Ask most people to specify a protection geotextile and you'll get a number in grams per square metre. Ask why that number, and the answer is usually that it's what the last job used.
Mass per unit area tells you how much fabric you bought. It doesn't tell you whether that fabric will stop an angular stone punching through a geomembrane under sustained waste load. Those are different questions, and they have different answers.
Protection is a different job to separation
A general-purpose nonwoven does separation and filtration. It keeps subgrade out of drainage stone and lets water through. Those duties are governed by opening size and permeability, and a mid-weight fabric handles them comfortably.
Liner protection is a mechanical problem. A geomembrane sitting on a prepared subgrade and under a drainage stone layer is being pressed onto every particle below it and by every particle above it, at pressures that climb as the facility fills. The protection layer's job is to distribute those point loads across enough area that strain in the liner never reaches the point of puncture or stress cracking - and to keep doing it for the design life of the containment, not just through construction.
That requires thickness retained under load, filament continuity, and the ability to deform without tearing. Needle-punched continuous filament polypropylene delivers all three, which is why it dominates protection specifications. Staple fibre and thermally bonded fabrics behave differently under sustained compression, and generally belong in separation applications instead.
The tests that actually measure protection
This is the part that gets skipped. Protection performance has its own test suite, and it's not the set of properties on a general-purpose datasheet.
- Protection efficiency (EN 13719) - measures how much the protected geomembrane deforms under sustained pressure, typically reported at 300 kPa. This is the closest thing to a direct performance measure, and lower is better
- Cone drop (EN ISO 13433) - drops a standard steel cone through the fabric and measures the hole diameter left behind. A proxy for the damage a dropped object or angular stone will cause during construction
- Pyramidal puncture (EN 14574) - force required to push a pyramidal probe through the fabric, which correlates with resistance to angular aggregate
- CBR puncture (EN ISO 12236) - force to push a flat plunger through, giving a general puncture resistance figure
- Thickness at 2 kPa (EN ISO 9863-1) - the cushioning depth actually available
Look at those together across a manufacturer's range and one thing becomes obvious: performance does not scale linearly with mass. Doubling the grammage typically improves protection efficiency by a much smaller proportion. Specify on mass alone and you'll either under-protect the liner or pay for fabric that isn't buying you performance.
What should drive the selection
A protection layer should be selected from a protection design, not from a number carried over from the last project. The inputs are:
- Geomembrane thickness and polymer - a 1.0mm liner needs more cushioning than a 2.0mm one under the same load
- Subgrade condition - particle size and angularity of what the liner sits on
- Cover material grading - the drainage stone or waste directly above, which is usually the governing input
- Applied stress - height and density of the material the facility will hold at full capacity
- Allowable liner strain - the limit the design permits, typically well below yield
Where those are known, matching a grade is straightforward. Where they aren't, the design is the thing to commission before anyone specifies a fabric.
Installation decides whether any of it works
A correctly specified protection layer laid badly protects nothing.
- Subgrade preparation - flat, with no stones or objects protruding. Where the geotextile goes directly onto a geomembrane, no further preparation is needed
- Deployment - unrolled by hand on even ground. On slopes, lift the roll with a steel pipe through the core and pull the fabric down the face, or fix the leading edge at the toe and lift the roll up
- Overlaps - 300mm minimum. Heat-fusing overlaps with a propane torch stops wind displacement, but the fabric must be dry to fuse and must not be overheated
- Anchoring - plug the upper end on soil, or embed in a trench where it sits on a geomembrane. Sandbag the edges against wind
- Traffic - no vehicles on the exposed geotextile. Back-dump drainage gravel and spread with low-ground-pressure plant. Where site traffic is unavoidable, build a load-distributing haul path over 600mm thick
- Storage - dry, even ground, grades stored separately, and rolls covered completely if they'll sit more than two weeks in direct sun
The traffic rule is the one most often broken and the most expensive to break, because damage done to the liner during gravel placement is invisible by the time anyone would think to look for it.
One thing to watch when comparing datasheets
Protection geotextiles are published against both EN and ASTM test methods, and the figures differ because the tests measure slightly different things. Comparing one product's ASTM tensile figure against another's EN figure will produce a conclusion that isn't there. Check that you're reading the same column, and that the column matches the standard your specification cites.
Where we come in
We supply protection geotextiles across the full weight range and install them with our own crews. If you have the protection design, we'll match a grade to it. If you don't yet, that's the conversation to have before material gets ordered - it's a great deal cheaper than discovering the answer after the cell is filled.
Speak to our technical team about protection layer selection for your containment project.