In short
Greater Hobart sits on Permian mudstone, Triassic sandstone, Jurassic dolerite and pockets of Tertiary clay and basalt, so soils change from street to street. Clays weathered from dolerite are documented as expansive, in Mineral Resources Tasmania reports on houses in Lindisfarne, Lenah Valley and Taroona. A soil test classifies each site to AS 2870, from Class A to E or P, and the footings are designed for that class.
Key takeaways
- Hobart's ground varies over short distances: rock, sandstone, dolerite clays and fill can all appear within one suburb.
- Black and brown clays weathered from dolerite are documented as expansive and have cracked houses in Hobart.
- Only a soil test on your site gives its AS 2870 class. There is no Hobart-wide class to assume.
- Most damage on reactive clay starts with water: trees drying the soil, or leaks and poor drainage wetting it.
Two houses on the same Hobart street can sit on very different ground. One may be on dolerite rock, the next on a metre of black clay, and a third on old fill. That's why every new house, extension or granny flat starts with a site classification, and why cracking in older homes so often comes back to the soil.
Hobart soils: quick facts
- Rock types
- Permian mudstone, Triassic sandstone, Jurassic dolerite, Tertiary clay and basalt
- Documented expansive
- Clays weathered from dolerite (MRT reports)
- Standard
- AS 2870-2011 site classes A, S, M, H1, H2, E, P
- Common triggers
- Trees drying the soil; leaks and poor drainage wetting it
- Who classifies
- Soil scientist, Engineer – Civil, geotechnical engineer or engineering geologist
- Our fee
- Fixed quote per job, within 1–2 business hours
What soils and rock are under Hobart?
Greater Hobart is built on four main rock groups, and the soil on each one behaves differently. That's the main reason site classes vary so much across short distances.
- Permian mudstone and siltstone in the foothills and under parts of the city.
- Triassic sandstone on the valley floor and under the CBD. A 1987 investigation in Argyle Street found good sandstone 5.5–6.1 m down, under fill and a layer of dolerite and mudstone boulders in sandy clay.
- Jurassic dolerite on the higher hills and kunanyi / Mount Wellington. As it weathers it forms black and brown clays, and in Hobart black clay frequently overlies weathered dolerite.
- Tertiary clays and basalt in local pockets, which weather to clays of their own.
On top of these sit fill, old creek deposits and garden soil. Fill matters as much as the natural ground: a 1988 investigation in Lenah Valley found a house sitting entirely on fill up to 2.5 m deep.
Which Hobart soils are reactive?
Clays weathered from dolerite are the best-documented reactive soils in Hobart. Mineral Resources Tasmania's reports (from its predecessor, the Department of Mines) describe them on houses in three suburbs:
| Report | What it found | Cause and advice |
|---|---|---|
| Lindisfarne, 1987 | Black and brown dolerite-derived clays, highly plastic. These clays "are known to be expansive" and have cracked houses in Hobart and Launceston | Wet clay along the cracked walls, poor drainage and a neighbour's trees. A drain and path were recommended; underpinning the whole cracked area did not appear warranted |
| Lenah Valley, 1988 | A house on up to 2.5 m of fill over black clay, with a gravelly clay below | The clays were expansive and may cause structural problems to houses |
| Taroona, 1977 | Of 83 houses studied, 46 had no structural damage, 22 light damage and 15 major damage | Mainly expansive clay, moisture change and inadequate footings; less often settlement, peat or landslip |
Tasmania Department of Mines unpublished reports 1987/71, 1988/39 and 1977/35.
The Taroona study found the brown clay was very plastic and rich in montmorillonite, a strongly swelling clay mineral, and that serious damage often didn't start until many years after a house was built. Near Sorell, two 2025 consultants' reports lodged with development applications show why the class depends on more than the clay. Both found highly reactive clay over Tertiary basalt. One site was classed M, with basalt only 0.5–0.7 m down, and would be Class A with footings on the basalt. The other was classed P because of uncontrolled fill. There is no Hobart-wide soil class: each site has to be tested.
What do the AS 2870 site classes mean?
A site class describes how much the ground surface is expected to move as the soil gets wetter and drier, measured as the characteristic surface movement (ys). The class sets the footing design, so a higher class means stiffer, deeper or more carefully detailed footings.
| Class | Ground | Expected surface movement |
|---|---|---|
| A | Little or no movement from moisture change: most sand and rock sites | Little or none |
| S | Slightly reactive clay | 0–20 mm |
| M | Moderately reactive clay or silt | 20–40 mm |
| H1 | Highly reactive clay | 40–60 mm |
| H2 | Highly reactive clay | 60–75 mm |
| E | Extremely reactive clay | More than 75 mm |
| P | Problem site: soft soil, uncontrolled fill, trees, abnormal moisture or landslip risk | Needs a specific engineering design |
AS 2870-2011 Residential slabs and footings. Our site classification and footings service gives the class and designs to it.
Why do houses on reactive clay crack?
Houses on reactive clay crack when the soil under one part of the footing changes moisture more than the rest. Clay swells as it wets and shrinks as it dries, so uneven drying or wetting bends the footing and the walls above it. Even movement is rarely the problem; uneven movement is.
What moves a house? Four common causes
- Trees and roots draw moisture from the soil near a footing, so clay under that side shrinks.
- Reactive clay swells when wet and shrinks when dry, moving footings with the seasons.
- Leaking pipes or poor drainage soak and soften the ground under part of the house.
- Poorly compacted fill keeps settling under a footing long after the house is built.
The MRT reports show the same pattern again and again. In Lindisfarne it was a neighbour's line of trees and a garden that drained against the wall. In Taroona the listed causes were shrubs and trees, new drains, upslope building that changed the water regime, and broken service pipes. Cracks on reactive clay often open and close with the seasons, which is why a single reading can mislead. Our guide to cracks in walls explains how to measure them.
How do you get a soil test or site classification in Tasmania?
You get a site classification from a qualified site classifier: a soil scientist, a licensed Engineer – Civil, a geotechnical engineer or an engineering geologist. In the medium, medium-active and high landslip bands, it must be a geotechnical practitioner.
Desktop check
Geology maps, landslip bands, past reports and the proposed building layout.
Boreholes
Usually two or more, by hand auger or drill rig, logged for soil type, moisture and depth to rock. Fieldwork may be done with our geotechnical partner, [geotechnical partner].
Classification and footing design
The AS 2870 class, any problem-site factors, and footings designed for them, with a Form 35 for the building surveyor.
You need one for a new house, an extension, a granny flat and often a large deck or retaining wall. Underpinning is notifiable work that may require a site and soil test to AS 2870 and an engineer's design, so a site investigation is also part of underpinning design.
How do you reduce movement on reactive clay?
You reduce movement by keeping the moisture under and around the footings as even as possible. CSIRO's homeowner guide Foundation Maintenance and Footing Performance is the standard reference.
Do
- Fix leaking pipes, taps and downpipes quickly
- Grade the ground to fall away from the walls
- Keep paths and paving sealed against the house
- Water gardens evenly in long dry spells, away from footings
Don't
- Plant large trees close to the house
- Let downpipes or overflows discharge against the wall
- Build garden beds that pond water at the footing
- Remove a big established tree without advice: the clay can swell back
When should you call a structural engineer about soil?
- Before you build or extend, for the site classification and footing design
- Cracks are growing or open and close with the seasons. See our crack and foundation report (from $590 + GST)
- Doors stick or floors slope in one part of the house
- A big tree is close to the house, or one has just been removed
- You've been quoted for underpinning and want an independent view first (from $390 + GST)
Find the cause before the repair
We are engineering only, so we have no repair to sell. Send the address, photos or plans and you'll have a fixed quote within 1–2 business hours. Local detail is on our Lindisfarne, Lenah Valley and Taroona pages. Get a fixed quote.
Need an engineer?
Need a soil test or footing design?
Send the address and your plans. We reply within 1–2 business hours with a fixed quote.
Sources
- Tasmania Department of Mines, Unpublished Report 1987/71: Investigation of house cracking at 21 Cottesloe Street, Lindisfarne (W. R. Moore)
- Tasmania Department of Mines, Unpublished Report 1988/39: Report on drilling at 89 Doyle Avenue, Lenah Valley (W. R. Moore)
- Tasmania Department of Mines, Unpublished Report 1977/35: House damage in the Taroona area (C. J. Knights)
- Tasmania Department of Mines, Unpublished Report 1987/29: Foundation investigations at Kings Car Park, Argyle Street, Hobart (D. J. Sloane)
- T. Webster, The geology beneath the Hobart CBD (AusIMM Tasmania)
- Sorell Council, development application 5.2025.183.1, 24 Federation Drive, Sorell (advertised documents, 2025). Site and soil assessment: Class M, Class A if founded on basalt
- Sorell Council, development application 5.2025.185.1, 62 Friendship Drive, Sorell (advertised documents, 2025). Site and soil assessment: Class P because of uncontrolled fill
- Standards Australia, AS 2870-2011 Residential slabs and footings. Site classes and characteristic surface movement
- CSIRO Publishing, Foundation Maintenance and Footing Performance: A Homeowner's Guide (formerly BTF 18)
- Department of Premier and Cabinet and State Planning Office, Fact Sheet: Landslip Hazards in the Tasmanian Planning Scheme and Building Act 2016 (15 January 2026). Who may prepare an AS 2870 site classification