Two residential sites may look almost identical on the surface, yet require completely different foundation designs because of the soil beneath them. One of the most common reasons is the presence of reactive clay soils, which can expand and contract as moisture levels change throughout the year.
Reactive soils are found in many parts of Sydney and Adelaide, making them an important consideration for residential construction. Understanding how these soils behave before construction begins allows engineers to recommend footing systems appropriate for the site's ground conditions, helping to reduce uncertainty during design and construction.
What Is Reactive Soil?
If you've been told your site contains reactive soil, it simply means the ground has the potential to change volume as its moisture content changes. This behaviour is most commonly associated with clay soils, which expand when they absorb moisture and shrink as they dry.
The amount of movement depends on several factors, including:
- The type and plasticity of the clay
- Seasonal moisture variations
- Vegetation and tree root activity
- Surface drainage around the site
- Local climate and environmental conditions
Not all clay soils behave the same way, and not every site experiences the same level of ground movement. This is why a site specific geotechnical investigation is important before foundation design begins.
Understanding what reactive soil is is the first step. The next step is determining how reactive the site is and what that means for the proposed development.
Why Does Reactive Soil Matter for Footing Design?
Buildings rely on stable foundations to perform as intended over their design life. Where reactive clay soils are present, seasonal changes in ground moisture may cause the soil to expand or contract, resulting in ground movement that engineers must consider in foundation design.
If these movements are not appropriately accounted for, they may contribute to issues such as:
- Cracking in masonry walls
- Uneven floor movement
- Sticking windows and doors
- Distortion of finishes
- Differential movement between different parts of a structure
It's important to note that reactive soil does not automatically lead to building damage. Many homes are successfully constructed on reactive clay sites every year. The key is ensuring the footing system is designed for the site's expected ground movement and complies with the relevant engineering standards.
This is why understanding the site's ground conditions before construction begins is essential to residential development.
How Is Reactive Soil Classified?
Once a geotechnical investigation has been completed, engineers assess the site's expected ground movement and assign a classification in accordance with AS 2870:2017 – Residential Slabs and Footings.
The classification provides a consistent way to describe the reactivity of the site's soils and helps guide footing design.
The most common classifications include:
- Class S – Slightly reactive soils with relatively low ground movement potential.
- Class M – Moderately reactive soils that require footing systems designed for moderate ground movement.
- Class H1 – Highly reactive soils with a high movement potential.
- Class H2 – Highly reactive soils with a greater movement potential than H1 sites.
- Class E – Extremely reactive soils requiring specialised engineering consideration.
- Class P – Problem sites (uncontrolled fill, soft soils, abnormal moisture conditions, or other geotechnical constraints) that require project-specific engineering.
The classifications defined in AS 2870:2017, including Class M, H1, H2, E, and P, don't indicate whether a site is suitable for development.
Instead, they describe the site's expected ground movement or geotechnical conditions so engineers can recommend an appropriate footing system.
Because ground conditions can vary significantly even between neighbouring properties, a site classification for reactive soil should always be based on a site-specific geotechnical investigation rather than on nearby developments.
How Does AS 2870:2017 Influence Footing Design?
AS 2870:2017 – Residential Slabs and Footings provides the framework engineers use to design residential footing systems for different site classifications.
The standard establishes requirements for classifying residential sites and designing footing systems to accommodate the expected ground movement associated with different site conditions.
Using the findings from a geotechnical investigation, engineers consider factors such as:
- Site classification
- Soil profile and reactivity
- Foundation loads
- Building layout
- Local ground conditions
This information helps engineers determine an appropriate footing system in accordance with AS 2870:2017, taking into account the proposed development and the site's expected behaviour.
By designing foundations to suit the site's actual ground conditions, engineers can help improve the long-term performance of residential structures while supporting compliance with the relevant Australian Standards.
What Footing Types Are Used for Reactive Clay Soils?
There is no single footing system that is suitable for every reactive clay site. The appropriate solution depends on the site's classification, the expected ground movement, the proposed building, and the recommendations provided by the geotechnical and structural engineers.
Depending on the project, engineers may recommend footing systems such as:
- Stiffened raft slabs
- Waffle raft slabs
- Strip footings
- Pier and beam systems
- Other engineered footing solutions designed for the site's conditions
The objective of reactive clay soil footing design is not to eliminate ground movement. Instead, it is about designing foundations that can accommodate the expected movement while supporting the structure's long-term performance.
For this reason, footing selection should always be based on a site-specific geotechnical investigation rather than assumptions about nearby properties or previous developments.
How Can Builders Reduce the Risk of Ground Movement?
While ground movement cannot always be prevented, builders and property owners can take practical steps to help manage the effects of reactive soils throughout construction and the life of the building.
Good practice includes:
- Completing a geotechnical investigation before design begins
- Designing footings in accordance with AS 2870:2017
- Managing surface drainage to minimise excessive moisture variation
- Considering the location of trees and large vegetation near foundations
- Following engineering recommendations during construction
- Maintaining consistent moisture conditions around completed buildings where practical
These measures work together to help foundations perform as intended under expected site conditions.
Why Site Investigations Matter Before Construction
Reactive soils cannot be accurately identified through a visual site inspection alone. While some sites may appear similar at the surface, their subsurface conditions can vary significantly.
A geotechnical investigation provides the information engineers need to assess:
- Soil profile and subsurface conditions
- Soil reactivity
- Bearing capacity
- Groundwater conditions where it's relevant
- Site classification under AS 2870:2017
- Factors that may influence footing design
This information allows foundation recommendations to be based on measured site conditions rather than assumptions, supporting more informed engineering decisions before construction begins.
Why Choose BHM Geotechnical?
BHM Geotechnical provides practical geotechnical investigations that help builders, developers, architects, and homeowners understand the ground conditions beneath their projects.
Our investigations combine field testing, laboratory analysis where required, and practical engineering advice to support footing design for residential developments across Sydney and Adelaide.
Our team provides:
- Residential geotechnical site investigations
- Site classification reports in accordance with AS 2870:2017
- Bearing capacity assessments
- Foundation and footing recommendations
- Clear engineering reports that support planning and design
- Responsive advice throughout the project lifecycle
Every investigation is tailored to the site's conditions, providing practical recommendations that help projects progress with greater confidence.
Frequently Asked Questions
What is reactive soil?
Reactive soil is soil that changes volume as its moisture content changes. This behaviour is most commonly associated with clay soils, which expand when wet and shrink as they dry.
Is reactive clay bad for building?
Not necessarily. Many homes are successfully built on reactive clay sites every year. The key is understanding the site's ground conditions and designing the footing system to suit the expected level of ground movement.
What is the difference between M Class and H Class soil?
M Class indicates moderately reactive soil with moderate ground movement potential, while H1 and H2 indicate highly reactive soils with greater movement potential. These classifications help engineers determine appropriate footing designs in accordance with AS 2870:2017.
Does reactive soil always cause cracks in a house?
No. Reactive soils do not automatically cause structural damage. However, if expected ground movement is not appropriately considered during foundation design, it may contribute to movement that affects parts of a building over time.
What footing type is used for reactive clay soil?
The appropriate footing system depends on the site's classification, soil conditions, building design, and engineering assessment. A geotechnical investigation and structural design are used to determine the most suitable solution for each project.
Do I need a site classification report for reactive soil?
For most residential developments, a site classification report provides important information about soil reactivity and expected ground movement. Engineers use this information to support footing design in accordance with AS 2870:2017 and the site's specific conditions.
