--- title: "What is Filtration in Geotextile?" date: 2022-05-22 author: "Pawan Khatri" url: https://oceangeosynthetics.com/what-is-filtration-in-geotextile/ --- May 22, 2022 | [Geotextile](https://oceangeosynthetics.com/category/geotextile/), [Insights](https://oceangeosynthetics.com/category/insights/) # What is Filtration in Geotextile? ![](https://oceangeosynthetics.com/wp-content/uploads/2022/05/filtration-300x300.jpg) ## What is Geotextile? The word [geotextile](https://oceangeosynthetics.com/products/geotextile-manufacturers/) joins “geo”, from the Greek for earth, and “textile”. A geotextile is a permeable fabric used with soil, rock or other construction materials in civil engineering. Geotextiles belong to the technical textiles family. Most are made from polypropylene or polyester, either woven from tapes and yarns or formed as a nonwoven felt of bonded fibres. Their openings are small enough to hold back soil particles while letting water through, and that combination is what makes them useful for filtration. ## Table of Contents ## Geotextile for Filtration ![](https://oceangeosynthetics.com/wp-content/uploads/2022/05/filtration-300x300.jpg) Filtration is one of the main functions of a [geotextile](https://oceangeosynthetics.com/products/geotextile-manufacturers/) in earthworks. A filter geotextile lets water flow across it, out of the soil, while holding the soil particles in place. Without it, flowing water carries fines out of the soil and into drains, gravel layers or the sea, which leads to clogged drains, voids and settlement. Both woven and nonwoven geotextiles can filter, though needle-punched nonwovens are the usual choice. Two properties govern filtration: the size of the openings (apparent opening size, ASTM D4751) and how easily water passes through the fabric (permittivity, ASTM D4491). Thicker nonwovens also carry water within their own plane, so they can drain sideways as well as filter. Filter geotextiles are laid both vertically, as in trench drains and behind retaining walls, and horizontally, as under roads and riprap, to deal with drainage problems along roads and structures. ## How a Non-Woven Geotextile Filter is set up A geotextile matched to the soil grading lets a small amount of the finest soil pass through when water first starts to flow. In the process it builds a natural filter zone in the soil next to it. Close to the fabric the water moves fastest and carries the fine and medium particles through. A little further back the water has less energy and can move only the finest particles. Further back again the soil stays put. What remains is a graded filter in the soil: coarse particles against the geotextile, then coarse and medium, and finally coarse, medium and fine. Once that zone forms, the soil holds itself in place and the flow settles to a steady rate. ## Do Geotextile Filters Clog? All filters clog to some extent, whether they are geotextile or graded stone. A filter geotextile is chosen to be far more permeable than the soil it protects; against fine sands and silts it can pass 1,000 times more water than the soil. If such a geotextile lost 99% of its flow capacity, it would still pass ten times what the soil delivers. So the useful question is whether it will clog badly enough to matter, and that depends on the site conditions. ## Usual Conditions In usual conditions, a correctly specified filter geotextile will not clog to the point of failure. “Usual” here means soil and water conditions that do not fall into any of the categories below. ## Unusual Conditions The following conditions cause problems for geotextile filters, and for traditional stone filters as well: Uniformly graded fine cohesionless soils, where nearly all particles are the same size, such as loess, rock flour, pulverised fuel ash (PFA) and stone quarry fines. Dispersive clays, which break down into individual fine particles in water over time (most clays do not). Gap-graded cohesionless soils, with missing particle sizes, combined with high-energy water flow. Highly alkaline groundwater, where the water slows as it passes through the geotextile and deposits calcium, sodium or magnesium precipitates. Liquids with high suspended solids, such as turbid river water or dredged slurry, or with a high microorganism content, as in bioreactor filter beds, old mine workings releasing orange ochre, and agricultural wastes. High-energy or reversing flows, as in tidal zones and coastal works. In these cases, get specialist advice on the filter design. A specialist geotextile often gives good filtration performance. In others the geotextile may clog, shortening its working life, and it may need periodic replacement. For the numbers behind filter selection, our article on [how water passes through geotextile fabric](https://oceangeosynthetics.com/does-water-go-through-geotextile-fabric/) explains permittivity and apparent opening size, with a worked example and the conditions that make a fabric clog. ## Ocean Non Wovens: filtration geotextile manufacturer in India ![](https://oceangeosynthetics.com/wp-content/uploads/2022/05/ocean-global-1-300x300.png) Ocean Global manufactures and supplies geosynthetics in India, including geotextiles, geogrids, [geomembranes](https://oceangeosynthetics.com/products/ocean-geomembrane/), geocomposites and geonets. The company has more than 21 years of experience in the field. For an example of filter geotextile in use, see the [non-woven geotextile at the Sultanganj intake pump house](https://oceangeosynthetics.com/non-woven-geotextile-for-intake-pump-house/). 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Of note, over the past… [See More](https://oceangeosynthetics.com/geosynthetic-clay-liner-vs-compacted-clay/) [![](https://oceangeosynthetics.com/wp-content/uploads/2020/11/Geomembrane26-300x225.jpg)](https://oceangeosynthetics.com/geomembranes-the-multipurpose-geosynthetics/) [Geomembrane](https://oceangeosynthetics.com/category/geomembrane/), [Insights](https://oceangeosynthetics.com/category/insights/) [Geomembranes, the multipurpose Geosynthetics](https://oceangeosynthetics.com/geomembranes-the-multipurpose-geosynthetics/) What are geomembranes? 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