The Ultimate Guide To Geotextile Fabric

Geotextile | Definition

Geotextiles are permeable textile materials used with soil. Most are made from polyester or polypropylene. They separate soil layers, filter water, drain, reinforce, and control erosion.

Different polymers and manufacturing processes give geotextiles different properties, which is why one product family covers so many civil construction applications. Geotextiles sit within the wider geosynthetics family, alongside geogrids, geonets, geomembranes and others used in geotechnical and environmental engineering.

Geotextile | Types

Geotextiles can be grouped by the fibre they are made from (natural or synthetic) and by how the fabric is made (woven, nonwoven or knitted). The fibre types come first:

Natural Fibers

Natural fibres come from plant, animal and mineral sources. Plant fibres such as jute have high strength and stiffness with low breaking extension, but they absorb water, creep under sustained load and biodegrade, so natural geotextiles suit temporary jobs such as erosion control while vegetation establishes. Mineral fibres are brittle and are rarely used for geotextiles. The natural fibres used for geotextiles include jute, sisal, flax, hemp, abaca, ramie and coir. Ocean supplies jute geotextile and coir geotextile.

Synthetic Fibers

Geotextiles made from synthetic polymers are geosynthetics. Geotextiles can be made from natural or synthetic fibres, so the two terms overlap but are not the same: natural geotextiles such as jute and coir are not geosynthetics, and many geosynthetics, such as geomembranes and geogrids, are not geotextiles. Four polymer families supply most geosynthetics: polypropylene, polyester, polyamide and polyethylene.

Polypropylene

Polypropylene (PP) is the most widely used polymer for geotextiles because of its low cost, adequate tensile properties and chemical inertness: it resists acids and alkalis across the pH range found in soils. Its low density means a low cost per unit volume. Its weaknesses are sensitivity to UV light (it needs stabilisers and should be covered soon after laying), loss of properties at high temperatures, and higher creep than polyester under sustained load. Store it out of the sun and follow the installation guidance.

Polyester

Polyester (PET) is used for its good creep resistance and better strength retention at high temperatures. It suits geotextiles that carry sustained high stress, such as reinforcement. Its weakness is hydrolysis in strongly alkaline conditions: avoid it where pH is above about 9, for example against fresh concrete or in lime- or cement-treated soil. Polyester staple fibre (PSF) geotextiles are economical and are made from PET chips, often from recycled PET.

Polyamide

Polyamide (PA), mainly Nylon 6 and Nylon 6,6, is used for some geotextiles and erosion mats, though far less than PP or PET because it costs more and absorbs water. As with the other polymers, the grade is chosen on strength, creep behaviour, fabric structure and finishing requirements. Only synthetic polyamides are used; natural polyamides such as silk are not.

Polyethylene

Polyethylene (PE), mostly HDPE, appears in geotextiles mainly as woven tape fabrics and coated woven sheets. It has good tear and puncture resistance and chemical resistance, can be heat-welded, and resists UV when carbon black or stabilisers are added. It expands and contracts more with temperature than PP or PET, which matters for exposed sheets.

Differences between Woven and Non-woven Geotextile

Picking the right geotextile starts with the difference between the two main types. In general, woven geotextiles are stronger for their weight, while nonwoven geotextiles have higher flow rates and absorb more water. A summary:
PHYSICAL ATTRIBUTES: Woven fabrics feel smooth and plastic-like; nonwoven fabrics feel like felt.
MANUFACTURING PROCESS: Woven geotextiles are made on a loom from tapes or yarns; nonwoven geotextiles are made by bonding fibres together, usually by needle punching or heat.
REFER: Wovens are usually specified by tensile strength (kN/m); nonwovens by mass per unit area (grams per square metre, GSM).
STRENGTH: The crossing yarns of a woven fabric give it high tensile strength at low strain, which suits reinforcement and stabilisation.
ELONGATION: Nonwoven geotextiles stretch much further before breaking (more than 50%) than woven geotextiles (between 5% and 25%).
WEIGHT: Weight is rarely the main specification for wovens, because they are chosen for strength and separation. For nonwovens, weight (GSM) is the main specification.
COST: For separation and filtration, nonwovens are usually the economical choice and are quick to produce. Where high tensile strength is needed, wovens give more strength per rupee.

6 Popular Applications Of Geotextiles

Geotextiles are used in roads, drains, harbour works, breakwaters and many other civil engineering jobs. The same fabric can do different jobs depending on where it is placed. These are the six functions engineers specify most often.

Separation

The geotextile is laid between two different materials to keep them from mixing: subgrade and aggregate, two types of soil, or old and new pavement. On a road, it stops the aggregate base from punching into a soft subgrade and stops fines pumping up into the base. Separation and stabilisation often happen together, but they are different functions.

Stabilization

In stabilisation, the geotextile is laid on a soft, usually wet, subgrade. It separates the subgrade from the aggregate above and lets pore water escape upward into the free-draining layer. As the subgrade drains it consolidates and gains strength, so the base above performs better.

Reinforcement

In reinforcement, the geotextile itself carries tensile load, rather than helping the soil get stronger as in stabilisation. It is placed within the soil mass, for example in layers in a reinforced soil wall or steep slope, or at the base of an embankment over soft ground. The idea is the same as steel in reinforced concrete: the soil takes compression and the geotextile takes tension.

Filtration

Filtration is related to stabilisation in that water passes out of the protected soil. Here the aim is to let water out without letting soil particles go with it. The geotextile replaces or supplements a graded aggregate filter, for example around a perforated drain pipe, behind a retaining wall, or under riprap on a riverbank.

Drainage

In drainage, water flows within the plane of the geotextile itself and is carried to an outlet. Thick needle-punched nonwovens, and geocomposites made from a drainage core with geotextile on both faces, are used behind walls, under slabs and in landfill caps.

Moisture Barrier

This is the opposite of the functions above. A paving-grade nonwoven is saturated with asphalt tack coat (bitumen emulsion) so that it blocks water instead of passing it. Laid between an old pavement and a new asphalt overlay, it seals out surface water and acts as a stress-relieving interlayer that slows reflective cracking.
Ocean Geosynthetics supplies geotextiles for each of these functions. Contact us or browse the products on our website.

Ocean Geosynthetics also has solutions for potable water storage infrastructure, including lining systems for dams and reservoirs that help prevent leaks. We also provide Oasis floating covers for potable water reservoirs, which protect the water surface, help keep the water clean and reduce loss through evaporation.

How Thick Is Geotextile

Thickness depends mostly on how the geotextile is made. Woven slit-tape fabrics are typically well under 1 mm thick. Heat-bonded or chemically bonded nonwovens are about 0.5–1 mm, while needle-punched nonwovens are thicker, commonly around 3 mm, and get thicker as GSM rises. Thickness is measured under a small, fixed pressure (ASTM D5199 uses 2 kPa) because the fabric compresses under load. Thickness and GSM are different things: GSM is mass per unit area, and two fabrics of the same GSM can differ in thickness.
GEOTEXTILE TYPES

By manufacturing method, geotextiles fall into 3 categories:

1. Woven Geotextile
2. Non-Woven Geotextile
3. Knitted Geotextile

WOVEN GEOTEXTILE

Woven geotextiles are made on looms in the same way as ordinary woven fabrics, from flat tapes, monofilaments or multifilament yarns. They have two sets of parallel yarns: the warp runs along the length of the roll and the weft runs across it.

NON-WOVEN GEOTEXTILE

Nonwoven geotextiles are made from continuous filaments or short staple fibres laid in a web and bonded thermally, chemically or mechanically (needle punching), or by a combination.
Thermally and chemically bonded nonwovens are thin, about 0.5-1 mm, while mechanically bonded (needle-punched) nonwovens are much thicker, around 3 mm.

KNITTED GEOTEXTILE

Knitted geotextiles are made by interlooping yarns, as in ordinary knitting. They are far less common than woven or nonwoven types and are often combined with other techniques, such as weaving.

Besides these three geotextiles, other geosynthetics include geonets, geogrids, geocells, geomembranes and geocomposites, each with its own uses.

PUNCTURE RESISTANCE

Puncture resistance is the ability of a geotextile to stop stones or other objects pushing through it. Needle-punched nonwovens stretch and conform around sharp points, which helps them survive the placing and compaction of fill. Slit-tape wovens carry high tensile loads, but a sharp stone can split the tapes, and that damage is often hidden under the fill. One puncture test (the CBR puncture test, ASTM D6241 / ISO 12236) pushes a 50mm wide plunger into the geotextile and measures the force in kN (higher is better). Another (the cone drop test, ISO 13433) drops a cone onto the geotextile and measures the hole size in mm (lower is better).

TENSILE STRENGTH

Woven fabrics get their strength from crossing yarns, so for the same weight they are stronger in tension than nonwoven fabrics.

Nonwoven geotextiles elongate much more (over 50%) than woven geotextiles (between 5% and 25%) before breaking.

LIFE EXPECTANCY

Geotextiles are used in water conservation works, dams, road construction and many other fields, often in places where they cannot be replaced. Buried and protected from sunlight, polypropylene and polyester geotextiles are routinely designed for service lives of more than 50 years. The actual life depends on the polymer, its stabilisers, the soil chemistry and the temperature; exposed fabric degrades far faster.

PROPERTIES AND TEST METHODS

Geotextile properties are usually grouped as physical, mechanical, hydraulic, and endurance and degradation properties. Each group covers a different aspect of how the geotextile behaves. Tests also divide into index tests and performance tests. An index test describes the product in a standard way and is used for comparison and quality control, but its values cannot be used directly in design. A performance test models how the geotextile will behave in the actual engineering system.

Physical properties describe the geotextile as manufactured, measured by standard index tests. Typical physical properties are specific gravity, mass per unit area (ASTM D5261), thickness (ASTM D5199) and stiffness.
Mechanical properties describe the geotextile’s strength and its behaviour under different loads. Typical mechanical properties are compressibility, tensile strength, tear strength, puncture strength and seam strength. Several tests exist for each, most designed to represent conditions during installation in the field.

Tensile strength is one of the most important properties of a geotextile. It is measured by clamping opposite ends of a sample in a test machine and pulling it until it breaks. Both the force and the elongation are recorded, so the load-strain curve can be plotted and the modulus calculated. The tensile tests differ mainly in the type of clamp and the size of the specimen. The grab tensile test, ASTM D 4632, is the one most often specified and reported. It tests a specimen 100 mm wide by 200 mm long, gripped across only the central 25 mm of its width. The wide-width test (ASTM D4595 / ISO 10319) grips a 200 mm wide strip and gives strength in kN/m for design.

Other strength tests check that the geotextile can survive installation, which often puts the highest stresses on it in its whole life. Tear strength and puncture strength are the usual ones. Tear strength is measured by cutting a small slit in a trapezoid-shaped specimen and pulling it apart in a tensile machine (ASTM D4533). Puncture strength is the force needed to push a probe through the geotextile. ASTM D 4833 is a pin puncture test that older specifications quote; current geotextile specifications such as AASHTO M288 use the CBR puncture test, ASTM D6241, instead.