Groundworks & Foundations: Foundations

ABOVE:Trenchfill foundations are one of the most commonly used systems for new houses. Trenches are dug and then filled with concrete

Simple

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.

ABOVE: Piled foundations were the perfect solution for this waterside home on the Thames, as they allow the house to move slightly and water to travel underneath should any flooding occur... Read more about this self build log home

Rafts

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 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. (abbeypynford.co.uk)

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.

Foundation Types

A quick guide to the most common types of foundation

Strip Foundation

A shallow version of a typical strip foundation; the wide strip footing is the slab beneath the concrete pillar

Trenchfill Foundation

A rough foundation constructed by filling an excavated trench with concrete, great for shallow foundations

Raft Foundation

A continuous reinforced concrete slab which supports the load over its whole surface

Piled Foundation

Groups of piles are driven into the ground, then connected by a large concrete ‘pile cap’. Great for deep foundations

How They Work

In basic terms, the purpose of a foundation is to distribute the weight to be carried over a sufficient surface as to prevent the subsoil from spreading and avoid an unequal settlement of the structure. This particular example is of a concrete strip foundation. The strip footing must be considerably wider than the wall it supports in order to be structurally sound. The depth of foundations varies with the character of the subsoil, but any brick wall below ground, such as this wall base, should be built with cement mortar. A drainage membrane prevents the intrusion of water onto the foundation wall. A suspended slab is supported at the wall base over the hardcore. A subsoil drain is laid beneath the ground in gravel to dry out damp soil and lead seepage water away from the foundations to a public drain.

Further reading:

• Groundworks & Foundations: Excavations
• Groundworks & Foundations: Ground Floors
• Groundworks & Foundations: Drainage
• Groundworks & Foundations: Rainwater Management

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