How Thick is a Geotextile?

Geotextile is a permeable synthetic fabric used with soil to separate, filter, reinforce, protect or drain. It is used in roads, harbours, landfills, embankments and many other infrastructure projects.

How thick a geotextile is depends mostly on how it is made. Woven and heat-bonded fabrics are thin, typically well under 1 mm. Needle-punched non-wovens are much thicker, about 3 mm for common grades, and get thicker as mass per unit area (GSM) goes up. GSM is a weight, not a thickness, so two fabrics of the same GSM can have different thicknesses. Thickness is measured to ASTM D5199 (or ISO 9863-1) under a small fixed pressure of 2 kPa.
GEOTEXTILE TYPES

Geotextiles are made of polymers such as polyester or polypropylene. By the way the fibres are put together, they fall into 3 categories:

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

WOVEN GEOTEXTILE

Woven geotextile is the most common type. It is made on looms like ordinary textile fabric, from two sets of yarns or tapes at right angles: the warp runs along the length of the roll and the weft runs across it.

NON-WOVEN GEOTEXTILE

Non-woven geotextile is made from continuous filament or short staple fibres, bonded by heat, chemicals or mechanical needle punching, or a combination.

Heat-bonded and chemically bonded non-wovens are thin, about 0.5-1 mm. Needle-punched non-wovens are bulkier, around 3 mm, which is why they are preferred where cushioning or in-plane drainage is needed.

KNITTED GEOTEXTILE

Knitted geotextiles are made by interlooping yarns, as in ordinary knitting. They are far less common than woven or non-woven types.

Besides geotextiles, the geosynthetics family includes geonets, geogrids, geocells, geomembranes and geocomposites, each with its own uses.

PUNCTURE RESISTANCE

Puncture resistance is a geotextile’s ability to resist stones and sharp objects being pushed through it. Non-woven geotextiles stretch and deform around a stone instead of splitting, so they cope well with the damage caused when fill is dropped and compacted on them. Woven geotextiles can carry a high puncture load, but they stretch little, so a sharp point can split them and that damage is easy to miss during installation. Two common tests:

• CBR puncture (ASTM D6241 / ISO 12236): a 50 mm diameter plunger is pushed through the fabric and the peak force is recorded in kN (higher is better).
• Cone drop (ISO 13433): a steel cone is dropped onto the fabric and the diameter of the hole is measured in mm (lower is better).

TENSILE STRENGTH

Woven geotextiles are made of interlaced yarns, so for the same weight they are generally stronger and stretch less than non-wovens.

Non-woven geotextile has much greater elongation at break (over 50%) than woven geotextile (between 5% and 25%).

LIFE EXPECTANCY

Geotextiles are used in long-life works such as water conservation, dams and road construction. Buried and protected from sunlight, a good polypropylene or polyester geotextile is expected to last more than 50 years.

Service life depends on the polymer, the additives (especially UV stabilisers), how long the fabric is exposed before it is covered, and the chemistry of the soil. Polyester can degrade faster in strongly alkaline soils, for example next to fresh concrete or lime-treated ground. Polypropylene is more vulnerable to UV and oxidation, so it relies more on stabilisers and prompt cover.

PROPERTIES AND TEST METHODS

Geotextile properties are usually grouped as physical, mechanical, hydraulic, endurance and degradation properties. Tests also fall into two kinds. Index tests compare products and check quality, but they do not give values that can be used directly in design. Performance tests show how the geotextile is expected to behave in the actual soil system.

Physical properties include specific gravity, mass per unit area (ASTM D5261), thickness (ASTM D5199) and stiffness.
Mechanical properties describe how the geotextile resists load: compressibility, tensile strength, tear strength, puncture strength and seam strength. Most of these tests are designed to reproduce the stresses of installation.

Tensile strength is one of the most important properties. In a tensile test, the two ends of a specimen are clamped in a testing machine and pulled apart until it fails. Force and elongation are recorded throughout, giving a stress-strain curve from which the stiffness (modulus) is worked out. The tests differ mainly in clamp type and specimen size. The grab tensile test, ASTM D4632, is the one most often specified and reported. It uses a specimen 100 mm wide by 200 mm long, gripped across only the central part of its width. Wide-width tensile (ASTM D4595 / ISO 10319) uses a 200 mm wide specimen and is closer to design conditions.

Other tests check that the geotextile survives installation, which is often the heaviest loading it sees in its life. The usual ones are tear strength and puncture strength. Tear strength is measured with the trapezoid tear test (ASTM D4533): a small cut is made in a trapezoid-shaped specimen and the force needed to propagate the tear is recorded. Puncture strength is the force needed to push a probe through the specimen. Older specifications used ASTM D4833 (index puncture with an 8 mm probe); most current geotextile specifications use the CBR puncture test, ASTM D6241.