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Raft slab on bored piers

Raft slab on bored piers

Raft slab on bored piers

I have a residential house project brief calling for a raft slab / slab-on-ground with perimeter edge beams only (no internal beams).
The internal area of the slab is proposed to be supported on bored piers only, extending some depth into the underlying clay foundation.

I am seeking advice on the recommended depth the piers should extend into the foundation material in order to provide resistance to seasonal moisture changes of the clay foundation material. This, I believe, should be at least the depth of expected seasonal/long-term moisture change (ie. height of suction) - eg. 1.8-2.7m (6'-9') minimum or possibly deeper. But a previous design I have viewed shows only 1.2m (4') embedment in to the ground and only approx. 0.6m (2') into a very stiff clay layer. I also believe this approach is similar to the one that should be taken when dealing with seasonal/long-term moisture changes associated with trees near foundations.

Any comments would be appreciated.

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