Why a cushion geotextile is the cheapest insurance on a liner

Every lining project has a line item that gets deleted when the budget is tight, and it is almost always the geotextile. The membrane is obviously essential; the fabric under it looks like an optional extra. It is not, and the consequence of deleting it does not appear for several years, which is precisely what makes the false economy so easy to commit.

What a cushion layer actually does

A geomembrane under a head of water is pressed down onto whatever is beneath it, hard. Every stone, every angular piece of gatch, every bit of construction debris becomes a point load. The sheet does not puncture immediately — polyethylene is tough — but under sustained load it thins locally, and over months and years that thinning becomes a hole.

A nonwoven needle-punched geotextile spreads those point loads. It is thick, compressible and permeable, so it distributes the pressure from a single stone across a much larger area of membrane, and it deforms around the stone rather than transmitting its shape.

When you need one

You can lay a geomembrane straight on the subgrade when the subgrade is fine-grained, graded, compacted and genuinely stone-free — clean sand that has been rolled and hand-picked. That is a real condition and plenty of UAE sites meet it.

A cushion layer is needed when any of the following is true:

• The subgrade is gatch, gravel, crushed rock or wadi material.
• The site is reclaimed ground, or made ground with debris in it.
• The water depth — and therefore the pressure on the membrane — is significant.
• The basin will be drawn down and refilled often, cycling the load.
• The liner is being installed over an old lining or an old concrete surface.
• Anything granular will be placed on top of the membrane.

In practice that covers most projects in Ras Al Khaimah, Fujairah and the inland gravel plains, and a good share of everything else.

Selecting the mass

Mass per unit area is the usual specifying parameter because it correlates with thickness and with protection performance. On critical projects the protection is verified by test rather than assumed — a cylinder test that loads the actual membrane-and-geotextile combination against the actual subgrade material.

Protection above the liner too

The same logic applies in the other direction. If gravel, drainage stone, concrete or a soil cover is going onto the membrane, it needs protecting from that as well. On landfill work the protection geotextile above the liner is frequently heavier than the cushion below it, because a placed drainage stone layer is a far more aggressive loading than a prepared subgrade.

Installing it properly

• Overlap adjacent rolls by at least 300 mm, more on slopes.
• Sew or spot heat-bond the overlaps so the fabric cannot separate while the membrane is deployed over it.
• Anchor it into the same trench as the membrane so it cannot slide down the slope.
• Do not leave it exposed for weeks — polypropylene geotextiles degrade under UV faster than a stabilised geomembrane does. Cover it with the membrane promptly.
• Keep it clean and dry. A geotextile full of sand is stiffer and protects worse.

What geotextile is not

It is not a barrier. Geotextile is permeable by design — it filters, separates, drains and protects, and water passes straight through it. Occasionally someone orders geotextile expecting it to line a pond. It will not. The barrier is the geomembrane; the geotextile is what keeps the barrier intact.

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