Did you know that different soil types can affect the construction of your new home?

The type and reactivity of the soil on your block of land is an often overlooked factor - one that can surprisingly impact your new build.

If your home is built on a poor foundation due to reactive soil, you’re at risk of structural damage over time. So, what are the different soil types for building, and how can you take measures to mitigate the risks of reactive soil? In this guide, we’ll answer all your questions about the various types of soil for construction.

What are the Different Soil Types for Building?

Soil classifications are listed in the Australian Standard AS 2870/2011 – Residential slabs and footings. Soil types are determined as either S, M, H1 or H2 — slightly, moderately, highly, or extremely reactive.

• Slightly reactive: May experience slight ground movement when moisture levels change.
• Moderately reactive: Moderate ground movement from moisture changes. Good drainage is an essential aspect of the design on these sites.
• Highly reactive: High ground movement from moisture changes. Class ‘H’ is often broken up into two groups: H1 and H2. In addition to adequate drainage, there may be special requirements protecting pipes from ground movement. H2 requires extra engineering.

The word ‘reactive’ refers to the way soil reacts when the moisture changes. For example, reactive clay soil changes in volume as it absorbs and releases moisture. More highly reactive soils have a greater chance of shifting in large amounts over time, causing problems for the foundation of your home.

While reactive soil sounds worrying, even frightening, there are ways around construction sites with high degrees of reactivity — which are more common than you may think. In fact, approximately 20% of all sites in Australia have moderately to highly reactive clay soil, and here in Townsville, it’s common for construction sites to have highly reactive soil. 

Soil Types and Bearing Capacity

Bearing capacity is another factor affecting the soil’s impact on a build. Soil’s bearing capacity is the maximum average contact pressure between the construction’s foundation and the soil before shear failure, resulting in soil rupture and bulging.

In simple terms, bearing capacity is how much weight the soil can support before giving way. It is a figure used in the design of home foundations.  

What Does This Mean for Your House?

Why in many cases, ‘soil type’ is far from the minds of first-home buyers or even experienced property investors, it’s one of the most important elements you should consider. Overlooking the reactive properties of the soil on your site can lead to structural complications later on, such as:

• Wall cracks
• Sinking concrete
• Moving house piers
• Bouncy floorboards
• Dips and uneven surfaces 

Seasonal and Abnormal Soil Conditions 

Seasonal changes may include soils drying out or becoming moist quickly, which can result in extreme volume changes and soil movement. Meanwhile, abnormal soil conditions may occur before, during, or after construction:

• When an existing building is demolished or removed
• When trees are removed from the site
• When trees remain nearby or within the site
• When there is inadequate or improper drainage during construction
• With lack of maintenance of site drainage
• When trees are located too close to the footing
• Through plumbing leaks
• With excessive or incorrect watering of grounds near the building
• Through air conditioning and hot water system outflow

How to Identify Soil Type and Avoid Ground Movement

Before your house is built, we recommend getting a soil test on the block of land. Testing the soil before building will help identify any potential additional costs involved in preparing the foundation. Your builder can work with you to determine the options available to you before building commences.  

If you are building on highly reactive soil, you may need to hire a structural engineer. The resulting added requirements for your home may inflate labour and material costs but save you money in the long-term by reducing the risk of structural damage. 

A structural engineer can identify the footing system needed to match normal wet and dry conditions through the depth, reinforcement, and spacing of edge and internal beams. These measurements will be determined by the type of construction and the classification of the soil, so it’s a good idea to hire a structural engineer after the soil test.

Over the life of your house, practise good ongoing management to reduce the risks of damage. If you notice any structural problems, tend to them immediately.