Demolishing and it’s Effects on Soil Class and Construction Costs


Soil Class and Construction Costs

Understanding soil types and their classifications is crucial for anyone wanting to understand costs of a new build or a knock down rebuild project. In Australia, soil classes play a significant role in determining construction costs, footing types, and engineering requirements. Whether your project involves demolishing an existing home or starting from scratch, the soil classification can have a substantial impact on your budget and the overall success of the build. This article delves into the various soil types in Australia, their implications on construction, and how demolishing and rebuilding can influence soil class and ultimately affect costs and engineering decisions.

Understanding Soil Types in Australia

Deciphering Soil Classifications

In Australia soil types are categorised into classes ranging from Class A to Class P. Class A represents stable, non-reactive soils such as sand and rock, which typically require standard footing systems—leading to lower construction costs. Moving up the scale, Class S and Class M soils, which include silt and clay, respectively, are slightly more reactive to moisture changes and need more consideration in footing design to prevent structural damage. Class H and Class E soils are highly reactive clay soils that expand and contract with moisture variation, necessitating specialised footing solutions, which increase both complexity and costs. The most problematic is Class P, often associated with sites that have been disturbed or have unusual moisture conditions.  These sites often require significant site-specific engineering solutions. Understanding the soil class is essential to forecast construction costs and design appropriate footings for your new build or knock down rebuild project.

Influence on Construction Costs

The type of soil on your property has a direct effect on construction costs. Non-reactive soils such Class A or S require minimal special considerations, leading to standard construction costs. As you move to more reactive soil types, like Classes M, H, and E, the costs increase due to the need for more complex footing systems designed to manage soil movement and maintain structural integrity. The highest costs are typically associated with Class P soils.  These classes may require extensive ground treatment, customised engineering designs, and even ongoing maintenance solutions to mitigate the effects of soil instability. These additional measures are crucial to preventing future issues but do result in higher upfront expenses. Therefore, accurately identifying the soil class before starting your new build or knock down rebuild is vital to budgeting effectively for the ensuing construction costs.

Knock Down Rebuild: Soil Implications

The Impact on Soil Class

Demolishing an existing structure can significantly alter the soil class of a site. The machinery and activities involved in the demolition process can compact the soil or change its moisture content, potentially increasing its reactivity. This means a site that was once classified as Class A or S could be reclassified to a more reactive class, leading to a reassessment of construction requirements. Additionally, the removal of a building can disrupt drainage patterns, which may affect the soil’s moisture levels and, consequently, its classification. It’s imperative to conduct a new soil investigation post-demolition to determine the current soil class accurately. Failing to do so could result in designing footings that are inadequate for the new soil conditions. Therefore, the impact of demolition on soil class is a critical consideration in the planning stages of a knock down rebuild project.

Adjustments in Engineering and Footing Design

When the soil class changes due to a knock down rebuild demolition, engineering solutions and footing designs must be adjusted accordingly. For a newly reactive soil classification, engineers may recommend deeper or more robust footing systems to accommodate potential ground movement. This could include the use of reinforced concrete slabs, piers, or screw piles that are designed to reach stable ground below the reactive soil layer. On the other hand, if the soil becomes less reactive post-demolition, it may be possible to scale back on some of the previously required engineering measures, potentially reducing costs. However, it’s essential not to overlook the impact of any remaining debris or changes in the soil’s composition. The key to successful footing design lies in a thorough assessment of the current soil conditions and applying the appropriate engineering principles to ensure the longevity and safety of the new structure.


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