My Wrekin

Geotextile FAQs

What is a Geotextile?

A geotextile is a man-made synthetic textile that can be used in a wide variety of Civil Engineering applications. In order to be classed as a geotextile it must be permeable and it must separate the soil layers either side of it.

Why use a Geotextile?

Fundamentally the use of a geotextile will save money. By correctly selecting a quality product, manufactured in controlled traceable conditions, this will be achieved by a product which delivers consistent performance, that can be rapidly installed with a reduction in the use of raw materials. Furthermore, inclusion of a geotextile will be cost effective against soil or other construction materials with a carbon footprint that is much lower than traditional solutions.

How are Geotextiles manufactured and does this make a difference?

Geotextiles fall into two broad groups being either woven or non woven. Within these 2 groups there are also more subtle variances such as whether a non woven geotextile is thermally or mechanically bonded (or varying combinations of both). The method of manufacture will affect the properties of the finished product and how it performs.

For example, in the majority of cases a basic woven (such as FasTrack 609) or non woven (MultiTrack NW8) is suitable to use as a separator. However, woven geotextiles are generally better at providing additional reinforcement to the soil where as non woven geotextiles provide greater permeability and superior filtration.

What functions can a Geotextile perform on my project?

Depending on the type and grade of product used a geotextile can perform the functions of separation, filtration, reinforcement, and protection. To a lesser extent a geotextile can also provide erosion protection or drainage (where fluid/ gas travels in the plane of the product rather than directly through it although this is often better provided by other geosynthetics)

What ranges of Geotextile do Wrekin offer, how do they differ?

Wrekin offer extensive ranges of both woven and non woven geotextiles so, unlike many suppliers, can offer the best product for a particular application rather than the ‘next best’ based on what they have to offer. Our woven geotextile range is called FasTrack and comes in Standard Grade (SG) and High Flow (HF) Our non woven geotextile range is called MultiTrack and comes as non woven (NW), Superior Needle Punched (SNW), and ‘coloured’ Needle Punched (VNW). The various ranges utilise thermal and mechanical bonding to different extents to produce products best suited to their expected application. Irrespective of the method of manufacture or extensive ranges enable us to offer the best product for any given application.

How are Geotextiles specified?

Geotextiles can be specified either by a number of performance properties (e.g. tensile strength, CBR puncture resistance), function (e.g. separation or filtration), or trade name and grade (e.g. Terram 1000).

The most common specification method in the UK is by trade name for which Wrekin have a range of equivalent, often superior, products against the established trade names.

Can Wrekin assist in choosing the correct geotextile for a particular scenario?

Yes. Regardless of which of the above specification methods has been used, Wrekin can provide technical assistance to select the correct product.

What are the key properties to assess to correctly specify a Geotextile?

The key properties will depend on what application the product is to be used in which will dictate which function or functions the geotextile needs to perform. In turn this will govern the relative importance of mechanical properties such as tensile strength, elongation, puncture resistance and cone drop, or hydraulic properties such as permeability and pore size.

What is tensile strength?

Tensile strength is measured by clamping a sample of product and pulling it in opposing directions. The strength, normally at break, is then recorded in kN/m. Certain applications may require strengths to be reported prior to the product breaking.

What is elongation?

Elongation is a measure of how much a sample of product has stretched from its original length when it is loaded. This is recorded as a % increase. Elongation values will vary widely between different geotextiles and can be an important factor in selecting the correct product.

What is CBR puncture resistance?

This is a measure of a product’s resistance to pushing a plunger through a clamped sample. The load required is recorded in Newtons. The sample’s vertical displacement from its original position can also be shown as the CBR displacement.

What is cone drop?

Cone drop measures the product’s resistance to installation damage and sharp stones in the ground. A weighted metal spike is dropped onto a clamped sample and the diameter of the hole is recorded in mm.

What is permeability and how does it differ between geotextiles/types?

Permeability is the measure of the rate at which water will flow through a product. It is measured in litres/m2/sec or m/s. For example, the permeability of MultiTrack NW8 is 110 litres/m2/s, this is the same as 110 x 10-3m/s With the exception of specific High Flow woven geotextiles, non woven geotextiles will offer superior permeability.

What is the Geotextiles ‘Pore’ or ‘Opening’ Size?

The pore size defines the size of the largest particle that can pass through the geotextile and defines its ability to trap particles and prevent their passage. Smaller and more consistent openings will be found in NON WOVEN geotextiles which is why they generally provide superior filtration performance.

Why do woven geotextiles have a higher tensile strength than non woven geotextiles?

This is only partly true and may also not be relevant to the application. The reason why woven geotextiles appear to have a higher strength is that the manufacturing process uses straight yarns of polymer. The product therefore takes up load more quickly when stressed than a non woven product where the fibres in the finished product are not straight. This difference in the manufacturing process also accounts for the difference in elongation performance between a woven and a non woven geotextile and also the fact that a non woven geotextile exhibits a more isotropic behaviour.

Are all geotextile tests standardised?

The correct test methods for geotextiles in the UK are defined by EN ISO. However, different countries have historically developed their own tests and even if you think you may be comparing like-for-like results this may not necessarily be the case. For example, the EN ISO test for recording tensile strength tests a particular width of sample at a defined loading rate. This may be different, and hence give different results, from the American (ASTM) testing or the German (DIN) testing. European tests are now standardised but you may still see specifications referring to old/historic tests such as BS or DIN. Similarly if a specification has been copied from an overseas project it may refer to an incorrect test method.

Is the Geotextile’s weight important?

No, weight is an identification property and, unlike mechanical or hydraulic properties does not affect how the geotextile will perform in-situ. Despite this, weight is frequently incorrectly included in the specification for a geotextile. This is very misleading as different types of geotextile of the same weight will have very different mechanical and hydraulic properties.

Should I use a geotextile manufactured from recycled fibre?

In the vast majority of applications only geotextiles manufactured from virgin polymer will offer a suitable product in terms of performance, cost effectiveness and design life. However in certain applications, only a short design life (e.g. <5yrs) may be required, and geotextiles manufactured from recycled fibre may provide a suitable solution. Wrekin offer their MultiTrack VNW range for this purpose.

Is the Geotextiles thickness important?

Thickness is fundamentally an identification property and as such does not directly contribute to the geotextiles performance. If specified on its own, thickness is meaningless. Even if specified in conjunction with another property, such as puncture resistance or weight, it can be misleading and prevent the approval of a suitable product or mean that an unsuitable product is approved. For example, products manufactured from virgin fibres will tend to achieve a puncture resistance using approximately half the weight of fibre required to achieve the same puncture resistance when recycled fibre is used. Generally the recycled fibre cannot be needled to the same degree as virgin fibre so will result in a significantly thicker product.

Which type and grade of Geotextile should I use for my project?

Fastrack SG609 – SG40/40: Standard separation and filtration geotextile with grade dependant on ground conditions and size of granular fill.

Fastrack HF180 – HF1300: Geotextile where high water flow rates are required

MultiTrack NW6 – NW45: Separation and filtration geotextile with high permeability and superior filtration properties. Required grade is dependant on ground conditions and size of granular fill.

MultiTrack SNW14 – SNW140: Separation and filtration geotextile with high puncture resistance to provide protection.

MultiTrack VNW200 – VNW2000 : Medium performance geotextiles manufactured from coloured fibres. Suitable where a thick cushioning layer and a high elongation are key design requirements.

What is the life expectancy of a Geotextile?

Provided that the geotextile is installed in accordance with installation instructions and it is not adversely exposed to degradation mechanisms such as heat or chemicals we are confident that our geotextiles will have a service life in excess of 60 years. This is in accordance with other manufacturers. If you come across a product with a design life in excess of 60 years then please contact us as there may well be caveats to this apparent extra longevity.

How soon should I cover the Geotextile on site?

To avoid the detrimental effect of UV degradation we recommend that our geotextiles are covered as soon as is practicably possibly and certainly within 7 days. If it is not possible to cover the geotextile within 7 days then please contact us to see if another product with higher UV protection may be more suitable.

How can Geotextiles be joined?

There are several ways in which geotextiles can be joined such as overlapping, sewing, gluing / taping, or leistering. Of these, the most commonly used method in the UK is overlapping. As a rule we would suggest an overlap of 300-500mm to ensure a full overlap is maintained once any irregularities in the sub grade and post construction settlement have been accounted for. In certain applications, such as underwater, the overlap may need to be increased.

Can Geotextiles be installed underwater?

Yes. However, when installed in water the majority of geotextiles will float hence specific consideration needs to given to the installation method as this may, for example, require divers, methods to sink the geotextile, and bespoke lengths of geotextile to avoid jointing/cutting the product underwater.

Can Geotextiles get damaged and is one type more susceptible than another?

All geotextiles can get damaged during the installation process especially if the installation guidelines are not adhered to. Installation stresses can be one of the most severe a product will be subjected to hence if instructions are not followed then the effect on a product’s properties can be detrimental.

It is often believed that woven geotextiles are more susceptible to installation damage than non woven geotextiles. Although this is a widely debated point there is little actual evidence to support this.

Can I drive on the Geotextile before covering it with stone?

No. This will cause significant damage to the product.

What types of Polymer are used to manufacture Geotextiles?

The main polymers used in the manufacture of geotextiles are polypropylene, polyester, polyethylene, polyvinylidene and polyamide. Of these the most commonly used is polypropylene.

Different polymers will have different properties such as melting point, specific gravity, resistance to acid and alkali, creep resistance and flammability and these may be relevant in specifying the correct product for an application.

How can holes in the Geotextile be repaired?

If the Installation Instructions are followed then this should not be an issue. However if required the best remedial measure would be to overlay the affected area with a ‘patch’ of the same product ensuring an overlap of at least 300mm around the hole/s.

What grade of Geotextile should I use with which grade of stone fill?

Should you require we can offer more advice but as a ‘rule of thumb’ the larger the stone size the more robust the grade of geotextile that is required. For example, for a stone with a maximum size of 200-250mm we would recommend our MultiTrack NW26 whereas for stone with a maximum size up to approximately 50mm either our MultiTrack™ NW8 or FasTrack™ 609 should be adequate.

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