Owner-builders · Builders · Designers
Soil tests, site classification and footing design in Hobart
Before a new house, extension or slab goes ahead, someone has to classify the ground under it. We test the soil, give the AS 2870 site class and design footings that suit it, with a Form 35 for your building surveyor.
Engineering only
We inspect, diagnose, design and certify. We never sell the repair.
Fixed quote in 1–2 business hours
Get a fixed quote for a soil test
Send the address, a site plan and what you plan to build. Your quote says how many boreholes the site needs.
Fixed quote for your job. No obligation. Or call 03 6146 4747
In short
A soil test, or site classification, measures how much the ground can shrink and swell and gives it an AS 2870 site class from A to P. Your slab or footing design depends on that class. In Hobart, clays weathered from dolerite are documented as expansive, so we test each site rather than assume a class. Priced by fixed quote.
- Fee
- Fixed quote per job, agreed before we start
- Site classes
- A, S, M, H1, H2, E or P under AS 2870-2011
- You receive
- Site classification report, footing design and a Form 35
Overview
What is a soil test and site classification?
A soil test for building is a site investigation that finds out what the footings will sit on, and a site classification turns the result into an AS 2870 class. AS 2870-2011 Residential slabs and footings sorts sites by how much the ground surface can rise and fall as clay soils wet and dry, which it calls the characteristic surface movement.
On site, boreholes are drilled by hand auger or drill rig where the building will go. Each one is logged to AS 1726-2017: soil type, colour, moisture, strength, depth to rock and any groundwater or fill. Where the clay's reactivity needs measuring, samples go to a laboratory for shrink-swell testing. The engineer then classifies the site and designs footings to suit.
When you need one
- A new house, granny flat or garage on a slab or footings
- An extension with new footings, or a second storey on existing ones
- Underpinning, which the Director's Determination notes may require a site and soil test to AS 2870
- A slab for a carport, shed or studio that your building surveyor needs designed
Key takeaways
- A site class comes from boreholes and testing, not from the suburb name.
- Classes run from A (little or no movement) to E (more than 75 mm), plus P for problem sites.
- Hobart's ground changes over short distances: dolerite clay, sandstone, mudstone and basalt.
- Wind class is site-specific too, so we assess it with the soil.
AS 2870 site classes
What do the AS 2870 site classes mean?
Each site class describes how much the ground surface can move as the soil moisture changes. The higher the class, the stiffer and deeper the footings need to be.
| Class | Ground | Surface movement (ys) | What it means for footings |
|---|---|---|---|
| A | Little or no ground movement from moisture changes: most sand and rock sites | Little or none | Footings sized for strength rather than ground movement |
| S | Slightly reactive clay | 0–20 mm | Lighter slab beams and strip footings |
| M | Moderately reactive clay | 20–40 mm | Stiffer slab beams, more care with drainage |
| H1 | Highly reactive clay | 40–60 mm | Deeper, stiffer beams; masonry walls need articulation |
| H2 | Highly reactive clay | 60–75 mm | As for H1, with stiffer footings again |
| E | Extremely reactive clay | More than 75 mm | A very stiff slab, or piers to more stable ground |
| P | Problem site: soft soils, uncontrolled fill, trees or abnormal moisture | Not classed by movement | An engineer's design for the specific problem |
Site classes and movement ranges from AS 2870-2011. The footing notes are general engineering practice; your design follows the class and conditions found on your site.
How is the class worked out?
The engineer combines the borehole logs with the clay's measured or estimated shrink-swell behaviour and the depth over which soil moisture changes in the local climate. The result is the characteristic surface movement, which sets the class from S to E.
Class P overrides the movement bands. A site with uncontrolled fill, soft ground, large trees nearby or abnormal moisture, such as a leaking pipe or a spring, is a problem site, and its footings are designed for that condition rather than taken from standard tables.
A higher class doesn't mean a site can't be built on. It means the footings must be designed for more movement, and drainage around the house matters more over its life.
Characteristic surface movement by site class
- Class S0–20 mm
- Class M20–40 mm
- Class H140–60 mm
- Class H260–75 mm
- Class Eover 75 mm
Hobart's ground
What are Hobart's soils like for building?
Hobart's soils change over short distances because the rock beneath them does: Permian mudstone and siltstone in the foothills and under the city, Triassic sandstone on the valley floor and in the CBD, Jurassic dolerite on the higher hills and kunanyi, with local Tertiary clays and basalt. No published map gives a site class for the whole city, so every site is tested.
Dolerite clays
Clays weathered from dolerite are documented as expansive. Mineral Resources Tasmania reports link them to cracked houses in Lindisfarne and record expansive clays in Lenah Valley.
Taroona
A 1977 MRT report put house damage in Taroona down mainly to expansive clay, moisture change and inadequate foundations.
Sorell basalt clays
Consultant reports for local developments in Sorell found highly reactive clay over basalt, yet one site was classed M (basalt close to the surface) and another P because of uncontrolled fill.
Rock and fill
Sandstone, mudstone or dolerite close to the surface can make a site Class A, while old fill or a removed tree can make it Class P.
Our guide to reactive clay soils in Hobart explains why houses on clay crack and how to look after the ground around your footings.
Footing design
Which footings or slab will my site need?
The right footing depends on the site class, the slope, the depth to rock and the building. These are the types we design most often for Hobart homes.
Stiffened raft slabs
Concrete slabs with deepened edge and internal beams, sized to the site class. Waffle slabs suit flatter, lower-class sites.
Strip and pad footings
For brick and masonry walls, timber floors on piers, and posts carrying beams or decks.
Bored piers and screw piles
To carry loads down through reactive clay or fill to firmer ground, often on steep or Class P sites.
Stumps and piers
For raised timber floors on sloping blocks. See restumping for existing houses.
Footings for extensions
New footings matched to the existing house so old and new parts move together, or jointed so they can move apart.
Form 35 and stage checks
A Form 35 with every design, and an inspection of excavations or reinforcement before the pour if your surveyor wants one.
Wind class is site-specific
Tasmania sits in non-cyclonic wind Region A, but the wind class for your house depends on the site's terrain, shielding and topography under AS 4055. There is no single wind class for Hobart, so we assess it with the soil and give both to your designer.
How it works
How does a site classification work?
- Step 1
Send the address and plans
A site plan showing where the building goes, plus any earlier soil reports for the lot.
- Within 1–2 business hours
Get a fixed quote
It says how many boreholes the site needs, whether a drill rig is needed and when you'll receive the report.
- On site
Drill and log the boreholes
Boreholes logged to AS 1726-2017, with samples for laboratory testing where needed. Drill-rig work may be carried out with [geotechnical partner].
- On the date in your quote
Site classification report
The AS 2870 site class, the soil profile and the wind class, ready for your designer and surveyor.
- If you need it
Footing or slab design
Drawings and a Form 35, so your builder can price and build the footings.
Soil test and footing design
Priced for your site and building once we have the address and plans, and fixed before anything starts.
- Boreholes logged to AS 1726-2017
- AS 2870 site class and a site-specific wind class
- Slab or footing design with a Form 35, if you need it
- Desktop footing design Tasmania-wide from an existing soil report
Before the soil test
How can you prepare for a soil test?
A little preparation makes the fieldwork quicker and the result more useful for your design.
Do
- Peg or mark the corners of the planned building
- Find underground services, such as stormwater, sewer and power, before anyone drills
- Clear access for a drill rig if the quote includes one
- Tell us about old fill, removed trees, springs or earlier buildings on the lot
Don't
- Assume your neighbour's site class applies to your lot
- Cut or fill the building platform without telling us first
- Order a footing design before the site class is known
- Leave a slope or landslip band unmentioned: it may need a landslip report too
Related services
Often needed alongside a soil test
Extensions & renovations
Structural drawings for extensions, renovations, second storeys and granny flats, working with your architect or building designer.
Learn more
Underpinning advice, design & certification
An independent second opinion on an underpinning quote, then design, stage inspections and certification. We never sell or install underpinning.
Learn more
Landslip reports
Landslip hazard reports for building in a Landslip Hazard Code band, prepared to AGS 2007 with an AS 1726-2017 site investigation.
Learn moreFAQ
Soil test and site classification FAQs
Still unsure? Call 03 6146 4747 or send photos for a fixed quote.