How Many Boreholes Does A Development Site Need?

Three is the fewest boreholes that shows which way a rock surface dips under a site, and a job with a basement usually needs more, placed at the corners, near the neighbours and under the heaviest loads. The real test is whether you can see how the ground changes across the footprint and below the deepest part of the excavation.

No single number suits every job. We plan each investigation under AS 1726 by working backwards from the questions the design has to answer, which means the building, the ground and the neighbours set the count between them.

Why three is the floor

One borehole tells you what is under that spot, and two give you a line, with no idea what the ground does either side of it.

Three points define a plane, which finally shows how the rock surface tilts across the site. That matters in Sydney, where the top of the sandstone can drop away over a short distance, because it decides whether the basement sits in rock at one end and in clay at the other. That split changes everything below ground.

What pushes the number up

A basement. Every face of the excavation needs its own information, so we want boreholes near each wall line rather than bunched in the middle.

Neighbours close to the boundary. A borehole near a shared boundary tells us what the ground is doing beside the footing we have to protect.

A big or irregular footprint. A long site has more room for the ground to change from one end to the other.

Heavy, concentrated loads. A tower core or a line of transfer columns deserves a borehole of its own, because the footing design rests on what is directly below it.

Mixed geology. Where the first results disagree, the honest response is more holes, not a bigger assumption.

A small low-rise job on known ground, with no basement, sits at the other end. That still gets an investigation, sized to what the building asks of the ground, and the [site classification basics](/blog/site-classification-soil-investigation-basics) cover the residential version.

How deep they go

Depth matters as much as the count, and a borehole that stops at the planned basement floor tells you nothing about what the footings will sit on.

So the holes go below the bottom of the excavation and below the founding level, far enough to see what carries the load. On a site with piles, that means past the expected pile toe. On rock, we core it, because a drill bit grinding through sandstone doesn't tell you where the joints and weak seams are. A core laid out in a tray does, and the reasoning behind that is in [founding in Sydney sandstone and rock sockets](/blog/founding-in-sydney-sandstone-and-rock-sockets).

The hole also has to reach far enough to show what sits under the good layer. A hard band of rock with soft clay below it can look like a founding level for about a day.

Tests in and out of the hole

A borehole is more than a log of colours. In-situ tests such as the standard penetration test give a strength reading as the hole advances, and cone penetration testing gives a continuous profile in softer ground.

Samples then go to the lab for moisture, plasticity, strength and consolidation testing, plus the chemistry that decides durability. On low-lying land, that includes the checks for [acid sulfate soils](/blog/acid-sulfate-soils-on-sydney-development-sites), and on Western Sydney shale it includes salinity.

Groundwater readings

A water level taken from an open hole on drilling day isn't a groundwater reading, because the drilling water and slow seepage both distort it.

For a basement, we want standpipes left in selected holes and read over time, ideally through wet weather. That reading sets the shoring type, the dewatering plan and the uplift on the slab, so a single guess can cost more than the whole drilling programme.

When the building is in the way

Access is the practical limit on many Sydney sites. An existing building, a tight driveway or a low ceiling can keep a drilling rig away from the spots you most want to test.

In that case we drill what we can reach, design conservatively and say where the gaps are. A second stage of drilling after demolition then fills them in, while the design can still change. We would rather put that on the table early than have the excavator find it.

What a borehole can't tell you

A borehole samples a tiny fraction of the ground. Say the log shows clay to six metres, and the excavator finds sand and water at four. Nothing was wrong with the log, because the ground between two holes was never measured.

Which is why the report needs [reading properly](/blog/understanding-geotechnical-investigation-reports), and why an engineer needs to inspect the ground as it opens up. We design the investigation and a specialist drilling contractor runs the rig, so the person who needs the answers is the person setting the programme.

How we approach it

We start with the architect's basement plan, the likely footing loads and the geological map, then mark up where a wrong assumption would hurt the most. The boreholes go there first.

Drilling is one of the smallest bills on a project, and it's the only one that tells you what the rest of the bills are going to be.

ACSES provides geotechnical engineering, including site investigation design, across Sydney. Talk to us about a project.

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