Foundations text

Foundation Design

There are some new and interesting developments in foundation design which offer an alternative to the way we have been constructing foundations for the last few hundred years. At least so it seems. It would be interesting to look at these new developments in the context of the traditional ways of supporting a building.

Let’s consider what we expect from a foundation. It has to be able to spread the load so that the ground can support the load. It has to be stable so that it will not move around. Sometimes the foundations are used to anchor the building so that it will not overturn – this is particularly true of taller, lightweight structures such as timber frames houses.

A photo of pale stone foundations with a small set of stairs.
Image courtesy of Wiki Commons stock images.

So how do we achieve this? Spreading the load is not difficult except where the soil is very soft, and that is not usually the case in our area. Ensuring that there is no movement is more difficult as our clay in the South East of the UK is prone to shrinkage and heave caused by changes in the moisture content in the clay. For this reason, the minimum founding depth is usually a metre and much deeper if trees are nearby. Holding a building down sometimes has to be considered but by the time you have dealt with the other criteria, this holding down or overturning aspect can be shown to be resolved.

The way we spread the load of the building can be dealt with in a number of ways. The usual way is to dig a trench, fill it full of concrete and then build the load-bearing walls off of this. This is called a trench fill foundation (or footing as builders like to call it) where the concrete almost comes to the surface, or a strip foundation if the trench is only partly filled with concrete and then masonry is built up to the ground level. Sometimes we dig a series of holes which are filled with concrete and then beams span between. These ‘pad foundations’ as we call them require less excavation and soil to be taken from the site, and less concrete, but require additional structural elements above.

The above techniques account for 90% of low rise buildings in the UK whereas for the remaining 10% the solution is usually a piled foundation. Crudely, piles are either driven in or a hole is drilled in the ground and then filled with concrete. The piles will give intermittent support just like the pad foundations mentioned above, and so beams have to be used to span across the top to support the buildings walls. Where the hole in the ground is first created and then filled with concrete it is classed as a replacement pile, and where a steel element is driven into the ground it is called a driven pile. Further sub categorisation is made and they are described as short bored or deep bored piles.

Now this neatly brings us onto the first new innovation in the UK for many years. We now have a worm-screw type of foundation which could be described as a large steel screw and this is screwed into the ground where it becomes the support. It reduces the amount of spoil that has to be removed from site and can be installed in any weather.

Another new type of foundation is that promoted by Advanced Foundation Technology Ltd as advocated by Kevin McLoud of Grand Designs. Basically, this seems to rely on removing some soil and replacing it with a material that will not be affected by freezing conditions. I confess to not understanding how this deals with the shrinkage caused by changes in the moisture content of clay and so I will remain skeptical for now, but clearly in areas where the ground is affected by changes in temperature only, this could be effective.

Clearly the type of foundation your building designer or engineer chooses will be based on individual factors pertaining to your project, and the industry is notoriously conservative for not taking up new ideas but it will be interesting to see how these new ideas are taken up.

Written by Tony Keller – Building Tectonics Ltd

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