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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?
- 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.
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.
Related Calculators
- Soil Resistivity Calculator -- measure soil electrical properties using the Wenner method.
- Seismic Geophone Calculator -- determine subsurface layer depth and velocity from refraction data.
- Oil Recovery Factor Calculator -- estimate recoverable petroleum reserves.
- French Drain Design Calculator — design drainage systems that often use geotextile fabric.
- Permeameter Calculator — measure soil permeability for filtration design.