Foundations
77
“You can never entirely eliminate the risk of
cost overruns below ground.”
Risk
Whatever you do, you can never entirely eliminate the risk of cost overruns
below ground. You may find features like wells or mine shafts that lay in your
way. It is, therefore, essential that you hold back some contingency funds for
unforeseen eventualities. The good news is that whilst foundation costs can
sometimes double, they rarely treble: there is an upper limit at which some
solution can usually be found.
Basements
If your ground conditions appear difficult, you might do well to consider
building a basement. If you are expecting to spend, say, €30,000 on getting
out of the ground, then you are maybe halfway towards the cost of a basement
and you may find that you are able to add considerably more value to the
house than the additional cost of a basement build.
Foundation Types
The standard routine is to place as little solid concrete as possible into
trenches (min 250mm) and then to build up from this in blockwork until
ground level, where the walls switch to brick or stone or whatever the chosen
external cladding should be. This is known as a strip foundation. A widely
used alternative is to fill the trenches with ready-mix concrete to just below
ground level (trenchfill) which saves labour but adds to costs. Just above
ground level, the footings are topped with a damp-proof course and then the
ground floor is fixed.
Engineered
If the ground is deemed to be difficult, there are a number of options used —
all of them considerably more expensive than straightforward foundations.
These are referred to as being ‘engineered’ because they usually involve the
skills of a structural engineer in determining what is the best solution for the
site.
The simplest is just to dig the foundation trenches deeper and then fill with
a much greater depth of concrete, sometimes fitting sheets of polystyrene
beside the trenches to act as a slip membrane.
However, there comes a depth (around 2.5m deep) beyond which it
becomes impractical and dangerous to work, and the amount of concrete
needed to fill the space becomes prohibitively expensive. If the site requires
deep foundations in more than a couple of spots, then it is now usual to use a
different approach, most often piling, occasionally using concrete rafts.
Piling
Piling contractors are increasingly being used on housing sites as their
services are gradually becoming more competitive. Some housebuilders
now use piled foundations on every site because the costs are predictable.
Piles are driven into the ground and then filled with concrete, and the whole
foundation gets topped with a ground beam to build off.
Green Options?
Builders looking for sustainable solutions worry about the large amounts
of concrete used in foundations and floors. However, attempts to substitute
other materials, notably lime for cement, have not been conspicuously
successful to date. The most sustainable option would be to return to the way
the Victorians built foundations, which was to build walls up off the ground
itself, but this is now felt to be inadequate for modern construction methods.
Despite decades of experience and millions of homes having been built with
concrete, foundation failures remain a significant problem. Generally, it is
better to err on the side of caution and to use as much concrete as is felt
necessary to prevent any subsequent problems.
Winter 2016 Home & Build
Winter 2016.indd 77
08/01/2016 4:32 p.m.