Why Canberra blocks differ: weathered rock, clay and site classes

What rock is under Canberra?

Old, folded and faulted rock, much of it deeply weathered. The ACT sits in the Lachlan Fold Belt, described in a 1986 review as "folded and faulted metasiltstones, metasandstones, and acid volcanics, intruded by a major granitic pluton, with some limestone lenses".

The standard geological map of the city was published in 1980, with notes from the Bureau of Mineral Resources, now Geoscience Australia, in 1981. Summaries of that map place the main rock units roughly like this:

Background from the 1980 Geology of Canberra, Queanbeyan and Environs map, as commonly summarised. The Deakin Fault runs through Narrabundah, Griffith, Forrest and Yarralumla.

Geology tells you what's down there in general. It doesn't tell you how the soil on your block will move, so it's never a site class. We work across both districts that sit mostly on the Canberra Formation: see our pages for the Inner North and Gungahlin.

How deep is the weathered rock under Canberra?

Often deep, and rarely even. Weathering slowly turns hard rock into softer, clay-rich material, and under Canberra it reaches a long way down in places.

Geoscience Australia's notes on the map record more that matters for building:

• Cavities in limestone forced a footing redesign at the Treasury Building.
• High water tables and seepage caused problems "near Canberra Grammar School and at Ainslie".
• Poor drainage in the alluvial basins at Isabella Plains and Lanyon. At Isabella Plains it was solved "by digging deep drainage channels and dewatering the ground before urban development".
• Shallow hard rock: "where hard rock occurs near or at the surface the necessary blasting adds to excavation costs".
• Fault zones: the 1986 review found "deep but variable weathering", reaching "at least 70 metres depth in places".

The upshot for a house: Canberra's rock is often deeply and unevenly weathered, and rock can be near the surface on one part of a block and metres down on another. Footings that bear on rock at one corner and on deep clay at another have to be designed for that difference.

What do the ACT soil landscape maps show?

They show broad soil types and their limitations at 1:100,000 scale. That's useful background, but too coarse for a single block. The ACT Soil Landscapes dataset has 55 map units, each with its own report. These are the ones under most Canberra suburbs:

Source: ACT Soil Landscapes, mapped for the ACT Government at 1:100,000, with a report for each unit on eSPADE. The map can be viewed on ACTmapi.

Two points matter when you read it. A "foundation hazard" is a mapped limitation for land use, not a rating of your block: a mapped hazard is not a site class. And the Williamsdale report notes that "a considerable range in profile form occurs within a small area", so the soil can change within a short distance.

What are AS 2870 site classes?

Site classification sorts a block by how much its soil is expected to move as it wets and dries, and flags problem sites. It's set under AS 2870, the standard for residential slabs and footings, and the footing design follows from it.

Plain-English summary of the AS 2870 classes. The engineer or soil tester sets your block's class from what's found on site.

There's no official map of site classes for Canberra or any suburb, so nobody can tell you your class from your address. Published reports for new-estate blocks show controlled fill and highly reactive classes, and a single block can range from rock to deep clay. Every site needs its own classification, reassessed after any cut or fill. Our guide to building on new-estate blocks looks at those reports in detail.

What does the certifier need about the soil?

The founding depth, and what the footings sit on. For houses and class 10 structures, form AF2025-18 asks for footing and slab details, including dimensioned plans, the "nominated founding depth and description of founding material", reinforcement and cover, slab preparation and concrete strength. If those details are on the engineer's plans, the certifier notes "Engineering Plans" on the checklist.

No ACT document requires a geotechnical report by name. What the certifier needs is footing design for your site's soil, and that design starts with knowing the soil, usually through a site classification from a soil test on the block.

When to call a structural engineer

Call an engineer before footings are designed for a new house, knock-down rebuild, extension or granny flat, and when cracks suggest existing footings are moving.

• Footings, slabs and site classification: from $1,290 + GST ($1,419 inc GST). We arrange the soil test and pass on its cost, then provide footing calculations and drawings your certifier and builder can use. Your fixed quote says if a site visit is needed.
• Knock-down rebuild engineering: a fixed quote from your plans, covering the site classification, footings and structure for the new house.
• Cracks in an existing house: start with our guide to cracks in walls in Canberra.

We're engineers only. We don't dig, pour or build, so the footing design is set by your site rather than by what a contractor would rather sell.