Piled Raft Foundations And How They Work

A piled raft is a concrete raft and a group of piles designed to share a building's load, with the raft bearing on the ground and the piles placed mainly to control settlement. It suits a heavy building on stiff ground, where a raft alone has enough strength but would settle too far.

That makes it a settlement design more than a strength design. The piles are there because of how far the building would move, and that changes how many you need and where they go.

How the load is shared

A conventional piled footing is designed as if the piles carry the whole building, and any help from the slab is ignored. A piled raft counts the ground under the slab. Load reaches the soil by pressure under the raft and by friction and end bearing along the piles, and each one changes how the other behaves.

You don't choose the split at the start. It comes out of the analysis, and it moves with the pile layout, the raft thickness and the stiffness of every soil layer below.

Ground that suits it

Stiff clays and dense sands suit a piled raft, because the ground right under the slab has real strength to offer. The raft carries a good part of the load, and a smaller number of piles holds the settlement to what the building can live with.

The design falls apart where the ground can walk away from the slab. Soft clay and loose sand near the surface offer little to share. Uncontrolled fill and ground that is still consolidating can settle out from under the raft, which leaves the piles holding a load they were never sized for.

Across much of Sydney, sound sandstone sits within reach and a footing on rock is the simpler answer, as set out in [founding in Sydney sandstone and rock sockets](/blog/founding-in-sydney-sandstone-and-rock-sockets). A piled raft earns its place where the rock is too deep to reach sensibly and the soil above it is stiff.

Where the piles go

A tall building doesn't load its raft evenly. The core and the heaviest columns push down hardest, so a plain raft wants to dish towards the middle of the tower.

Putting piles under those loads reduces the dishing, which is the [differential settlement](/blog/differential-settlement-and-what-causes-it) that cracks walls and tilts lift shafts. A few well-placed piles under the core can do more for the building than an even grid across the whole footprint. The same logic helps where a tower rises off a lighter podium and both share one raft.

The analysis behind it

Hand calculations get you a concept and no further. The final design needs a model of the layered ground, the raft and the piles working together, because the interaction between them is the whole point.

We run that modelling in programs such as Plaxis and Repute, and the output gives us the settlement across the raft, the load on each pile and the bending in the slab. The model is only as good as its soil stiffness values, so the investigation has to measure stiffness at depth, a harder job than classifying the soil. That difference is covered in [bearing capacity versus settlement](/blog/bearing-capacity-versus-settlement).

Standards and pile capacity

The piles are designed under AS 2159 and the raft under AS 3600, and neither one hands you a piled raft recipe. That is where first-principles engineering does the work, and it is why we have a second senior engineer pull the model apart before the design is issued.

Some piles in a piled raft are allowed to work close to their geotechnical capacity, which would look wrong on a conventional footing. It works because the raft picks up the extra load if a pile reaches its limit. The structural capacity of that pile still has to be there, so we check each one for the load the model gives it, plus a margin for the model being wrong.

Water and uplift

A raft below the water table has water pushing up on it, and early in construction there isn't much building sitting on top to push back. In that stage some piles act in tension, holding the slab down rather than holding the building up. The piles need reinforcement for that load case too, and the mechanics are in [basement tanking and hydrostatic uplift](/blog/basement-tanking-and-hydrostatic-uplift).

Proving it on site

Test piles confirm the capacity and stiffness the model assumed, before the production piles are all in the ground. Then settlement monitoring runs as the building rises, so the survey marks on the raft can be compared to the prediction floor by floor.

How we approach it

Our principal geotechnical engineer, Dr Le Duc Ta, wrote his University of Sydney PhD in 1997 on pile groups, rafts and piled raft foundations in layered soils, and his in-house tools sit alongside the commercial software. He works alongside the structural team detailing the raft, so a change in pile layout reaches the slab reinforcement across a desk.

A piled raft asks the engineer to count on the ground, so the engineer had better know that ground.

ACSES provides geotechnical engineering, including piled raft design, across Sydney. Talk to us about a project.

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