Geotextile Permittivity
Calculate a geotextile's permittivity, ψ = k_n ÷ t, from the cross-plane permeability k_n (m/s) and the geotextile thickness t (m); the result, in s⁻¹, is the permittivity. Permittivity characterizes the geotextile's ability to let water pass PERPENDICULAR to its plane (through the fabric), and is the key property in the FILTRATION and cross-plane DRAINAGE functions. It is defined as permittivity (not simply permeability) because a geotextile's thickness is small, variable and hard to measure precisely under load — so permeability is normalized by thickness, giving a property (ψ = k/t) measurable directly from flow per unit area and gradient, without knowing the thickness. In a geotextile filter (replacing the traditional graded sand filter in drains, behind retaining walls, under riprap), the geotextile must be permittive enough to let water pass freely (without damming and building pore pressure), but with pores small enough to RETAIN the soil particles (without clogging or letting soil escape — the retention criterion). The balance between permittivity and retention is the heart of geotextile filter design. Enter the cross-plane permeability and the thickness.
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Permissividade de geotêxtil
A permissividade de um geotêxtil é ψ = k_n ÷ t, a partir da permeabilidade normal ao plano k_n e da espessura t; o resultado, em s⁻¹, é a permissividade. Ela caracteriza a capacidade do geotêxtil de deixar a água passar perpendicularmente ao seu plano (atravessando o tecido), e é a propriedade-chave nas funções de filtração e drenagem transversal. Define-se como permissividade (e não simplesmente permeabilidade) porque a espessura de um geotêxtil é pequena, variável e difícil de medir com precisão sob carga — então normaliza-se a permeabilidade pela espessura, obtendo uma propriedade (ψ = k/t) medível diretamente pela vazão por unidade de área e gradiente, sem precisar conhecer a espessura. Em um filtro geotêxtil (que substitui o tradicional filtro de areia graduada em drenos, atrás de muros de arrimo, sob enrocamentos), o geotêxtil deve ser permissivo o suficiente para deixar a água passar livremente (sem represar e gerar poropressão), mas com poros pequenos o suficiente para reter as partículas de solo (sem colmatar nem deixar o solo fugir — o critério de retenção). O equilíbrio delicado entre permissividade e retenção é o coração do projeto de filtros geotêxteis, que substituíram em grande parte os filtros granulares por serem mais finos, fáceis de instalar e de desempenho controlado. Informe a permeabilidade normal e a espessura.
Related Tools
Geotextile Transmissivity
Calculate a geosynthetic's transmissivity, θ = k_p·t, from the in-plane permeability k_p (m/s) and the thickness t (m); the result, in m²/s, is the transmissivity. Transmissivity characterizes the geosynthetic's ability to convey water WITHIN its own plane (longitudinally, like a planar drain), and is the key property in the DRAINAGE function. While permittivity measures flow THROUGH the geotextile (perpendicular), transmissivity measures flow ALONG it (parallel). It is the fundamental property of drainage geocomposites and geonets — products with a 3D open core (geonet) between filtering geotextiles, used to drain water replacing gravel layers: drainage behind retaining walls, under landfills (leachate and gas collection), in roads, sports fields and gardens (subsurface drainage), and in foundations. Transmissivity depends strongly on confining PRESSURE (the more compressed, the less space for water to flow and the lower θ) and on gradient, so it is specified at the work's real load conditions. Times the gradient and width, it gives the drained flow. Enter the in-plane permeability and the thickness.
Geocomposite Drain Flow
Calculate the drainage flow of a drainage geocomposite, q = θ·i·b, from the transmissivity θ (m²/s), the hydraulic gradient i (dimensionless) and the drain width b (m). This is the practical application of transmissivity: it estimates how much water a drainage geocomposite (geonet between geotextiles, or drainage geotextile) can convey in its plane, to check whether it adequately replaces a gravel layer or conventional drain. The flow is the product of transmissivity (the drain's in-plane 'conductivity' at the work's confining pressure), the hydraulic gradient (the head-line slope driving the flow) and the drain width (the drainage front). It is Darcy's law applied to in-plane flow in the geosynthetic. This calculation is essential to size drainage systems with geocomposites: gas and liquid drainage in landfills and mining, drains behind walls and cutoffs, green-roof and buried-structure drainage, and road and railway drains. The flow the geocomposite provides is compared with the design flow (the water to drain, with a safety factor); if insufficient, a higher-transmissivity geocomposite is chosen or the width increased. Enter the transmissivity, hydraulic gradient and width.
Geosynthetic Tensile Stiffness
Calculate a geosynthetic's tensile stiffness (secant stiffness modulus), J = T ÷ ε, from the tensile force per unit width T (kN/m) and the corresponding strain ε (dimensionless, or ε/100 if in %); the result, in kN/m, is the stiffness. Unlike conventional materials, where stiffness is Young's modulus (stress/strain, in Pa), in geosynthetics the 'stress' is expressed per unit WIDTH (kN/m, since thickness is ill-defined and variable), so the stiffness J is also in kN/m. Tensile stiffness is fundamental in soil reinforcement design because geosynthetics only mobilize force when they DEFORM (stretch): the higher the stiffness J, the smaller the deformation needed to reach the required reinforcement force. This is crucial because reinforced-soil structures have ALLOWABLE deformation limits (a wall cannot bulge too much, an embankment cannot settle excessively) — so design is often controlled by stiffness (deformation) rather than strength (rupture). Modern reinforcement geosynthetics (polyester or HDPE geogrids) have high stiffness to limit deformations. Stiffness is measured in the wide-width tensile test, usually at a reference strain (2%, 5%). Enter the tensile force and the strain.
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