Stiffened raft - footings in the ground but footings and slab poured together
Footing slab - footings in the ground but slab poured separately from the slab
Waffle rafts - slab poured on the ground level with polystyrene pods separated so beams typically at 1.1m form between the pods
Stiffened slab with deep edge beams
Section 4 of the code allows engineers to design any slab system, but they have to comply with certain differential deflection depending on the type of wall construction...clad frame (the most flexible) through to full masonry (the most inflexible)
The uncertainty with anything in the ground is the cost uncertainty for everyone, who knows how well the trenches will be dug, will rain collapse the trenches, will they be over excavated, will soft pockets be found. They are also potentially unsafer from a worker point of view to construct.
Along comes waffle and gives everyone a simpler and more cost certain outcome with similar if not better structural performance - this is the main reason waffle has been so successful as a slab system.
The reason they are so popular are they are cheaper than stiffened raft.Due to so many failures particularly in the western suburbs engineers have been starting to do hybrid slabs with an excavated deepened edge beam in an attempt to stop moisture ingress under the slab.
The waffles have several major problems.
*Granular bedding material (used to be scoria) which allows moisture to spread under the slab.
*The block has to be perfectly flat usually causing drainage issues especially in highly reactive sites.
*You don't get to check if the soil test results are correct as there is no excavation.Soft spots, tree roots and obstructions are
never exposed during trench excavations.
*"E" reactive sites are often called "H2" because once it is an "E" you can't use a waffle.If you constantly call sites "E" you will lose volume builders because they only want waffles.You may say well to bad the site is what it is and I agree with this but the fact is it influences what geotech's are prepared to say so waffles are put on "E" sites.
You can build E waffle, no law says you can't. It just must be engineered, extra concrete, extra steel, piers etc and the usual drainage requirements.
All this anecdotal talk is fun but where is the long term scientific evidence?
We all know the waffle slab disaster of the late noughties when the longest drought in Victorian history broke etc and all the waffle pods heaved and Metricon getting sued etc, but that all coincided with Metricon becoming the biggest project builder in Australia, so of course they had the biggest exposure. Plus they were building on poorly classified land, which hadn't been tested thoroughly enough. Since then the standards have been upgraded to find a compromise that suits builders and engineers.
Let's see the results of a proper scientific survey before we try and scare people eh?
Sweetswisssteel
Here is 3 Phd's written of the topic with local conditions I have been involved in 2 of them.
Investigation of expansive soil for design of light residential footings in ...https://researchbank.swinburne.edu.au/f ... Thesis.pdf
New Post-Construction Site Characterisation Models for Low-rise ...
Assessment of the reactivity of expansive soil in Melbourne ...https://researchbank.rmit.edu.au/eserv/ ... 78/Zou.pdf