Physical Properties of Bioretention Soils
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Physical Properties of Bioretention Soils
Scott WikstromCity of Walnut Creek
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Stormwater Treatment
Rainfall
x
Tributary Area
x
Soil Infiltration (Biotreatment)
=
Clean Water
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Why 5? (in/hr)
• Compromise between treatment and loading rate
• Treatment area: 4% of tributary impervious area
• Equivalent Rainfall of 400-600 inches per year
• Seems to work!!!
• Adopted into Municipal Regional Permit
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Soil Infiltration Rate
• Gravel = high infiltration
• Sandy Loam = Just Right!
• Clays = low infiltration
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Sandy Loam or is it Loamy Sand?
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Soil Mechanics 101
• Three types of soil: Sand, Silt, and Clay
• Silt is just very small Sand
• Finer than 0.0029 inches (#200 sieve)
• Clays are different
• Finer than 0.0029 inches (#200 sieve)
• Thin, flat plate-like shape
• Negative electrical charge attracts and bonds to water
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Soil Infiltration (Permeability) Ranges
• Gravel ~ 10,000 inches / hr
• Clean Sand ~ 50 inches / hr
• Fine Sand / Silt ~ 1 inch / hr
• Silty Sandy Clay ~ 0.05 inches / hr
• Clay ~ 0.001 inches / hr
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Stormwater Treatment
Rainfall x Tributary Area x Soil Infiltration
= Clean Water
Rainfall +/- 100%
Area +/- 20%
Infiltration +/- 10,000%
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Soil Variability
• Vertical Variation
• Horizontal Variation
• Parent Rock Material
• Deposition Method
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Contra Costa Soil Spec – Part I
• Several installations using on-site soils failed
• Soil Supplier mix worked well
• Well graded mix of two local soils
• Some “Topsoil”
• Amended with Compost
• How to Verify?
• Hard to replicate throughout Bay Area
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Contra Costa Soil Spec - Part II
• Target: 8-12 inches per hour
• Goal: 5-50 inches per hour
• Limit fines content, especially clay
• Readily available – not a proprietary soil blend
• Consistent mix
• Testing and verification considerations
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Soil Gradation Curves
0
10
20
30
40
50
60
70
80
90
100
0.0010.010.11
Particle Diameter (inches)
Per
cen
t F
iner
by
Wei
gh
t
Mix B Composite
ASTM C33 - Fine Aggregate
Recommended Mix
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Contra Costa Soil Spec – Part III
• Performance Spec
• Minimum 5 inches per hour infiltration rate
• Support Healthy Plant Growth
• Two Mixes provided: Fine Sand with Compost
• ASTM C33 Fine Aggregate “Concrete Sand”
• Contra Costa Clean Water Program Verification
• Inspectors pull tags at jobsite.