The Importance Of Footings
When it comes to building, getting the design of the foundations right from the beginning is arguably one of the most critical aspects to your design because it can either make or break your building.
A strong foundation is required for a building to stand and survive for generations to come. At CGI Solutions, we are experts in house footings and it is our business to provide the solid foundation necessary to support structures of all shapes and sizes.
Below we identify the varying foundations we have to work with here in Australia and the different footings we can employ to ensure your structure remains solid and stable.
What Do We Mean By Foundations?
The foundation serves as a base to support the columns and walls and transfer the full load of the structure to the underlying base.
Structural engineers will always want to know what material they are working with at the base in order to determine the foundation design.
Different forms present different options and challenges.
A building foundation is first made by digging a trench into the ground. You will need to dig deep to reach the subsoil. Subsoil is very important to the structural integrity of your building and is arguably more important than topsoil.
Different founding materials will behave differently depending on your loadings and conditions, so you need to know what the sub-soil structure is and test the whole site.
Founding Materials
For example, in Australia we find that clay is often a material we have to work with as a base on house footings. Clay has a high bearing capacity, although once it gets wet, it will swell up.
The silt is sticky and softer. It expands and contracts and as a result has a poor bearing capacity. For this reason, it is probably the worst base material to work with.
Sand and gravel make for fairly course material. The challenge here is that as your bear down soil particles may move away.
As you would probably imagine, rock is the best material that you can get because it is really firm and solid. It is stable, even with moisture and has a high bearing capacity.
The different geological properties of rock mean that depending on the type of rock you are working with they will have a varying degree of strength. Sandstone – very common in Australia – is a fairly soft sedimentary rock compared to the hard Igneous basalt. So you really do have to know what rock type you are working with as the approach to the foundation and structural design will vary. That said, founding on any type of rock is good news as it has the highest baring capacity and will be less expensive to develop on.
Shallow Or Deep Foundation Systems
Shallow Foundations:
Shallow foundations normally range from 1 meter to 3 meters deep from the natural grade level. They are more cost effective than deep foundations since a lesser volume of excavation is required.
In principle, there are three different types of footing options available to the shallow foundation.
What Do We Mean By Footings?
A footing is a foundation that is constructed under the base of a wall or column. The purpose is to distribute the weight of a building over a large area.
Footing is placed directly below the lowest part of the structure it supports.
Pad/Isolated footing system
This is one of the most common footing types used in Australia.
It is usually shaped as a square, rectangular or even circular slab of concrete that is used to support an individual column.
Perfect for supporting light structures such as residential and medium story buildings.
Pad footing in particular is an isolated pad with a point.
Pad footing will add weight to the soil, so you need to consider the load over, but also the self-load of the footing system itself when employing this footing design.
Strip/Spread footing
Strip footing or spread foundation is used to provide a continuous level to a wall or structure. Essentially, it is a beam that is cast in thick ground and transfers the load to the ground in a strip or linear format.
Strip footings are commonly used as the foundations to load bearing walls. This continuous strip provides a base for walls. Ordinarily, the size and position of the strip are related to the overall width of the wall built on top and must be three times the width of the wall.
The concept is that the load is transmitted at 45 degrees from the base of the wall to the soil.
Most of the time we will use concrete to provide the foundation and reinforced concrete is particularly effective.
Because it is commonly used as a shallow foundation system, it’s important that the soil itself should have a good loading capacity.
For this reason, strip or spread footing is often used for low-rise to medium-rise residential buildings.
Raft Footing
A raft foundation is mainly used in weak soil areas. It is good because you can modify the raft based on the soil type that you have. It also means that there are minimal to no footings internally. These footings are easy to build and bear on the soil below it.
Raft systems are often employed in bigger structures.
A slab might be integrated into the design and some structural engineers will recommend using waffle pods or stiffened rafts to balance differential movement.
Deep Foundations:
Generally speaking, if you have really poor founding material, you will need to go down deeper.
Another reason to dig a little deeper is because your loadings are too high for a shallow foundation.
The cost implication is significantly higher with a deep foundation.
Screw piles
Screw piles are typically made out of high-strength steel and used to screw as deep down into the foundation of the structure as possible.
These monster ground screws also go by the name of helical piers and are giant galvinised steel shafts that are designed to be loaded as soon as they are installed. So the process can be fairly quick.
They work in a variety of soil conditions and will last a very long time.
You will need specialised machinery to install them and it can be expensive.
Driven piles
Very similar to screw piles are driven piles.
Designed to accommodate bigger structures, it is a bigger system and requires a big hammering motion to hammer down into the bedrock.
Subsequently, driven piles can be very noisy to install and may have an impact on adjacent structures.
Existing residents may well notice the reverberations and noise.
These make for good residential slabs and footings when construction is required over several stories.
To Conclude
It is possible to build using a combination of different footings.
However, it is important to note that only in rare circumstances should you want to mix a shallow foundation with a deep foundation.
Footing size and type will vary based on the overall bearing capacity and varied height to which you want to build.
If you would like to know more about the right footings and foundation for your property, please feel free to contact us at CGI solutions and we will be happy to talk through your options and take a look at the site.