How to choose the right geotextiles?

Solutions & Applications · Geosynthetics

Geotextile is the cheapest material in the project and is usually the first to be cut back in the tender. It is also the material which, if chosen incorrectly, will block drainage within three years and necessitate excavation. The cost difference between the right and the wrong roll is a few cents per square metre; the difference in consequences is an entire project.

Step 1 — What function are you requesting?

The EN ISO 10318 standard defines six distinct functions. A geotextile usually fulfils more than one — the NW Standard covers four — but one is the primary function, and this determines the choice:

Function	What it does	Critical dimension
Separation	Prevents the mixing of two layers with different grain sizes	CBR puncture resistance
Filtering	Allows water to pass through, retains fine-grained particles	Characteristic pore opening O₉₀
Drainage	It transports water within the plane of the material	Hydraulic permeability
Reinforcement	It absorbs tensile stresses in the ground	Tensile strength & elongation
Protection	Geomembrane protected against punctures	Mass per unit area & CBR
Corrosion control	Stabilises the ground on slopes and embankments	Mass & tensile strength

Step 2 — The two factors that determine the choice

O₉₀ — the characteristic pore opening

It is the grain size at which 90 per cent of the pores are retained. It defines the balance between two opposing risks:

• Very high O₉₀: fine particles penetrate the fabric and migrate into the drainage material, which becomes blocked from the inside. Failure is gradual and not visible.
• Very low O₉₀: the fine particles are retained on the surface of the fabric and form a watertight layer. The failure here lies with the fabric itself.

The correct O₉₀ is derived from the particle size distribution curve of the natural soil — not from the weight of the roll. Two geotextiles of the same weight may have very different O₉₀ values depending on the manufacturing method.

However, within the same series there is a constant relationship, and it is inverse. An example from the five types of ESHA-DTF:

Type	O₉₀ (μm)	Vertical permeability (×10⁻³ m/s)	CBR (N)
DTF 150	70	80	300
DTF 200	65	75	400
DTF 300	50	65	600
DTF 400	50	60	900
DTF 500	45	50	950

It reads as follows: the heavier the fabric, the finer its filtration — but the less water it allows through. From 150 to 500 g/m², the O₉₀ falls from 70 to 45 μm, but permeability decreases by 38 per cent. The notion that ‘heavier is therefore safer’ is a fallacy: in high-flow filtration applications, overspecifying the weight reduces hydraulic performance.

Strength, on the other hand, varies linearly with weight — CBR from 300 to 950 N. For this reason, the selection is based on two criteria simultaneously rather than just one.

CBR — static penetration resistance

It determines whether the membrane will survive installation, which is almost always the most gruelling stage of its life: dumping of aggregates from a height, spreading with a crawler-type roller, and compaction with a vibratory roller. A fabric with the correct O₉₀ that has been torn during compaction serves no purpose.

The rule of thumb is as follows: the coarser and more angular the overlying material and the softer the underlying soil, the higher the CBR required.

This gives rise to a rule that defies intuitive logic: weight is determined by the mechanical stress during construction, not by the function the product will subsequently perform. For example, from the NW Standard range:

Application	Approximate weight
Building drainage filter	200–300 g/m²
Road construction separation	300–400 g/m²
Geomembrane protection	400–600 g/m²

Step 3 — Woven or non-woven

	Non-woven (needle-punched)	Woven (strips)
Structure	Random fibre orientation, needling	Weaving of strips onto a mesh
Permeability	High — three-dimensional porosity	Low — openings only at junctions
Extension	Large — adapts to the terrain	Low — high strength at small deformation
Main use	Filtration, protection, separation	Reinforcement, separation under load
Typical projects	Drainage systems, flat roofs, landfill sites, retention basins	Road construction on soft ground, heavy-duty car parks

This difference also affects the method of installation, and this is where mistakes are often made:

• Non-woven — fitted loosely. Not stretched. It must be able to follow the surface contours and settle without developing stresses that would cause it to tear.
• Reinforcement fabric — requires tensioning. Its high tensile strength is only activated under deformation; if laid loosely, it offers no benefit until the ground settles sufficiently for it to be stretched.

Coverings are also a matter of substrate rather than convention: 30 cm on a stable substrate, up to 100 cm on soft ground, where movement during laying is greater.

Step 4 — Polymer: PP or PET

The choice of polymer is not a commercial one. It is determined by the chemical environment in which the material will be used:

• Polypropylene (PP): resistant to alkaline environments. It is the safe choice for use in contact with fresh concrete, lime-based or cement-stabilised soils and mortars.
• Polyester (PET): higher modulus of elasticity and lower creep — an advantage for long-term reinforcement. However, in a strongly alkaline environment, its durability is compromised, as polyester is susceptible to hydrolysis at high pH levels.

The scope of application is explicitly stated in the product data sheets: ESHA-DTF is designed for a service life of ≥ 50 years in natural soils with 4 < pH < 9 in accordance with EN 13438, and polypropylene is selected for use in contact with fresh concrete or a calcareous substrate.

The rule for the building site: check the pH before the weight. In projects involving cement stabilisation or contact with concrete, the question ‘PP or PET?’ comes before ‘how many grams?’.

The exception that proves the rule: the flat roof

The whole rationale behind O₉₀ is based on the assumption that there is fine-grained soil to be retained. This is not the case on a flat roof. There, the requirement is reversed: the geotextile must allow water to drain away as quickly as possible, without retaining anything above the waterproofing layer.

The FIBRANfilter SF32 is designed precisely for this purpose, with O₉₀ = 140 μm — the largest in its class — and a water permeability of _VH50 = 70·10⁻³ m/s. It bears the CE 0751 marking in accordance with ten harmonised standards and has a projected service life of 100 years.

Reading the text in reverse is equally important, as the leaflet itself states: at 140 μm, it is not suitable as a filter for fine-grained soils. For basement drainage, road construction or landfill sites, a geotextile with a lower O₉₀ value should be selected. There is no such thing as a ‘good geotextile’ — there is only the one suitable for the specific interface.

Step 5 — Ultraviolet radiation

All geotextiles in the range are UV-stabilised in accordance with EN 12224, but this stabilisation has a time limit on exposure — and the stated service lives apply subject to this condition. The FIBRANfilter SF32 brochure states this explicitly: the geotextile must be covered within two weeks of installation in order for the projected 100-year durability to apply.

The practical implication is simple and is often disregarded: the fabric should be covered within the window of the brochure, not when the aggregates arrive. Roller blinds left unrolled during a Greek summer have lost some of their strength before they are even loaded.

The ARGYNET range

Product	Type	Its unique selling point
NW Standard	Non-woven PP, needle-punched	The broader range: 80–600 g/m², CBR up to 7.4 kN, O₉₀ 90–128 μm · manufactured in Greece
SNW Superior	Non-woven PP, heat-treated	Filtration geotextile: pore size from 60 μm with a minimum flow rate of 45 l/m²·s
PNW Protection	Heavy-duty PP non-woven fabric	CBR up to 14 kN and weights up to 2,000 g/m² — the highest in the range
CNW Medium	Non-woven PP, coloured	The only colour feature: colour-coded zones
ESHA-DTF	Non-woven PET	Five standard codes ranging from 150 to 500 g/m² with nominal values per type
NWR-100	Recycled non-woven PET	100 per cent recycled fibres; low creep under permanent load
FIBRANfilter SF32	Non-woven PP	O₉₀ 140 μm — the largest; for flat roofs, not for groundwork
WG High Flow	Woven PP, tapes	The only thing that really delivers: 42–105 kN/m at low elongation

This category features filters by geotextile type, CBR mechanical class, polymer type and primary function — that is, searching by project dimensions rather than by product name.

CNW Medium is worth noting: the colour is not aesthetic. In a multi-layer installation, visual differentiation allows the supervisor to confirm at a glance that the correct material has been installed at the correct level — a check that would otherwise require sampling.

Design series

1. Main function: separation, filtration, reinforcement or protection. Specify the type before each number.
2. Grain size analysis of natural soil: this determines the required O₉₀.
3. Installation conditions: particle size distribution and angularity of the sub-base, compaction method — these determine the CBR.
4. Chemical environment: soil pH, contact with concrete or stabilised materials — specifies PP or PET.
5. UV exposure time: plan to cover the surface within the time limit specified in the leaflet.
6. Documentation: technical data sheet, declaration of performance and CE marking — the documents required by the supervising engineer.

Non-woven Fabrics — Filtration, Separation, Protection

Woven Fabrics — Reinforcement & Separation under Load

When drainage is required

If the design specifies drainage rather than filtration, the material belongs to a different category. Dimpled drainage membranes feature conical profiles of 5 to 20 mm in HDPE or recycled HIPS, with or without a pre-bonded filter geotextile — that is, the geotextile is part of the product, not an alternative to it. The perforated SN4 pipes drain water away from the site, where the membrane ends.

The height of the cones is not selected on the basis of strength but on the available layer thickness and water management — a factor which, in a green roof, determines whether the roots will be able to breathe.

When soil stabilisation is required

Geotextiles provide reinforcement through friction and separation; geogrids provide reinforcement through the interlocking of aggregates within their openings. In embankments, road pavements on soft subgrade and reinforced slopes, the two materials often co-exist: the mesh absorbs the stresses, whilst the geotextile keeps the fine particles out of the base course.