What types of foundations are used in UK construction?

Foundations play a key part in a building’s structural integrity. Not only do they support the weight of a building, they also stabilise it against natural forces such as extreme high winds. The ultimate goal is to transfer any loadings on the building to the ground. Soil or Geotechnical Investigations are required to understand the existing ground, its parameters and its limitations. 

Several construction foundation types exist within the UK, each offering their own unique benefits. Here, we explain the key differences between building foundation types and the circumstances in which you should apply them.  

Strip foundations  

Strip foundations are one of the most common house foundation types in the UK. They’re typically used in low-rise constructions, on soil that demonstrates good load-bearing qualities.  

This foundation type is created by pouring a concrete base to a depth of at least 225 millimetres. A foundation wall is then built on top of the cured concrete. If local areas of high loads or point loads are present, pad elements can be incorporated into the strip footings.  

Reinforcement is often utilised to ensure sufficient load spread if necessitated by design.  

Strip foundations can be constructed fairly fast, making them a common choice for projects where budget and time are a consideration, assuming the ground conditions and proposed building loadings are suitable.  

Trench fill foundations 

Trench fill foundations tend to be constructed of mass concrete with the bottom of the concrete taken through the weak upper levels of soil to bear into the firmer strata below. They’re formed by excavating a trench and filling it with concrete. 

Trench foundation widths vary according to ground conditions and applied loads. Typical trench foundation widths range from 450mm to 600mm and above (to suit standard bucket widths on excavators). While trench foundation depths are governed by factors such as ground conditions, the presence of drainage systems, and nearby trees. The effect of trees in clay soils on the depth of foundations will be discussed in a further blog or for further reading refer to NHBC chapter 4.2 . 

Pile foundations 

Pile foundations are formed by driving or boring steel or concrete columns into the ground. The piles effectively work as stilts to support the structure off the much stronger ground at deeper levels. Piles get resistance in 2 ways; end bearing or skin friction.  A slab (or beam elements) span between the piles, helping to support the weight of the structure and mitigate any movement.  

Pile foundations are typically used in areas of weaker ground or soils where the zone of influence requires excavations to be in excess of 2.50m for safety and economic reasons. This is largely because it is difficult to construct other types of foundation at these depths.  

Unlike trench or strip foundations, piled foundations are suitable where the shallower soils do not have good bearing capacities, as well as sites that suffer from surface instability.  

For lightly loaded and low-rise buildings mini-piles or screw piles can be utilised. These are installed with much smaller rigs and sometimes hand tools in the case of screw piles. They are also good when access to a site is very restricted. 

Raft foundations  

Raft foundations consist of a reinforced concrete slab that occupies the same dimensions and shape as the building it’s supporting. They will often have beam thickenings under the loadbearing lines and at the perimeter. They are generally cast at ground level with a ‘boot detail’ to enable the cladding to cloak the edge of the slab. A raft is designed to be sufficiently stiff to enable the slab to move should any ground movement occur, due to this there are limitations on overall size and proportions of rafts.  

Raft foundations are used when it’s necessary to spread the load of a building over a larger area, usually because the bearing pressure of the soil is low. Rafts would be formed using reinforced concrete, the depth of the raft varying subject to ground conditions and applied loads. They are more expensive to construct, both in terms of material and labour. 

Pad and beam foundations  

Pad and beam foundations are one of the most straightforward construction foundation types to build. They’re commonly used with lightweight structures, on sites where it’s acceptable to have a shallow foundation, such as houses with timber frames. 

This type of foundation is useful in areas with unstable soil or where the building needs to be raised above the ground, such as in tree root protection areas. 

It may also be used where deep foundations are required due to the influence of trees; pads could be spread out and dug at intervals, with a shallower excavation required between the pads to facilitate the installation of ground beams. 

Foundations for basements 

Where a building is being formed partly or wholly in the ground, a basement retaining structure is required. 

There are various methods of forming a basement, which we will cover under a separate blog. 

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In conclusion, selecting the right foundation for your project comes down to site specifics – load arrangement, ground conditions, site constraints etc. 

Foundations for your construction project 

For more information about what foundations are most suitable for your project, talk to dbstructural’s team of trusted structural engineers.