Soil Test - Rock Found

Hi guys,

I am planning to build a house in QLD and got the soil test report from the builder today. It says the site classification is 'S' class, but they also found rock. Below is the bore log sheet. They have drilled in 3 different places and it basically says they found "Weather rock" at 300mm. Is it common for them to not drill through the rock to find out how deep it is? The Lab test section says "N/A" for everything.

They also have a different lot address and different builder name in their report, but the bore log sheet has the correct builder name. I have raised this query with the builder.

The additional notes they have added in the email is - This one has rock at 300mm:
Note: Rock has been identified on the soil report in one or more bore hole starting at 300mm. In the event that during construction <builder> encounter rock that is required to be removed, the Client will be notified immediately to approve a 'COST PRICE' variation to bear the cost of rock breaking and removal to continue work onsite.

Can anyone please explain if this the following has sufficient information? (especially the lab test section)












Any help is greatly appreciated.

Residential soil tests are usually done with auger drilling which can't penetrate bedrock-standard practice because it is usually adequate to meet regs.

The information provided looks fine-they can't do any testing because there is no soil to test. Its an A site not an S site though.

Its looks likely rock excavation will be required quite shallow so service trenches/water/power etc will require minimum cover and may require rock excavation. Maybe confirm with them what the red line is-underside of slab or bottom of thickening beams/footings. The underside of slab and footings should be at the same depth on this site with no edge beams.

You might be looking at a little bit of rock excavation but a lump sum claim or some such thing to mobilise the equipment required to excavate that rock.

I'm not all over contract wording but I think you can only issue a variation where unforeseen ground conditions are encountered. Moderately strong as per the logs (not AU standard term) would be considered hard rock requiring rock excavation equipment.

worldofmud Thank you for the reply. I will make sure to ask the builder about the red line.

How deep do builders usually cut the site to construct the slab on site with rock? I tried searching online but there isn't any information about this.

Albert142

The minimum required to accommodate the floor slab. I think you only need 100mm for the slab thickness but 300mm for footings at the sides to comply (this can only be done on a Class A site). But the 300mm is detail excavation not cut so it's a curious e-mail.

Who ever did the soil report really has done the minimum required and left unanswered questions. Don't suppose you are allowed access to the site (armed with a 10mm diameter steel rod and lump hammer...)

worldofmud

Oh okay. No, I am not allowed access to the site, as it is unregistered. We were given access only due to the soil test clause in our contract.

Its frustrating when they take your money and do as minimum as possible. I will have a talk to the builder and see what they have to say about this.

To be fair to the soil testers, they have done what they were asked most likely. I looked at the borehole locations from a geotech. investigation point of view and the locations would be inadequate for that type of investigation but pretty standard for a residential soil test.

It's likely the builder who requested this level of works, and site class to AS2870 is the minimum required to satisfy DA/building requirements, but not normally the required level to control costs on any site never mind a rock site.

If you do contact the builder, perhaps query if there would be a difference in price (to you) where a site is classed A rather than S.
















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