If you are building on a deep fill site in Adelaide, the standard footing design is almost certainly not going to cut it. Getting this wrong leads to cracked slabs, differential settlement and failed inspections, all of which are expensive to fix after the fact.
This guide breaks down what deep fill sites actually are, why they are classified as Class P under AS 2870, and what experienced builders need to know before they pour a single slab on one.
A deep fill site is any lot where the ground has been built up using imported or excavated material, typically to level a sloped block, reclaim low-lying land or prepare a subdivision. The depth of this fill can range from a few hundred millimetres to several metres below finished ground level.
What makes deep fill different from natural ground is compressibility. Even when fill has been engineered and compacted correctly, it behaves differently from undisturbed soil. When fill is uncontrolled, placed without proper engineering specification, the risk increases substantially.
Under AS 2870-2011, deep fill sites are treated as Class P (Problem sites) unless a Geotechnical Engineer certifies that the fill was placed in accordance with an appropriate specification. Standard deemed-to-comply footing designs do not apply.
Adelaide’s building landscape includes a significant number of deep fill sites, particularly in:
The challenge in Adelaide is that a site-wide classification issued by a developer may not reflect variations at an individual lot level. Two adjacent lots in the same estate can behave very differently if the fill depth changes significantly between them.
The core problem with deep fill sites is settlement, specifically differential settlement. When different parts of a slab sit on fill of varying depth or density, they move at different rates as the fill compresses over time.
This creates the conditions for:
Conventional stiffened raft slabs are designed to handle reactive clay movement, not the vertical compression that deep fill produces. They rely on the ground providing lateral restraint and consistent bearing capacity, neither of which is guaranteed on a deep fill site.
A standard residential soil test is not sufficient for deep fill sites. The investigation needs to go much deeper to characterise the full fill profile and the natural soil below it.
A thorough geotechnical report for a deep fill site in Adelaide should include:
Without this information, it is not possible to design a compliant or reliable footing system for the site.
On a deep fill site, the footing system needs to do one thing above all else: bypass the fill entirely and transfer the building load into competent natural soil or rock below.
Screw piles, also called helical piers, are purpose-built for exactly this. The pile is rotated through the fill profile using hydraulic torque equipment until it reaches and locks into the load-bearing stratum. The torque measured during installation directly correlates to the pile capacity, giving the engineer real-time verification that each pile has achieved its design load.
A full set of screw piles for a residential slab can typically be installed in a single day. There is no excavation, no soil disposal, no concrete trucks and no curing time. The slab can follow the very next day.
Compared to conventional bored concrete piers, screw piles have a clear advantage on deep fill sites in three specific situations:
For builders working with us on deep fill sites in Adelaide, the typical process looks like this:
The torque log matters. It is your paper trail confirming that every pile achieved its design capacity, which is essential for inspections and for the engineer sign-off.
Working on a deep fill site in Adelaide?
We have installed screw pile foundations on deep fill sites across Adelaide and South Australia. Contact our team for a quote or to talk through your geotech report.
Not every site gives you room to work. A narrow side passage, a rear lot with no vehicle access, an extension being built while the family stays in the house, a site bordered by retaining walls on three sides. These are real constraints that rule out conventional foundation methods before you have even picked up a shovel.
Screw piles were built for exactly this kind of work. Here is how they perform on restricted access sites and why more builders in South Australia are specifying them as the default solution when space is tight.
Restricted access is not just about whether a large excavator can fit through the gate. It covers a range of physical constraints that affect how foundation work can be carried out safely and efficiently:
The standard approach to residential foundations relies on excavation: trench digging for strip footings, auger drilling for bored piers, or excavation for pad footings. Each of these requires equipment with a meaningful footprint, soil spoil to be removed from site, and in the case of bored piers, concrete trucks and pumping gear to follow.
On a restricted access site, these requirements stack up quickly into a problem. The excavator needs room to manoeuvre. The concrete truck needs road access. The spoil needs somewhere to go. When access is constrained, each of these steps costs more time, more money, and introduces more risk to surrounding structures and landscaping.
Bored piers present a specific additional problem. In sandy or loose soils, the sides of a drilled hole can collapse before concrete is placed, causing the pier to blow out in diameter and become difficult to certify. In reactive clay, wet conditions can compromise the bore. Both outcomes are more likely when the equipment operator has less room to work with.
A conventional piling rig typically needs 2.5 to 3 metres of clear working width and several metres of headroom. A screw pile rig can operate in as little as 1 metre of width and under 2 metres of headroom, opening up sites that would otherwise be impossible to foundation with standard equipment.
Screw piles remove most of the physical constraints that make conventional foundations difficult on tight sites. The installation process is straightforward: a compact hydraulic drive head, attached to a small excavator or dedicated piling rig, rotates the pile into the ground. No digging, no concrete, no spoil.
The practical implications for restricted access work are significant:
Across South Australia, screw piles are regularly used in the following restricted access scenarios:
Even with screw piles, a restricted access site requires upfront planning to run smoothly. Before installation, the following should be confirmed:
Working on a tight access site in SA?
We have the rigs and the experience to work on restricted access sites across South Australia. Tell us your access constraints and we will confirm what is possible before you commit to a design.
Tree protection laws in South Australia tightened significantly in May 2024, and a lot of builders are still catching up. If your project sits near a mature tree, whether on the same lot or an adjoining property, there is a real chance that tree is now protected under the new regulations and that your standard footing approach will not get planning approval.
This guide covers what changed, what it means on site, and how screw piles have become the go-to foundation solution for projects near protected trees.
The SA Government introduced new regulations under the Planning, Development and Infrastructure Act 2016 that took immediate effect from 16 May 2024. The changes were significant and caught many builders and developers off guard.
The key updates were:
Penalties for undertaking tree-damaging activity without approval can reach $120,000. Any work that damages a regulated or significant tree, including excavation that severs roots, requires development approval from the relevant council.
This is where builders need to pay close attention. Under the PDI Act 2016, a tree-damaging activity is not limited to cutting a tree down. It includes any activity that could damage the tree, and that definition has been tested in case law.
For a construction context, the following can all constitute a tree-damaging activity:
The practical takeaway is that a conventional footing system, with its requirement for excavation, trenching and concrete placement, is often incompatible with building near a protected tree.
The root system of a mature tree extends well beyond what most people expect. Research consistently shows that up to 90 percent of a tree’s roots lie within the top 600 mm of soil, and they radiate outward far beyond the canopy edge.
For planning and arboricultural purposes, a Root Protection Zone is typically calculated based on the tree’s trunk diameter at breast height. The council’s assessment of your DA will consider whether your proposed works intrude on this zone and what measures are in place to protect the tree.
Builders working near regulated or significant trees in SA should expect the council to require one or more of the following:
An Arboricultural Impact Assessment is not optional on most projects involving regulated trees. Submitting a DA without one, where one is clearly required, will stall the assessment process and push out your programme.
Traditional strip footings, pad footings and even bored concrete piers all require varying degrees of ground excavation. Near a protected tree, that excavation creates two specific problems.
First, the physical act of digging severs roots. Major roots above 25 mm in diameter are critical to the tree’s structural stability and water uptake. Severing them can trigger decline, increase wind throw risk and in some species cause rapid deterioration.
Second, concrete placed near tree roots creates a barrier that restricts oxygen and water movement through the soil. Compaction from machinery causes the same problem by reducing soil porosity. Both outcomes are harmful to tree health and are likely to be flagged by the arborist reviewing your DA.
On a reactive clay site, there is an additional complication. Trees actively dry out the soil around them, creating seasonal moisture variation that drives heave and shrinkage. A conventional slab or strip footing that relies on consistent bearing capacity at shallow depth will be affected by this movement for the life of the building.
Screw piles have become the preferred foundation system for projects near protected trees, both in SA and across Australia. The reason is straightforward: they require no excavation and cause minimal soil disturbance.
The pile is rotated into the ground using hydraulic torque equipment. Rather than removing soil, the helical plates displace it. The pile shaft is slim relative to the footprint of a conventional footing, and installers can adjust pile positions to work around visible surface roots. When a major root is encountered during installation, the pile location can be shifted without redesigning the whole foundation.
Screw piles are widely accepted by arboricultural consultants and planning authorities as a compliant foundation solution within Root Protection Zones. Having screw piles specified in your Tree Protection Plan significantly strengthens your DA application.
There are additional structural advantages that matter specifically on tree-affected sites:
For any project near a regulated or significant tree in SA, the DA process will typically involve the following steps:
Getting the arborist and engineer working together early in the design process saves time and avoids costly amendments to the DA after lodgement.
Building near a protected tree in SA?
We have installed screw pile foundations on tree-sensitive sites across South Australia. Our team works alongside arborists and engineers to keep your DA on track and your tree intact. Get in touch to discuss your project.
Something is shifting on construction sites across South Australia.
Builders who have been pouring concrete footings for years are making a change. Not because they have been told to. Not because of a regulation. But because they have been burned enough times to know that on a reactive clay site, a deep fill lot or a tight inner-Adelaide build, traditional footings carry risks that screw piles simply do not.
If you have been hearing more about screw piles on the tools or from other builders in your network, here is the honest reason why. Five of them, in fact.
Ask any builder who has been working in Adelaide for more than a few years and they will tell you the same thing. The clay here moves. It swells when it rains and it shrinks in summer. That movement is what cracks slabs, shifts footings and generates the kind of callbacks that eat into your margin and your reputation.
Traditional concrete footings sit in that reactive layer. When the soil moves, the footing moves with it.
Screw piles work differently. They are driven down through the reactive clay to stable bearing soil below. The structure above is supported from a layer of ground that does not move, which means the slab stays flat and the footings stay put.
For builders working in Adelaide’s middle and outer suburbs, where reactive clay soils are almost universal, this alone is reason enough to make the switch.
If you have ever had a pour window fall apart because of a wet forecast, you already understand this one.
Concrete has requirements. The ground needs to be stable and dry. The pour needs to happen at the right time. The cure needs to run its course before loading. In an Adelaide winter, or during an unseasonably wet spring, that sequence can add days or weeks to a program.
Screw piles can be installed in the rain. There is no curing period. Once the piles are in, the structure can be loaded the same day. The program keeps moving regardless of what the weather is doing.
For builders running tight schedules and fixed completion dates, that predictability is worth a significant amount.
One of the most common frustrations builders share about concrete footings is the variation. The geotech report says one thing. The soil does another. The excavation goes deeper than planned. The concrete order increases. The cost blows out.
Those variations are not always the builder’s fault, but they are the builder’s problem. Clients do not want to hear about soil surprises. They want the price they were quoted.
With screw piles, the pricing is fixed from day one. The engineering design is based on the geotechnical report and the structural load requirements. The installation price is agreed before work begins. If something unexpected happens on site, that is managed within the scope of the original agreement, not passed on as a variation to the client.
For builders who want to protect their margin and their client relationships, fixed pricing is not a small thing.
The easy sites have been built on. What is left in Adelaide’s established suburbs often comes with complications.
Heritage overlays that restrict excavation. Significant trees with council protection zones that cannot be disturbed. Lots with no side access for a concrete pump. Sites where the neighbour’s house is close enough that traditional excavation creates a real risk of movement.
Concrete footings struggle on all of these. Screw piles handle them well.
The installation equipment is compact enough to work in tight access conditions. The process produces minimal vibration, which matters when you are working close to existing structures or within a tree protection zone. There is no spoil to remove, which eliminates a significant logistical headache on small inner-city lots.
As the development landscape in SA moves toward more infill and more challenging sites, screw piles are simply better suited to the work that is actually available.
This one does not get talked about enough.
When you use an engineered screw pile system installed by a certified provider, you receive a Certificate of Compliance on completion. That certificate is issued under AS2159, the Australian Standard for piling design and installation. It documents exactly what was installed, to what specification and to what bearing capacity.
Certifiers know what it means. Engineers accept it. It sits cleanly in the project file and it does not generate questions down the track.
Compare that to situations where footing depths were varied on site, where verbal approvals were given and not documented, or where the footing type diverged from the engineer’s specification without a formal variation. Those situations create risk that can resurface years later.
Clean documentation is not exciting. But it is the kind of thing that protects a builder’s licence and their professional reputation over the long term.
The builders Anchorpile works with across South Australia are not switching to screw piles because they want to try something new. They are switching because they have had a problem with a concrete footing on a reactive site, or a program delayed by weather, or a variation claim that cost them a client relationship.
Screw piles solve real problems that SA builders face on real sites. The technology is not new. The materials are engineered to last 50 years. The process is certified under Australian Standards. And the pricing is fixed so you know exactly what you are committing to before work begins.
If you are still on the fence, the best thing to do is talk to someone who has made the switch. Or talk to us. We will give you a straight answer on whether screw piles are the right solution for your next project and what the cost will look like.
Send through your geotechnical report and site details and Anchorpile will come back to you with a fixed-price proposal.
Anchorpile is a division of IdealCorp. Engineered screw pile supply and installation across South Australia. AS2159 certified. Certificate of Compliance on every project.
If you have been building in South Australia for any length of time, reactive clay is not a new problem. It is the default condition on most Adelaide building sites, from the inner suburbs out to the growth corridors in the north and south.
The clay expands when it absorbs moisture. It contracts when it dries out. That cycle of movement is what lifts slabs, cracks footings and generates the kind of structural callbacks that are expensive to fix and hard to explain to a client.
Understanding why reactive clay behaves the way it does, and why the traditional response to it often falls short, is the first step to choosing a foundation system that actually holds up over time.
Reactive soils contain clay minerals that change volume depending on their moisture content. The most common of these in South Australia is montmorillonite, a highly expansive clay mineral found extensively across the Adelaide Plains and surrounding regions.
When moisture enters the soil, the clay particles absorb water and swell. When the soil dries out during summer or drought, those same particles shrink. The result is a ground surface that moves up and down across the seasons, sometimes by several centimetres in highly reactive sites.
This movement is not uniform. Different parts of a site can have different moisture conditions at the same time, particularly around the edges of a slab where vegetation, paving or roof drainage affects moisture distribution. Differential movement is what causes the most damage.
Australia uses a classification system for site reactivity. The geotechnical report for a site will typically assign one of the following classifications.
A Class S site is a slightly reactive site with minor surface movement expected. A Class M site is a moderately reactive site, common across much of Adelaide. A Class H1 or H2 site is a highly reactive site where significant movement is expected. A Class E site is an extremely reactive site requiring specialist engineering.
The majority of residential building sites across Adelaide fall into the Class M to Class H range. This means that for most builders in SA, reactive clay is not an edge case. It is the standard condition they are designing and building for on every project.
The conventional response to reactive clay in residential construction has been to engineer the slab and footing system to accommodate the expected movement. This typically involves using a thickened edge slab, increased reinforcing steel, or a waffle pod raft, designed to move as a rigid unit rather than crack under differential soil movement.
This approach works on lower reactivity sites when the engineering is done well and the slab is maintained properly. But it has limitations.
The footing sits within the reactive layer. The soil movement acts directly on the footing, and the slab has to resist that force through its structural rigidity. Over time, particularly when drainage conditions change around the building, that resistance can be overcome. Trees planted close to the structure, changes to paving or landscaping, and periods of extreme drought or heavy rainfall can all shift the moisture balance enough to cause movement that the slab was not designed for.
When a slab on a reactive site cracks or moves, the repair is disruptive and expensive. In some cases the damage is significant enough to require underpinning, which is a far larger intervention than getting the foundation right at the beginning.
Screw piles take a fundamentally different approach. Rather than designing a footing that sits in the reactive layer and tries to manage the movement, screw piles are driven through the reactive layer entirely and locked into stable, non-reactive soil below.
The depth required to reach stable bearing soil varies across SA. In many established Adelaide suburbs, the reactive clay layer extends to between 1.5 and 3 metres below surface. In some northern and southern growth areas, it can be deeper. The geotechnical report for the site will specify the depth at which competent bearing material is encountered, and that becomes the minimum installation depth for the pile.
Once the pile is in stable ground, the structure above is essentially decoupled from the reactive surface layer. The clay can expand and contract through the seasons as it always has, but that movement is no longer transmitted to the building. The pile passes through the clay but is not supported by it.
This is the core engineering principle behind why screw piles perform reliably on reactive sites where concrete slab systems struggle.
Beyond the structural logic, screw piles offer several practical advantages specific to clay conditions in SA.
Installation does not require excavation into the clay, which means there is no spoil to manage or remove. Clay spoil is heavy, often classified as reactive material and can be difficult to dispose of cost-effectively. Eliminating it simplifies the project.
There is no concrete to pour into a clay-heavy excavation, which removes the risk of water infiltration compromising the concrete during or after placement. Clay soils can behave unexpectedly during excavation in wet conditions, and keeping the foundation process above ground rather than in the soil reduces that exposure.
The installation process is also fast. A full residential pile set can typically be completed in a single day, and the structure can be loaded immediately after installation. There is no curing period and no waiting on weather conditions to clear before work can proceed.
Every Anchorpile screw pile installation in SA follows a consistent engineering process.
The starting point is the geotechnical report for the site. The report provides the soil profile, the reactive soil classification, the depth to competent bearing material, and the bearing capacity of the foundation stratum.
From that report, the Anchorpile engineer specifies the pile diameter, helix configuration, minimum installation depth and target installation torque for each pile location. The specification is tied directly to the structural loads from the building design and the actual ground conditions at that specific site.
During installation, the crew monitors and records the torque as each pile is driven. When the pile reaches the specified minimum torque at or below the required depth, it has achieved the designed bearing capacity. Those records form part of the project documentation.
On completion, a Certificate of Compliance is issued under AS2159, the Australian Standard for piling design and installation. That certificate documents what was installed, to what specification and to what verified bearing capacity.
The answer for most SA residential and commercial sites is yes, and the starting point is always the geotechnical report. If you have one, send it through. Anchorpile will review the site conditions and come back with a fixed-price proposal and a clear recommendation on the right pile specification for your project.
If you do not yet have a geotechnical report, we can point you in the right direction.
Anchorpile is a division of IdealCorp. Engineered screw pile supply and installation across South Australia. AS2159 certified. Certificate of Compliance on every project.