Understanding foundation systems and soil types is key for anyone looking to build or extend their home. The cost of building foundations is one of the biggest variables, so anyone who wants to keep control of their budget and schedule needs to work out the most appropriate foundation system to use as early as possible.
Identifying the most cost-effective solution will depend largely on the ground conditions on site, including the type of soil, aggregate or rock and its key constituents.
The best way to achieve this prior to starting work on site is to instruct a geotechnical study which involves digging or boring trial holes around the site of a proposed new building, principally to establish the load bearing capacity of the soil at differing depths.
Samples are collected and tested for plasticity (shrinkability), pH levels, sulphates, moisture contact (seasonal water table) and other factors that will allow the foundation solution best suited to the site to be determined.
Neglecting to investigate the ground conditions on your site is one of the earliest and costly mistakes a self builder can make.
What Soil Types Might I Find on my Plot?
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A geotechnical study will reveal the type of ground on your site and will most likely be one of, or a mixture of the following:
Limestone, granite, sandstone, shale and hard solid chalk are all rocks that have a high bearing capacity. The rock may simply need to be stripped back and levelled off to build from.
Providing the chalk is not too soft, foundation widths of 450mm for low-rise buildings are generally acceptable. The depth of the foundation must be below any frost action, a minimum of 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 the possible presence 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 (for instance if the gravel is submerged), the bearing capacity will be 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.
There are three types of clay in the UK, classified by their plasticity, which is how much their volume can change due to their water content.
- Clays with the highest plasticity (and so the highest risk) are generally found in the South East of England, stretching up through the East Midlands to the Humber in the North and down to Bath in the West
- Medium plasticity clays are found in the rest of the South East, across the Midlands and up beyond the Humber Estuary towards the North East. They also turn up in some isolated areas in the North West of England near the coast
- The rest of England and Wales generally have low plasticity clays but even these still carry some risk.
The first 900-1,200mm layer of clay is subject to movement due to expansion and shrinkage from seasonal variation in moisture content, so it is generally necessary to excavate foundations to a depth where the amount of moisture present remains stable.
Strip, trench fill or pad foundations must be cast at a minimum depth of 750mm in low plasticity clays, 900mm depth in medium, and British Standard 8004 recommends a minimum depth of 1m for foundations in the highest risk areas. If there are, or were, trees nearby, depths of up to 3m may be necessary. The depth necessary will depend on the type of tree, as species have different water demands, with tall broad-leafed species such as poplars having the greatest impact.
Where there are or have been trees (removing trees does not remove the risk of expanding clay) engineers may recommend reinforcing foundations with embedded steel. Foundations in clay soils can also be protected to some extent from damage due to expansion (heave) by lining the trenches with compressible material such as clayboard.
Firm clay over soft clay
In cases where clay becomes softer as you dig down, a traditional strip foundation is sometimes acceptable, but it is important not to over dig as this may increase the stress on the softer clay beneath. A common solution is to dig wide strip foundations with embedded steel reinforcement, however an engineered foundation such as a reinforced raft or piled foundation may be necessary in some cases.
Peat and loose waterlogged sand are very poor subsoils. If stripping back the peat can uncover suitable loadbearing ground of at least 1.5m depth, strip foundations may be suitable. Otherwise a reinforced raft foundation will likely prove to be the most economical solution.
Where ground has previously been excavated and filled, it is generally necessary to dig down to a level beneath the area of the fill. It is important therefore to find out the depth of ‘made up ground’. As with all previously developed sites, it is vital to check for possible contamination before disturbing any material.
Sloping sites require stepped foundations. Guidelines are given in the Building Regulations.
Do I Need a Soil Survey?
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.
Strip and Trenchfill Foundations
An engineer’s design based on calculations of the building’s loads and the loadbearing capacity of the ground will indicate the foundation solution to be used. Excess spoil for all foundations is usually carted away to landfill.
The standard foundation solution in most parts of the UK is the strip foundation, also known as strip footings.
The topsoil is scraped back (and usually stored for reuse) and trenches dug down to a depth at least 450mm, which is enough to put the foundations below frost action. The trenches are then filled with concrete to a minimum 150mm below the surface ground level.
The walls are then built up to just below finished ground level in masonry (concrete blockwork or engineering bricks) with the outer leaf switching to the selected external facing material for the walls or plinth, typically brick or stone, just below ground level.
Approved Document A of the Building Regulations (England and Wales) defines the minimum widths for strip footings based on the type of ground and loadbearing wall.
As such, there are several ‘types’ of strip foundation:
Where strip foundations need to be at a lower level to reach soil with suitable loadbearing capacity, a wider trench can be dug to work within, and the strip foundations dug and poured to the required width starting at this lower level. Masonry walls can then be built up to just below ground level before the trenches are backfilled in layers to the finished ground level using clean subsoil or other material as specified.
Where the soil is soft or of a low loadbearing capacity, wide strip foundations can be used to spread the load over a larger area, reinforced with steel so that the loading per square metre is reduced.
Where it is not possible to construct conventional strip footings because the new building is hard up against an existing structure, or access to the adjoining land is not available, it may be possible to use an offset strip foundation rather than a more expensive piled foundation or a reinforced raft foundation. Typically 750mm wide and 450mm deep with a layer of A193 mesh placed in the bottom of the concrete with at least 50mm cover to the steel. The solution is generally suitable for single-storey structures.
A widely used alternative to strip footings is the trenchfill foundation, where the trenches are filled with concrete (typically ready-mix concrete for speed) to a depth just below ground level. This allows the first course of the external facing material (typically brick or stone) to be laid up to damp proof course. This is a quick solution compared to laying masonry but is generally more expensive due to the amount of concrete required to fill the full width of the trench. Steel reinforcement may be added in areas close to trees.
Engineered Foundation Systems
Where the required depth of foundations is more than 2.5m it becomes impractical to use conventional strip or trenchfill foundations unless a basement storey is planned. In these cases it is more cost-effective to look at alternatives such as a concrete raft or piled foundations.
A raft foundation is a reinforced concrete slab cast over a compacted hardcore sub-base that spreads the load of the building over a larger area of ground to overcome ground conditions with a lower load-bearing capacity.
Raft foundations are usually designed by a structural engineer, taking into account the ground conditions assessed in a geotechnical survey or at least inspection of trial holes by the engineer.
The raft design usually has an ‘edge beam’ around its perimeter and under any areas carrying large point loads. This consists of a cage of steel reinforcement that will need to be carefully assembled on site. Internal stiffening beams are sometimes needed too. These beams transfer the building loads through the rest of the slab and then evenly over the ground.
(MORE: Foundations for difficult sites)
This solution is used to support the point loads of buildings constructed using a steel frame or a timber post and beam frame. Pads of concrete, usually cast in situ, are placed beneath the position of each post of the frame and the posts are linked together at ground floor level to spread the load evenly. Pad spacings, size and depth are designed according to the design load of the building and the ground conditions.
Pad foundations can be well suited to sites where excavation needs to be kept to a minimum and can also be a cost-effective solution when overcoming a sloping site by suspending the ground floor on a post and beam frame.
Where ground conditions are poor, variable or unpredictable, piled foundations are usually the solution. There are essentially three types of pile: those that are cast in situ in holes that are bored or augered; those that are precast and driven into the ground by a piling rig; and those that are cast in situ within a steel tubular mould or ‘shell’ driven into the ground. Piles support the building load by transferring it down onto lower strata of soil or rock, or through friction with the ground around them, or by a combination of both ground support and friction.
A reinforced suspended concrete floor slab, or a reinforced concrete ring beam, is then cast over the top of the piles, linking them all together so that the load of the
building is spread evenly. For a post and beam structure, the piles or groups of piles are capped with a concrete pad.
Screw Pile Foundations
Local planning authorities will often allow structures to be built around trees within a root protection area on the proviso that the structures are supported by screw piles.
These are a relatively new foundation solution consisting of slender, hollow steel shafts with a small number of steel helices (or screw threads) welded to them. The piles are screwed into the ground until they achieve sufficient friction to support the required load.
This can be a cost-effective way of building on sloping ground too, as the steel piles can be left above ground and linked together with tension wires or rods, and topped with a steel ringbeam or grillage to build from.
Factors to Consider
What Might Affect my Choice of Foundation System?
Where the foundations are affected by tree roots (or their previous removal), you may be required to employ a fairly deep trench filled with concrete but with a compressible material to one or both sides of the external trenches to counteract any heave or expansion in the ground.
Water pipes must enter the building at a depth of at least 750mm but no more than 1.35m below ground. If that means that they pass through a concrete foundation then they must either be laid prior to pouring or, better still, a duct installed for them to be pushed through later.
If sewage pipes leaving the building have to be deeper than the top of the foundation concrete then they should also be ducted; they cannot be trapped within the concrete and must be able to move freely.
Electricity and gas don’t usually need to be ducted or installed at this point as they are normally surface mounted. Finally, the building and warranty inspectors will have to approve the excavated foundations prior to any concrete being poured.
(MORE: How to bring electricity to site)
Before You Purchase Your Plot
What If I Haven't Yet Purchased My Plot?
If you haven’t yet purchased your plot, take a look at this building plot checklist to make sure you exercise due diligence regarding various factors (including soil type) as best you can before committing to a purchase.
You can either check with your local authority or building inspector, or undertake a soil investigation. A soil investigation could cost as little as £500, but would flag any major issues before you start, which could save you £1,000s.
It is always a good idea to allocate at least 10% of your budget for a contingency fund should you encounter any unforeseen issues with your soil type.
Michael is HB&R’s Head of Content and Product Development. Michael is also, Chair of the National Custom and Self Build Association (NaCSBA), presenter of multiple property TV shows and author of Renovating for Profit (Ebury). Michael is a regular in the seminar theatres and Advice Centre at the Homebuilding & Renovating Show.
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