Soil and footings costs - experience or advice please

We have put in an offer on a lot in the Adelaide Hills. The agent gave us a copy of the soil tests done by the developer and it looks mostly H2 -D apparently. We are looking to build a home that is around 240sqm and our footings estimate provided by a building quote is 23K - the quote also states it's 'above standard'. We have forwarded the report on to the potential builder to seek advice, we have one more day that we can pull out of the contract if we need to owing to a huge increase in costs owing to this. Can anyone offer any advice or had this experience?

Do you know much else about your block?

How far down to rock? You might be able to determine this from the report you were given?

See if you can get a geotechnical survey for your block or section of the estate.

You basically want core sample details for your block or for samples near your block. This will give you some idea of depth down to rock which could drive site costs.

For my build I found out that nearby core samples indicate rock at 1.5m to 1.8m range. This meant I was comfortable with builder's allowance of 2.0m per pier. This depth also means they are unlikely to strike rock when trenches are dug for pipes, etc.

Check that your builder's allowance for piers is generous enough for depth down to rock. Some will say Xm per pier and others will say an allowance of say 100m of piering. An allowance per pier is easier to get right.

If there is any kind of slope, there may also need to be drop-edged beams. For my 2-storey house the builder allowed 5sqm for drop-edged beams. The fall on my block (left-to-right) is only 0.5m so their allowance should be enough.

Thank you for your reply. I have the entire soil report so will cut and paste info to do with the borehole closest to our site. I apologise if it is not the right info, I have only become familiar (if you could even call it that!) with these terms in the last two days since we got the report. I have no idea how to interpret these things but here goes

"2. The northern half of the site (BH4 and BH5) comprised an upper layer of high plasticity, dark brown / black silty clay up to a depth of 0.9 m underlain by grey, mottled orange brown high plasticity clay to a depth of 2.8 m to 3 m. The grey, mottled brown clay is assessed to be residual clay. In BH4 extremely weathered siltstone was encountered below 2.8 m depth whilst in BH5 siltstone was not encountered until 3 m depth. Refusal to push tube drilling was encountered in BH4 at 3.4 m depth.

The upper dark brown / black clay was moist and of hard consistency. The residual clays were relatively moist (greater than plastic limit) and of stiff consistency. The clays encountered during the investigation were typically dry and of stiff to hard consistency. DCP blow counts of about 2 or more blows per 100 mm penetration were recorded.
Groundwater was not encountered in any of the boreholes. Seasonal variations in groundwater levels must be anticipated. Groundwater seepage could be anticipated in excavations depending on the water levels in the creek / swale."

Our site is closest to BH5 and about 10m from the hole according to the site map at it's closest point. The contour plan indicates that each line on the site is equal to 0.5m and we have two running across the site from front to back so I would assume a 1m difference??? Does any of this help??

I am no builder, but if you need piers under your slab down to rock then each pier will be in the 3.0m range.

Make sure that your site costings allow for piers to 3.0m (best case) to 3.5m (worst case) depth for each pier. On these guesses if your slab needed 40 piers then you would need 120 to 140 lineal metres of piering.

The former owners of my block had I plans for a 26sq double-level home and that had approx 40 piers. Single-level homes generally need more piers because the slab would be bigger.

H-class soils mean that the soil is highly reactive. That may mean clay. Clay expands and contracts due changes in moisture levels. That's why piering is often needed under slabs for support.

One other thing - if you like the location you could always use the possibility of higher site costs to try and haggle/negotiate the price down.

Thank you again for your advice and information. We will check out how much this might cost tomorrow. It sounds like we need to move fast as we have until tomorrow night to pull out of the deal if it looks too expensive

So how much did it cost?