The results of a soil test are rarely front of mind for someone building their own home. Rather, excitement builds as they - together with their building designer - draft floor plans and elevations for their new dream home and see it come to life on paper (or on the computer screen in beautiful 3D rendered images).
However, the very nature of the land you build on is perhaps the biggest variable of a building project and the one aspect that is critical you get right before kicking off the construction phase. That’s where soil testing and site classification come in. They provide the information needed to understand what’s happening underground and how to build on it.
But not all soil test results are the same.
Even though all soil tests for residential building projects must conform to the same Australian Standards AS 2870 (for single dwelling houses, townhouses and similar structures) there are differences in how tests are done on site, in the quality and readability of the soil test report, and potentially even differences in the final site classification given for the block.
To ensure a solid start to any building project - and minimise problems or delays later in the build - it’s wise to first focus on getting a quality soil test completed by a reputable engineering company.
Before we get into what makes for a quality soil test and site classification report, let’s cover some fundamentals.
Soil testing (for building construction) and site classification are essentially two sides to the same coin. They allow you to understand your soil before you start planning your build or designing the foundations.
The Soil Test has two components:
The field component includes the extraction of soil samples, documentation of the existing soil strata and collection of field data, such as bearing capacity - thereby determining the suitability of the soils to place load upon it e.g. weight of dwelling.
It is also a requirement as part of the site investigation, to document other factors that may affect the long term stability of the designed foundations, e.g. trees, drainage, slope, existing structures, etc.
The lab component involves undertaking laboratory tests so as to determine the soil's capacity to swell and shrink in the course of the life of the dwelling under normal site conditions.
The Site Classification is the determination as per AS2870-2011 of the characterisation of the site to a number of specific requirements within the Australian Standards, so as to classify the site, e.g. A, S, M, H1, H2, E or P.
The information collected during the Soil Test (field analysis, site features and lab testing) provides the necessary information so that an accurate classification can be determined per the standard - which in turn determines the foundation design (for each site classification e.g. A, S, M, etc, there are specific designs and design guidelines for foundations and footings).
For residential construction, the proposed building envelope will require a field investigation undertaken to obtain data relating to site features such as:
As part of the process, the soil strata is documented so as to identify the different soil types, colour and consistency, with the use of a 4WD drill rig or alternatively a hand auger when sites are inaccessible by vehicle.
On-site tests may consist of one or more of the following;
Once site samples have been transported to a laboratory, the soil may be tested to assess a range of indicators. It should be noted that STA has its own laboratory that is NATA accredited (not all soil labs are).
Reactivity— how much the soil is going to heave (expand) or settle (shrink back down) when we add or remove water. This is calculated using a formula for characteristic surface movement, as well as the soil suction zone depth for the region in which the site is located.
Salinity— the level of salts in the soil. High salinity may cause corrosion of steel reinforcing within concrete. To account for this, additional concrete cover, a double vapour barrier and/or an increase in concrete strength may be required to protect the steel and mitigate corrosion.
Permeability— the speed at which moisture (or water) moves through a soil and what volume of water the soil can hold. This is used for wastewater design such as septic systems.
More specifically, the following list describes some of the main tests undertaken to determine the above soil characteristics:
STA also has the ability to undertake other soil testing requirements when deemed necessary.
The depth of the bore hole drilled on site are determined as a function of a number of factors such as:
A “suction zone” is the depth below which the soil holds its moisture content through cycles of weather events over an extended period of time (years). These suction zones are documented within AS2870-2011 and are the result of historical climate and rainfall mapping.
For a region with more regular rainfall, the moisture level will be more constant and therefore the region will have a shallow suction zone. For extremely dry, variable climates, the suction zone is much deeper.
For example, Airlie Beach has a shallow suction zone, so the bore holes are set at the minimum of 1.5 metres. Whereas Adelaide’s suction zone is up to 4.0 metres. In Brisbane and coastal South East Queensland, suction zones normally range between 1.5m to 2.3m.
A number of factors from the above list of soil tests determine the site class:
AS 2870 provides six classifications of sites by the amount of reactivity (potential site movement), as shown below in the extracted Figures 2.1 and 2.3.
Clearly, an A class site is ideal as there is no reactivity, however the site works (such as cut and fill) may present issues where shallow rock is encountered. As you move down the classification table, more complex and expensive foundations are required.
For Classes M, H1, H2 and E, deep-seated moisture changes characteristic of dry climate suction zones at 3.0m or more can lead to an additional “-D” designation, which may result in classifying them as M-D, H1-D, H2-D or E-D. However where the site has shallow rock, it will not incur the additional -D classification.
Causes for a Class P site class, usually described as a problem with the site.
As part of the soil test report, there will be an explanation within regarding the reasons for a Class P determination.
The site classification is presented in a formal report which generally forms part of a building application. The report is commonly made up of the following elements:
Understandably, some engineers’ reports are more user friendly than others, in the way the above information is presented and in making the most important information readily available - rather than having to trawl through pages of data to find the site classification or wind rating.
Following soil testing and with a site classification determined, an engineer is able to develop an engineering design for the foundation, which is incorporated into, or attached to, architectural/designer drawings.
Foundation Design variations may include:
It is a government legislative requirement that all new residential construction require a soil test and site classification in order to design fit for purpose foundations.
Further soil tests (and a possible subsequent re-classification of the site) may be required when:
While AS2870 allows some rare exceptions as to who determines site classification and designs foundations, a better result for the building project can often be achieved by engaging the same team to undertake these services from start to finish - and ideally have all services performed under the supervision of a Chartered Engineer.
You want your build to run on time - to avoid both an under-engineered foundation (risking structural problems) or an over-engineered foundation (adding unnecessary costs). There are several factors to look for in choosing an engineer for your project. Here’s why builders choose STA:
STA is a one-stop engineer with fully integrated services and deep understanding of residential construction.
With our own NATA-accredited soil testing laboratory, we’re experts in soil compositions, the impact they have on foundation design, and how to engineer site-specific foundations for structurally sound buildings. With almost 30 years’ experience, and regional soil profile maps, we’re uniquely equipped to deliver more accurate, comprehensive results.
We have a team of specialists in geotechnical, foundation, structural and hydraulic engineering to see you through every phase of the job, and we’re also committed to continuously improving our processes and client communication. Our easy-to-follow reports lead the industry and will help you quickly get to the heart of costings and a building schedule.
To set off on the right foot with your project, call us on (07) 3071 7444 (QLD) or (02) 4032 6450 (NSW) to book a soil test and site classification, or just to ask some questions. We’re here to help you get that first step right. You can also request a quote online.