The term foundation in building a structure such as a house, refers to the very first part of the building in which everything else will be built upon. Foundations are also referred to as the 'Base', as it is th first thing you need to build.

Foundations can be made of different materials, and built using different designs and methods to suit both the structure to be built, and the type of soil the building is to be built upon.

The most common type of foundation used in building houses in Australia is a concrete slab. This can also be referred to as a shallow foundation, or more commonly 'footing'.

The purpose of the foundation or footing is to transfer the weight of the building to the soil below. The weight of walls and roof is referred to as a 'load', and depending on the soil type will depend on the design of the footing design. The design will allow adequate transfer of these loads to the soil so the building does not collapse, and to try and minimise movement so brick walls don't crack.

Before foundations/footings can be designed, the type of soil you plan to build on must be determined. This is done by a Geo-Tech engineer that goes to the building site, drills bore holes and takes samples of the soil to determine the soils characteristics. These reports can be quite detailed, but the main piece of information an Architect or Building Designer needs to know is the Soil Class.

Soil Borehole Samples Example Photo
Soil Borehole Samples

The reason a soil test is important is to determine the soils ability to evenly bear the weight (load) of the building. Soils are often referred to as reactive, and the more reactive the soil, the more likely it is to swell and shrink in wet and dry weather, particularly as seasons change. This reaction can cause cracking in brick work, bowing of floors on bearers and joists, and potentially major structural failure of the foortings are not designed adequately for the soil class.

In Australia, there is an Australian Standard that is used in designing concrete footings (AS 2870 - Residential Slabs & Footings), and in this document it lists the main soil classes. (Refer Image below)

Australia Soil Classes
Table from AS 2870 (Australian Standard - ResidentialSlabs & Footings)

The table above lists soil classes on the left (A, S, M, M-D, H, H-D), and the additional information on the right relates to depth and thicknesses of the concrete, and what types of reinforcement bars and mesh is to be used. This chart is for one type of concrete slab.

Getting back to Soil Classes, ideally Class A & S is best as it is cheapest to build as less concrete and steel reinforcement is needed for the slab.

The higher the Soil Class, the more expensive it becomes as digger trenches will need to be dug, more concrete required, possible higher grade concrete, and more steel.

Soil Class A and H-D Footings

Comparrison Class A Soil to Class H-D Edge Beam Depth

There is another Soil Class not shown on the chart. This Soil Class is classified Class P. This is the worst class in terms of cost to build on. Class P requires a Structural Engineer to design the foundations, and often the concrete slab in theses cases is being supported by what are called Piles rather than just the soil. Class P soil is usually unstable, and this can be because of 'Fill' on the site, or the requirement to remove soil and fill in cases such as sites that used to be old petrol stations where the soil will be contaminated with petrol and oils so must be removed before building on.
Another reason why a site might be Class P is if a lot of vegetation / trees has been removed. The soil may technically be another class such as M for instance, but the removal of a lot of vegetation means the soil is now unstable as the vegetation stabilised the soil.

Types of Footings >>>


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