ABSTRACT
The foundations of the house are probably one of the most important parts of the property because without an adequate base the property will quickly become unstable. But the foundations are only as strong as the substrata of ground upon which they are built. Therefore, the substrata underneath the foundations should comply with the following:
(a) Be strong enough to sustain the loads put on it by the building. (If it is not, then a different type of foundation or wider foundation is needed to spread the load.)
(b) Not contain sulphates or other deleterious matter likely to destroy the concrete in the foundations. (Alternatively, the concrete in the foundations must be capable of resisting the effects of the sulphates or other deleterious matter.)
(c) Not be susceptible to the effects of frost action. (Clay can expand if affected by frost. That is why gardeners like to ridge up clay soil in cold weather so it breaks up. If the subsoil is of a type susceptible to frost action then the underside of the concrete foundations must be built sufficiently deep that the soil beneath them will not freeze.)
However, foundation design can present a problem for anyone designing small domestic extensions because, unlike larger projects, you, the designer, are very much on your own. There are no other consultants such as engineers on hand to make soil tests and to advise you about ground conditions in the area. On page 36 of Approved Document A (of the Building Regulations), the
subsoils upon which your foundations are likely to be built are classified into seven types (e.g. rock, clay, sand) and there is also a list of field tests that you can carry out. For instance, silt is defined as ‘fairly easily molded in the fingers and readily excavated’. Well, that’s great, except that even a lay person knows that foundations
are built below ground level. So, what is the point in attempting to assess the subsoil condition from ground level? You could of course visit your client armed with a pickaxe and spade
and start by digging trial holes in their garden (trial holes are pits about
and are but in my experience, most homeowners do not want their builder/surveyor digging up their gardens months in advance of work starting on site, especially as they know that they will have enough disturbance once work actually starts. In any case, even if you dig trial holes, it does not necessarily mean that
they will give the full picture. It is not unknown for the original builders to construct houses on sites that are far from perfect. Whilst doing so, they often disguise problem ground and it is only when a new extension is being built that the defect is exposed. There was a typical case in my office a few years back, when a new
extension was in the process of being built. Everything went according to plan until the builder began digging in the rear garden area and discovered the remains of an old pit. After he’d been digging for a while, he realized that the original house builder had decided that if he filled the pit with demolition rubble and old timber it would save him money. By doing so, he created a problem for the second builder because not only did he have to dig the new footings deeper than normal (normal strip footings cannot be built on filled ground), but he was forced to clear out the buried debris and remove it, which costs money. The fact is, when dealing with a small domestic extension, the best that
you can do is make an assessment of the likely ground conditions, design for those conditions, and advise your client that the foundations by their very nature are subject to review once construction work starts on site. However, you do not have to make your assessment totally blind.Without
a doubt, your client will have done some gardening and if he or she has dug down to any depth will know what type of soil is there. Other people in the street may have had extensions built and may tell you what the ground was like. If you know any local builders or the local BCO they may also be able to advise you. So, ask the following questions:
(a) What is the soil like – is it clay, sand, rock? (b) Did the neighbours have any problems when they built their extension
and how deep did they need to built their foundations?
