Your soil test report decides what slab or footings your home gets, and that has a big effect on the cost. A highly reactive clay site can add thousands to the slab compared with a stable sandy one. So it's worth knowing what the report is telling you.
Most Queensland homeowners don't get a full geotechnical investigation. They get a site classification report under AS 2870, the Australian Standard for residential slabs and footings. Here's how to read one.
What the report is for
A geotechnical engineer drills boreholes on your block, logs the soil layers and tests samples. From that they work out how much the ground will move as it gets wetter and drier, and how much load it can carry. Your structural engineer then designs the slab and footings to suit.
Site class: the number that matters most
The headline result is the site class. It's mainly about how reactive the soil is, meaning how much a clay soil swells when wet and shrinks when dry.
- Class A: mostly sand and rock. Little or no ground movement from moisture changes.
- Class S: slightly reactive clay. Characteristic surface movement (Ys) up to 20 mm.
- Class M: moderately reactive clay. Ys from 20 to 40 mm.
- Class H1: highly reactive clay. Ys from 40 to 60 mm.
- Class H2: highly reactive clay. Ys from 60 to 75 mm.
- Class E: extremely reactive clay. Ys over 75 mm.
- Class P: problem site. This covers soft soils, uncontrolled fill, collapsing soils, landslip risk, mine subsidence, or anything else that means the standard designs don't apply. A Class P site needs specific engineering design.
You may also see a "-D" suffix, such as M-D or H1-D, which means the moisture changes reach deeper than usual.
The higher the class, the deeper and stiffer the slab beams need to be, and the more steel and concrete go in. Moving from Class M to Class H2 or E is a real cost jump, and a Class P result can mean piers, piles or removing fill.
The Ys value
Ys is the characteristic surface movement: the engineer's estimate of how far the ground surface could move up and down over time, in millimetres. It's what sets the site class. If your Ys is near the top of a class range, ask your engineer how that affects the design.
Other things in the report
- Soil profile (borehole logs): the layers found at each borehole, such as topsoil, clay, sand, rock or fill, and how deep they go.
- Fill: if the report finds fill, it matters whether it is controlled fill (placed and compacted in layers with test results to prove it) or uncontrolled fill. Uncontrolled fill often pushes a site to Class P.
- Groundwater: a high water table affects excavation, slab moisture and footing design. Your engineer will tell you whether drainage or a different footing is needed.
- Allowable bearing pressure: how much load the ground can safely carry. This matters for pad and strip footings and for piers.
- Soil chemistry: acidic or saline soils can affect concrete durability, so your engineer may specify a higher concrete exposure class.
- Rock: shallow rock may mean extra excavation cost, or it may let footings sit on rock.
The lab tests you might see
On larger or problem sites you may see laboratory results as well. In plain terms:
- Particle size (sieve and hydrometer): how much gravel, sand, silt and clay is in the soil.
- Atterberg limits and plasticity index: how the soil behaves as its moisture content changes. Higher plasticity generally means a more reactive clay.
- Shrink-swell tests: measure how much a clay sample shrinks and swells, which feeds into the Ys calculation.
- Compaction (Proctor) test: finds the maximum dry density and optimum moisture content for a soil. It's used to check that fill on your site has been compacted properly, typically to a set percentage of that maximum.
- California Bearing Ratio (CBR): used for driveways and pavements, not for designing house footings.
Why your site affects the cost
Site costs are one of the biggest variables in a new home price. Before you sign:
- Make sure the soil test has been done on your block, not assumed from the estate.
- Check that the quote's slab design matches the site class in the report.
- Check whether site costs are fixed or provisional.
- Ask about extra costs for rock, fill, retaining or a high water table.
Questions to ask about your soil report
- What site class is my block, and what's driving it?
- Is there fill on the site, and is it controlled?
- Did the boreholes find rock or groundwater?
- What slab or footing system does the engineer recommend?
- What do I need to do with drainage and landscaping to protect the slab once I move in?
That last one matters more than people think. Reactive clay sites need good drainage and consistent moisture around the house, so keep water away from the slab and don't plant big trees close to it.
If you're watching the site works, my Earthworks checklist covers what to check during cut, fill and compaction.
C4A - Earthworks Quality Inspection Checklist
$59.00 AUD
Last updated 28 Oct 2026 · Revision 3 · Free updates for life
Checklist Type: Minor
Form Factor: Fillable, print-ready PDF (A4, 22 pages)
When to use: After the site is cut or filled, before the set-out
A 64-point checklist to inspect your building platform after earthworks: position, levels, cuts, fill, compaction, retaining walls and drainage, before the set-out and slab.
