Geotextile Filter Calculator

Example Data Table

Formulas Used

• Opening upper limit (retention): O95_max = F_ret × D85
• Opening lower limit (clogging screen): O95_min = F_clog × D15
• Suggested target opening: O95_target = √(O95_min × O95_max)
• Seepage flux estimate (Darcy): q = k × i (reported as L/s/m²)
• Minimum permittivity screen: ψ_req = R × (k / t) where t is thickness in meters

How to Use This Calculator

1. Enter soil gradation values, especially D15 and D85.
2. Provide soil hydraulic conductivity and an expected gradient.
3. Set the design factors to reflect your project requirements.
4. Optionally enter a candidate fabric O95 and permittivity.
5. Submit to view targets, checks, and export-ready results.

Professional Guide: Geotextile Filter Selection

1) Why filtration matters in construction drainage

Filters protect drains, riprap, and granular layers by stopping soil migration while allowing water to pass. Poor filtration can cause piping, sinkholes, or loss of support beneath pavements and structures. This calculator helps you screen key parameters early, then confirm with project specifications and product submittals.

2) Soil gradation data you should use

Reliable gradation comes from representative sampling and lab sieve analysis. D15 indicates the finer fraction controlling clogging risk, while D85 indicates the coarser fraction that influences retention. If results vary across borings, base inputs on conservative percentiles and document assumptions for review.

3) Retention screening using opening size limits

The upper opening limit, O95_max, is computed as a factor times D85. Using a higher retention factor yields a larger allowable opening, but may increase the chance of soil loss in gap-graded materials. Start with typical factors and tighten them for sensitive foundations, embankments, or fine sands.

4) Clogging and blinding considerations

The lower opening limit, O95_min, is tied to D15 to reduce blinding from very fine particles. If soils contain silt or organics, clogging risk can rise even when gradation looks acceptable. Consider supplemental criteria such as gradient ratio testing, longer term flow monitoring, or staged filter layers.

5) Flow capacity: linking soil k, gradient, and permittivity

For a quick capacity check, seepage flux is estimated with Darcy’s relationship q = k·i. The calculator then screens minimum permittivity ψ using thickness and a user-set ratio R. Increasing R provides a safety margin when gradients are uncertain, drainage length is long, or fines are expected.

6) Candidate fabric checks for submittals

Entering a product O95 and ψ provides a pass/check flag against the computed targets. Treat a “CHECK” outcome as a prompt to review manufacturer data, test methods, and installation constraints. Also capture mass per unit area as a traceable identifier for QA/QC and field verification.

7) Installation factors that affect performance

Wrinkles, torn fabric, and poorly prepared subgrades can create concentrated flow paths and local erosion. Overlaps, seams, and anchorage details matter, especially under riprap or behind retaining walls. Keep the fabric clean, avoid excessive tension, and prevent punctures during placement of aggregates.

8) Reporting and decision workflow

Use the CSV export to attach calculations to design notes and the PDF export for site records. Record soil source location, test date, and the chosen factor set for traceability. Final selection should align with governing specifications, approved product lists, and engineering judgment. On site, verify rolls, lot numbers, and storage conditions. Photograph subgrade prep, overlaps, and termination details. If groundwater is aggressive, consider chemical resistance notes. Recheck after major rain events to catch early erosion signs before acceptance inspections.

FAQs

1) What is O95 in geotextile filters?

O95 is the apparent opening size where about 95% of openings are smaller. It helps screen retention and compatibility between soil particles and geotextile pore structure.

2) Which soil percentiles are most important here?

D15 and D85 are commonly used for quick filter screening. D15 relates to clogging potential, while D85 supports retention limits. Use project testing and conservative values when uncertain.

3) Why does the calculator ask for hydraulic gradient?

Gradient influences seepage demand. Higher gradients raise flow rates and may require higher permittivity. Enter a realistic design gradient based on drainage head, slope, and seepage path length.

4) What does “permittivity” mean for geotextiles?

Permittivity (s⁻¹) is a normal-to-plane flow measure, roughly conductivity divided by thickness. A higher value generally means water can pass through the fabric more easily under similar conditions.

5) If my candidate fabric fails the check, what should I do?

Review soil variability, confirm units, and compare with specification limits. Consider alternate fabrics, protective bedding layers, or additional testing such as gradient ratio to assess clogging risk.

6) Can one fabric work for all soils on a project?

Sometimes, but not always. Different borrow sources and zones can have different gradations and fines content. Group soils into classes, then select fabrics that satisfy the most demanding conditions.

7) Does this tool replace engineering specifications?

No. It is a screening calculator to speed decisions and documentation. Always confirm with contract specifications, product test standards, and the engineer-of-record’s requirements before procurement.