Education Corner: Foundations & Floor Slabs

  • Creation Date: Tuesday, 14 May 2019

How Do I..?

09 March 2019

Edu foundationsYour foundation (also called a strip footing) is not just a trench in the ground filled with concrete. It is the item on which your home will stand or fall. Foundation settlement is the major cause of the vast majority of cracks that occur in a house over time and it is important that it is properly done. It is something that is easy to check on before casting any concrete and it is definitely worth checking.

The first point to note is that the trench should be a minimum of 550mm deep which allows 250mm concrete thickness (200mm minimum) with the top of the foundation an average of 300mm below ground level. The foundation should be a minimum of 600mm wide. Trenches for internal non-loadbearing walls can be 400mm wide. The concrete for foundations normally has a minimum10MPa to 15MPa strength mix.

Things to watch out for when inspecting your foundations are changes in soil type or moisture content over the area of your house. Different soils have different bearing capacities which means that foundations will settle at different rates for different soils, and as a result, so will your house.

Soils which have a high clay content, for instance, are treated differently with regard to foundation design compared with sandy soils. Saturated soils also act very differently to dry soils. The depth to which your foundation is excavated is also dependant on how much topsoil is present on your property. Your footings must be founded on undisturbed virgin ground. Where roots or plant matter are still visible, then the trench must be made deeper to get below that level.

There are situations where it would be prudent to get an engineer to independently check the trenches or foundation design before your builder starts casting the concrete. Doing this may cost you a small consultation fee but if there is a problem, it would be easy to fix at that stage rather than having to deal with it after your house is built which could then become very expensive.

In the photograph, you will note that there is no difficulty with the foundation trench but the contractor has made a lot of extra work for himself by piling the soil from the trench on top of the grass. If decent compaction of the filling under the floor slab is to be achieved then the grass has to be removed as it will eventually decay and the soil above it will settle in time and so will the floor slab.

The casting of your floor slab is also a simple procedure provided that the rules are followed. The biggest problem with concrete surface beds (floor slabs cast on the ground) is that the soil under them is more than often than not, badly filled and poorly compacted which leads to the settlement of the slab in places over time.

It is easy to check on what is being dumped into the space behind the walls of your house once the builder starts filling that area. Only clean compactable sand should be placed in 150mm layers and correctly compacted using water and the correct compaction equipment.

Particular attention should be paid to the space directly against any wall from the foundation up. This is the area that is most sensitive to settlement and is often the area that has most of the site rubble and rubbish dumped in it which is not to be permitted as this causes voids and soft spots all over the place. As with foundations, a minimum 10MPa concrete strength mix is required.

Note that in terms of the Energy Efficiency Regulations, an insulating layer of polystyrene needs to be placed on the inside of the foundation walls and under the floor slab.

There are many factors to consider when designing a foundation, foundation walls and the floor slab but when properly done, will ensure a solid start to your project.