Ground engineering · Ground improvement
Granular filter design (Terzaghi) calculator
Retention D15f/D85s ≤ 4–5 and permeability D15f/D15s ≥ 4–5 checks, with uniformity guide.
Standard: Terzaghi filter rules · USBR / Sherard
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Inputs in the worked example
The live calculator starts with these values; change any of them and every result, check and diagram updates instantly.
| Input | Value | Unit |
|---|---|---|
| Base soil D15 | 0.02 | mm |
| Base soil D85 | 0.6 | mm |
| Filter D15 | 0.3 | mm |
| Filter D60 | 2 | mm |
| Filter D10 | 0.2 | mm |
| Retention limit | 4 | |
| Permeability limit | 4 |
Worked example — results
| Result | Value | Unit |
|---|---|---|
| D15f/D85s | 0.500 | |
| D15f/D15s | 15.00 | |
| Filter Cu | 10.00 |
- ✓ Retention
- ✓ Permeability
- ✓ Filter Cu ≤ 20 (segregation)
Method, formulas and assumptions
- D15f/D85s = 0.3/0.6 = 0.5
- D15f/D15s = 0.3/0.02 = 15
- Filter Cu = 2/0.2 = 10
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For preliminary design and education. Check every result against the current standard and your own engineering judgement before use.