In order to reduce the effects of water on the strength and stiffness properties of the subgrade below pervious pavements a geotextile separator layer should always be provided over moisture sensitive or fine grained subgrades. This will enhance the performance of the pavement structure in two ways:
- Prevent pumping of finer soils into the open graded aggregate of the unbound layers.
- Provide enhanced resistance to deformation of the subgrade under load, by providing a tensile resistance.
Geogrids or geocellular confinement systems may also be incorporated into the granular layers in order to increase the stiffness and therefore reduce the required thickness of the layers. They can also be used as separation layers to prevent migration of fine particles between unbound layers and play an important role in removing pollutants.
Designers need to take careful consideration of geotextile properties with respect to the selection and specification of geotextiles. Many designers refer to layers within the construction that require specific properties merely as "geotextiles". The variability in performance with commercially available geotextiles is vast. They can vary in thickness from a few microns to tens of mm, can be manufactured from a diverse range of raw material (for example polyethylene, polypropylene, polyesters) and be any blend of the foregoing with various mixtures of virgin or recycled material. Geotextiles can be woven, non woven, needle punched or thermally bonded all with different pore sizes and permeability.
All these aspects give rise to a huge variance with regard to physical properties and performance of geotextiles, together with UV resistance, durability and robustness during installation. All too often designers specify a geotextile based on a popular brand name alone without due consideration of the required material properties.
Guidance will provide recommendations on the specification of geotextiles to ensure that they provide adequate filtration to prevent migration of fine soil particles, together with appropriate permeability so they do not limit flow of water in the system.
The geotextile must also be able to resist the punching stresses caused by loading on sharp points of contact and have sufficient strength to resist the imposed forces from traffic or other loading.
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