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

Screw pile foundations

Screw piles are a type of foundation, which are thought to have been invented in the 1830’s made from wrought iron with a view to supporting lighthouses and piers. Today, we make them using galvanised steel, meaning that the steel has been previously treated so that it will not corrode as quickly, we have also widened the structures in which this technology is used, so they are not exclusively used for lighthouses and piers now.

Put simply, it resembles a pipe with the way it looks, and it has one or more helices attached to it, how many and the spacing between each one is determined during the design stage. Helical is derived from the word ‘helix’ which means spiral, so the helical part of this structure is the spiral which is used to ‘screw’ the pipe into the ground. Each pipe has its helical flights specifically designed to suit individual ground types.

Special machinery is used to wind them into the ground, much like screwing a normal screw into some wood with a drill, but on a much larger scale. This type of foundation has no curing time, and therefore allows builders to begin building onto it as soon as the installation is complete, plus it can be installed in cold temperatures which would help lower any potential downtime. Once the pile has reached the target depth (which again, is determined in the design stage specifically for each project) it will remain permanently in its place, and it is usually filled with concrete.

The bit of the structure which protrudes from the ground provides the connection to the building structure above, and can, in some cases increase the capacity of the pile. This interface between the pile and the building is a very important part of the design, and this requires a lot of care. The connection is designed by considering the building and the ground into which the piles are screwed into. The most common way to connect a screwpile to a structures foundation is to cast the head of the screwpile into the buildings concrete foundations. Fixing them in place is relatively easy, through steel reinforcing bars, which are cast into the piles and protrude out into the footing of the building.

They are very quick and easy to install, plus there is no excavation to cart away which will save you money, especially if there were contaminated ground to take away. They are also good for small areas as there is no set size, and everyone is made specifically, plus they can be removed and re-used at any time.

Written by Jade Turney – Building Tectonics Ltd.

Foundation and extensions.

Foundations and extensions to houses.
There is a lot of confusion about foundations and extensions, especially where the requirement is to build over an existing building. Often the question is whether the existing foundations will support the load of the additional building. There are two very simple things to remember about foundations. 1) The width of a foundation and the nature of the soil below dictate how much load it can take. 2) The depth of a foundation is only important to stop the foundation moving due to fluctuations in the ground below.

In Milton Keynes, where I practice, the clay is usually supportive enough so that even a narrow foundation can support two storeys and therefore the foundation put in to support your single storey building will normally be able to take the load from two storeys. Thus you can often build over without worrying about the foundations.

In respect of the depth of a foundation, if you are building on clay, as in Milton Keynes, you have to take the foundation down to a depth where it will not be effected by seasonal shrinkage, ie dry summers ( remember those) causing the clay to crack and dry, even down to 900 millimeters. This effect is made much worse by the presence of trees where the ground can desiccate / shrink even at depths of 3 meters plus due to the trees sucking up so much moisture. This type of problem will cause cracking in a three storey house or a garden wall alike and thus logically no matter how much the load, if you want to avoid any type of movement you have to go deep on clay. Of course nobody invests a lot of money in a garden wall foundation so no wonder they crack and fall over so often.

It is not the same where building on sand. Sand does not suffer from shrinkage in the same way and so a much shallower foundation may be okay. However the foundation may have to be wider to take even a modest load – it depends on the type of sand.

Therefore most building can be successfully built over, using the existing foundations but, if there is evidence of movement such as cracking then it would be unwise to build over since that will probably crack too. All of the above is a generalization and you need an expert to advise, but you should not be put off exploring extending upwards because of a fear regarding the foundations. Talk to us and we will try to advise on the best strategy.

Written by Tony Keller – Building Tectonics Ltd.