Filter Layer Gradation Calculator

Example Data Table

This example illustrates a quick screening using typical limits.

Always confirm criteria with your governing specification and material behavior.

Formula Used

This calculator evaluates two widely used gradation checks:

• Retention check: D15(filter) / D85(base) ≤ Limit
• Permeability check: D15(filter) / D15(base) ≥ Minimum

It also computes a recommended band for D15(filter):

• D15(filter)_min = Minimum × D15(base)
• D15(filter)_max = Limit × D85(base)

How to Use This Calculator

1. Obtain gradation curves for the base and proposed filter materials.
2. Read D15, D50, and D85 in millimeters.
3. Enter the values for both materials in the form.
4. Keep default criteria or adjust to match your project specification.
5. Press Submit to view ratios, pass/fail, and recommended D15 range.
6. Use the CSV/PDF buttons to export your latest calculation summary.

Professional Article

Filter layers are the quiet workhorses of drainage and earthworks. When a drain blanket, French drain, or retaining wall heel zone lacks a compatible filter, fine base particles can migrate into voids, clogging flow paths and raising pore pressures. A practical gradation check starts with characteristic grain sizes read from a particle-size distribution curve. The most common values are D15, D50, and D85, meaning the particle size at 15%, 50%, and 85% passing by mass.

This calculator applies two screening rules that balance retention and permeability. Retention aims to stop base movement, so the filter should not be excessively coarse relative to the base. A typical expression compares D15 of the filter with D85 of the base. Permeability aims to keep water moving, so the filter should not be too fine relative to the base; a common comparison uses D15 of the filter versus D15 of the base. Together, these ratios give a first-pass check before you confirm specifications, soil sensitivity, and construction tolerances.

Example dataset: Base D15 = 0.150 mm, Base D50 = 0.300 mm, Base D85 = 0.600 mm. Proposed filter D15 = 1.000 mm, Filter D50 = 2.000 mm, Filter D85 = 4.000 mm. Using a retention limit of 4 and a permeability minimum of 4, the retention ratio is D15f/D85b = 1.000/0.600 = 1.667, so retention passes because it is ≤ 4. The permeability ratio is D15f/D15b = 1.000/0.150 = 6.667, so permeability passes because it is ≥ 4.

The calculator also reports a recommended D15 band based on your criteria: D15f(min) = Minimum × D15b and D15f(max) = Limit × D85b. For the example, the band is 4×0.150 = 0.600 mm to 4×0.600 = 2.400 mm, and D15f = 1.000 mm sits inside the band. If your selected criteria produce a minimum higher than the maximum, the inputs are mathematically inconsistent and should be reviewed.

In practice, gradation shape and durability matter. A filter that meets D15 ratios can still perform poorly if it contains excessive fines, is gap-graded, or breaks down during compaction. Field moisture control, segregation during placement, and compaction energy can change hydraulic behavior. Use these results as a documentation step: capture inputs, record pass/fail verdicts, and attach the exported CSV or PDF to your quality file. A well-graded filter protects drains, reduces maintenance risk, and extends service life.

FAQs