Foundation Systems And Soil Types

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

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.

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 soak ways 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.
• Gravel and sand – Dry compact gravel, or gravel and sand subsoil’s 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.

Foundation Types

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.