How many boreholes does a house block need? Six myths about site investigation depth and spacing

One hole in the middle of the block, two metres down, and the soil test is done. That is what many owner-builders picture when they hear "site investigation", and for a simple lot on uniform ground it is not far off. The trouble is that the number of holes, how far apart they sit and how deep they go are the exact decisions that separate a classification you can build on from one that misses the thing that later cracks the slab.

Most beliefs about borehole numbers are half-right. They come from real sites where they held true, then get applied to sites where they do not. Here are six of them, and where each one breaks down.

Myth 1: One borehole is enough for a house

On a flat lot, one geological unit, no fill, no slope, a single well-logged hole can genuinely represent the ground under a modest house. AS 2870 allows a classification to lean on regional knowledge and a minimum of investigation where conditions are consistent.

Where it breaks down is variability. Li et al. (2016) tested 47 sites across Melbourne and found wide scatter in soil reactivity within a single mapped geological unit. If reactivity varies that much between sites on the same geology, it can vary across a single block too, especially a cut-and-fill block where one corner is exposed clay and another is imported material. One hole in the natural corner tells you nothing about the fill corner.

What to do with it: treat one hole as the floor, not the standard. The more the ground varies across the lot, or the less you know about it going in, the more holes you need. A desktop study is the cheapest way to find out which kind of lot you have before you book the rig.

Myth 2: Two metres is deep enough

For a slab-on-ground house on stiff residual clay, the zone that drives footing design is shallow. The depth of seasonal moisture change (Hs), the layer that shrinks and swells with the weather, is often within the top couple of metres, so a two-metre hole can capture the reactive profile that sets the AS 2870 class.

It breaks down badly on soft ground. Hargreaves (2005) describes South East Queensland lowland estates from Noosa to Coolangatta built on soft compressible deposits where performance is controlled by material well below the surface, and where an initial investigation may need to extend beyond 30 metres. A two-metre auger on that ground finds a crust and stops short of the layer that matters. The same logic applies to the soft coastal clays of northern New South Wales; the Ballina clay research site was sampled with continuous boreholes precisely because the interesting behaviour sits deep in the profile (Pineda et al. 2014).

What to do with it: let the geology set the depth, not a default. Alluvial floodplains, back-swamp deposits and reclaimed land need holes that reach competent material. Residual clay over rock often does not.

Myth 3: More holes always means a better answer

It is tempting to think that if one hole is risky, ten must be safe. Spend enough and you eliminate the surprise.

Investigation spend follows a curve of diminishing returns. Past a point, extra holes on a small, uniform lot mostly confirm what the first few already showed. The skill is not maximising holes; it is placing them where the ground is likely to change. That is why establishing the origin of the soils first matters so much, and it is a point the reactive soils and building damage work keeps returning to: knowing whether you are on residual clay or layered alluvium tells you how far apart two holes can sit before they stop representing each other.

What to do with it: spend the effort on placement and depth, not raw count. Two well-sited holes on a variable block beat five clustered on the good half.

Myth 4: The boreholes down the street cover my block

Nearby borehole logs are the closest thing a desktop study has to ground truth, and on consistent geology they are genuinely informative. A log from three lots away that shows rock at one metre is a strong hint about your lot.

The distance at which that hint holds depends entirely on how uniform the ground is. River-laid soils are layered and can change over short distances as old channels and swamps cross a floodplain. Residual soils that formed in place from weathered rock grade unevenly into the rock below, so depth to refusal can swing metres across a single street. A neighbour's log tells you the unit and the likely range; it does not tell you where your lot sits within that range.

What to do with it: use nearby logs to scope the investigation, not replace it. They tell you what to expect and how deep to drill, which is exactly the job of a desktop report before anyone mobilises a rig.

Myth 5: The sampler and hole size don't change the result

Many assume a hole is a hole. Auger it, log it, done.

How the sample is taken changes what the laboratory sees, particularly in soft clay. The Ballina clay campaign used a whole range of samplers, from 50 mm open tubes to 89 mm fixed-piston samplers, precisely because disturbance during sampling can distort the mechanical properties measured afterwards (Pineda et al. 2014). A crushed or remoulded soft-clay sample reports the wrong strength. For a reactive stiff clay the concern is different but real: the sample has to be intact enough to run a valid shrink-swell index (Iss).

What to do with it: on soft or sensitive ground, the sampling method is part of the scope, not an afterthought. Ask what will be recovered and how, not just how many metres will be drilled.

Myth 6: A desktop study can tell me how many holes I'll need without a site visit

A desktop study genuinely narrows the question. It settles which geological unit is under the lot, which AS 2870 climate zone applies, whether the block sits on a floodplain or a filled gully, whether groundwater is likely to be shallow, and what the nearby logs show. From that, it can recommend a sensible investigation: roughly how many holes, how deep, and whether to budget for shrink-swell testing or expect soft ground.

Where it stops is the compliant number for your certifier. The desktop scopes the investigation; it never substitutes for the field site classification, and nothing in it is engineering advice for a specific lot. The final call on hole count and depth belongs to the geotechnical engineer who signs the classification, informed by what they actually find when the rig arrives.

What to do with it: run the desktop first so the drilling brief is right the first time. A forensic reading of an estate on the Nerang River floodplain would have reframed every borehole log on that site; getting the geology straight before the holes go down is what makes the holes mean something.

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Before you brief a driller or price a soil test, generate a desktop report for the address to see the geology, nearby bore logs and likely classification the investigation should be built around, or see a sample report to check what it covers.

LayeredGeo compiles geology, soils, groundwater and site data into an automated geotechnical desktop report for any address in Queensland, New South Wales or Victoria.

Sources

• Jubert Pineda, Laxmi Prasad Suwal and Richard Kelly (2014). Sampling and laboratory testing of Ballina Clay. Australian Geomechanics, Volume 49, Number 4 (Dec 2014). geomechanics.org.au

About this article. Published by LayeredGeo and written from the published research cited in the Sources section above. It is general information about how property and ground conditions are assessed in Australia, not engineering, planning, legal or financial advice, and it is not specific to any property. Check anything that matters against the source dataset or a suitably qualified professional before you rely on it. If you spot something wrong, tell us at hello@layeredgeo.com.au and we will fix it.

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