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Soil Classification: Why Do Soil types matter for Construction?

2 August 2023

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Did you know that the type of soil beneath your feet holds the key to the success or failure of your architectural masterpiece? Whether it involves towering skyscrapers in bustling cities or the foundation of modest homes, the type of soil plays a crucial role in determining the structural integrity and stability of any building. 

In Sydney, prospective homeowners should be aware of the unique soil types found in each landscape and neighbourhood. From sandy to clay-rich soils, each type has its distinct characteristics that can impact the foundation of your home. Being aware of the soil type in Sydney can help determine the best foundation for your home and mitigate the potential risks involved.

So, let’s dig deep into the world of soil classification to understand the significance of various types of soil for construction to ensure safety, resilience and long-lasting structure that can withstand the trials of time.

Understanding Soil Types

The classification of soil determines the suitable foundation type and building design. Soil types are classified based on soil movement, which is usually related to the ability of the soil to shrink or swell. As per Australian Building Code Board, soil classes are defined as.

Soil Class Foundation 
Class AMostly consist of sand and rock, and no significant ground movement is anticipated.
Class SSlightly reactive clay sites, but there will only be minimal ground movement due to changes in moisture.
Class MModerately reactive clay or silt sites may experience moderate ground movement due to fluctuation in moisture.
Class HSites are highly reactive and may undergo significant ground movement due to fluctuations in moisture levels. 
Class EThese sites are extremely reactive and can undergo extreme ground movement due to changes in moisture levels. 
Class PA problematic site includes soft soils like clay or silt, loose sands, landslips, mine subsidence, collapsing soils, soils prone to erosion and reactive sites with abnormal moisture conditions or sites.

Sandy Soil

Sandy soil has a gritty texture and large particle size. It can be a good choice for building homes and residential houses, especially in places with low seismic activity and stable ground conditions. Sandy soil drains well, which helps to prevent water damage to the foundation of a building. However, building in areas that are prone to earthquakes or high winds, sandy soil may not be the best choice for foundations or tall structures due to its low cohesion. Proper soil preparation is crucial before building a secure foundation, including compaction and stabilisation. Additionally, building on sandy soil can pose risks such as erosion or instability during heavy rainfall, so it’s important to take these factors into account.

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Loamy Soil

Loamy soil is a type of soil that contains a balanced mixture of sand, silt and clay particles. It is often considered the ideal soil type for construction due to its versatility and stability. Loamy soil retains moisture well while also providing excellent drainage, which makes it suitable for a wide range of construction applications. Building homes and residential houses on loamy soil is particularly advantageous due to its ability to provide a stable foundation. The balanced composition of loamy soil reduces the risk of settlement or subsidence, which can compromise the structural integrity of a building. 

Silty Soil

Silty soil can be used for building houses, but it requires additional preparation and consideration compared to other soil types. Silty soil has a high-water retention capacity, which can lead to issues with drainage and stability. This can be problematic for building foundations, as waterlogged soil can cause settling and subsidence over time. Therefore, it’s important to take steps such as compaction and stabilisation to ensure a solid foundation for the house.

Additionally, the building process in silty soil may not be suitable for areas with high seismic activity or where there is a risk of landslides or erosion. Its fine-grained nature can make it more susceptible to erosion and instability during heavy rain or wind. In such cases, a more stable soil type may be a better choice for building houses.

Clayey Soil

Clayey soil is known for its high plasticity and ability to retain water. While it offers good cohesion, it can be problematic in construction due to its expansive nature, its low permeability and poor drainage. During wet periods, clayey soil expands and during dry periods, it contracts, leading to foundation movement and affecting the stability of buildings. Building a house on clayey soil may require additional preparation, such as using proper foundation designs and implementing moisture control methods.

Peat Soil

Peat soil is a unique type characterised by its organic content and high water retention capabilities. Peat soil is unsuitable for construction projects due to its low bearing capacity and poor stability, which can result in settlement issues and structural damage. Construction of houses on peat soil requires thorough site inspection and engineering solutions, such as soil stabilisation techniques. It may also be at risk of erosion and landslides during heavy rainfall or other weather events. It’s best to choose a more stable soil type for construction projects, particularly for building homes and other residential structures.

Rocky Soil

Rocky soil is a type of soil that contains a high concentration of rocks, stones, and boulders. It can be challenging for construction projects due to the difficulty of excavation and grading the soil.  Building a home on rocky soil can be advantageous in areas where the soil is prone to erosion or flooding. The rocky soil can help to stabilise the foundation and prevent water damage to the building. However, it’s important to carefully assess the soil conditions and potential risks before proceeding with construction.

Soil types in Sydney and Canberra

Sydney’s soil is mainly categorised as sandy loam or clay loam. Due to the region’s humid subtropical climate and high rainfall, there can be soil erosion and stability concerns. Coastal and inland areas of Sydney have sandy soil, which allows for good drainage but may not have the best load-bearing capacity. Certain regions of Sydney have clay-rich soil, which can expand and contract depending on moisture levels. Shale-based soil can be found in some parts of Sydney, which offers relatively stable ground.

In Canberra, the soils are typically classified as clay loams or clay soils. Due to the temperate climate with low rainfall and high evaporation rates, the soil can become dry, and water may be scarce. Some areas may also have low fertility or high alkalinity due to the presence of limestone or other minerals. Canberra’s geology is unique and includes granite-based soils that possess specific engineering properties. Sandy loam soils are prevalent in Canberra, providing good drainage and a balanced combination. Gravelly soils can be found in certain areas of Canberra.

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The Importance of Soil Classification in Construction

Determining the classification of soil is a vital component of construction planning, as it helps determine the suitability of a site for building and enables the appropriate design and construction of foundations. By identifying important factors such as soil strength, compressibility and bearing capacity, proper soil classification ensures the stability and safety of the building. 

It’s crucial to accurately classify soil during the planning and design stages of construction to avoid failures like foundation settlement, slope instability and building collapse. Collaborating with geotechnical engineers and soil scientists can ensure that soil classification is done properly and appropriate foundation design and construction methods are implemented to mitigate risks associated with the different soil types.

Soil Test Systems 

When planning to build a house on a new block of land, it is crucial to perform soil testing to determine the soil’s reactivity and other geotechnical conditions at the building site. These tests are necessary for stability and safety reasons and help classify the foundation soil based on its tendency to shrink or swell with moisture change. An endorsed building engineer designs the slabs and footings, considering the soil classification and expected movement. This meticulous approach ensures that the appropriate foundation design is determined to withstand normal variations in the soil.

Summing Up!

Selecting the proper soil is crucial for building a sturdy foundation. Conversely, choosing the wrong kind of soil can result in catastrophic outcomes that may be expensive in terms of time, finances, and even human lives. The most suitable soil for building a house is one that is secure, well-drained and capable of bearing loads. It is, therefore, vital to comprehend the soil’s condition and behaviour during the construction process.

At Dhushan Construction, we will guide you through soil testing and classification that comply with Australian standards. This helps to identify the soil types present on your construction site and ensures that they meet your specific requirements. With our assistance, you can build your dream home in Sydney that is both functional and aesthetically pleasing while also being long-lasting. Let us help you lay the groundwork for your dreams. Contact Us now!

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