Soil Test ?

Hi all,

Do we need a soil (geotechnical) report/test before building a house? Our architect said we don't need one - i heard he said that's only required if my land is in acid zone. Not sure if that is true. If we need one when should it need to be done?

Hi Constant,

The answer is technically yes you should get one, but in practice your architect is correct, you probably don't need one.

Let me explain. Project Home builders are looking for ways to keep costs as low as possible so they design and build their structures to the absolute bare minimum that the Australian Standards require. That is why you always see people in this sector referring to soil tests.

They attract customers with all the shiny sparkly things that interior designers do and consumers don't think to ask about the strong and waterproof things behind the walls that actually matter.

So when you see their base prices, they are nearly always based on an "M" class slab and therefore they need to get a soil test done to see if there is any variation to that so they can pass those extra costs on to the customer ( site costs), for piering, extra steel and concrete etc.

Then if they pier, they will try and use screw piers rather than concrete to again save money.

In your case you are going down the custom build route and it sounds like your architect has experience working with custom builders similar to us, who are used to building higher quality structures.

In our case, as with other builders we have to give a 6 year structural warranty and we do not want to come back and fix any issue. Therefore we spend more money up front by over constructing our slabs and always piering to rock with concrete piers. We also have a specialist foreman onsite overseeing the slab pour.

Therefore, we don't normally get soil tests done for every site. We do occasionally get geotech studies done, for instance if we are building a large retaining wall next to a pool and you need to know the strength and depth of the rock and it's capability for withstanding lateral movement, but not for the reactivity of the clay.

Just make sure when you are selecting your builder, ask them about their process in constructing the slab and structure and who supervises that aspect of the build and how they maintain quality.

So for us we have one person that does the structures and 2 people who do finishes so we can maintain that consistency.

What you might also find is that your structural engineer will design to an M class and then put a notation on the drawings for the builder to get the test later. So in that case you are putting back on the builder.

I hope that long explanation helps.

Stay safe in the rain

Simeon

A few points.

Yes you need a soil test. Good idea to get one before even buying the land. It should also come with a land survey so, will tell you the fall of the block.
If the test comes back that it is worse then M class then an engineer needs to design the foundation.

A few more things....
No, concrete piers are not superior to screw piers. Used for different applications.
No concrete piers are not only designed to find rock for support. Some sites they bore down to a depth so that the static friction or suction of the pier supports the foundation.
Yes generally your builder will quote based on the house being on a slab, but whether it be M class is dubious.
No your framing material sizes has nothing to do with your foundation design. Usually wind rating and loading. Larger or wider timber framing sizes are used to accommodate the insulation. Usually a 70x35 stud at 600cts for partition walls which allows the plasterboard to flex more which is good or bad but not structurally weak at all.

Builders are not a charity. At the end of any blurb should be the disclaimer... we are in it for the money. So the variation is where they make it.

I don't think your architect gave you good advice.

Ardo

You have totally misread and miss-interpreted my post.

I will put it in dot points to make it easier for you:

1. Technically it is always a good idea to get a soil test and have as much information as possible. I finished the post by noting that the engineer will most likely put a notation on the drawing requiring one which the OP can then put back on the builder. So the architect could be coming at it from the perspective of your custom build being a design and construct scenario where the builder is responsible for confirming that the design is correct for the site.

2. If you have a builder who always builds the slab to a P standard then unless there is some unique site condition, ie you are on a landfill, I don't believe soil classification tests are necessary. If all we are testing for is soil reactivity, then why not just put some extra steel in and some extra concrete for insurance purposes? Have you seen how many cases some of the big builders fight, especially in Victoria when peoples homes have structural issues and they fight them all the way. Big money wins most of the time.

3. Concrete piers are far superior to screw piles. Come back to me when you have had a home sink by 50mm (when working for a project home builder) that is on a P site and the engineer had specified screw piles and tell me you would not have been better with concrete piers bearing to rock. Until that has happened to you, and you are responsible for the structural warranty there is no argument. Concrete piers cost us 4 times the price of screw piles, we choose spend more so customers get better quality and we dont have future structural issues.

4. Framing - No half decent carpenter would ever build their own home with 70x35 at 600mm centres, everyone of them would use 90x45 at 450mm centers for structural walls and 600mm for non load bearing. Enough said. My house is framed with 90 x 45, held together with over 2,000 75mm-100mm bugles, dynabolted to the slab at 600 centres, and I tell you what, whenever those southerlies hit my bedroom wall at night, I sleep like a baby!

5. It's the same with sarking, Australian Standards only require you to sark the trusses. The only project home company that I have personally seen who sarks walls in Metricon as that came out of Victoria. You cant tell me that having insulation in the wall cavity which is designed to be a wet cavity is good building practise. We sark and tape all our walls to keep that separation and also create an air barrier for better thermal comfort.

Ardo, we are talking about two different styles of building. The sausage factory style versus the custom method.

Off to work now.

Simeon

Perhaps you should read my post.

1. Not technically. It's a must. Most if not all councils require it.
2. I said that whether a builder will quote a price based on a M class slab is dubious. Unless they specify what it is then to assune that is a m class mistake.
3. Concrete piers are not superior to screw piers. As I said it depends on application. Plenty of concrete piers sink, or lift because they are squeezed out of the ground. Problem is builders choose a side and rubbish another method. Concrete piers are about 50-75 a metre based on a 450mm metre and screw pier is about 80-90 or standard 8kn. But the deeper you go the screw pier is cheaper.
4. Clearly I said partition wall. Did not mention load bearing. And I did say larger or wider timbers. F7, F11, mgp10, mgp12....
5. Sarking? Oh you could mean vapour permeable membrane perhaps? Or impermeable membrane, or anticon blanket. And yes the standard is the bare minimum. Insulation is about removing convection. Reflective wraps reflect heat. Which one does 'sarking do?'

Let's now debate steel v timber frames. Yellow tongue in bathrooms v cement sheet, raft v waffle... etc etc.
Many ways to skin a cat.
I'm at home on the pooper. Bit of youtube, bit of study, might go to work, might not..

Sorry to OP.

Sorry re soil test.
It is a requirement that a soil test is done before construction. Whether you do it or the builder, it gets done by someone. Just note some builders won't accept your soil test because they trust their own engineers. If you want to do get it done, you really should know where you want your house build so they can localise the testing area.