Geotextiles

Polybrane geotextiles are part of a wide range of woven polypropylene materials supplied by Synthetic Packaging Ltd. in Ireland.

Synthetic Packaging are proud of our reputation of excellence in the supply of both woven and non-woven Geotextile products are used in various applications in multiple projects. As a subsidiary of Thrace Plastics Co. S.A group we liaise with various International manufacturers within the group to supply the most up to date products available to our customers. Additionally our experienced staff can assist all customers with all queries in relation to Geotextiles.

Synthetic Packaging supply a range of both woven and non woven Geotextiles. Both ranges have varying characteristics and can be used in the above applications. The choice of woven or non woven typically depends on the application and the relative design requirements.

Polybrane®
Polybrane geotextiles are part of a wide range of woven polypropylene materials supplied by Synthetic Packaging Ltd. in Ireland.
Their mechanical and hydraulic properties make them ideal for a wide range of geotextile applications including:
- separating the sub base and sub grade (fig.1)
- filtering in longitudinal drains (fig. 2) and horizontal drainage layers
- reinforcing over weak sub grades (fig. 1)

The geotextiles used in each of these different applications have varying characteristics that are key to their success. In order to summarise these characteristics, the European Standards Committee (CEN) has documented the required characteristics for geotextiles and other geotextile-related products used in the construction of various structures. A separate document covers each structure type and stipulates within the functions, characteristics and test methods to be used in each application. One document, BS EN 13249:1998, lists the mechanical and hydraulic properties important to the functions of Separation, Filtration and Reinforcement when used in the construction of roads and other trafficked areas. Table 1 below lists those properties which should be considered.

If the geotextile is to perform a separation or reinforcement function with water present, the filtration function must also be taken into account. It can be seen that the units marked with a - and also units of thickness or mass per unit area have not been identified as being important to a geotextiles' performance.

These main characteristics, which are relevant to both woven and nonwoven geotextiles used in separation and filtration applications, are discussed as follows:

MECHANICAL PROPERTIES
When comparing woven and nonwoven geotextiles of equal weight, woven geotextiles demonstrate:-
- greater CBR Push through resistance
- greater tensile strength

Most geotextiles used in civil engineering applications, including the Polybrane range of geotextiles, are manufactured from polyproplene. Polypropylene can be modified to enhance the characteristics of the geotextile, but the main reason that Polybrane fabrics are stronger, lies in the technologies used in their manufacture.

In the case of non woven geotextiles, the strength of the fabric depends on the degree of which the fibres can be interlocked. Consequently, a non woven fabric will fail when the breaking strain of the mechanical or thermal bonds within it is exceeded, whereas the woven fabric continues to resist stress until the tapes themselves are broken.

In a woven geotextile such as a Polybrane the fibres or tapes are aligned in two principal directions and it is the tape itself which resists the stresses directly, giving these geotextiles their characteristic high tensile strength. Although tensile strength is usually only measured in the two principal directions, similar tensile strengths are recorded when the geotextile is tested at any other angle.

Typical Stress/Strain Curves for Geotextiles with a Mass per Unit area of 120g/m2

LOWER ELONGATION AT BREAK

WORKING STRENGTHS AT MINIMAL DEFORMATION
Woven geotextiles typically have a lower elongation at breaking point than non-woven geotextiles. This is mostly due to the method of manufacture. The bond between fibres in a needle punched non woven is not destroyed instantly when stress is applied - instead the fibres move apart and the material can commonly extend by 40% to 80% before it breaks. In woven geotextiles, the fibres begin to absorb the stress immediately without excessive stretching of the geotextile and elongation at break is therefore lower - 5% to 30%. Thermally bonded non woven geotextiles typically have an elongation at break somewhere between that of woven and needle punched non woven products.

On most projects it is desirable for the geotextile to minimise surface deformation. Tensile strength therefore needs to be as high as possible and the elongation needs to be minimal. As Polybrane geotextiles offer high tensile strength at low elongation (fig.4) their use should offer maximum reinforcing effect with minimal deformation of the construction layers.

The actual working strain induced in a geotextile is usually quite small - it is rarely greater than 5% so the tensile strength at this degree of extension is often the most relevant measure of a geotextile's strength.

INSTALLATION DAMAGE

CBR TEST, CONE DROP TEST, DAMAGE TEST

Geotextiles of all types can be damaged during installation. The placement of aggregate upon a fabric can cause damage but this can be minimised by good installation practices. Various tests have been developed which seek to model installation damage in the laboratory.

The CBR puncture test gives an indication of the ability of the geotextile to withstand slow puncture initiation. It enables the selection of a geotextile with sufficient robustness to minimise installation damage and ensure the required properties are maintained for the service life of the product (see fig. 3a).

The cone drop test indicates the likely performance of a geotextile when submitted to installation stresses. It provides evidence of the suitability of the geotextile to withstand possible damage in separation and filtration functions, especially when the aggregate placed onto the geotextile is sharp or angular in shape.

Damage tests are now being developed to simulate the forces which act on a geotextile during installation and attempt to quantify their effect. By simulating the effect of any damage done during the installation process, these tests enable a geotextile specifier to make an accurate assessment of the required working strengths of a geotextile and compare this to the performance of a particular grade.

Through CBR and cone drop testing it is evident that Polybrane geotextiles offer a high resistance to static and dynamic puncture. After simulated damage testing, it is found that at working strains Polybrane geotextiles retain a high proportion of their tensile strength.

HYDRAULIC PROPERTIES

PORE SIZE DISTRIBUTIONAND WATERFLOW

Soil particle size and distribution will differ on each site and for the geotextile to function correctly, the pore size distribution of the geotextile must be carefully matched to the soil conditions. If the pore size is too large all the soil particles will pass through the geotextile; if it is too small, the surface of the geotextile will quickly become clogged and nothing will pass through.

The main purpose of the geotextile is to foster the creation of a stable, natural soil filter in the soil itself (fig. 5). The geotextile must hold back the larger particles in the soil, which in turn will hold back smaller particles. In this type of system where the soil is able to create its own natural filter, water can flow through the geotextile with a minimum degree of clogging of the geotextile pores.

If a geotextile with too small a pore size is used the openings may be blocked even by small soil particles and significant areas of the geotextile will be unable to allow water to pass through. This problem is often more severe when the geotextile has a thick three-dimensional structure and soil particles can become trapped within the geotextile itself. These properties are characteristic of nonwoven geotextiles, particularly needle punched products and thus it is recommended in design that the coefficient of permeability for such fabrics should be as much as 100 times that of the soil.

In a woven geotextile which is less prone to clogging, the design coefficient need only be 5 to 10 times that of the soil to achieve the same long-term permeability performance. For this reason it is clear that the required minimum waterflow value normal to the plane of the geotextile will often be different for woven and nonwoven products. Consequently it is not unusual to find woven geotextiles with a waterflow of 15 l/m/sec competing favourably with nonwoven geotextiles with waterflows a magnitude higher. Both users and specifiers realise that 15 l/m/sec of water passing normal to the plane of a woven geotextile is a significant quantity of water (90cm depth every minute) and will generally offer a very large factor of safety over what is required in service.

The apparent opening size of Polybrane woven geotextiles (150 µm to 500 µm) has been designed to retain the larger particles in the filter zone. The pore size of a nonwoven geotextile is generally smaller
(40 µm to 150 µm). As the most efficient filtration results from the formation of a natural soil filter, the spread of pore size distributions available in the Polybrane range makes them ideally suited to initiate the natural filtration process.

Assured Quality & Service
The information contained herein is to the best of our knowledge accurate but since the circumstances and conditions of use are beyond our control, we do not accept any liability for any loss or damage, however arising, which results directly or indirectly from the use of such information.

For further information contact Synthetic Packaging Ltd. Clara, County Offaly, Ireland.
tel: 353-506-31282 fax: 353-506-31224
e-mail: sales@syntheticpackaging.com