What is a piled mattress?
A piled mattress is a foundation system in which an embankment or road structure on soft ground is carried by piles, with a granular layer — the “mattress” — spanning between them, reinforced with geosynthetics. The reinforcement acts like a hammock between the pile caps and transfers the weight of the fill into the piles, so the soft subsoil carries almost nothing. It is also known as a geosynthetic-reinforced piled embankment.
What it is used for
On peat and soft clay, a conventional sand embankment keeps settling for years. The usual answer is preloading and waiting — sometimes one to two years. A piled mattress is the fast alternative, used mainly for:
- Roads and railways where no residual settlement is acceptable
- Transitions to structures such as bridges and culverts, where a settling road next to a non-settling structure creates a bump
- Sites next to existing buildings, because an unsupported embankment can drag neighbouring foundations down with it
- Tight programmes, since no consolidation period is needed
How it is built up
- Piles — usually precast concrete, sometimes grouted or screw piles — driven to the bearing sand layer, typically on a grid of roughly 1.5 to 3 m centres.
- Pile caps that increase the bearing area.
- The mattress: a well-compactable granular layer, in practice often 0.5 to 1.0 m thick.
- Geosynthetic reinforcement — usually one or two layers of geogrid, in two directions, directly above the pile caps.
Points of attention
- Design is specialist work. In the Netherlands, CUR publication 226 is the governing guideline; in Germany, EBGEO. Both are recognised design documents for this system.
- A piled mattress costs more to build than preloading, but saves construction time, imported fill and future maintenance caused by ongoing settlement.
- The edges are critical: at the toe of the embankment, spreading forces have to be taken by the reinforcement or by an edge structure.
- Compaction quality and correct geogrid overlaps determine whether the system performs as calculated.
Related terms
- Geogrid
- Geotextile
- Ground improvement
- Pile foundation
- Driven pile
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