Geotextile filter criteria calculator.
Three filter tests in series: retention, permeability, anti-clogging. Per FHWA-NHI-07-092 (Holtz et al., 2008) and BS 8006. Enter soil grading characteristics, get required O95 (AOS), required permittivity ψ, gradient-ratio target, and recommended nonwoven geotextile grade.
What each output means.
Rule 1: Retention
The fabric must hold back the protected soil. Express as O95 (ASTM D4751) ≤ multiplier × D85 of the soil. Stable cohesionless soil uses O95 ≤ D85. Internally unstable or gap-graded soils tighten this to O95 ≤ D50. Dynamic (reversing flow) conditions tighten further to O95 ≤ 0.5 × D85.
Rule 2: Permeability
The fabric must let water through faster than the protected soil. Express as ψ × t (effective k of geotextile) ≥ 10 × k_soil for standard; ≥ 100 × k_soil for critical or high-fines. For Malaysian residual soils with k around 10⁻⁶ m/s, this is rarely the binding rule; on clean sands and gravels it can be.
Rule 3: Anti-clogging
The fabric must remain open through service life. Gradient-ratio test (ASTM D5101) on the soil-geotextile sandwich; ratio < 3 (or < 2 for critical) means flow stays adequate. Long-term flow capacity (LFTL) testing is the higher-confidence variant for dispersive or unusual soils.
Starting spec.
For typical Malaysian residual soil with 20-40% fines, k around 10⁻⁶ to 10⁻⁷ m/s, the calculator usually returns: O95 ≤ 0.10-0.18 mm, permittivity ≥ 1-3 s⁻¹, nonwoven PP 250-350 gsm. This matches the STRATA datasheet for grades commonly stocked through Starwall for Malaysian drainage and filter applications.
Other tools.
Need a gradient-ratio test on your project soil?
WhatsApp the gradation curve and project parameters; we'll match it to the right STRATA nonwoven grade.