Ground in Ballarat: basalt, alluvial clays and old mine workings

Ballarat's ground is mixed. The west of the city sits on a basalt plateau laid down by volcanic flows, while the centre and east grew on an alluvial basin in the Yarrowee catchment that was dug over in the gold rush. Old shafts still open up from time to time. For a home owner the practical question is how the ground under your house moves with the seasons, and whether drainage is making it worse.

Two kinds of ground

Volcanic flows from Mount Buninyong and Mount Warrenheip formed the basalt plateau under western Ballarat. Elsewhere, alluvial sediments and reworked mining ground sit closer to the surface. Each behaves differently when wet.

Mine shafts and sinkholes

Gold era shafts were rarely recorded. City of Ballarat caps one or two shafts a year on public land, and a large shaft opened in Sebastopol in 2016. A visual inspection cannot see underground, but subsidence, sudden cracking or a depression in the yard are worth reporting quickly.

How movement shows up

Diagonal cracks off window and door corners, doors that bind seasonally, and floors that slope are the common signs. We record widths and directions so a later visit can show whether movement is continuing.

Water decides a lot

Soil held wet against a footing behaves differently from soil a metre away. Ground falling toward the house, blocked stormwater and garden beds against the wall all concentrate water where the footing least needs it.

For new builds

The soil report for your lot sets the AS 2870 site classification and the slab or footing design follows it. No single class applies across Ballarat, so read your own engineering.

More about Ballarat

Written by our local inspection team from work carried out in Ballarat. Licensed building practitioners, Building & Plumbing Commission registration CDB-U 53425.

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Ground in Ballarat: common questions

What soil class is Ballarat?

There is no single answer. The site class is set for each lot from its soil report, and the city ranges from basalt plateau to old alluvial ground.

Could my house be over an old mine shaft?

It is possible in parts of the city because many shafts were never recorded. If you see a sudden depression or rapid cracking, contact council and get the house assessed.

Are cracks in a Ballarat home always serious?

No. Fine cracks are often shrinkage. Wider cracks, cracks that grow, or cracks with binding doors point to movement and are worth an investigation.

Can poor drainage cause cracking?

Yes. Uneven wetting and drying of the ground under footings is a common cause of movement, which is why we report on surface fall and stormwater.

What does a defect investigation not include?

Destructive investigation, geotechnical testing, engineering certification and rectification work. Where an engineer is needed, the report says so.