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Why is soil testing important when building a new house?

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Soil testing takes place in the first stage of building a new house, where a series of soil samples are taken from your block of land. The soil profile of each home site varies from place to place, so it is compulsory to have a soil report, even before your house plans are decided on. Some builders manage this for you, so check upfront. Make sure you have the results of your soil test prior to signing any building contract, as the outcome can significantly affect your build costs.

Engineers from Geo-Concrete Labs Getting Soil samples

The main reason is to understand how ‘reactive’ the soil is, and ensure that there aren’t any chemical or physical conditions on the site that might damage your house. Soil reactivity refers to how much the soil on the site is likely to move, expand and contract (normally as a result of changing moisture content) and is graded by class.

The soil bearing capacity tells us the weight the soil can support per unit area and determines the type of footings or slab subfloor that can be built on your site. If the soil is unstable, then the footings may be to be placed deeper into the ground, or a different type of foundation may need to be used.

A difficult site – one where there is expected to be higher than average surface movement – will cost considerably more to build on. These costs can sometimes be significant, so best to be well prepared before you start building.

What are the different types of soil classifications?

  • Class A – stable, non-reactive: the possibility of very little or no ground movement as a result of moisture change (often sand and rock sites)
  • Class S – the possibility of slight ground movement (often clay sites)
  • Class M – the possibility of moderate ground movement (often clay or silt sites)
  • Class H – the possibility of high ground movement (often clay sites)
  • Class E – the possibility of extreme ground movement
  • Class P – problem sites: ground movement as a result of moisture change may be very severe, you will need to consult a structural engineer before building a new house.

Generally, Class ‘A’ and ‘S’ only require a basic slab with footings, however, all other classes are likely to require the slab to be further reinforced.

How is a soil test performed?
Geotechnical engineers (also called ‘geo techs’) use a special piece of equipment to drill down into the ground and extract soil. By determining what kind of soil is at various depths, they will be able to classify the site and determine the bearing capacity of the soil.

Going for a knockdown rebuild?
Keep in mind that if you’re doing a rebuild, you’ll need two soil tests – prior to demolition and then again after. After demolition, you’ll also need an asbestos clearance certificate. An A-class licensed asbestos assessor, independent from the person or business responsible for the removal work, must inspect the site and your clearance certificate will verify the site is free of asbestos.

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