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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.
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.
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% |
Particle Size 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) |
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