Earthworks Perth Hills

kelsey · Jun 18, 2020 4:25 pm

Hi everyone,

I have a question about a block we are looking to build on. I’ve never built before, and while I’ve done some research I don’t have a construction background.
We have got several really conflicting quotes for the earthworks on our strata block in the Perth hills. It’s a flat 395m2 block in a strata. And they have left us confused about potential earthworks costs and we want to be somewhat sure of our budget.

The geo tech report has come back saying Class A soil with slightly clayey lateritic gravel. It also has rock refusal as several points from between 300mm-600mm across the entire strata.
The recommendation was mesh in the footings as some minor settlement may occur.

One guy said that because of the good class A soil, we would simply need to bring in more soil and compact to put the slab/footings down. We are in mains sewerage and all manholes and services have been installed/connected. He thinks this should make the earthworks costs quite low.

The other guy has said because of the rock it will likely need breaking to put the footings in, which could blow out the budget. This guys thinks an estimate of $15k earthworks is on the low side.

I am a touch confused about who is correct. Is it possible to just bring in extra soil (which will cost a bit I know) compact and that will be enough for the footings if the rock is 600mm down? Or is it likely to cost more because of the rock refusal?

Thank you!

alexp79 · Jun 18, 2020 5:18 pm

you really don't need very deep footings for Class A.

And yes, you can bring in some sand and compact it in case if you want to lift your house foundation a little bit, but even that might not be really needed if you are doing waffle pod slab, as most builders do those days. You would probably only need probably 100 mm of sand around building envelope + 1 meter perimeter if you go for conventional rafted slab.
You are doing full masonry (double brick)?

kelsey · Jun 18, 2020 5:52 pm

alexp79

you really don't need very deep footings for Class A.

And yes, you can bring in some sand and compact it in case if you want to lift your house foundation a little bit, but even that might not be really needed if you are doing waffle pod slab, as most builders do those days. You would probably only need probably 100 mm of sand around building envelope + 1 meter perimeter if you go for conventional rafted slab.
You are doing full masonry (double brick)?

Thanks for replying! I didn’t think we needed very deep footings. We are doing double brick, does that effect it?

alexp79 · Jun 18, 2020 6:20 pm

Full masonry would need 500 mm deep footings on Class A while articulated only 400 mm.
So probably 200 mm of fill in the worst case, depending on where you have rock at 300 mm.

chippy · Jun 18, 2020 10:05 pm

Typical footing on class A is 280 high 300 wide.
350mm below ground level minimum.
If you need to bring in fill and compact that's fine, you just need to check if you will then need retaining walls.

alexp79 · Jun 18, 2020 10:10 pm

AS 2870 specifies 400 mm deep footings for articulated masonry and 500 mm for full masonry on Class A (for rafted slabs). Waffle pods will be shallower, yes.

chippy · Jun 18, 2020 10:23 pm

alexp79

AS 2870 specifies 400 mm deep footings for articulated masonry and 500 mm for full masonry on Class A (for rafted slabs). Waffle pods will be shallower, yes.

Typical detail

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alexp79 · Jun 18, 2020 10:33 pm

So the total depth of internal beam is 450 mm on your drawing.
But yes, if you place slab on top of the natural ground (although it would still require some smoothening), you would only need to excavate to around 350 mm deep.