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Parking Forest Questions from the City of Gresham
 

Parking Forest Questions from the City of Gresham

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Gresham is considering using structural soil under their sidewalks to improve tree health and increase tree canopy but staff had a number of questions.

Gresham is considering using structural soil under their sidewalks to improve tree health and increase tree canopy but staff had a number of questions.

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  • 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.

Parking Forest Questions from the City of Gresham Parking Forest Questions from the City of Gresham Presentation Transcript

  • 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
  • About Me
  • About You
  • Why was I invited here today? Tualatin Riverkeepers’ Vision: the Parking Forest • Increase canopy in parking lots • Manage stormwater • No loss of parking spaces
  • Two Sites Watershed base map courtesy of Tualatin River Watershed Council Redevelopment Retrofit
  • Structural Soil (dry) The Parking Forest A Close Up (12 feet wide) Pavement outside of Parking Forest Re-paved areaTree Structural SoilAmended native clayey soilUndisturbed native clayey soil Crushed aggregate Wheel stop
  • THPRD Sunset Swim Center Redevelopment with Pervious Concrete
  • THPRD Sunset Swim Center in Plan Redevelopment with Pervious Concrete
  • PCC Sylvania Lot 10 Retrofit with Impervious Asphalt
  • PCC Sylvania Lot 10 in Plan
  • PCC Sylvania Lot 10 Drainage Area 6133 sf 1919 sf 754 sf 615 sf 2815 sf 6103 sf Parking Lot Sidewalk <50% Grass HSG C Woods, Good HSG C
  • Question How do structural soils work?
  • Trees Need to Access Soil to a Depth of 3 Feet Minimum Photo Source: gratefulmommy.com
  • 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)
  • Structural Soil Composition Source: Cornell University – CU Structural Soil - http://www.hort.cornell.edu Structural stability (similar to porous pavement aggregate): Tree health:
  • Question • What is different about structural soils than regular construction practices and soils in sidewalk/planter strip applications?
  • 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.
  • 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
  • Additional Benefit for Tree Health • Shallow depths of structural soil = little or (more likely) no fertilization
  • Assembly Procedure
  • Assembly Procedure
  • Assembly Procedure
  • Assembly Procedure
  • Assembly Procedure
  • Assembly Procedure
  • Assembly Procedure
  • Are there studies or examples showing the benefits? • Been around since the 1990s • Bend, OR
  • Olympia Results… After 8 Years Control Trees Planted in Compacted Native Soil Trees Planted in Structural Soil
  • 4 minute video from Cornell Horticulture (developed CU Structural Soil) • “Excavation of tree growing in CU-structural soil”
  • Palo Alto, CA is hooked Jan 29, 2004 presentation by Dave Dockter, Managing Arborist , Planning Division
  • From Dave Dockter’s Presentation
  • From Dave Dockter’s Presentation
  • 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?
  • Materials are Commonly Available • Rock • Clay • Optional amendments (Our mix included mycorrhizae, biota, biochar, Stabilizer = hydrogel and moisture holding element) • Hydrogel
  • 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)
  • Choosing Adequate Depth for Structural Stability • Our geotechnical engineer determined an appropriate pavement depth based on:
  • 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.)
  • 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
  • Tree planted in amended native soil
  • What are the implementation issues for using structural soils? Construction observation • Moisture • Compaction • Depth of lifts • Final depth
  • Question • What are the vendors/sources of structural soils? What different materials and designs are available? How do they range in cost?
  • Vendors & Costs • Sunmark Environmental,$105/cy delivered • Olympia DIY, $24/cy (delivered?)
  • 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.as hx Source: City of Olympia -Structural Soil Demonstration Project
  • Vendors & Costs • Sunmark Environmental,$105/cy delivered • Olympia DIY, $24/cy (delivered?) • CU Structural Soil (coming soon), national average $58.25/cy (not delivered)
  • Question • How can the stability of the adjacent road subgrade and curb be protected during the installation of structural soils?
  • Protecting Existing Infrastructure • Trees don’t seem to really care where the soil is, as long as they can get their roots to them.
  • Protecting infrastructure • Gresham staff idea: In retrofits, could set structural soil back from the curb.
  • Question • What different materials and designs are available?
  • 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 Pavement Jim 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
  • 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?
  • SE 182nd Avenue
  • Retrofit
  • New Development
  • 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?
  • Specs & Designs to review • I’ve consolidated everything I could find, plus my own details and specs, here: http://www.parkingforest.org/specs.htm
  • 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!
  • Thank You! Sustainability for all the places between the buildings 503.334.8634 www.greengirlpdx.com greengirl@greengirlpdx.com a certified women business enterprise