Geotextile fabrics are an essential class of materials in civil construction. (It’s vast! For a good technical introduction on it, check out Geotextiles in Construction). Their primary purposes applicable to this blog post are to provide: separation, drainage, filtration and reinforcement. We’ll look at each of these and how they apply to permeable paving.
What is Geotextile Fabric?
What we’re looking at in this post is non-woven geotextile fabric – we use Bidim Green from Geofabrics.
This is a “nonwoven” geotextile fabric, (as opposed to a traditional woven material), which gives exceptional tensile strength for maximum performance. It is supplied as a big roll of fluffy fabric! As a bonus, the material we use is also made from recycled plastics.
Geotextile fabric is designed to be inert in the environment, and being manufactured from plastics means it will not rot or decompose, providing maximum lifespan for the project.
How is Geotextile Fabric installed?
Geotextile fabric is installed in between different layers of materials in earthwork projects. The bottom layer, or subgrade, is first prepared according to the project requirements. Then, the geotextile fabric is rolled out and then the top layer is then spread over it.
How does Geotextile Fabric provide separation and reinforcement?
Separation is the most important role geotextile fabrics provide. By being installed in between different layers of materials, it prevents the two materials from mixing and preserves the characteristics of each material.
Geotextile fabric is installed under literally every single new road that is constructed in Australia! When water gets under a road surface, it softens the soil and makes the crushed rock road base mix with the soil beneath. This mixing causes the road base to sag and can cause road pavement surface failures such as potholes.
By installing the geofabric, the fabric prevents the two materials from mixing, and the tensile strength of the fabric helps the pavement subbase “bridge” over any soft spots. Engineers would describe the inclusion of geofabrics into pavement subgrades as being just as essential as including reinforcing steel in concrete.
Geotextile Fabric for Permeable Paving
So what does this have to do with permeable paving? Everything! When we install our permeable paving, laying geotextile out is the first thing we do after excavating the site to a suitable subgrade. On top of the geotextile fabric we then lay any screenings sub base required, and then our permeable concrete and resin bound pavement is then installed.
Why is Geofabric important for Permeable Paving?
The geofabric provides separation, meaning that when the permeable paving drains water through to the subgrade, it helps with the following:
- On all soils, it prevents the rock from being able to settle down into the subgrade beneath, preventing a possible cause of pavement settlement or cracking
- On silty clay soils with slow drainage rates, any build up of water can lift up fine silty mud out of the clay, but the geotextile fabric prevents the silt from rising up through
- On hard clay soils with soft pockets or over soft trenches, the tensile strength of the fabric prevents the rock from settling into the soft soil and helps the subgrade bridge soft or unstable areas
- On soft soils or sand, it helps prevent the subsoil from being able to move with water infiltration
As we discussed before, civil contractors put geotextiles under roads to prevent failure due to water ingress. And with permeable paving, the pavement is designed to achieve water ingress!
How does Geotextile Fabric achieve drainage and filtration?
Nonwoven geotextile fabrics are designed to be highly permeable to water but a barrier to fine particles. Subsurface drainage systems utilise geotextile fabrics in their design as a filter layer between soil and the drainage material (eg. fine rock in a trench) to keep the fine soil particles out of the drainage system, whilst still allowing water in.
Nonwoven geotextile fabrics are also highly absorbent like a sponge, so they can assist with drainage by drawing water along them and disrupting capillary action of water in the subgrade. In road construction, the capillary action of the fabric is used to draw water out of the subgrade and into drainage systems to prevent water damage.
How does Geofabric help with Permeable Paving?
Firstly, the absorption properties can help on steep sites where (in heavy rain events) water will be moving downhill under the permeable pavement, as the geofabric will help wick excess water out of the subgrade, helping to minimise disturbance to the subgrade due to the movement of water. And the fabric also helps stabilise the fine surface subgrade material in contact with the geofab. And as with any pavement, preventing degradation of the subgrade structure is the key to long term pavement stability.
On reactive clay soils, it is often necessary to install subsurface drainage to minimise the build up of excess water. In a “traditional” subsurface drain, the drainage trench is lined with geofabric before being backfilled with free-draining material. But with a permeable pavement on clay soil, the entire pavement area is free-draining material! So in order to maximise the performance of the subsurface drainage the entire subgrade under the permeable pavement should be lined with geotextile fabric. That way, clay silt cannot be carried into the drainage with the passage of water.
Is Geofabric the secret to successful Permeable Paving?
The correct use of geotextile fabric under permeable paving is just one of the many little nuances that needs to be done right in setting up for a structurally sound permeable pavement.