When building a home, you’ll hear about soil tests, Geotechnical Reports and soil reactivity. It all boils down to this: if you are building a home, you must have a soil test.
It’s one of the things spoken about often at House Builders Brisbane. You will also hear it discussed by pretty much every home building company in Australia. If you are going to build a home, no matter what type, you need to have an independent soil test undertaken by a qualified geotechnical engineer.
So, what will this produce?
In its simplest form, it’s called a Geotechnical Report and this is what is produced by the qualified engineer.
Soil is a naturally reactive substance and different soils react in different ways under different conditions. Structural engineers and house builders know this and so the process of determining what type of soil you have and how reactive it is, comes from the Geotechnical Report.
This process should always be done as early as possible – even before you purchase the land. Because it will identify any extreme elements that could severely affect the cost of building your home. In addition to the soil reactivity information the Geotechnical Report will also highlight any hidden chemical or physical conditions on the building site that could affect your costs or pose a long-term threat to your house building project.
Ok, so what is soil reactivity?
Soil reactivity refers to how much the soil on the building site is likely to expand, contract, shift and settle (normally as a direct result of changing moisture content). This is a real issue, its why the report and the engineering specifications that derive from it are mandatory for all construction. The structural engineer will consider the reactivity of the soil and weight this against what you are planning to build and will provide detailed structural foundations as a result.
Reactive soil can cause a lot of damage to a house, especially if the home that was built was done so with the wrong type of engineered foundations. House cracking is a major concern when reactive soil is identified in the Geotechnical Report. Check out the picture in this article as an example.
What are the soil classifications?
Soil reactivity is generally graded in terms of the following classifications:
Stable, non-reactive. Most sand and rock sites. Little or no ground movement likely as a result of moisture changes.
Slightly reactive clay sites. May experience slight ground movement as a result of moisture changes.
Moderately reactive clay or silt sites. May experience moderate ground movement as a result of moisture changes.
Highly reactive clay sites. May experience a high amount of ground movement as a result of moisture changes.
Extremely reactive sites. May experience extreme amounts of ground movement as a result of moisture changes.
Problem sites. The ability of the soil to evenly bear a load is very poor. Sites may be classified as ‘Class P’ as a result of mine subsidence, landslip, collapse activity or coastal erosion. Ground movement as a result of moisture changes may be very severe.
Class A, S and M are generally considered ok and if there are no other concerning issues then you should not be concerned.
Class H and E will need investigation and analysis by the structural engineer relative to what you are planning to build.
If you are building on a Class P site you will need to consult a structural engineer very quickly. The costs and risks must be considered early in the feasibility of the project.
What does the Structural Engineer do with the Report?
The Geotechnical Report that lands with the structural engineer indicates the physical properties of the soil on your site, the stability of natural slopes, the chemical composition of the soil and various other details. From all of this information the structural engineer will work with your builder and building designer to design the type of footings or concrete slab subfloor that can be built on your site. Once final engineering plans are produced then the structural engineer will eventually come to the site and inspect the actual foundations prior to the pouring of concrete and sign off on them.