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Soil Testing: the Foundation of every Building Project

October 29, 2020

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.

What’s the difference between site classification and soil testing?

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

The Soil Test has two components: 

  1. Field soil analysis to collect soil, data and site measurements
  2. Laboratory tests on the soil samples taken

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

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).

technician crumbling soil in hands beside 4wd vehicle
On site drilling for soil samples using an auger mounted on the back of one of our vehicles

Field Soil Analysis

For residential construction, the proposed building envelope will require a field investigation undertaken to obtain data relating to site features such as:

  • slope
  • trees
  • overland flows 
  • existing structures

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;

  1. Bore holes drilled, documented and samples taken.
  2. Dynamic Cone Penetrometer (DCP) which enables an allowable bearing capacity to be determined.
  3. Pocket Penetrometer (PP) which is used to undertake soil strength tests on cohesive clays.
  4. Shear Vane Test which is used in the determination of the shear capacity of the in-situ soil
  5. pH and Salinity Tests used in the determination of an exposure classification for concrete, screw piles, etc.
  6. Identification of groundwater.
soil samples in a laboratory
Field samples in the STA NATA accredited laboratory, ready for processing.

Laboratory Soil Testing

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:

  1. Shrink Swell
  2. Liquid Limits
  3. Linear Shrinkage
  4. Moisture Content
  5. Atterberg Limits
  6. P Values

STA also has the ability to undertake other soil testing requirements when deemed necessary.

What determines the Bore Hole Depth?

The depth of the bore hole drilled on site are determined as a function of a number of factors such as:

  • Climate Suction Zone
  • Trees
  • Existing infrastructure 
  • Placed site fill or 
  • Site characterisation e.g. a swamp

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.

Climate Suction Zones: Shallow vs Deep

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.

Determining the Site Classification

A number of factors from the above list of soil tests determine the site class:

Site Reactivity

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.

AS 2870-2011, Table 2.1 Classification based on site reactivity
AS 2870-2011, Table 2.1 Classification based on site reactivity

AS 2870-2011, Table 2.3, Classification by characteristic surface movement (ys)
AS 2870-2011, Table 2.3, Classification by characteristic surface movement (ys)

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.

Deep Seated Moisture Change (Class -D)

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.

Problem Sites (Class P)

Causes for a Class P site class, usually described as a problem with the site.

  • Identified uncontrolled fill: Extra soil that has been placed on your block that either hasn’t been compacted properly or that doesn’t have the paperwork (compaction tests) to show that it has been placed and compacted properly.
  • Soft soil: Soil could be soft because it is loose or unusually moist. Soft soil may not be strong enough to support the weight of your new building without extra precautions being taken.
  • Abnormal moisture conditions: If the soil tester has identified the potential for abnormal soil moisture changes on your block.
  • Identified landslip or mine subsidence areas.
  • The removal of existing structures that will require consideration when undertaking the foundation design.

As part of the soil test report, there will be an explanation within regarding the reasons for a Class P determination.

The Soil Test Report

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:

  • Site information
  • Client supplied information
  • Findings
  • Fieldwork and laboratory test results
  • Subsurface conditions
  • Site classification
  • Comments & recommendations
  • Glossary of terms
  • General notes
  • If asked for may also include a wind classification

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.

Architecturally designed house with surrounding trees
The proximity of trees and other site factors such as drainage, slope and existing structures may affect the long term stability of the designed foundations.

How this affects Foundation Design

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:

  • A  conventional stiffened raft concrete slab system (strong and stiff enough to resist soil movement
  • A waffle raft concrete slab (stiff enough to limit deflection due to soil movement)
  • The use of piles and/or piers in conjunction with one of the above designs
  • Strip footings supporting masonry / in-fill slabs
  • Elevated floor systems supported on isolated footings (typically timber or steel floor systems)

When is soil testing and site classification needed?

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. 

  • The soils are the foundation of the proposed structure, on which the entire build relies to perform over the service life of your home.
  • The type of soil you have on site will determine the materials and designs used in your build.
  • The site classification may dictate the method, complexity and cost of your foundations.

Is more than one soil test and/or site class needed on the same block?

Further soil tests (and a possible subsequent re-classification of the site) may be required when:

  • You have undertaken significant cut and filling of the block since the time of your original soil test. Although the fill soil may have been compacted, a new test may be needed.
  • You change the location of the proposed building on the site. This is particularly important on large sites.
  • Where structures such as houses have been removed.
  • Where abnormal features have been identified during construction.

Who should do site classification and foundation/footings design?

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.

What to look for in an Engineer – and why choose STA

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:

  • Experience – With almost 30 years experience in soil testing, site classification and foundation design with thousands of tests through our lab every year, STA are experts in this field.
  • Regional Soil Data – With decades of data across hundreds of thousands of soil tests, we’ve developed local and regional profiles of soil compositions, along with wind ratings, bushfire assessments, and other “site estate specific” data. We can understand an estate as a whole rather than a single site, which gives us a better chance of identifying potential site features, like fill, boulders, etc.
  • Number of bore holes – For a normal sized dwelling it is mandated a minimum of two bore holes are drilled within the envelope of the structure. STA typically undertake three bore holes.
  • NATA Lab – STA has its own NATA accredited laboratory where we thoroughly test and analyse materials. Because we’re in full control over the testing process, we produce extremely reliable data for every project.
  • Diversity of engineering skills – STA aren’t just soil testing experts. We are experienced in and provide engineering services for foundation design, structural engineering, building hydraulics, compliance inspections and geotechnical investigations. We are construction engineering specialists.
  • Clear reporting – We are constantly reviewing and improving our reports for readability and user friendly layout, helping builders and contractors get the information they need faster.
  • Responsiveness and communication – We make it easy for you to get answers from the right people – whether for a general query, a quote, soil test results and engineering designs. We provide direct access to our technical teams enabling faster response and our systems utilise email and SMS to alert clients to updates as they happen.
  • ISO 9001 – STA Consulting Engineers is ISO 9001 accredited ensuring a greater level of quality control for the project.

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.

Book a soil test and site classification

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.

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