Building on solid ground - Construction foundation essentials

If you have ever sat on a deck chair on the beach you will know that point loads on soft ground results in sinking. This is not really an issue when relaxing on holiday, however if the foundation on your house starts sinking that’s more of a concern!

The obvious solution would be to avoid building where the ground is soft and try to build only on rock to keep your building safe and stable. Although, this is a good practice, normally we choose where we want to build a building for other reasons and the ground condition is given little consideration. So, what can you do if the ground you want to build on isn’t made of rock? Lay foundations, or “footings”.

Foundations, put simply, are man-made “rocks” laid in the ground which give the building’s walls something firm to bear on. As you descend deeper into the ground during excavation, it generally gets firmer, therefore foundations are laid underground on firm ground and take the place of soft ground at shallower depths. In very old structures the foundations would often be rocks imported to the site and laid in trenches or pits before building walls or columns on top of them. By the 1800s these rocks were replaced by large brick footings that were often laid under the wall and extended out with the building’s perimeter (these portions of the brick foundation were often called “toes” or “footings” because of their resemblance to ankles and feet in cross-section).

Today, foundations are almost always made with concrete or reinforced concrete (a steel bar encased in concrete). Its widespread use is due to the speed of construction, the ability to be formed in any shape required, its enormous strength under compression, and high tensile strength when reinforced. Concrete is made from three materials (with additives often added to change its properties); cement, water and aggregate with the latter being adjusted for different applications but generally it is a mix of 10-40mm stone and grit/sand. The concrete is poured in as a thick liquid which then cures into a rock-like product, this typically takes between 14 to 28 days depending on the ratio of the mix (higher cement content will allow the concrete to cure faster).

Foundations come in all shapes and sizes depending on the application. For example, garden wall “footings” might be only 300mm deep and 100mm either side of the wall, while large-scale engineering projects (eg. skyscrapers, bridges) will be significantly larger . The sizes can run into the tens of thousands of cubic meters of concrete, for example the Burj Khalifa had 45,000m3 of concrete in its foundations alone!