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AS2870 Building Raft Foundations on top of ground

AS2870 Building Raft Foundations on top of ground

AS2870 Building Raft Foundations on top of ground

Was wondering if there are any issues building a raft foundation system on top of the building pad (similar to a waffle pod slab) rather than excavating the foundations into the building pad?
The pad would effectively have to built up to underside slab level with 200-300mm of suitable fill material and then the internal slab beams constructed through this material.
Not entirely sure of the reasoning behind this approach (although it will be easier to construct the internal beams through fill material rather than natural material) but this approach has been raised with us recently and was hoping to get some others thoughts.
The requirements of Figure 5.7 (c) would need to be adhered to also.

RE: AS2870 Building Raft Foundations on top of ground

I'd expect the issue would be that it would be uneconomic to build up an ensure suitable compaction of fill material. It isn't just about having the right fill it is ensuring it doesn't consolidate over time.

Surely it would just be simpler to form and pour a simple rectangular slab? Sure more concrete, but it would be quicker, easier, simpler and more robust.

RE: AS2870 Building Raft Foundations on top of ground

No problem, often it is requested because of flood levels.

RE: AS2870 Building Raft Foundations on top of ground

Yes not a problem, AS2870 even allows upto 400mm of fill to be placed in 200mm layers; rolled and compacted. this would be deemed as controlled fill. However I would generally just put some backhoe piers in through the fill so you are not relying on workmanship

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