Foundation options

Your bespoke home will have unique requirements, but so will the ground you’re building on. Tim Doherty looks at the different foundation solutions

Tim Doherty helps find the best solution for your plot

Foundations spread the weight of a building over the ground to maintain stability and prevent unexpected movement in the superstructure. They are generally wider than the ground wall thickness and anchored deep down where the subsoils are more compressed and have a higher bearing capacity.

Key principles

Site levels, vegetation, sub-soil and building’s weight need to be considered to design the appropriate foundations. It is commonplace for contractors to include a standard type and depth in their quotations and to worry about the actual depths when the job starts. However, prudent self builders will evaluate site variables in advance to understand the most suitable foundation before work begins.

Remember that the foundations need to carry the load of the structure itself, as well as you, your furniture and the weather (ie snow on the roof). The weight of your building is important, but the bearing capacity of the subsoils is, too. Different makeups have different capabilities. With soft silty or sandy clay, you’re likely to need a reinforced and engineered solution. Compact sandy and granular soils should support most houses at modest depths and average widths, with a safe bearing capacity of circa 100kn/m² (a measure of force used to calculate loads). Firm and stiff sandy clays are likely to call for extra depth and/or width, as their bearing capacity might be reduced to 70 or 80kn/m². Rock at shallow depths often means the base of the external wall will need no thickening.

Vegetation on site will also have a bearing, with nearby trees influencing both foundation type and depth (see box, right). If you’re demolishing a building to start again, it’s unlikely that any existing foundations will be used. Consequently, these will need to be removed, which can churn up the site, affect sub-soil compaction and make it more difficult to excavate neat trenches.

Calculating building weight

Establishing the structural load produces what are called line loads, which enables the designer to match bearing capacity of subsoils to foundation widths. Let’s assume a two storey house with a ground bearing slab and a pitched roof. Your structural engineer needs to calculate the dead load of a 1m length of wall, intermediate floor and roof, which is the weight of the materials measured in kg. This will include the anticipated volume of foundation concrete,

any substructure masonry, the superstructure brickwork, blockwork and insulation, half the intermediate floor weight (joists, chipboard and plasterboard) and half the roof structure and covering (half because the weight will be spread over the foundations on both sides). Design changes that add extra weight include steeper pitched roofs, extra storeys and suspended ground floors.

If you’re interested in the science of it, here’s an example of how this might look. It’s probable that you will end up with a total weight of somewhere between 2,800 and 3,500kg per linear metre; for now we’ll call it 3,000kg (three tonnes). This needs to be converted into newtons through multiplying by 10 (the actual figure is 9.81, but we’re rounding up here) and dividing by 1,000 to give us

kn (kilonewtons). So our building dead load (the measure of just the building without anything inside it) per m as a measure of force is 30kn. We also need to add the loads of people standing on the intermediate floor and the potential for snow to rest on our roof, both of which are a standard domestic design allowance of 1.5kn/m² multiplied by their lengths. The floor is likely to be 2.5m and the pitched roof 3m to its centre point which, at the 1.5kn/m² rate, adds another 8.25kn. This brings our total line loads to 38.25kn per m run of foundation trench.

If we have good granular soils with a bearing capacity of 100kn/m², our foundation width could be around 400mm. If the soil was semi-firm clay with a bearing capacity of 70kn/m², the foundation width would need to be 550mm.

Strip foundations

For many builds, strip foundations are the most effective solution. Trenches are dug to a specific width (commonly 450mm or 600mm as these are standard bucket sizes for diggers) and usually to a depth 1m. The excavation must

reach a point where the soil is hard enough. After the building inspector has approved the base, a predetermined depth (say 200-250mm) of concrete is poured into the excavation to form a strip at the bottom. Sub-structure foundation blockwork (or brickwork) is built on top of the concrete strip up to DPC (damp-proof course) level, usually at the same thickness as the external wall itself. The cavity is densely filled and the two skins effectively act as one monolithic structure on top of the concrete and underneath the superstructure wall.

It’s still a popular choice in Scotland, but less so in England and Wales, with one of the drawbacks being the difficulty in laying blockwork in a deep, narrow trench. But it should be cost effective as there is less material going in the ground and less excavation to cart away from site.

Some engineers might want to strengthen the base concrete by adding some reinforcement. This will improve its resistance to ground tension, which can arise from slight movement in the sub soils.

Trench fill or deep strip

A common solution because of ease of installation, the same trench as above is excavated but instead of only installing a strip of concrete at the excavation base, the whole trench is mass filled with concrete up to, or close to, the top. You pay more in material costs for muck away and concrete, but it’s quick, so you’ll making savings in terms of labour. There is still some below DPC brick and blockwork to set out but this is easier to install.

Most houses need foundations between 1m-2m deep, and trench fill can easily cope with that, so it’s often a good option if you’re unsure what the conditions will be. The maximum depth strip foundations and trench fills can go to is 2.5m to 3m, beyond which the costs become quite considerable and other engineered options might be more sensible.

Engineered raft

If your soil is mainly quite soft or there could be some voids in the ground, overall loads will need to be spread further and wider. In these cases a raft foundation might be the best solution. This is a concrete slab reinforced with steel that acts as foundations and floor slab. This is precision work, so the engineer will specify the width and depth of the foundation elements, the thickness of the slab and the size, diameter and frequency of reinforcing bars and mesh.

The contractor prepares the site by digging down to the required level, which will be more under load bearing walls. Compacted hardcore will be positioned under the raft and a layer of sand (blinding) then put over to create a level platform, followed by a damp proof membrane (DPM).

Around the perimeters, a temporary formwork is built up to contain the concrete pour. This could be plywood or metal with stakes, braces and rams to hold it in position. The steel reinforcement is then put in, which is held up off the ground by spacers and tied together to create the right positioning for the thickness of the slab. This is difficult work and requires accuracy to ensure all reinforcement has the sufficient concrete cover and that the bars are spaced correctly and in exactly the right depth of the slab.

Once complete, the concrete is poured and after 28 days will have cured. Thereafter the external walls are positioned directly on the raft at the perimeter and internal loadbearing walls through the interior as necessary.

Piles

Some sites are too unstable even for engineered rafts and instead piles would be required. These are designed to travel through (or certainly into) the superficial deposits (the top layers of soil that aren’t good enough to support the building load) until you hit something stable enough. Piling is generally thought to be expensive, and much of it is, but there are many different types. Cost comes down to installation complexity, the size of equipment required and the ultimate depth of each pile.

The first decision is to choose whether the piles are to be driven or bored. Driven piles are pushed into the ground, displacing and compacting the soils. Bored piles see the soil excavated by the drilling corkscrew, leaving a hole behind. This is filled with concrete and, depending on design, could also be reinforced with a steel cage driven into the wet concrete. The right solution for your project is usually selected by the engineer based upon site location, soil type, acceptable noise parameters and strength.

The second is whether the piles need to be end bearing, where the load bearing capacity is at the toe of the pile. Land friction might be enough with the combined surface area of all the piles sufficient to anchor the building. Alternatively, where hard strata is at an easy depth of say 5-6m, end bearing piles might be preferable. Usually the piles are joined together via concrete caps, which provide more surface area to put on a ring beam or structural slab. Mini piles are also available for sites with difficult access, wound into the soil by a tractor rig. These are often used for small extensions or lighter weight buildings.