Drafting in Sydney: What is Geotechnical Report and why do I need one?

"How reactive the soil on your site is will determine how your house will need to be built."

Many of our clients asked: "what is a geotechnical report?", "What does it do?"

What is a geotechnical report?

A geotechnical report is a report prepared and tested by a geotechnical engineer (sometimes called ‘geotechs’ or even ‘mud doctors’). The purpose of the report is to allow us, (particularly the structural engineers to understand how ‘reactive’ the soil is, and to ensure that there aren’t any hidden chemical or physical conditions on the site that might damage your house.

When do I need it?

The general answer will be: When the council/certifying authority requested one. 

However, with jobs that a bulk excavation is required, a geotechnical report is mostly required. 

At other times, a structural engineer may still request one to ensure a safe footing/foundation is to be designed to comply and work functionally.

What is the difference between a Soil Report and a Geotechnical Site Investigation Report?

A Soil Report describes the soil profile of your site after laboratory testing, a Site Classification in accordance with Australian Standards 2870 will be provide.

A Geotechnical Investigation Report will provide the above information as well as giving desciption of the site conditions and all the information required to design an appropriate footing system. Sometimes, recommendations is also given to the end users.

What is soil reactivity?

Soil reactivity refers to how much the soil is likely to move, expand and contract on the site as a result of changing moisture content (can be weather dependent). This can be a significant issue to what is being built, especially where concrete slab subfloors are concerned, or building storeys house with heavy weight loading. Reactive soil can easily cause a lot of damage to a house, especially if the design specifies the wrong type of concrete slab subfloor.  

What should I do if I am planning to extend my current home or building a new house? Do I need to get a geotechnical report first?

Speak to a residential drafting design consultant first, let them understand the nature of your land and what your council's requirements are, prior to ordering any reports.