Knowing the soil type found on your building plot with undoubtedly inform your choice of foundation system, but while many areas of a building project can be planned and prepared for in advance, your planned approach for your foundations may need to change once you break ground.
This uncertainty means that a contingency fund is vital to ensure that you can deal with any unforeseen issues without the whole project grinding to a halt.
Here we explain the soil types you may encounter on your site and the foundation systems you can choose from.
- hard solid chalk
These rocks 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.
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.
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 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.
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 require stepped foundations. Guidelines are given in the Building Regulations.
Identifying Soil Types
If you don’t already know the soil type of your plot, a good starting point is to call your local authority Building Control department. They can give you an idea of the typical soil type in the area you are building, and the sort of foundation that is appropriate.
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.
Soil investigations can prove very useful but are not a prerequisite. Most sites start without a formal soil investigation, relying instead on either the knowledge of the designer, or the local expertise of the building inspector.
The process involves holes being dug at various points on the site and extrapolating the findings in each hole to assume the subsoil conditions throughout the site.
What are Strip Foundations?
The standard routine is to place as little solid concrete as possible into trenches 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.
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: 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: 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.
What are Trenchfill Foundations?
A widely used alternative is the trenchfill foundation, where the trenches are filled with ready-mixed concrete to just below ground level. Steel reinforcement may be added in areas close to trees. While this method saves on labour, it will add to the overall cost of your foundations. Just above ground level, the footings are topped with a damp-proof course and then the ground floor is fixed.
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.
If you have a difficult plot where the ground is deemed to be difficult, then standard strip or trenchfill foundations are unlikely to suit. There are alternative options, but they are considerably more expensive.
Digging deeper trenches and filling with more concrete, and potentially adding sheets of polystyrene beside the trenches to act as a slip membrane can be a simple solution.
But if you are having to dig beyond 2.5m in depth, then this solution becomes impractical. Not only will the amount of concrete needed to fill the trench become prohibitively expensive, but working at that depth can prove dangerous.
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.
(MORE: Foundations for difficult sites)
What are Raft Foundations?
A raft is an alternative to piled foundations as it can be less expensive
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.
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.
What are Piled Foundations?
Short bore piles are typically 2–3m long and can be reinforced with steel
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.
Short bore pile and beam: 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.
What are 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.
For a how-to guide on getting started with your foundations, check out our Beginner’s Guide.