Foundations in the construction of any building, whether commercial or domestic, are so important for its structural integrity that we have even adopted a phrase from the construction industry in everyday conversation – we speak about ‘getting the foundations right’ for all kinds of projects, not just buildings. Whilst that phrase might be well-understood in the English language, we often don’t stop to think about its origins, and indeed, how vital it is to get foundations right in building projects of all types and sizes.
In this article, we will discuss exactly what foundations are, what the different types of foundations are and when they are used, and what needs to be considered when planning and building foundations, including foundation depth and materials used within foundations.
What Are Foundations?
All types of buildings rely on foundations to provide a firm base for the structure above ground, and to spread the load of the building evenly. Without foundations, buildings would be at risk of structural failure and even collapse. Whilst foundations cannot be seen once a building is complete, they play a vital role in the structural integrity of a building. There are many types of foundation used in various kinds of construction projects, but they all share the same basic features – using reinforced concrete below ground to provide strength and stability to the upper parts of the building.
What Is The Difference Between Foundations And Footings?
People often use the terms foundations and footings interchangeably, and it’s particularly common for some types of shallow foundations to be referred to as footings. However, the term footings is sometimes also used to refer to the blockwork or brickwork that sits on the foundations and below the level of the damp proof course.
Different Foundation Types
There are many different types of foundations and the particular type to be used will depend on the nature of the project. Clearly, a domestic single-storey house extension has very different requirements, in terms of load bearing and resistance to external forces, than a multi-storey car park or a high-rise commercial tower in a built-up city centre. As well as the type of building being constructed, structural engineers and construction professionals will also take into account the local ground conditions, climate, site access and many other factors when deciding on the best type of foundations and their physical properties.
When considering the various types of foundation, they are often split into two groups – shallow foundations and deep foundations. Pad footings, strip foundations and raft foundations are all examples of shallow foundations. Deep foundation types include pile foundations, caissons and diaphragm walls.
Shallow Foundation Types
Shallow foundations are typically used when there is a relatively low load-bearing requirement, for example in domestic property builds and extensions. The key types of shallow foundation include:
Also known as pad foundations, this type of foundation is composed of circular or rectangular pads, on which columns are positioned to support loads in a localised area. This type of foundation is often used in commercial property such as warehouses, in order to provide columnar support for very large roof spans.
When it comes to residential house foundations, strip foundations are the most common type of foundation used, and they are generally what people are most familiar with. Strip foundations consist of a trench dug to a specified depth and width, which is then filled almost to the top with reinforced concrete. It’s vital, as with any type of foundation, that the correct steel reinforcement is used, and that it is correctly positioned in the trench to ensure adequate and even coverage when the concrete is poured.
This type of foundation involves a reinforced concrete slab poured over a large area, sometimes covering the entire footprint of the building. Foundations like these are typically used when the ground conditions are poor or inconsistent, or when site analysis suggests that settlement or heave is a possibility. Some complex building designs rule out the use of pad or strip foundations, and in these scenarios, raft foundations are often the chosen solution.
When building raft foundations, mesh reinforcing sheets are typically used, as these offer a cost-effective means of providing the required reinforcement over a large surface area. Mesh panels are available with flying ends – where one of the end struts is missing to allow for overlapping panels without a build-up of layers. Supports are also used to ensure that the mesh reinforcement panels are positioned at the correct depth throughout the slab footprint.
Raft foundations differ from traditional concrete slab foundations in that they do not involve any footings below the raft slab – a raft foundation literally ‘floats’ on a layer of compacted hardcore on the ground surface, whereas with a standard concrete slab foundation, the slab sits on top of reinforced concrete footings that are spaced throughout the area to be covered. This makes raft foundations relatively inexpensive and straightforward to implement, since there is no need for expensive and time-consuming trench excavation.
Deep Foundation Types
Unsurprisingly, deep foundations are used in larger-scale construction projects, where the load-bearing capacity of the building will be significant. Deep foundations include:
Pile foundations are long cylinders of reinforced concrete which extend deep into the ground to provide the required strength and stability for the building supported. There are two main methods of pile construction – driven piles and bored piles. In driven piles, a prefabricated pile is delivered to site and driven into the ground – with soil being displaced downwards or sidewards to make way for the pile itself. In bored piles, the hole for the pile is excavated, and the pile is then poured. In some cases, the boring and the pouring take place simultaneously – these piles are known as continuous flight auger (CFA) piles.
A caisson is a very specific type of foundation, which is typically used for building structures under water, such as dock walls, breakwaters and sea defences, and bridge substructures and abutments.
Caissons resemble boxes in structure, and are generally pre-fabricated and then floated out to the construction site before being sunk below the water in their final position.
Diaphragm walls are extremely deep foundations that act as a foundation wall or retaining wall. With multiple vertical panels constructed one after the other, diaphragm walls require a special bentonite slurry to be used during the excavation to prevent the walls of the deep trench from caving in. Trenches for diaphragm walls are typically 10 to 30 metres deep, and often significantly deeper, allowing for extensive sub-ground and above-ground construction to take place, sometimes simultaneously.
Foundations Depth Guide
It’s essential to accurately work out how deep foundations need to be in order to fully support the load of the building being constructed, and also to ensure that construction of the foundations follows the plans and calculations meticulously. There is no hard and fast rule as to how deep building foundations should be, since so much depends on the type of building being constructed, the ground on which it is being built, the load-bearing requirements of the build, the environmental and climate pressures exerted on the structure and many other factors.
For house foundations, for example, where trench or strip foundations are being used, the depth of the concrete foundation should be at least 1 metre, and could be significantly more than that in difficult conditions. The actual excavation would need to be more than 1 metre, to allow for brickwork and blockwork to be added to the top of the foundation to build it up to ground level. There must be at least a further 150mm between the top of any concrete cavity fill and the damp proof course.
For domestic builds such as house extensions, the local building regulations inspector will need to sign off on the house foundations, ensuring that they meet with the guidelines contained in The Building Regulations 2010 legislation, but also the specific rules contained within the British Standards for foundations (British Standard 8004:2015 Code of practice for foundations).
For other types of foundations and for commercial builds, calculations for the depth and width of the foundations are generally made by a structural engineer and written into the detailed plans for the build – these calculations are beyond the scope of this article.
As can be seen from the overview given in this article, building foundations that are fit for purpose can be a complex business, especially in larger construction projects or on sites with access or ground condition difficulties. The foundations for a routine house extension may well be a simple matter, but nonetheless, the same care and diligence should be applied at the planning and construction stages to guarantee perfect execution and a build that is free of structural problems. From choosing the right type of foundation through to ensuring the foundations are built to the right depth and with the right concrete reinforcement materials, it pays to get it right, first time. Call us today to discuss your next foundations build, and to find out about our comprehensive range of steel reinforcement products.