A foundation is a structure upon which a building sits that transfers loads from the weight from walls and columns via footings to the earth.
This structure on the soil is the footing and the soil under the footing is the foundation. The footing has to be designed to spread the building load over the soil foundation material and is sized to suit the strength of the soil under. Footings are designed in consultation with a structural engineer and will vary depending upon a number of factors:
- Weight of building
- Wall construction type and height
- Soil type
- Slope of the block
- Budget
- Drainage requirements on the block
There are five main types of footings:
Strip Footings
A Strip Footing is a relatively small strip of concrete placed into a trench and reinforced with steel. The footing supports the load of the exterior walls and any interior wall that is load bearing or supports a slab such as for a bathroom. Strip footings can be used for both traditional timber and concrete floors. They are one of the most common footings used in Australia.
Concrete Pad Footings
A concrete pad footing is the simplest and cost-effective footing used for the vertical support and the transfer of building loads to the ground. These footings are “isolated” i.e. there is no connection between them. They are also reinforced.
Holes are dug into the ground and fitted with a reinforcement cage then filed in with a concrete mix to ground level. Concrete pad footings are used to support lightweight timber-framed houses.
Pole Construction (Post and Concrete)
For this type of footing, a hole is dug into the ground. A pole is then placed into the hole and ready-mixed concrete is backfilled around the pole. Pole construction footings are one of the least expensive footings types, especially when constructing a pier/footing on sloping land but engineer’s details will be required for the builder and certifying authorities.
Grout Injected Piles
Where it is impossible for a footing to be constructed, a pile which is both pier and footing is used. This method is only used in unstable or potentially unstable soils such as mud flat estuary areas and beachfront. Grout injected piles are “isolated” footings and/or piers, which are cement grouted (not concrete) and steel reinforced.
The piers are installed by inserting a cork like a screw (Metal Auger) attached to a Backhoe into the ground. The Auger screws all the dirt out of the pier hole. Once all the dirt is removed the grout is injected through the end of the rotating Auger into the hole. As the hole fills with grout the rotating Auger is slowly removed ensuring no dirt collapses back into the hole. The Auger machine drills out the pier holes with minimal disturbance to adjoining soil and structures.
Timber Piles
Timbers piles are a more cost-effective method of constructing structural piles. Timber piles are long timber poles that are hammered deep into the ground by a pile driving rig (big hammer). The piles are driven into the ground their full length or until the pile hits bedrock. If the pile hits a floating boulder it will skew in the ground but the pile will be amply stable to support a floor structure. Pile driving vibration can disturb adjacent buildings, resulting in cracking, failure and even collapse.
QBCC Footings Failure Information
Wikipedia Entry for Shallow Foundation
*Disclaimer: We do not provide professional advice nor services related to any infrastructure planning and building.
Please refer to your local city council for more information.
