While seemingly innocuous, the soil type could have a dramatic impact on your project. Michael Holmes analyses the various ground conditions and foundation solutions, revealing their cost
A Guide to Soil Types
A good starting point is to call your local authority Building Control department. They should be willing, informally, to give you an idea of the typical soil type in the area you are building, and the sort of foundation that is appropriate.
Usefully, most local authorities produce a fact sheet on typical foundation solutions for different soil types commonly found in the area.
Another useful source of information is the Building Regulations Approved Document A: 2004 which lists seven types of soil plus subsoil conditions and practical field tests to help you identify soil type (download from planningportal.gov.uk).
Rock
Rocks such as limestone, granite, sandstone, shale and hard solid chalk have a high bearing capacity. The rock may simply need to be stripped back and levelled off to build from.
Rock can be impervious, so topsoil is likely to require drainage as it is not possible to build soakaways to dispose of rainwater or surface water. Off-mains drainage options will also be very limited.
Chalk
Strip foundations are commonly used in chalk. Providing the chalk is not too soft, widths of 450mm for low-rise buildings are generally acceptable. The depth of the foundation must be below any frost action (700mm). If the chalk is soft it will need to be excavated until firm chalk is reached.
Chalk soils can be prone to erosion so be wary of hollows or caves.
Gravel and sand
Dry compact gravel, or gravel and sand subsoils are usually adequate for strip foundations. Generally a depth of 700mm is acceptable, as long as the ground has adequate bearing capacity.
If the water table is high (i.e. the gravel is submerged), the bearing capacity is halved, so it’s important to keep the foundations as high as possible. A shallow, reinforced, wide strip foundation may be suitable.
Sand holds together reasonably well when damp, compacted and uniform, but trenches may collapse and so sheet piling is often used to retain the ground in trenches until the concrete is poured.
Clay
The first 900-1,200mm layer of clay is subject to movement due to expansion and shrinkage depending on moisture content, so it is generally necessary to excavate foundations to a depth where the moisture content of the clay remains stable. British Standard 8004 recommends a minimum depth of 1m for foundations But if there are, or were, trees nearby, depths of up to 3m may be necessary.
In clay, prior to concreting the foundations, the trench is often protected from heave by lining it with a compressible layer (e.g. Clayboard).
Firm clay over soft clay
A traditional strip foundation is sometimes acceptable but it is important not to overdig as this may increase the stress on the softer clay beneath. A common solution is to dig wide strip foundations with steel reinforcement — however an engineered foundation may be necessary.
Peat
Peat and loose waterlogged sand are very poor subsoils. If the peat can be stripped back to find suitable load-bearing ground of at least 1.5m depth, strip foundations may be suitable. A reinforced raft foundation will likely be required.
Filled ground
Where ground has previously been excavated and filled, it is generally necessary to dig down to a level beneath the area of the fill.
Sloping sites
Sloping sites require stepped foundations. Guidelines are given in the Building Regulations.
Which Type of Foundation Should You Choose?
A quick guide to the most common types of foundation
Strip Foundation
A continuous strip of concrete supporting load-bearing walls. For a single storey building strip foundations will typically be 450mm wide and at least 200mm deep, and for two storeys 600mm wide and 200mm deep.
Deep Strip Foundations: Deep strip foundations: Where strip foundations need to be at a lower level to reach soil with suitable bearing capacity, a wider, deeper trench can be dug to work in, and the strip foundations dug and poured at a lower level. Walls are then built up to ground level in masonry.
Wide Strip Foundations: Wide strip foundations: Where the soil is soft or of a low load-bearing capacity, wide strip foundations can be used to spread the load over a larger area, reinforced with steel so that the loading per m² is reduced.
Trenchfill Foundation
Due to the high cost of labour, deep strip foundations have largely been replaced by trench fill. Trenches are dug to a depth where the subsoil provides sufficient load-bearing capacity, and the whole trench is filled with concrete. Steel reinforcement may be added in areas close to trees.
Compared to deep strip foundations, trench fill minimises the width of the dig and the labour and materials required for building masonry below ground level, offsetting the cost of the additional concrete.
Raft Foundation
A reinforced concrete raft or mat is used on very weak or expansive soils such as clays or peat. They allow the building to ‘float’ on or in the soil. A raft is used where the soil requires such a large bearing area that wide strip foundations are spread too far, making it more economical to pour one large reinforced concrete slab. A raft is an alternative to piles as it can be less expensive.
Piled Foundation
Short bore pile and beam: Where the ground conditions will not support strip foundations and the depth of trench fill foundations become uneconomic, or ground conditions make them unsuitable, a series of columns (piles) can be bored and cast in-situ, or precast piles driven into place until they reach stronger strata.
Short bore piles are typically 2–3m long and can be reinforced with steel. Each pile is then connected at the top by a precast horizontal beam of reinforced concrete. A suspended reinforced concrete ground floor can then be built using precast components, or cast in situ.
Friction Piles: A similar concept to short bore pile and beam used in situations where there is no suitable bearing stratum at an acceptable depth. Friction piles rely on skin resistance against the soil.
Pad Foundations
Used when isolated loads need to be supported, for instance to support the columns of a steel or post and beam frame house. The load is concentrated on a small area.
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.
Typical Costs for Foundations
NB: The strip footings and trench fill costs both include all work to oversite and slab, so the costs are directly comparable on a like for like basis with the stage a raft foundation takes you to. Trench fill foundations work out slightly cheaper than traditional strip foundations, and are much faster to build.
The cost of deeper strip foundations can rise by 30–40% if the trenches require planking and strutting to support them whilst footings are being constructed. As soon as ground conditions require a dig of around 2m, a reinforced raft becomes a cheaper option.
Just because they are not visible once the house is built, doesn’t mean they are not vitally important. Mark Brinkley explains all about foundations
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.
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.
DIY Surveys
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 an 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.
Simple Foundations
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.
Engineered Foundations
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.
Piled Foundations
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.
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
Raft Foundations
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.
