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USGA Recommendations
for Gravel When Intermediate (Choker)
Layer Is Not Used

Performance Factors Recommendations
Bridging Factor D15 (gravel) ≤ 8 x
D85 (rootzone)
Permeability Factor D15 (gravel) ≥ 5 x
D15 (rootzone)
Uniformity Factors D90 (gravel) / D15 (gravel) ≤ 3.0
No particles larger than 12 mm
≤ 10% particles smaller than 2 mm
< 5% particles smaller than 1 mm

The 3 - 4" layer of 2 - 9.5 mm Gravel is placed above the drain tile.  The 12" Rootzone Mix is placed above the gravel.

 

USGA Recommendations
When Gravel and Intermediate (Choker)
Layer Is Used

Material Description
Gravel:
Intermediate Layer is Used
≤ 10% particles larger than 1/2" (12 mm)
≥ 65% particles between 1/4" (6 mm) and 3/8" (9 mm)
≤ 10% particles smaller than 2 mm
Intermediate Layer Material ≥ 90% particles between 1 and 4 mm

The 3 - 4" layer of 1/4 - 3/8" Gravel is placed above the drain tile.  The 2 - 4" layer of 1 - 4 mm Intermediate (Choker) Layer is placed above the gravel.  The 12" Rootzone Mix is placed above the intermediate layer. 

 

Particle Size of USGA Rootzone Mix

Name Particle Diameter
mm
Recommendation
(By Weight)
Fine Gravel 2.0 - 3.4 ≤ 10% total particles in this range including a maximum of 3% fine gravel (preferably none)
Very Coarse Sand 1.0 - 2.0
Coarse Sand 0.5 - 1.0 Minimum of 60% particles must fall in this range
Medium Sand 0.25 - 0.50
Fine Sand 0.15 - 0.25 ≤ 20% particles within this range
Very Fine Sand 0.05 - 0.15 ≤ 5% Total particles in this range shall not exceed 10%
Silt 0.002 - 0.05 ≤ 5%
Clay ≤ 0.002 ≤ 3%

 

Physical Properties of USGA Rootzone Mix

Physical Property Recommended Range
Total Porosity 35-55%
Air-filled Porosity (at 30 cm tension) 15-30%
Capillary Porosity (at 30 cm tension) 15-25%
Saturated Hydraulic Conductivity Minimum of 6 inches/hr (150mm/hr)