Guest Post

The Significance of Soil Examination in Home Construction

Building a new home is a major undertaking. As floorplans grow and block sizes shrink, builders are placing increasing importance on solid engineering and stable footings.

Soil testing is one of the most significant factors when building a new home. The soil you build on can have a major impact on the stability of a building, and improper construction can lead to critical failures.

In most Australian jurisdictions, soil tests are a requirement for obtaining building approval. In this article our expert house builders in Brisbane we will explore why this is the case and discuss the significance of soil examination and classification.

Why Soil Type Matters in Home Construction

The soil on your property can dramatically affect the long-term stability of anything you build. Different types of soil react differently under the footing of a building. They also have varying reactions to moisture, temperature and shifts in the ground.

Soils that are very reactive to these factors can be problematic for construction projects. They may be prone to erosion, poor drainage or excessive movement. This can cause the foundations of a building to crack or collapse, and repairing those issues is often incredibly costly.

In modern construction, we test soils to verify the long-term stability and safety of your property. If we don’t, issues with the soil that affect the foundation could lead to critical failures and serious injury.

How Soil Types are Classified

In Australia, soil classifications are laid out in AS2870-2011 – “Residential slabs and footings – Construction.” The Australian Standards set out a number of guidelines for designing footings based on the soil classification. These footing designs take into account three major factors:

  • Erosion. Highly reactive soils absorb water, causing them to change in volume and erode. Soils with a high sand content are more likely to erode. This typically requires modifications to the footing, such as deeper floor beams, thicker slabs and additional reinforcement to prevent cracking.
  • Load Bearing Capacity. This is a measure of how well a soil supports loads that are applied to the ground. Bearing capacity has little relationship to soil reactivity. For instance, sandy soils often have low reactivity, but they also have low bearing capacity and typically require compaction prior to laying a slab.
  • Seasonal Shifts. Australia’s dry and stormy seasons mean our soils often change throughout the year. Accounting for prolonged dry and wet seasons prevents house footings from being damaged by outlier events. These sorts of problems may be addressed with reinforcement, or the property’s drainage system may require additional consideration.

Under AS2870, soil receives a letter grade – A, S, M, H, E, P. A and S class are suitable for normal slab construction. Soils with a higher reactivity (H to P) need to be evaluated by an engineer, and they may require compaction or earthworks during site preparation.

The residential footing code provides more detail on these soil classifications:

Classification Foundation Surface Movement
A – Acceptable Little ground movement due to moisture changes (sand and rock) 0mm
S – Satisfactory Slightly reactive, slight movement with moisture change (clay) 0-20mm
M – Moderately Reactive Moderate site movement due to moisture change 20-40mm
H – Highly Reactive Highly reactive to moisture change (clay) 40-60mm
E – Extremely Reactive Extreme reactivity due to moisture change (clay) 60-75mm
P – Problem Soft soils, silt, loose sand and soft clay – sites that may collapse, soils that are likely to erode, sites with abnormal moisture conditions N/A

Factors Affecting Soil Classification

The letter grade recorded in your soil report is obtained by taking several samples from across the property. Samples are assessed in a lab to determine their composition, bearing capacity and other factors, such as:

  • Particle size – particle size affects the strength and load bearing capacity of soil. Smaller, well-graded particles tend to be more stable.
  • Plasticity – the plasticity of a substance refers to how easily it changes shape or moves when under pressure. Higher plasticity soils are easier to shift, which typically makes them weaker and more prone to settling and changing shape under the loads produced by a house.
  • Organic matter content – it’s normal for soils to contain other types of organic matter, such as leaf debris. High organic content makes soils less stable and less suitable for building.
  • Moisture content – the amount of water retained by soil affects its strength and durability. Soil that is excessively wet or excessively dry can present a problem for construction projects.

When Should Soil Testing Be Conducted?

Soil tests are usually carried out before construction commences. Engineers may also order soil testing during construction to check changes in the surface, modify build plans or to verify that construction has been completed properly.

In an ideal scenario, home buyers should conduct soil testing during the purchase phase. If a buyer intends to build on a property, paying for a soil test will verify that the block is suitable, which can save significant expense during construction.

There’s no need to specifically avoid properties with reactive soils – in some cases, those issues can be addressed with simple engineering changes. However, having this information up front can help budget for additional earthworks and footings expenses, rather than incurring those costs unexpectedly.