using structural soil in sidewalk areas, then pedestrian traffic would require even less structural soil depth, still on a wet, uncompacted soil condition. In this way, the uncompacted native soil is available to the tree for rooting and we can save a whole bunch of money on structural soil!
Silva Cells are an commercially produced support structure made of plastic for under pavement. The matrix is filled with a soil mix appropriate for trees. Silva cells have an advantage over structural soils in that 92% of the volume can be filled with planting medium, compared to something on the order of 22% for structural soils.