How to Choose the Maximum Aggregate Size for Concrete
Maximum gravel size depends on member dimensions, bar spacing and placement method. Learn the common rules and how this choice affects cement use and workability.
Industrial applications
Drainage and Filter Materials: Selection Criteria and Terzaghi's Rule
A drainage layer must pass water and hold back soil. Learn Terzaghi's filter criteria, material cleanliness and how to place French drains and geotextiles.
A drainage layer has two apparently opposing jobs: it must pass water easily while holding back particles of the adjacent soil. Striking this balance depends above all on the grading of the material.
Filter criteria
Terzaghi’s classic rule for granular filters has two conditions. D15 and D85 are the sizes that 15 and 85 percent of a material passes.
- Retention: the filter’s D15 should be less than 4 to 5 times the soil’s D85, so soil does not pass through the filter.
- Permeability: the filter’s D15 should be more than 4 to 5 times the soil’s D15, so water passes freely.
Design guides have refined this rule for different soils, and the designer sets the final grading.
Material properties
| Property | Reason |
|---|---|
| Clean, low fines | Stops the water path clogging |
| Suitable grading | Provides both retention and flow |
| Durability | Stability in constant contact with water |
Rounded natural gravel and pea gravel are common options for this use.
French drains and geotextiles
In trench drains, geotextile is usually wrapped around the gravel to act as the filter. The fabric must suit the adjacent soil, because the wrong fabric clogs. A pipe and suitable fall take collected water to the discharge point.
Common mistakes
- Using unwashed material that later clogs the drain
- Contaminating the layer with soil during spreading
- Providing no outlet for the collected water
- Accepting material on appearance alone
See the requirements of this field on the Drainage and filtration page.
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