Will be interesting for you guys to evaluate my approach, which is a bit different from what I have seen in Australia so far.
I have M class clay soil on P classified site due to a lot of vegetation and tree roots in the 200-300 mm topsoil (most of the big trees have been removed back in September, though). The soil tests are showing that starting from around 500 mm and below there is very stiff to hard brown clay with almost zero plasticity - DCP count starting from 8 (200 kPa) and all the way up to 30-35 for the bedrock at around 1.8 m-2.5 m deep.
As I am using ICF-type of a system to build my house, my reinforced concrete footings are only 200 mm deep and 400 mm wide and reinforced with just one layer of L11TM-300, however, above they are connected through reinforcement with 120 mm wide and 300 mm high concrete-filled blocks made out of light concrete with high degree of thermal conductivity, so they are also acting as a formwork to my 100 mm SL82-reinforced slab. Then they are all connected through steel reinforcement with wall, so my footings and walls perform as one composited system.
I excavate and remove around 900 mm of top clay soil and replace it with ~750 mm of crushed sandstone and 120-150 mm of crusher dust for the top layer (under slab), compacted in 200 layers to tested to at least 96% Proctor and then excavate and pour my footings in this controlled fill. This means that my footings are sitting on very well draining and well compacting fill, there is no point loading, so even if what gets under, some minor deep clay movement here and there under them is not that much of a concern.
I put 50 mm of extruded polystyrene directly under my slab wrapped into the builder's film mainly for under slab insulation.
In addition, I add Radmyx (https://www.radcrete.com.au/product/rad ... erproofing) to the concrete to waterproof my footings and my slab for life, so not really concerned about moisture damage to the concrete if water gets near or directly under the footings.
Also, planning replacing all my horizontal steel re-inforcement in footings and slab with HelixSteel micro-rebars (it will be pre-added to the concrete at the plant), but vertical starter bars will stay standard.
Will be interesting to hear your opinion, guys.
When I first started out in Perth and we got clay the consutling engineer I worked for would recommend removing 600mm and replace it with sand. We would specify that the surface of the underlying clay was graded and try and get the water away from under the house....but on a flat block where we couldn't do this, I was not happy with potentially building a dam under the home.