When I enquired with the council as to what type of trees they were and how high they were expected to grow, I realised that eventually a couple of parts of the dwelling would come within 75% of their mature height (which is around 12 metres).
The engineer then re-specified a number of piers - 7 in all - which now go down to 2.5 metres. These are the piers that are within that "75%" influence zone.
I actually found it interesting that my slab has a very large number of piers. The house I built 20 years ago just had one. This house has 22 of them - and it is a tiny house (only around 130 m2) sitting on M-D soil. That said, I actually asked them to upgrade the slab two "half classes" above what the soil report demanded, even though it obviously cost me more money. So I asked for the engineer to design an H1-D slab. I personally feel that the current Australian standards for residential construction are not necessarily ideal and instead are a compromise between mitigating the liklihood of problems occurring down the track versus what people are prepared to pay.
Perhaps some will say I am being overzealous but the cost difference wasn't that large and if it reduces the chance of doors sticking or other issues years down the track I think it is a small price to pay here and now. Anyway, if any builder said to me they wanted to upgrade the footings or slab, I would not hesitate to agree to this.
FULLY agree,is a small cost compared to problems you will face.