--- title: "Does Water Go Through Geotextile Fabric? How Drainage Fabric Works" date: 2022-01-14 author: "Pawan Khatri" url: https://oceangeosynthetics.com/does-water-go-through-geotextile-fabric/ --- January 14, 2022 | [Geotextile](https://oceangeosynthetics.com/category/geotextile/), [Insights](https://oceangeosynthetics.com/category/insights/) # Does Water Go Through Geotextile Fabric? How Drainage Fabric Works Yes. Water goes through geotextile fabric, and for drainage fabric that is the whole point: the fabric is porous, so water passes while soil grains stay behind. A needle-punched nonwoven geotextile lets water through across its thickness and also along its plane. A woven geotextile passes water too, but usually much less. The only common “fabric” that stops water is a geomembrane, which is a solid plastic sheet rather than a textile. How much water a particular drainage fabric passes, and whether it keeps passing it for years, depends on the grade, the soil against it and how it is installed. ## How water moves through drainage fabric A nonwoven geotextile is a mat of polypropylene or polyester fibres locked together by needle punching. Between the fibres is open space, usually most of the fabric’s volume, and those connected pores are the paths water follows. Two directions matter: - **Across the fabric (cross-plane).** Water passes from the soil on one side to the gravel, pipe or drain on the other. This is the filter function, and it is what most drainage fabric is bought for. - **Along the fabric (in-plane).** Water travels sideways inside the fabric. Thick nonwovens carry a small in-plane flow, which helps vent gas under pond liners, but for real drainage capacity along a layer you need a [geocomposite drainage layer](https://oceangeosynthetics.com/what-is-a-geocomposites-drainage-layer/) with a ribbed core. Woven fabrics made from flat slit-film tapes have few open paths between the tapes, so their flow is lower. Use them where strength and separation matter most, and nonwovens where water must pass freely. ## Measuring it: permittivity and opening size Two laboratory numbers on a datasheet tell you how a drainage fabric handles water. **Permittivity** (symbol ψ, unit s⁻¹) is the cross-plane flow capacity. It is measured to ASTM D4491, and in India to IS 14324, which BIS revised in 2024 to align with ISO 11058. Permittivity equals the fabric’s permeability divided by its thickness, so it lets you compare fabrics of different thickness directly. The BIS standard itself makes that point: comparing geotextiles by permeability alone can mislead. **Apparent opening size** (AOS or O₉₅, in mm) is roughly the size of the largest particle the fabric lets through. It is measured to ASTM D4751, or by dry sieving to IS 14294:2024. A smaller AOS holds back finer soil. For example, the filter geotextile on Ocean’s MoRTH geocomposite has a permittivity of 0.50 s⁻¹ and an AOS of 0.220 mm. Ocean’s full range of [geotextiles](https://oceangeosynthetics.com/products/geotextile-manufacturers/) covers nonwoven PP and PET from 90 to 1000 GSM, plus woven, coir and jute. ### Worked example (illustrative) These figures are assumptions to show the arithmetic, not values for a particular product. A nonwoven drainage fabric with a permittivity of 0.5 s⁻¹ and a thickness of 2 mm has a permeability of 0.5 × 0.002 = 1 × 10⁻³ m/s. A silty sand behind it might have a permeability around 1 × 10⁻⁶ m/s, a thousand times lower. The fabric is nowhere near the limit; the soil controls how fast water arrives. Under a head of just 50 mm across the fabric, one square metre would pass 0.5 × 0.05 × 1 = 0.025 m³ per second, or 25 litres per second. Real drains carry a tiny fraction of that. Clean fabric is rarely the bottleneck, which is why the questions that matter are about keeping it clean. ## Choosing a drainage fabric that keeps passing water A filter fabric has two jobs that pull in opposite directions. Its pores must be small enough to hold the soil, and open enough that water passes and fine particles do not collect inside it. The checks most designers use, set out in the FHWA geosynthetics design manual by Holtz, Christopher and Berg, are: 1. **Retention.** The AOS must be smaller than a multiple of the soil’s D₈₅ (the grain size that 85% of the soil is finer than). Fine silts need a tighter fabric than coarse sands. 2. **Permeability.** The fabric’s permeability should exceed the soil’s. For critical or severe conditions, such as dam drains or walls where failure is costly, designers ask for ten times the soil value. 3. **Clogging resistance.** Prefer nonwovens with a high porosity, and avoid the smallest possible AOS when the soil is gap-graded or has a lot of fines. 4. **Survivability.** The fabric has to come through installation intact. Coarse angular stone dropped on it needs a heavier grade than rounded gravel placed by hand. For subsurface drainage, AASHTO M288 also lists minimum permittivity by soil type: 0.5 s⁻¹ for soils with less than 15% passing the 75 micron sieve, 0.2 s⁻¹ for 15–50%, and 0.1 s⁻¹ for more than 50%. ## When water stops going through Drainage fabric rarely fails because it was too tight on day one. It fails because something changes in service: - **Blinding.** A thin cake of fines builds up on the soil face of the fabric. It is most likely with very fine, uniform silts and fabrics whose openings are too small. - **Clogging inside the fabric.** Particles lodge in the pore structure. Iron-rich groundwater can also deposit ochre in drains. - **Sunlight.** Polymer fabrics left uncovered lose strength. Cover them promptly after laying. - **Contamination during construction.** Muddy runoff washing over an open drain trench can clog the fabric before the drain is ever used. Keep trenches covered or protected in the monsoon. - **Wrong product.** A geomembrane or a plastic sheet laid where a filter fabric was specified will stop water completely. Our article on [filtration in geotextiles](https://oceangeosynthetics.com/what-is-filtration-in-geotextile/) covers how a filter fabric works. ## Where drainage fabric goes on Indian projects - **French drains and road edge drains.** A trench lined with nonwoven fabric, filled with clean aggregate and fitted with a perforated pipe. Ocean supplies [PVC geo pipes](https://oceangeosynthetics.com/products/pvc-geo-pipes/) for the drainage pipe. - **Behind retaining walls and abutments.** A filter layer against the backfill, draining to weep holes. See our [drainage of structures](https://oceangeosynthetics.com/solution/drainage-of-structures/) solutions. - **Sports fields and landscaping.** Separating the root zone from the drainage gravel. A 120 GSM nonwoven was used for a [football field in Mumbai](https://oceangeosynthetics.com/football-field-project-using-nonwoven-geotextile-in-mumbai/), and geotextile drainage work was done on a [landscaping drainage upgrade in Khorda, Odisha](https://oceangeosynthetics.com/landscaping-drainage-upgrade-in-khorda-odisha/). - **River intakes and banks.** A filter under stone or geobags stops the bank washing out through the gaps. Ocean nonwoven geotextile was used at the [Sultanganj intake pump house](https://oceangeosynthetics.com/non-woven-geotextile-for-intake-pump-house/). For roll sizes and GSM options, see [non-woven geotextile drainage fabric sizes and specs](https://oceangeosynthetics.com/non-woven-geotextile-drainage-fabric-sizes-specs-prices/). ## Frequently asked questions ### Is drainage fabric waterproof? No. Drainage fabric is a permeable geotextile designed to let water through while holding soil back. If you need to stop water, you need a geomembrane such as an HDPE liner. ### Does woven or nonwoven geotextile drain better? Nonwoven needle-punched geotextile usually passes far more water and resists clogging better, so it is the usual choice for drains and filters. Woven fabrics are chosen for strength and separation under roads and embankments. ### Which test tells me how much water a geotextile passes? Permittivity, measured to ASTM D4491 or IS 14324, gives the cross-plane flow capacity in s⁻¹. Compare that figure, not GSM, when choosing a drainage fabric. ### Can drainage fabric clog over time? It can if the opening size is wrong for the soil, if fines wash into an open trench during construction, or if iron-rich groundwater deposits in it. Matching AOS to the soil and protecting the drain during construction prevent most clogging. ## Need help picking a drainage fabric? Tell us the soil type, the drain detail and the aggregate you plan to use, and we will suggest a geotextile grade. [Contact Ocean Non Wovens](https://oceangeosynthetics.com/contact/) for a recommendation. 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