Foundations need to be dug according to a predetermined plan, and to have been accurately surveyed and set out. It is surprising just how often this is not done, and occasionally the ramifications can be very serious indeed, as completed houses turn out to be in the wrong place and subsequently have to be demolished.
Your plans should identify all the load-bearing walls and the width of the trenches to be excavated. The depth of excavation is harder to predetermine and this is routinely decided by the building inspector on site. This is where things can get a little bit tricky, because if you have a difficult site, the foundation trenches may have to go down two metres, sometimes even more, below ground, which is expensive and potentially dangerous.
What your building inspector or warranty provider is looking for is principally a good bearing on solid ground. However, you can never be certain just what lies beneath the ground until it’s opened up. This has led to professionals becoming more and more cautious about foundations and specifying loads more concrete or, increasingly, engineered or piled foundations.
Typical foundation work
|1 Digging out the trench to a level agreed with the building inspector on site before.|
|2 Concrete is laid to a depth of at least 150mm.|
|3 Blocks are laid up to finished floor level.|
Ground Surveys: Official vs Unofficial
Today, many people recommend that you undertake a professional ground survey before you start work. Trial holes are dug around the site so that a view can be taken on the best means of placing the foundations.
The professional you need for this work is a structural engineer and in hiring such a person, you are effectively placing the risk for the success of your foundations onto them, or more particularly, their insurance policy. Consequently, engineers tend to be ultra conservative in their assessments and recommend engineered solutions, such as rafts or, more likely, piling.
The problem here is that such foundation systems are very much more expensive than the more traditional methods. Instead of costing around £60/m², the cost spirals to over £100/m², and sometimes rather more than this.
Whilst not suggesting in any way that you should take on the risk of designing your own foundations, it is worth making an assessment of your site and the likelihood of it encountering problems. Things that engineers are typically looking for are the presence of large trees (easy to spot!), boggy ground and clay soils, all of which are observable without requiring trial holes.
Conversations with neighbours and local builders may well reveal a lot of background information that will help you form a picture of the chances of your site requiring specialist foundation work. A key person to seek advice from is the local building inspector.
Risk and Cost Control
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.
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.
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 foundation costs. Just above ground level, the footings are topped with a damp-proof course and then the ground floor is fixed.
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 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.
As the name suggests, a concrete raft is designed to ‘float’ on the ground beneath. The structure is made up of an extra-thick floor slab, strengthened by masses on steel reinforcing. Rafts have the advantage of providing the base of a ground floor solution, not just wall trenching, but they are reckoned to be rather more complex to construct.
There are hybrid systems available, such as Abbey Pynford’s Housedeck (BELOW), which combines a piled foundation with an insulated concrete floor slab.
Which Type of Foundation Should I Choose?
The decision to opt for engineered foundations is not always an easy one to make. If the ground conditions look like they may be difficult, it is a good idea to approach two or three specialist contractors to see what they suggest, and to get them to quote. Weather can also be an issue — ideally do this in the dry.
A Hybrid Foundation Construction
1. Stainless steel angle over piles 2. Steel is covered in concrete 3. Concrete set on finished slab 4. Follow-on trades can start
Abbey Pynford’s Housedeck is a hybrid foundation system that mixes the principles of piled and raft foundations, and also incorporates insulation and eliminates the need for a damp-proof course. It is less likely to be affected by bad weather than other foundation systems, and problems associated with excavations, unstable ground and ground water are virtually eliminated.
Follow-on trades are able to commence work immediately on a clean surface, significantly improving build time. It is also flexible in that any shape can be constructed, and allows you to work closer to trees than with other systems.
Green Foundation 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.
Foundation Methods: How They Work
- Raft foundations are thick, reinforced floor slabs, strong enough to build the house walls off.
- The ‘trenchfill’ method is the most common- it’s quick and easy but uses more concrete than strip foundations.
- The traditional method involves pouring concrete to a depth of no less than 150mm and then building up to floor level with bricks and blocks.