Building the foundation is not the first step in building a new home. Before that, it is important to understand the "ground" on which the home will be built. A site classification report becomes essential at this stage. The report is prepared after assessing soil conditions and classifying the site in accordance with AS 2870.
This is particularly important in Adelaide and many parts of South Australia, where reactive clay soils and deep seasonal moisture changes can significantly influence ground movement. A site classification report helps engineers understand how the site is likely to behave so foundations can be designed for local ground conditions.
In this blog post, we have explored how a site classification report delivers practical engineering insights, supports smart footing design, reduces construction risks, and helps ensure your new home stands on solid ground.
What Is a Site Classification Report?
A site classification report is a geotechnical assessment that classifies the soil beneath a proposed residential building site. Prepared following a site investigation and soil testing, the report evaluates how the ground is likely to behave under normal seasonal moisture changes. It identifies the site's characteristic surface movement in accordance with AS 2870 – Residential Slabs and Footings.
The report is an important part of the design process because foundation performance depends on the conditions beneath it. Different soil types respond differently to moisture changes, and these variations can influence footing selection and design.
Adelaide is known for areas containing highly reactive clay soils and a relatively deep soil-suction zone, where seasonal moisture changes can extend further below the ground surface than in many other parts of Australia. These local conditions make accurate site classification particularly important when designing residential footings.
A typical site classification report may include:
- Site observations and existing ground conditions
- Soil profile and subsurface conditions identified during the investigation
- Laboratory test results where they are required
- Soil reactivity classification under AS 2870
- Groundwater observations, where relevant
- Engineering recommendations to support footing design
How Does AS 2870 Site Classification Work?
Residential footing design is generally guided by AS 2870 – Residential Slabs and Footings, which sets out a standard method for classifying residential building sites based on expected ground movement.
To determine the appropriate classification, a geotechnical investigation is carried out to assess the site's subsurface conditions. Depending on the project, the investigation may consider:
- Soil type and composition
- Moisture conditions
- Depth of reactive clay layers
- Existing fill or disturbed ground
- Local geological conditions
- Surface drainage characteristics
Engineers evaluate the information gathered during field investigations and, where required, laboratory testing to understand how the soil is likely to behave under normal seasonal moisture changes.
Based on these findings, the site is assigned a classification ranging from Class A to Class P. The classification provides a consistent way to describe the site's expected ground movement and serves as an important input to foundation design.
While the classification is an important engineering tool, it does not determine whether a site is suitable for development. Instead, it helps engineers design footing systems that are appropriate for the site's ground conditions.
What Do M Class, H Class, and Other Soil Classifications Mean?
One of the most common questions homeowners ask after receiving a report is what their soil classification actually means.
The classifications describe the expected level of ground movement caused by changes in soil moisture, particularly in reactive clay soils. They do not indicate that a site is "good" or "bad." Instead, they help engineers understand how the ground may behave over time so the footing system can be designed accordingly.
The most common classifications include:
- Class A – Sites with little or no ground movement potential, typically consisting of stable soils.
- Class S – Slightly reactive clay sites with relatively low ground movement potential.
- Class M – Moderately reactive clay sites where seasonal moisture changes may result in moderate ground movement.
- Class H1 and H2 – Highly reactive clay sites with a greater potential for movement, with H2 generally indicating higher reactivity than H1.
- Class E – Extremely reactive sites that require specialised engineering consideration because of their high movement potential.
- Class P – Problem sites where conditions such as uncontrolled fill, soft soils, abnormal moisture conditions, or other site-specific factors require individual engineering assessment.
The classification itself is only one part of the assessment. Engineers also consider the proposed building, site topography, drainage, vegetation, and other site-specific factors before making foundation recommendations.
By understanding what the classification represents, homeowners can better appreciate how geotechnical investigations contribute to practical and reliable foundation design.
How Does a Site Classification Report Influence Footing Design?
Different soils respond differently to moisture changes. Reactive clays, for example, may expand during wetter periods and shrink as they dry.
A footing system designed without considering these characteristics may be more susceptible to movement over time:
- Expected ground movement
- Soil strength and bearing characteristics
- Foundation performance requirements
- Site-specific conditions that may affect construction
Based on this information, engineers can recommend a footing type for reactive soil or other ground conditions that is appropriate for the proposed home and consistent with relevant Australian Standards.
Selecting an appropriate footing system at the design stage helps improve construction planning and gives greater confidence that the foundations suit the site's conditions.
When Is a Site Classification Report Required?
For most new residential developments, a site classification report is completed before foundation design begins. It forms part of the geotechnical information used by structural engineers, designers, builders, and approval authorities during planning.
A site investigation may be required when:
- Building a new home
- Constructing an extension that requires new footings
- Developing multiple residential lots
- Building on sites with reactive clay or variable ground conditions
- Planning construction on filled or previously disturbed land
Depending on the project and local authority requirements, the report may also support the soil test for the building permit process by providing the geotechnical information needed for foundation design and regulatory approvals.
Completing the investigation early allows potential ground-related considerations to be identified before construction progresses, helping reduce design changes later in the project.
What Factors Can Affect a Site Classification?
Although the classification process complies with AS 2870, each site is different. Two neighbouring properties may even receive different classifications if their ground conditions vary.
Some of the factors that may influence a site's classification include:
- The type and depth of surface soils
- The presence and thickness of reactive clay layers
- Fill material or previous earthworks
- Natural drainage patterns
- Groundwater conditions where they are relevant
- Existing trees or large vegetation that may influence soil moisture
- Local geology and site topography
- The depth of the soil suction zone, which can influence how seasonal moisture changes affect reactive soils
Because every site is unique, classifications should always be based on a site-specific geotechnical investigation rather than neighbouring properties or previous developments.
Why Choose BHM Geotechnical for Site Classifications?
BHM Geotechnical provides practical engineering advice that helps builders, developers, architects, and property owners understand site conditions before construction begins. Our investigations combine field assessments, soil testing, and clear engineering recommendations to support informed foundation design and project planning.
Our team delivers:
- Geotechnical site investigations tailored to residential developments
- Site classifications prepared in accordance with relevant Australian Standards
- Clear reporting with practical engineering recommendations
- Responsive service for projects across Adelaide and South Australia
- Advice that supports safer, more efficient construction outcomes
Every investigation is focused on providing reliable site information that helps projects move forward with greater certainty.
Frequently Asked Questions
Is a site classification report mandatory for a new home?
A site classification report is commonly required before designing footings for a new residential home. It provides the geotechnical information engineers use to recommend an appropriate foundation system and may also form part of the documentation required during the building approval process, depending on the project and local authority requirements.
What is the difference between a soil test and a site classification report?
A soil test involves collecting and analysing soil samples to understand the site's physical properties. A site classification report uses the results of those investigations, together with site observations and engineering assessment, to classify the site in accordance with AS 2870 and provide recommendations that support footing design.
What do M Class and H Class soils mean?
M Class indicates moderately reactive clay that may experience moderate ground movement as soil moisture changes. H1 and H2 Class indicate highly reactive clay with a greater potential for movement. These classifications help engineers design footing systems that are appropriate for the expected ground behaviour.
Can two neighbouring properties in Adelaide have different site classifications?
Yes. Even within the same suburb, ground conditions can vary because of differences in soil profile, reactive clay depth, fill material, drainage, and site history. A site classification should always be based on a geotechnical investigation carried out on the individual property rather than nearby developments.
Does a site classification report determine the footing design?
No. A site classification report provides information about the site's ground conditions and soil reactivity. Structural engineers use this information, together with the proposed building design and other project-specific considerations, to determine an appropriate footing system.
How long is a site classification report valid?
A site classification report reflects the ground conditions at the time of the investigation. If site conditions change because of excavation, filling, changes in drainage, or if construction is significantly delayed, an engineer may recommend reviewing or updating the investigation to confirm that the original findings remain appropriate.
