H1 slab with Pier

Greeting everyone

Just confuse in regards to significant soil report/ cost site done by builder for block of land in north Brisbane.

House built area 260 single story

First report
Site class P due to previous tree and soil has low bearing capacity
Soil tyre 75 (ys)

Solution : E class slab and screw pier
Cost for E class slab 24 K, and Screw pier 32 K

Had another soil test with different company
Result
Site class P due to previous tree , does not mention soil low bearing capacity like previous report.
Slab recommended H1 and concrete pier, has not mentioned about any screw pier

Site cost
H1 was already included in quote, so for concrete pier site cost 21K

I am just curious how come two company has significant soil test report and recommendation for totally different slab classification.

Of course I would go with low cost slab recommendation, but just would like to know any one has come across like this situation.

Thanks in advance.

what is bearing capacity like?

They did not mention in soil report, just said soil has low bearing capacity, so it will require E class slab plus screw piering.

Poor bearing capacity should come with a number attached to it.

With properly designed foundation, a good structural engineer can put a literally 1000 ton house on a soil with 50kPa capacity with zero bored piers or screw piles.

Most of the house builds will never weight even 1/4th of that number at the full load.

A bit out of the box (i.e. BCA mandated) thinking (beyond standard waffle pod based or rafted slabs) will certainly be helpful.

As for you case, E and H1 soils will have significantly different soil reactivity figures, e.g. 40-60 mm vs 75 mm+.
Were you geo companies taking soil samples to the lab? They should be required to explain why there is such significant difference in soil reactivity, but if you are indeed on E class site, you certainly won't able to use waffle pod based slab there.

With any type of foundation though, I would definitely go for a proper perimeter drainage and at least 300 mm of soil replacement to stay on a safe side.

Hi alexp79 - We are looking at a new build and the engineers recommended a waffle slab with piers. The soil is H1. Have waffle slabs improved since some of the stories I've read, or is it still not recommended to build on H soil with waffle. Or do piers add the structural integrity needed to make it fine?

They also recommended extra slab height and steel reinforcement.

buildy

Obviously, raft slab is better option for H1.

But if there is not much leeway with the builder, the first thing you have to ensure is adequate drainage to be deployed/installed before the pour.

Ideally it has to be a perimeter french drainage around whole slab area and you have to make sure that soil on which slab will be built is inclining at a slight angle towards the drainage.

If you can replace 30 cm of soil under footings and slab with compacted sand or gravel, it would be also great for your slab.

Thanks Alex. I believe they are replacing some of the soil too. It's a volume builder so we might be stuck with whatever they offer, but I can check about a raft slab if it is indeed much better to use.