- Strip or trenchfill – a 600mm wide perimeter of concrete around the base of the property a metre or so underground.
- Raft – A slab of concrete sitting on the ground and knitted together with embedded steel.
- Piles – Posts cut very deep and filled with steel and concrete. The gap between the piles is bridged with a steel frame.
Let’s say a house weighs in at around 50 tonnes (that’s 50,000 kg and not a bad guess). Let’s say the foundation strip has an area of 50 square metres. That’s 1 tonne per square metre.
The more foundation area you have, the less weight it carries per square metre. This is very important on soft ground, not so on rocky ground.
Ground tends to change in unpredictable ways as you dig down. The top level (topsoil) is generally soft, compressible and organic. If you dig a hole to fifty feet in different regions of the country you will get different results.
In London, for example you may get 500mm of topsoil and then 50m of clay. In Wiltshire you may get a layer of topsoil followed by 1m of clay followed by 1m of chalk followed by flint.
As a rule you try to build on firm ground. You can do this by digging a trench down until you get some.
If you know through investigation that the firm ground is 10m down then you will decide to drive piles into the ground, as a trench would be impractical. If you know the ground is soft and boggy for 500m you will chose to fit a raft and float on the surface.
In most cases however you will simply build a concrete strip foundation. Ultimately, the person who is best trained to decide on approach is a structural engineer.
You commonly will not need to resort to this high level of help. Generally, the man from the council (building control) will know the area and will tell you or your designer what to do.
It’s important to note that the foundation is commonly well below ground, say 1m, even if the ground is firm close to the surface. There are two reasons for this:
The ground tends to be firmer down there.
Clay and frost don’t mix. Clay takes in water, expands, contracts and does all kinds of tricks. The deeper you are, the safer you are from clay movement. Since trees have an effect on clay soil movement, the presence of trees requires much deeper foundations.
Setting a strip foundation
This involves Levelling the area to accommodate the building, digging trenches for foundations, digging service trenches and filling with concrete.
Levelling the area to accommodate the building
Your builder will mark out the approximate position of the building with marker poles. He will also place poles at the extremities of the excavated area to include room for patios, paths, flat lawns, etc.
His excavator machine will use its front appliance (i.e. a blade like the front of a caterpillar or bucket like on a JCB), to push earth into a pile so that you have a good and firm playing field. Topsoil is commonly 300mm deep.
This tends to become firmer and more compact ground below this level (because it’s had weight from above applied to it for a long time).
Generally, firm ground is easy to recognise (as it’s firm!) and the person with the digger has done it enough times to recognise it. Your surveyor will help with this.
Setting out the foundations
At this stage you have created the terrain profile you want and removed topsoil from the area of the building. The next step is to mark out the actual foundation positions and set depth gauges.
Foundations need to be level within about 20mm and need to be aligned within about the same margin i.e. plus/minus 10mm. The reason for being level is that the building is constructed on the foundation and you certainly want this to be level.
It is possible to adjust the level as the foundation is built using thinner or thicker cement beds but this makes for more work so best get it right at concrete level.
The grey line (cement powder) gives the excavator a line to work to.
The corner boards, called profiles, are used to hold string lines that mark wall positions. These positions are worked out using a tape and simple geometry:
A rectangle has equal diagonals.
A right-angled triangle can be formed by sides in the ratio 3:4:5 with 5 being the long sloping side (or hypotenuse).
Check your builder’s diagonals are within about 15mm overall for the building.
Pouring the concrete
Once your trenches are in place (and the building control inspector has verified they are OK), your builder will pour concrete into them. Concrete is usually specified by your designer.
Its strength varies from C10 to C50, with yours likely to be in the range C15 to C30 depending on the application.
You will also be asked to specify ‘slump’. This refers to how wet the concrete is. It generally should arrive in a fairly stiff, hard-to-move state.
Key Tip: Most builders request water to be added on site, which makes the concrete flow around the trenches. This compromises its strength quite dramatically, however, and you are right to object to this practice.
Finished foundations – the top surface is smoothed to provide a good base for building.
Building foundation walls
About 24 hours after the concrete strip is fitted, the bricklayers can build up the foundation walls. They do this with a strong cement mix of around 4:1 sand to cement.
The foundation can be of solid construction with the cavity starting around ground level. Brickies commonly use large foundation blocks to do this as they are easier to lay. These blocks are made from a lightweight aggregate that provides insulation to the floor platform.
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Daniel Peacock