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Geotextile Calculator

Permittivity equals normal hydraulic conductivity divided by thickness

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Permittivity Equation

Permittivity measures how easily water flows perpendicular to a geotextile fabric, in units of inverse seconds. It equals the normal hydraulic conductivity divided by the fabric thickness.

Ψ = Kₙ / t

Transmissivity Equation

Transmissivity measures how easily water flows within the plane of the geotextile fabric, in units of cm²/s. It equals the in-plane hydraulic conductivity multiplied by the fabric thickness.

θ = Kₚ × t

How It Works

Geotextiles are permeable fabrics used in civil engineering for filtration, drainage, separation, and reinforcement. Two key hydraulic properties govern how water moves through them: permittivity (Ψ = Kₙ / t) measures flow perpendicular to the fabric, while transmissivity (θ = Kₚ × t) measures flow within the plane of the fabric. Higher values mean better drainage.

Example Problem

A nonwoven geotextile is 0.3 cm thick with a normal hydraulic conductivity of 0.15 cm/s. What is its permittivity?

  1. Ψ = Kₙ / t = 0.15 cm/s / 0.3 cm = 0.5 s⁻¹

AASHTO M288 requires a minimum permittivity of 0.1 s⁻¹ for most subsurface drainage applications, so this fabric easily qualifies.

When to Use Each Variable

  • Solve for Permittivity — when you know the normal hydraulic conductivity and fabric thickness, e.g., verifying a geotextile meets AASHTO M288 requirements.
  • Solve for Normal Conductivity — when you know the permittivity and thickness, e.g., back-calculating conductivity from a lab permittivity test.
  • Solve for Thickness (Permittivity) — when you know the conductivity and permittivity, e.g., determining the fabric thickness from test data.
  • Solve for Transmissivity — when you know in-plane conductivity and thickness, e.g., evaluating a drainage layer's capacity to move water laterally.
  • Solve for In-Plane Conductivity — when you know transmissivity and thickness, e.g., extracting conductivity from a transmissivity test.
  • Solve for Thickness (Transmissivity) — when you know conductivity and transmissivity, e.g., verifying fabric thickness from hydraulic test results.

Key Concepts

Geotextile hydraulic properties govern how water interacts with the fabric. Permittivity (Ψ = Kn/t) measures cross-plane flow capacity and is critical for filtration applications. Transmissivity (θ = Kp × t) measures in-plane flow capacity and is critical for drainage layers. Both properties depend on fabric thickness, construction (woven vs. nonwoven), and the applied confining pressure.

Applications

  • Subsurface drainage: selecting fabrics for French drains, retaining wall drains, and pavement edge drains
  • Erosion control: using geotextiles as filters behind riprap and gabion walls to prevent soil piping
  • Landfill engineering: specifying drainage layers in leachate collection systems
  • Road construction: separating subgrade soil from aggregate base to prevent intermixing and maintain structural integrity

Common Mistakes

  • Confusing permittivity and transmissivity — permittivity is cross-plane flow (filtration), transmissivity is in-plane flow (drainage)
  • Ignoring confining pressure — permittivity and transmissivity decrease significantly under load as the fabric compresses
  • Using woven geotextile where nonwoven is needed — woven fabrics have lower permittivity and are better for separation, not filtration
  • Not matching the apparent opening size (AOS) to the soil — an oversized AOS allows soil migration and clogging

Frequently Asked Questions

What is the difference between geotextile permittivity and transmissivity?

Permittivity describes how easily water flows through the fabric (perpendicular to the surface), measured in s⁻¹. Transmissivity describes how easily water flows along the plane of the fabric, measured in cm²/s. Filtration applications rely on permittivity; drainage layers rely on transmissivity.

How do you select a geotextile for a retaining wall drain?

Match the geotextile’s permittivity and apparent opening size (AOS) to the surrounding soil. The permittivity should be at least 10 times the soil’s hydraulic conductivity to prevent clogging, and the AOS should retain at least 85% of the soil particles.

What is hydraulic conductivity in a geotextile?

Hydraulic conductivity (K) is the rate at which water moves through the fabric under a given head gradient, typically reported in cm/s. Normal conductivity (Kₙ) applies to cross-plane flow; in-plane conductivity (Kₚ) applies to flow along the fabric.

What minimum permittivity does AASHTO M288 require?

AASHTO M288 specifies minimum permittivity values of 0.5, 0.2, or 0.1 s⁻¹ depending on the percent of soil fines passing the #200 sieve (<15%, 15-50%, or >50% fines, respectively). Soils with more fines require higher flow capacity to prevent clogging.

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