Why Are Reactive Soils a Concern?

In residential and commercial construction, ensuring that the foundation rests on soil with adequate bearing capacity is fundamental. However, in Australia, the primary concern often shifts from bearing capacity to soil reactivity.

Reactive clay soils, prevalent across much of Australia, including the Adelaide Plains, exhibit significant volume changes with moisture fluctuations. These soils expand when wet and contract when dry, leading to ground movement that can compromise the structural integrity of homes. This movement, driven by environmental factors such as seasonal rainfall, drought, and vegetation, can result in foundation damage, internal cracking, and long-term structural issues if not properly addressed during the design phase.

What is the Impact of Soil Movement on Structures?

The expansion and contraction of reactive soils impose environmental loading on buildings, similar in significance to wind or seismic forces. These movements can cause footings to shift, leading to structural distress. National building standards, while outlining various imposed loads, do not provide specific guidance on the dynamic loads caused by reactive soil movement. Therefore, assessing soil reactivity independently during site preparation is crucial.

A critical component of this assessment is the site classification report, which estimates the vertical movement expected at the site based on soil characteristics and environmental conditions. Classifications, such as “M” for moderate movement, guide structural engineers in selecting appropriate footing designs.

What Is a Site Classification Report?

A site classification report is a specialised assessment that determines the expected ground movement due to reactive soils. Prepared by qualified geotechnical practitioners under the guidelines of AS 2870, the report involves visual inspections, desktop analyses, and laboratory testing. The outcome is a clear classification of the site based on expected vertical ground movement, informing the structural design of the building’s foundation.

The site is then classified (e.g. Class M, H1, H2, or E), guiding the structural engineer in selecting a footing design that responds appropriately to predicted soil movement. Our team is ready to help interpret these results and incorporate them into your construction documentation.

The Role of Soil Bore Logging and Reporting

Soil bore logging is a critical part of understanding ground conditions. It involves drilling into the site to collect samples and determine the type and characteristics of soil at various depths. The purpose is to identify soil strata, detect reactive clay or fill material, and assess moisture content and groundwater levels.

Borehole logs are compiled into reports that describe soil types, consistency, plasticity, and potential reactivity all in accordance with Australian Standard AS 1726. This data allows us to determine how deep footings should be placed and whether additional testing is needed.

Is a Site Classification Report Necessary?

Yes, a site classification report is essential for any residential construction project in Australia. It provides critical data needed to design footings that accommodate the soil’s movement characteristics. Without this report, there is a high risk of structural performance issues arising in the future. Moreover, the report is a mandatory requirement for building permit applications and must be prepared by a qualified practitioner.

By providing accurate and locally-informed soil assessments, a site classification report allows structural engineers to design foundations that meet the National Construction Code requirements while avoiding over-engineered, and often unnecessarily costly, solutions.

At OB Engineering, we specialise in providing comprehensive soil and site reports and structural engineering solutions tailored to the unique soil conditions of South Australia. Our expertise ensures that your residential construction project is built on a foundation designed to withstand the challenges posed by reactive soils.

For professional advice and services on managing reactive soils in your construction project, contact OB Engineering today.