Reading groundwater and bore records: what they tell you about a site before you dig

You can learn a lot about a block from the surface. But some of the most important things about building on it happen below the surface, where you can't see them, and one of the biggest is water.

Groundwater, the water sitting in the soil and rock beneath a site, quietly shapes what you can build and how much it costs. A high water table can flood an excavation, weaken a footing, and turn a straightforward slab into a design headache. And the frustrating part is that you often don't find out until the digger's already on site.

The good news: there's a lot of public information about groundwater in Queensland and New South Wales, and a desktop report can pull it together before you commit.

Why groundwater matters for footings and slabs

Water under a block affects the ground in a few practical ways.

First, it changes how the soil behaves. Saturated clay is softer and weaker than dry clay, so it carries less load. If the water table sits close to your footing depth, the ground your house relies on may not be as strong as the classification alone suggests.

Second, it makes excavation harder. Dig below the water table and the hole fills up. That means dewatering (pumping the water out while you work), shoring, and delays, all of which cost money that wasn't in the original quote.

Third, water drives soil movement. On reactive clay soils, seasonal wetting and drying is what makes the ground swell and shrink under a slab. A shallow or fluctuating water table changes that moisture story, and that flows straight through to your AS 2870 site classification and footing design.

And finally, water and certain soils don't mix well. Where the water table intersects acid sulfate soils, disturbing the ground can trigger chemical reactions you'd rather avoid. Knowing the likely depth to water is often the first clue that acid sulfate soils are worth a closer look.

What bore records actually are

A bore is a hole drilled into the ground, usually to reach or monitor groundwater. Every time one is drilled, whether for water supply, monitoring, or investigation, it usually generates a record.

That record can include:

• The location and depth of the bore
• The standing water level, meaning how far below the surface the water sat when it was measured
• A driller's log describing the layers of soil and rock encountered on the way down
• Sometimes water quality or yield information

Both Queensland and New South Wales keep public databases of these bore and groundwater records. They're one of the few ways to get a real, measured look at what's under a specific patch of ground without drilling your own hole.

What bore records can and can't tell you

Bore records are genuinely useful, but they have limits, and it's worth being clear-eyed about both.

What they're good for:

• A sense of depth to water. If nearby bores all show water at a few metres, you have a strong hint about your own block.
• A picture of the geology. Driller's logs describe the soil and rock profile, so you can see whether a site is likely to be sand, clay, rock, or a messy mix.
• Context for a full investigation. A geotechnical engineer walks onto a better-informed site when they already know what the neighbours' bores found.

What they can't do:

• Speak precisely for your exact site. The nearest bore might be a street away, in a slightly different landform, drilled decades ago.
• Capture seasonal change. A water level measured in a dry September can be very different from the same spot after a wet summer.
• Replace boreholes on your block. For a footing design you can rely on, someone eventually has to test the actual ground.

So bore records are a strong indicator, not a guarantee. They tell you what to expect and what to watch for, which is exactly what you want before you spend money on the ground.

When the desktop picture is enough, and when it isn't

For early decisions, buying, feasibility, comparing sites, understanding likely constraints, a desktop review of groundwater and bore data is often plenty. It tells you whether water is likely to be a problem, whether the site might need dewatering, and whether the geology looks friendly or difficult.

Once you're committed and moving toward design, you'll usually still need boots-on-ground testing to confirm the classification and footing design. If you're weighing up which stage you're at, our explainer on what a geotechnical desktop study is and when one is enough walks through it.

The smart move is to do the desktop work first. It's cheaper, faster, and it tells you whether the expensive site investigation needs to include groundwater monitoring at all.

Common questions

How do I find groundwater bore records for a property in QLD or NSW? Both states publish groundwater and bore databases online, drawn from registered bores across the state. You can search by location to find nearby records, water levels, and driller's logs. The catch is that the data is scattered across government portals and the logs can be technical, which is why a desktop report that pulls the relevant records for one address saves a lot of digging.

Does a high water table affect my AS 2870 site classification? Indirectly, yes. The classification is mostly about how reactive the soil is and how much the ground moves, but groundwater influences soil strength and moisture conditions, which feed into footing design. A shallow water table often signals a site that needs more careful attention, and sometimes a Class P (problem site) outcome.

Can bore records replace a site investigation? No. They're a valuable head start, giving you likely depth to water and a picture of the local geology, but they measure other holes, not yours. For a footing design you can build to, a geotechnical engineer still needs to test the actual site.

Before you drill, dewater, or design a footing, it's worth knowing what the ground is likely to hold. Pop in your address or see a sample report.

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

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