Your slab and footings are designed around what's under the ground. If the soil test misses something, the design can be wrong, and fixing foundations after the house is built is about the most expensive repair there is.

Most standard soil tests for a house use two boreholes, usually at opposite corners of the house footprint. On a lot of sites that's fine. On many sites it isn't enough.

What the soil test actually does

A geotechnical engineer drills boreholes, logs the soil layers and tests samples. From that they give your site an AS 2870 site classification:

  • A: sand and rock, little or no movement.
  • S: slightly reactive clay.
  • M: moderately reactive clay.
  • H1 and H2: highly reactive clay.
  • E: extremely reactive clay.
  • P: problem sites, such as uncontrolled fill, soft soils, landslip risk or trees affecting the soil.

Your engineer uses that classification to design the slab and footings. Reactive clays swell when wet and shrink when dry, and the more reactive the soil, the stiffer and deeper the slab and footings need to be.

Why two bores can miss things

Two holes sample a tiny fraction of your building footprint. Soil can change dramatically within a few metres, especially on estates where the land has been cut and filled. Two bores can miss:

  • A pocket of soft soil or uncontrolled fill.
  • A change in clay type and reactivity.
  • A rock shelf under one end of the house.
  • Wet ground or a perched water table.
  • Old tree roots or the zone where a tree has been removed.

If the design assumes uniform ground and it isn't uniform, you can end up with uneven movement. That shows up as cracked walls and slabs, doors and windows that stick, and uneven floors. In Queensland we rarely have basements, but poor drainage combined with reactive soil can also mean water ponding against the slab edge, damp walls and mould.

When extra boreholes matter most

Ask about three or four bores, within the footprint and around its edges, if:

  • Your block is sloping.
  • It's on an estate with cut and fill, especially if you're on the fill side.
  • Trees have been removed from the site or there are large trees nearby.
  • The house is large, long or irregular in shape.
  • The first results come back as class P, or the two bores show very different soil.
  • There are signs of wet ground, soaks or overland water flow.
  • Neighbouring homes show cracking or movement.

On a flat, small block in an established area with consistent soils, two bores may well be enough. That's the engineer's call to make, not a salesperson's.

What extra bores won't tell you

More holes give a better picture of soil layers, reactivity and bearing capacity. But some things need specific testing, not just more holes:

  • Soil aggressivity (acid sulfate soils, or salts that attack concrete and steel) needs particular lab tests.
  • Groundwater may need monitoring over time, not a single reading.
  • Fill compaction needs records from the developer (a Level 1 compaction certificate) or specific testing.

If any of these are a concern on your site, ask the engineer what testing they recommend.

How to ask for it

  1. Before you sign the preliminary services agreement, ask your builder how many bores are included and where they'll be.
  2. Ask the geotechnical engineer how many bores they'd recommend for your site and house design, and get that recommendation in writing.
  3. Ask for the price of extra bores. It's usually small compared with the cost of the slab, let alone a foundation repair.
  4. When the report comes back, check the classification, the bore locations relative to your house, and any notes about fill, trees or water.
  5. If the house design or position changes later, ask whether the existing bores still cover the new footprint.

My C1 Before Buying Land and C4A Earthworks checklists cover the site questions to ask before and during this stage. You'll find them at constructor.net.au/checklists.

The bottom line

A soil test is one of the cheapest parts of your build and one of the most important. Don't assume two bores is the standard you have to accept. Ask the engineer what your site needs, and pay for it if they recommend more.