Soil Type Classification

In the grand scheme of building a home, soil type probably isn’t included in your list of top priorities. But it’s a vital piece of information that helps determine the very foundation of any house.

Before building anything, a soil test is necessary to find out how easy or complicated it is to build a home in your block of land. This solidifies the foundation of your house and saves you additional costs down the track.

Let’s dive deeper into the importance of soil types, how they’re classified, and what the soil classes mean. It’s handy to know these for when you receive those soil test results!

The importance of soil type

The soil on your chosen block of land determines the most ideal slab and footing design to build your home. To ensure your dream home is set up on the correct foundation, qualified professionals will drill holes on different sections of your block to perform a soil test. The result will classify your soil and advise your builder of your home’s slab and footing design requirements.

How soil types are classified

Soil types are generally classified according to their reactivity — the likelihood of soil movement as it gets wet and dries out. Your soil type will indicate any extra engineering costs needed. The more reactive soil is, the more expensive it becomes to build a house on it. And the earlier you find out how much it’s going to cost, the better informed your decision will be!

There are other factors related to reactivity that can affect the slab and footing design of your home. Being proactive (and not reactive!) about this can help mitigate certain risks involved with the type of soil that your block has.

Erosion

The higher the reactivity, the more likely it absorbs water and changes in volume. As water can sometimes cause erosion, engineers will outline measures that need to be taken to build on highly reactive soil. Some types of soil are more susceptible to erosion than others — such as sand — so even clay with a sand component can suffer from erosion.

Bearing capacity

The bearing capacity of soil refers to its ability to support the loads applied to the ground. It’s possible for certain soil types to have low reactivity but also have a low bearing capacity. For example, sand is very soft and would require proper compaction to take a heavy load.

Seasonal swelling and shrinkage

It’s also crucial to account for prolonged wet and dry seasons. When soil absorbs moisture, it can swell and create an upward force on the footings of a building. When soil dries out, it can take away from the support needed by the footing to maintain equilibrium. While this phenomenon may only get noticed if it goes on abnormally for weeks or months, it’s good practice to factor this in sooner than later. If the area you live in is prone to such periods, the engineer may recommend ways to prevent swelling or shrinkage, whether it’s installing a good drainage system or creating concrete paths to keep the soil moist.

The soil classifications

Your soil test results will return one of six letters (A, S, M, H, E, P) — each stands for a soil type. Most A-S classes will only require a basic slab supported on the ground with footings. Soils with high reactivity (H to P) require further evaluation by an engineer to ensure the stability and integrity of the slab and footings. Some may need stiffening while others need more intricate solutions such as closely spaced ribs.

soil-type-classification

IMAGE: Example of slab on ground (source: Build Right)

The Residential Slab and Footing Code (AS 2870) outlines the following soil classifications:

Class Stands for Foundation Surface Movement
A Acceptable Little to no ground movement from moisture changes (not reactive); most sand and rock sites 0mm
S Satisfactory Slightly reactive with only slight ground movement from moisture changes; clay sites 0-20mm
M Moderately Reactive Moderately reactive with moderate ground movement from moisture changes; clay or silt sites 20-40mm
H Highly Reactive Highly reactive with high ground movement from moisture changes; clay sites 40-60mm
E Extremely Reactive Extremely reactive with extreme ground movement from moisture changes; clay sites 60-75mm
P Problem Includes soft soils, such as soft clay or silt or loose sands; landslip; mine subsidence; collapsing soils; soils subject to erosion; reactive sites subject to abnormal moisture conditions or sites which cannot be classified otherwise. N/A

Getting your soil tested

A soil report and site inspection will follow upon confirmation of your build. If you’re ready to make your new home vision come true, book an appointment at your nearest display centre today.