If you've ever seen a retaining wall where the drainage aggregate has turned into a muddy mess, or where soil has migrated through the drainage layer and blocked the ag pipe — that's what happens when geofabric is left out.

Geofabric (also called geotextile fabric or filter fabric) is one of the least glamorous components in a retaining wall, but it's one of the most important. This guide explains what it does, why it matters and how to use it correctly.

What Is Geofabric?

Geofabric is a permeable synthetic fabric made from polypropylene or polyester fibres. It's designed to allow water to pass through freely while preventing the movement of soil particles.

In retaining wall construction, it serves as a filter layer between the soil and the drainage aggregate behind the wall.

What Does Geofabric Do?

1. Prevents Soil Migration

Without geofabric, fine soil particles wash through the drainage aggregate over time, gradually filling the voids and reducing drainage capacity. Eventually the drainage layer becomes clogged and stops working — leading to water build-up behind the wall and the hydrostatic pressure that causes walls to fail.

Geofabric acts as a filter: water passes through freely, but soil particles are held back.

2. Protects the Ag Pipe

Ag pipe (slotted agricultural drainage pipe) is installed at the base of the wall to collect and remove water. Without geofabric, soil migrates into the slots and blocks the pipe — often within a few years. Geofabric wrapping (a "sock") around the ag pipe prevents this.

FPM supplies ag pipe pre-wrapped in a geofabric sock for this reason.

3. Separates Soil Layers

In some applications, geofabric is used to separate different soil layers — for example, preventing drainage aggregate from mixing with the backfill soil above it. This maintains the integrity of each layer over time.

4. Extends Wall Lifespan

By keeping the drainage system functioning correctly over the long term, geofabric directly extends the lifespan of the retaining wall. A wall with good drainage can last 50 years or more. A wall with blocked drainage may fail in 10–15 years.

Types of Geofabric

Non-Woven Geofabric

The most common type for retaining wall applications. Made from randomly oriented fibres that are needle-punched together. Good filtration properties and high water flow rate. This is what FPM stocks.

Woven Geofabric

Made from woven fibres — higher tensile strength but lower filtration performance. More commonly used for road base separation and erosion control than retaining walls.

How to Install Geofabric in a Retaining Wall

What You'll Need

• Geofabric roll (non-woven, minimum 150g/m²)
• Ag pipe with geofabric sock (100mm diameter, slotted)
• Drainage aggregate (20mm blue metal or drainage gravel)
• Staples or pins to hold fabric in place during installation

Installation Steps

1. Prepare the excavated face. After excavating behind the wall line, clear any loose material from the cut face.

2. Line the excavated face with geofabric. Unroll the geofabric and pin it against the excavated soil face. The fabric should extend from the base of the excavation to above the top of the drainage aggregate layer. Allow enough overlap at the top to fold back over the aggregate later.

3. Place the ag pipe. Lay the sock-wrapped ag pipe along the base of the wall, on top of a thin layer of aggregate. The pipe should slope continuously to a drainage outlet — a minimum fall of 1:100 (1cm per metre) is recommended.

4. Add drainage aggregate. Fill behind the geofabric with drainage aggregate (20mm blue metal). Compact lightly — do not over-compact as this reduces drainage capacity.

5. Fold the geofabric over the top of the aggregate. Fold the top of the geofabric back over the aggregate layer to cap it. This prevents soil from the backfill above migrating down into the drainage layer.

6. Backfill above the geofabric. Backfill with compactable fill above the folded geofabric layer.

How Much Geofabric Do You Need?

For a standard retaining wall, you need enough geofabric to line the full height of the excavated face plus 300–400mm of overlap at the top.

For a 1.2m high wall with 1.2m of post depth, you need approximately 1.5–1.8m of fabric height per lineal metre of wall.

FPM stocks geofabric in rolls — contact us for pricing and to calculate the quantity you need for your project.

Common Mistakes

Leaving out the geofabric entirely. The most common mistake — and the one that causes the most long-term problems.

Using the wrong weight. Lightweight geofabric (under 100g/m²) can tear during installation and may not provide adequate filtration. Use minimum 150g/m² for retaining wall applications.

Not folding the top over. If the top of the geofabric is left open, soil from the backfill above will migrate down into the drainage aggregate over time.

Not wrapping the ag pipe. Even if you use geofabric on the wall face, an unwrapped ag pipe will eventually clog with fine particles.

Shop Geofabric and Drainage Products at FPM

FPM Building Supplies stocks non-woven geofabric and sock-wrapped ag pipe from our Epping warehouse. Our team can help you calculate quantities for your project and advise on the right products for your soil conditions. Contact us or visit us in store.