# What is a bentonite mat?
**A bentonite mat (also called a geosynthetic clay liner or GCL) is a factory-manufactured waterproofing sheet consisting of a layer of sodium bentonite clay granules sandwiched between two geotextile fabrics, which are needle-punched or stitched together.** When the bentonite absorbs water, it swells to form a dense, impermeable barrier that prevents water from penetrating foundations, basements and other underground structures.
## How does a bentonite mat work?
The working principle relies on the swelling behaviour of sodium bentonite:
1. **Installation** — the mat is rolled out against the outside of a foundation wall or under a floor slab
2. **Contact with moisture** — groundwater or soil moisture reaches the bentonite granules
3. **Swelling** — the bentonite expands to fill all voids, overlaps and minor imperfections
4. **Sealing** — the fully hydrated bentonite forms a continuous, watertight membrane
The self-sealing property means that small punctures or penetrations caused during backfilling are automatically repaired as the bentonite swells around them.
## Application
Bentonite mats are used in:
– **Basement waterproofing** — applied to the outside of below-grade walls before backfilling
– **Under floor slabs** — laid on blinding concrete before the structural slab is poured
– **Retaining walls** — protecting earth-retaining structures from groundwater ingress
– **Tunnel construction** — as part of composite waterproofing systems
– **Landfill liners** — preventing leachate from contaminating the surrounding soil
### Installation guidelines
– Overlap adjacent sheets by at least 150 mm, with loose bentonite granules sprinkled in the overlap zone
– Ensure the substrate is smooth and free of sharp objects that could damage the geotextile
– Do not install in rain, as premature wetting activates the bentonite before the mat is properly positioned
– Apply sufficient overburden (soil, concrete or another structural layer) to confine the mat and ensure effective swelling
### Advantages over liquid-applied membranes
– Fast installation with minimal specialist equipment
– Self-healing capability
– No curing time required
– Effective even with minor substrate imperfections
## Related terms
– Bentonite
– Waterproofing
– Geotextile
– Damp-proof course
– Foundation
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