In South East Queensland, the top metre or two of most blocks is topsoil, fill, loose sand or reactive clay that cannot safely carry a building. A screw pile bypasses that weak layer and founds the load on competent material below, and how deep that is changes suburb to suburb: dry coastal sand firms up with depth, sand over groundwater needs driving into coffee rock, reactive clay must be piled below its moving zone, and some hinterland and coastal ground is full of rock that stops a pile short. The soil report and the piling engineer set the target; the load cell on the machine proves every pile reaches it.
Why the soil decides everything
Screw piles suit ground with a low bearing capacity: ground that cannot hold a house's weight near the surface. That covers a lot of South East Queensland, which is why soil testing is now standard on nearly every build and so much of what used to be built on bare footings is piered today, largely for compliance. The important thing to understand is that your block can classify completely differently to next door. Your neighbour might need nothing while you need a full run of piles, because the ground changes over very short distances here.
None of this is guesswork. A geotechnical soil test is taken, the piling engineer reads it and specifies a depth and load, and we install to that. Increasingly, a soil report is mandatory before construction, which is the demand driver behind piling in the first place.
What a soil test for building tells you
A soil test for building is a geotechnical investigation of your specific block, carried out by a geotech engineer before design is finalised. Boreholes are drilled or a probe is pushed at several points across the footprint, and the resulting geotechnical soil report sets out four things that decide your footings:
- The layers. What sits at each depth, from topsoil and fill through sand, clay, coffee rock and rock, and how thick each layer is.
- The water table. How far down groundwater sits, which is what turns firm coastal sand into soft slop and changes the founding depth entirely.
- The site classification. The AS 2870 class (A, S, M, H1, H2, E or P) that describes how much the ground moves with moisture.
- The bearing capacity. What load the ground can safely carry, and at what depth, which is the number the piling engineer designs to.
Core Screw Piling does not carry out soil testing. We work to your report, and if you do not have one yet we can point you to the geotechnical engineers builders in this patch use. Soil test cost varies with the number of boreholes, access and how far the crew has to travel, so get the quote against your actual block rather than a rule of thumb. It is a small line next to the cost of getting the footings wrong.
AS 2870 site classifications, in plain English
Soil classification in Australia runs on AS 2870, the residential slabs and footings standard. Every site is classified by how much the ground moves with moisture:
- Class A / S: stable sand or rock, or slightly reactive clay. Most footing systems work here.
- Class M: moderately reactive clay, common across Brisbane, Ipswich and inland Sunshine Coast suburbs.
- Class H1 / H2: highly reactive clay that swells and shrinks significantly between wet and dry seasons.
- Class E: extremely reactive sites.
- Class P: problem sites: uncontrolled fill, soft ground, landslip zones and steep slopes. Much of the Sunshine Coast hinterland and many coastal blocks are Class P.
The more reactive or problematic the class, the stronger the case for screw piles, because they found the structure below the moving zone rather than inside it.
Types of house footings, and what each one needs from the ground
The classification and the layers below it are what push a job from one footing system to another. These are the types of house footings used across South East Queensland:
- Strip footings. Concrete trenches under load-bearing walls. Cheapest where the ground is stable and shallow, and the first system to crack on reactive clay.
- Waffle and raft slabs. A stiffened slab that rides on the surface and resists movement as one plate. Common on Class M and H sites, but only as good as the ground beneath it.
- Bored concrete piers. Holes augered down to a founding layer and filled with concrete. Proven and familiar, but they generate spoil, need cure time, and collapse in wet sand.
- Screw piles and screw pile footings. Steel piles wound down past the moving zone, proven by torque on the way in, ready to build on the same day. Best where the top layer cannot carry load and access or weather is against you.
- Stumps and steel posts. Suit lightweight and elevated structures, and still need something competent underneath them, which is often a pile.
No system suits every block. The soil report decides which of these is right, and we will tell you when it is not us. Our honest side-by-side is in screw piles versus concrete and bored piers.
The four kinds of ground we pile in
Dry coastal sand
Around places like Sunrise Beach and Sunshine Beach the hills are all sand, dry, with no groundwater near the surface. There is nothing soft in it and it just gets harder the deeper you go, so we pile down two to three metres to the firm bed and found on that. You cannot sit a house on the loose sand up top, but the sand at depth carries load well once the helix develops in it.
Sand over a high water table, and coffee rock
Take the same sand closer to the water table and you hit groundwater a metre or so down, which turns it into soft slop that will not carry anything. There we keep driving, down through the slop into coffee rock, a naturally cemented, iron and organic bound sand layer common along the SEQ coast, or the hard layer below it, until the pile proves its load. Coffee rock is a good founding layer once you reach it.
Reactive clay
Away from the ocean, through the Gold Coast, Brisbane and the inland suburbs, the ground is clay or ordinary soil. Clay is solid, but it moves: it swells when wet and shrinks when dry, and that seasonal movement is what heaves houses up and down and cracks them. We pile down underneath the reactive zone and tie the structure to stable ground, so when the clay moves, the house does not move with it.
Rock and difficult ground
Some ground fights back. Airlie Beach is a good example: it is all hills, everything moves, and although you would expect sand on the coast it is actually full of rock. Rock makes for slower, harder driving that is priced accordingly, and we install in ground like this regularly. Where solid rock sits shallow enough to stop a pile short of design depth, we will say so and tell you a bored pier may suit better. See when screw piles are the wrong choice.
Area-by-area quick reference
| Area | Typical ground | Piling approach |
|---|---|---|
| Coastal Noosa / Sunshine Coast beaches | Dry sand hills, firming with depth | Pile 2 to 3 m to the firm sand bed |
| Low-lying coastal strip | Sand over a high water table, coffee rock below | Drive through the soft zone into coffee rock or the hard layer |
| Ipswich, Brisbane, Gold Coast, inland suburbs | Reactive clay (Class M to H) | Found below the movement zone, tie the structure to stable ground |
| Low-bearing Brisbane pockets (e.g. The Gap) | Low bearing-capacity soil, now piered for compliance | Piled to the engineer's design; varies block to block |
| Sunshine Coast hinterland (Woombye, Palmwoods, Maleny) | Sloping Class P sites, steep ground and shallow batters | Piles at natural ground level, no benching |
| Rocky ground (e.g. parts of Airlie Beach) | Shallow rock, hard driving | Case by case; a footing or bored pier may suit better |
How a soil report becomes a pile design
- Geotechnical soil test. A geotech engineer bores the site and reports the layers, the water table and the site classification.
- Piling engineer's design. A registered engineer (RPEQ) reads the report and specifies pile type, founding depth and the load each pile must carry, documented for your building approval (Form 15 where required).
- Installation to torque. We wind each pile down while a load cell reads the actual torque against the engineer's minimum. When it hits the number in competent ground, the pile is proven.
- Certification. Depth and torque are logged for every pile, supporting the engineer's Form 43/12 inspection certificate your certifier needs.
There is more on the equipment and the load cell in our For Builders page, and on the method itself in the screw piling guide.
What the ground means for your quote
Because founding depth depends on the ground, we quote to the design depth from the soil report, for example four metres, with a clear rate per extra metre if the ground makes us go deeper to hit the load. Harder ground, like the rocky sites above, is priced for the extra work up front. Either way it is a known number, not a surprise on the invoice.
Soil and ground condition FAQs
Why does my block need piles when my neighbour's did not?
Ground changes over short distances in South East Queensland. Two neighbouring blocks can return different soil classifications, so one needs a full run of piles while the other carries a standard footing. The soil report on your specific block is what decides it.
What is coffee rock?
Coffee rock is a naturally cemented sand layer, bound by iron and organic matter, common along the South East Queensland coast. It sits below loose sand and a high water table, and it is a good founding layer for screw piles once you drive down to it.
How deep will my piles go?
To the depth where the measured torque proves the engineer's load, typically around 2 to 6 metres for residential work in South East Queensland, but the soil report and the ground on the day decide it. Extensions are added until the pile gets there.
Are screw piles good in reactive clay?
Yes, this is one of their strongest cases. Reactive clay moves seasonally and cracks shallow footings. A screw pile founds below that moving zone and ties the structure to stable ground, so seasonal movement does not carry into the building.
Is there ground where screw piles are the wrong choice?
Yes, and we will say so. Solid rock at shallow depth, heavy uncontrolled fill full of obstructions, and very deep soft profiles with no competent layer within reach are all cases where another system can be better. See when screw piles are the wrong choice.
Sources and standards referenced
- AS 2870: Residential slabs and footings (site classification A, S, M, H1, H2, E, P)
- AS 2159: Piling: design and installation
- Queensland Building and Construction Commission (QBCC): licensing and certification