Geotextile road fabric is an ideal material for under-road base construction. A woven fabric over the subgrade soil layer helps preserve the integrity of the road and cuts the maintenance costs tied to "fallout" and potholes. Road fabric does two main jobs — separate and stabilize. Woven 200 lb geotextile fabric provides excellent base support for roadways, parking lots, and driveways. Below we cover separation and stabilization, plus reinforcement and filtration.

The Separation Function

Geotextile is a flexible, porous textile placed between different materials so the function of both can stay intact or improve (Koerner, 1994). For paved and unpaved roadways where granular aggregate sits on fine-grained soils, two damaging things happen over time without a geotextile separator:

First, fine-grained soils work up into the gaps of the granular aggregate, preventing proper drainage; the aggregate loses its rock-to-rock contact, which greatly diminishes the strength of the support layer and increases road failure.

Second, the granular aggregate pushes down into the fine-grained soil, decreasing the needed thickness of the aggregate layer. Choosing the right woven or non-woven geotextile prevents both failures and greatly improves road performance. Separation is a critical function of geotextile and a key driver of road longevity. Builders have long known how important a tight, well-graded aggregate is to minimize upward subgrade movement — but that's only partly effective, and tight aggregates can weaken because they can't drain. A separation geotextile lets you use a stronger, more free-draining aggregate with better AASHTO drainage performance.

The Stabilization Function

Roadways stay stable for a long time in two ways when geotextile is placed at the subgrade/aggregate interface.

First, when aggregate is compacted over the geotextile, individual stones seat into it, making impressions in the fabric and subgrade. That interaction locks the bottom of the aggregate into a fixed position, stabilizing the aggregate layer indefinitely.

Second, the subgrade is stabilized beneath the geotextile because the soil is loaded from the top while surrounding soil is held in place, preventing punching or shear failure. Shifting the soil-failure mode from local shear to general shear allows roughly 80% additional loading before the soil's strength is exceeded — which means a reduced structural section over a geotextile-stabilized subgrade. Using less base aggregate and/or pavement thickness over a geotextile saves money up front and preserves the whole structural section. Separation and stabilization working together is the leading reason roads built over geotextile cost less and last longer.

The Reinforcement Function

Reinforcement brings the geotextile's tensile strength into the soil to improve the system's overall strength (Koerner, 1994). In roadway applications, geotextile provides tensile reinforcement through frictional interaction with the base course, reducing applied stress on the subgrade and preventing rutting from an overstressed subgrade.

Both woven and non-woven geotextiles offer reinforcement, but woven usually has a higher tensile factor — high tensile strength at low strain — which generally makes woven the better reinforcement material. The benefit depends on how much system deformation is allowed: unpaved roads sometimes tolerate a substantial amount, where reinforcement helps significantly. Paved roads allow very little deformation, so reinforcement is generally not considered applicable there — but separation and stabilization remain important.

The Filtration Function

In roadway applications, geotextiles filter through their openings — retaining soil particles while letting water flow, which yields a free-draining pavement system. In these applications, filtration is closely related to separation. A geotextile is a good separator, but it won't always provide adequate filtration for both retaining particles and passing water.

Filtration is defined as the equilibrium geotextile-to-soil system that allows adequate water flow with minimal soil loss across the fabric's lifetime, suited to the application (Koerner, 1994). Chosen properly, woven and non-woven geotextiles improve pavement performance through filtration. Depending on site conditions, filtration can be as important as the other functions — particularly when the subgrade is very wet. If water is trapped in the subgrade when a load is applied, pore pressure builds immediately and weakens the soil, in severe cases creating an unstable, waterbed-like effect.

Overlay Asphalt Fabric

Fabric is also commonly used to repair or extend the life of existing distressed and cracked roads. The main role of asphalt-overlay paving geotextiles is to act as a stress-relieving membrane between layers of pavement. Bonded to the existing asphalt with saturated asphalt cement, the fabric reduces reflective cracking and deterioration that could otherwise lead to roadway failure.

• Waterproofs the lower layers, maintaining higher material strength
• Reduces cracking in the overlay by acting as a stress-absorbing membrane interlayer
• Increases structural stability through more stable subgrade moisture content
• Reduces material and demolition cost, decreasing construction time
• Increases roadway lifespan and lengthens maintenance intervals

Reference: Koerner, R.M. and Koerner, G.R., "Separation: Perhaps the Most Under-estimated Geotextile Function," Geotextile Fabrics Report, January 1994.