When foundations are appropriately designed to soil conditions, there are dual benefits for the builder and building owner alike. But too often buildings end up with an under-engineered foundation (which risks structural problems) or an over-engineered foundation (adding unnecessary costs to the building project). That’s why it’s important to work with foundation engineering consultants who are experts in both soil testing and foundation design, so you get the perfect foundation for your site every time.
When you work with STA, you can be confident that you’ll get a foundation design that’s based on accurate soil testing data and site classification results. We consider your structure and the specific conditions and features of the land to tailor a foundation design to suit each specific project and location.
"We at Kerdic have been in the construction building business since 1990. STA is one of the consulting firms we have continued to work with. Since I joined Kerdic in 2003, I have been regularly engaged the STA team (Foundation team and Structural team). They are extremely helpful in designing and drafting for our projects. I personally hope STA continues to help our projects in many years to come."
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Foundation engineering sits in between civil, structural, and geotechnical engineering. It’s all about the bits in between the soil and the building and includes the foundation or lower parts of the building structure. Some foundations are deep, like columns and piles used for large structures or poor soil quality at the surface. And some are shallow, like 1m deep footings used in most residential homes.
All solid structures need foundation engineering, from roads and bridges to residential homes, retaining walls and huge city apartment buildings. Foundation engineers design custom solutions that take into consideration both the site conditions and soil classification, and the building structure itself.
All buildings require foundation engineering services to ensure they get the right foundations to suit their soil type and site characteristics. Every building site is different, so there’s no one-size-fits-all foundation system. Custom foundation design ensures your building structure’s weight is evenly distributed and properly anchored (even in extreme weather conditions). Plus, it avoids uneven settlement, provides greater stability, and minimises movement that could damage the structure.
Foundation engineering is normally done just after soil testing and site classification is complete. Although, if you work with a full service engineering consultancy like STA, these services may be bundled together.
STA Consulting Engineers mostly work with the following foundation types, depending on the site conditions and build type:
Stiffened Raft Slab
Stiffened Raft Slabs include both Monolithic and Conventional (two-pour) Foundations.
Conventional Foundations are a common type of footing and slab foundation, where beams or slab thickenings are excavated into the ground. They’ve been used for many years in both Queensland and New South Wales, but especially Queensland. While Conventional can work for some buildings, it does come with a considerable drawback – it can be difficult to estimate the concrete quantities used due to “blow out” within the soils.
Monolithic Foundations are similar to Conventional Foundations, but the edge beam, internal beams, and slab are poured as one. This type of foundation system is only suitable for lower reactive sites.
Waffle Raft Slabs
The Waffle Raft Foundation system is the most common foundation used for residential structures. They’re a raft foundation system, designed to sit above-ground, which means minimal excavation into the soil. This allows for greater accuracy in estimating material quantities. Waffle slabs are created by pouring concrete into single-use plastic forms or expanded polystyrene blocks that are set on the ground. This concrete forms beams along the base of the foundation (with voids in between) and the concrete slab sits on top of these. This type of foundation is strong enough to allow for some soil movement and swelling (on minimal to moderately reactive sites), while drainage is managed effectively via the voids.
The “PolyVoid” system is an Australian Standard AS3600 compliant piled suspended slab design. The system sits on top of steel screw piles, supporting a fully suspended slab. The screw piles are engaged below a nominated zone of influence (Hs Zone), anchoring them in position for tension and compression loads when hogging/swelling occurs. This is ideal for clay soils which can shrink and swell considerably as moisture levels vary.
Strip Footings and Pad Footings
Strip footings are generally used to support subfloor walls when a house is constructed with an elevated floor frame. Pad footings, bored piers, or other piling options form the support for isolated columns and are located to support floor framing where walls are not present. This system is suited for sloping blocks, more traditional timber-framed Queenslander style houses, and verandah/deck framing support.
Other Void Former Systems
Alternative proprietary void former systems generally work in a similar way to the Waffle raft system in that forms are used to create air pockets under the slab. Made from recycled polypropylene, it can help reduce the volume of concrete, reinforcing steel, and labour required to install, and is much easier to transport than waffle pods. It’s also more environmentally friendly and can be a more cost effective alternative to polystyrene Waffle Pods.
The first step in any foundation design is soil testing. So, if you haven’t yet organised soil testing and site classification, your foundation engineering team will do this for you. Otherwise, we’ll refer to your existing reports and certifications.
Your foundation engineers will use the data to determine the best foundation system for your build. They’ll custom-design your foundations and footings based on their recommended system, to suit your site conditions and building design. Once everything is checked by another engineer, your designs and reports are sent through, so you can move on to the next step in your project.
Once the project plans are approved and your site is ready for building, your builder will refer to your foundation designs to ensure the right materials and techniques are used, and that the foundations are positioned appropriately.
STA uses all the industry leading software you’d expect for a results-focused engineering consultancy. For foundation engineering, the key software packages we use include:
We have the tools we need to provide the right information in the right format for residential projects. And we’re continually investing in new software to help us deliver more accurate, more efficient results for clients.
If your foundation needs screw piers (or screw piles), rather than concrete or timber piles, we usually recommend piles made from Australian 400 grade high tensile pipe with twin curved fins.
Other types of piles we use include bore piers, which are suitable for fair-to-good ground conditions, where no issues with rock/soil removal is anticipated and there’s no shallow water table.
Foundation engineering is about more than just the design process – client communication is critical to ensuring the project stays on schedule. Clients can request daily job progress reports via email so they can be confident we’ll deliver the designs and documentation they need, on time. Plus, our clients are always welcome to call or email their engineer if they have any questions or concerns.
Most of our foundation designs are delivered around 6 days after the soil report is available for the site. But this will depend on the scope of the project.
We believe that design documentation is fundamental to a successful project as this technical information is used daily by estimators, detailers, external clients, councils, and certifiers. STA can analyse, design, and produce span charts, compliance certificates, and other technical documents to help support your build.