Why do we have to dig such deep foundations?
If you ever get involved in working around the foundations of some pre-20th century buildings, as you might if you were building an extension on to a Victorian house, you will be amazed at how shallow and how basic the existing foundations appear to be compared with what we have to build off now. ‘How come this house is still standing?’ will be your first thought. Followed by ‘If these foundations have supported this house for so long, what am I doing messing around a metre further down below ground?’ Both good questions. Neither has a simple answer.
The history of foundations is vaguely interesting and a little instructive. The current idea of building up off slabs of concrete at depths of a metre or more below ground level is relatively new, very much a 20th century innovation. The Victorians used to step the walls out at the base, pyramid style, over a depth of just three or four brick courses, that way spreading the load of the house across a wider area. These below-ground courses were known as footings and the footings were effectively the foundations. Today we still refer to footings but we put concrete foundations under them. 21st century footings are simply the brick or blockwork sandwiched between the concrete foundations and the damp proof course which is usually installed at 150mm above ground level.
When concrete foundations were first adopted widely, in the 1920s, they were sold to builders as being a cheaper and quicker method of building-in adequate load bearing. In fact, the load imposed by a typical house (which maybe weighs around 100 tonnes in total) really isn’t that great – it’s similar (on a weight per area basis) to that imposed on the ground by your feet when you stand up. The problems arise from the fact that the ground has a nasty habit of shifting about which itself has a nasty habit of causing all manner of disruption on the very rigid structures placed on it. Building foundations out of concrete may sound very hard and solid but when the ground starts moving then they look as feeble and spindly as you could want.
The Building Research Establishment investigated the phenomenon of ground movement in the 1990s and found a site which regularly heaved up and down by 50mm over the course of a year, not to mention a 60mm sideways movement as well. Mostly it’s sites with large trees and clay soils but it’s also rather unpredictable so consequently the momentum is towards deeper and deeper foundations, which means more and more concrete. This usually works but we still have a large number (approx 1,000 a year) new houses that subsequently suffer from foundation failure.
The best advice is to follow the advice you are given as closely as you can – don’t cut corners. Readymix concrete and excavations are moderately expensive but nothing like as expensive as having to fix later failures.
As foundation requirements have grown deeper over the years, there have evolved two options about how to construct them – at least, on largely problem-free sites. One is to pour the minimum amount of concrete possible into the foundation trenches and then build upwards in brick or blockwork – this is known as strip foundations. The other system reverses this logic altogether and pours as much concrete as possible into the trench before starting on the bricklaying – this is usually called trenchfill. Both methods have pros and cons.
The traditional method, laying footings below ground level, is cheap on materials but heavy on labour; it is also slower. In contrast, trenchfill is much quicker but, of course, uses more concrete. When pouring concrete into clay soils, you will probably have to use the trenchfill method.
An important consideration when designing foundations is to set the level of the concrete so that it ‘works blocks.’ Blocks are laid either flat in courses of 110mm or in the vertical in courses of 225mm (ish). If you have surveyed the site accurately, you will know exactly where the damp proof course will be so you can ensure that you stop the concrete at 225, 450 or 675mm below that point. That ensures that you keep the fiddling about and cutting to a minimum. – provided, of course, that you pour your foundations level!