Foundations for Building

The Importance of Foundations in Properties

Most people are aware that the foundations of a property are essentially the most important part of any home. A well-built house on poor foundations is a recipe for disaster, and a guaranteed way to ensure that the property will soon be demolished. Choosing the foundation for a small extension or garage is a task that is typically carried out by a professional building contractor.

However, for more complex structures or buildings in unstable grounds, a structural engineer will be required to design a specific foundation. In any case, quality builders will generally know what is required. The building control officer will check the excavation to make sure the depth and quality of the base is sufficient before allowing it to be filled with concrete. Ultimately, the type of foundation will depend on the terrain. With this being said, there are three main types of foundation used in the UK, these are, strip, piled and raft. Here, we will explore all three in more detail.

Strip Foundations

Strip foundations are a tried and tested method used around the globe for centuries. They are the most common building foundation used in the UK, and they are arguably the cheapest. While their price tag is a huge advantage, the reason why they are usually considered is more to do with the solid and robust platform they provide.

The term strip referrers to the strip of concrete that runs underneath the load-bearing walls. However, this term can be misleading. The term strip implies that the foundations are thin and narrow and thus inherently weak, and this is simply not the case. The strip really refers to the shape of a ditch that is dug below the load-bearing walls, which is then filled with concrete for maximum strength.

Strip foundations

Strip foundations

The depth of the excavation will depend on the type of strip foundation that the property requires. Shallow strip foundations are acceptable for the majority of properties, but others may require deeper or wider strips. The technique that is carried out to create these types is basically the same as a traditional shallow strip, but the diameters and measurements will change accordingly.

To create a shallow strip, a building contractor will need to dig a ditch following the blueprints of the property until a suitable sub base has been reached. An even and horizontal building surface is essential, and so if the site is sloping, steps will need to be added. Once this has been checked, a minimum of 225mm of concrete will be poured into the excavation. Cavity walls are built up to ground level from this newly concreted trench or strip, which are then backfilled for strength and support. As already mentioned this technique is the basis for all strip foundations, but it will vary dramatically for each building.

Building foundations and excavations should only be performed by a professional with years of experience. Typically, a government regulated building inspector or structural engineer will make the decision as to what type of foundation is right for your property before any work is started. However, by reading this article you should now be familiarised with how this task is completed and the nature of the work involved. For additional information in this area, speak to a qualified building contractor who will be able to offer structural engineering advice.

Piled Foundations

Piled foundations are generally required for buildings that exert a great deal of load-bearing pressure due to their weight, or for buildings that are created on foundations where top-soil is unstable or prone to erosion. In scenarios like these, piled foundations are usually seen as a better option than a standard strip foundation.

Trenches are dug beneath each load-bearing wall, and within these piles are inserted below the ground level. The term pile is used to describe the long vertical column that ultimately transmits the weight of the property and provides it with more support. This weight is transmitted onto a more stable surface that can be found below the initial layer of topsoil and clay. If these layers are strong enough, this technique will not be required, but if a government registered building inspector deems otherwise than this method will have to be used. If the latter happens to be the case, then this article will give a brief overview as to how these can be created.

Piled Foundations

Piled Foundations

First, an experienced building contractor will need to be employed. A structural engineer will design the piling scheme based on local knowledge or deep augured soil sampling. This is not a job for a DIY enthusiast. The scheme must be submitted to the building control department for approval. Once all these formalities are complete, a construction company will be able to provide a quote, and when this has been accepted work can begin. This will involve a machine to auger deep holes, which are then reinforced with steel rods and filled with concrete. In some instances, the augured holes will require lining. Trenches are then excavated exposing the pile heads. These are incorporated into a concrete ground beam again with steel reinforcement. This beam is similar to a strip foundation but is not load bearing. A gap should be left so they are not in contact with the sub soil. This gap is usually filled with insulation. This Gap will allow the sub soils to swell and shrink in wet or dry conditions avoiding ground heave affecting the foundations. This ground beam creates a strong foundation for walls to be constructed on. Cavity walls can be built to ground level and then backfilled with concrete for additional support and strength. Once this stage has been reached, your property should now have solid foundations, which will have no problem in supporting the weight of the building.

The above is a brief overview as to how piled foundations are created. This of course is not accurate for all applications, and it can’t be applied to all buildings. It must be emphasised that piled foundations are not as common as typical strip foundations, and as a result it can be seen as specialist work. Not all building contractors will be able to carry out this task, but fortunately there are those who can. In any case, you should ask the company for structural engineering advice before proceeding.

Raft Foundations

Raft foundations are rarely used because they are more expensive than strip foundations. Furthermore, as the name suggests they are designed like a raft and will move up and down from season to season. Therefore, the structure has to be built independently from other buildings. However, this type of foundation is essential for properties that are built on softer grounds or are in areas that could experience abnormal levels of movement. For example, buildings that are in close proximity to trees, or mining districts will probably have to have raft foundations.

A workman is hard at work building a raft foundation for an extension.

A workman is hard at work building a raft foundation for an extension.

The type of foundation that you propose building will need is based on a decision that is made by a structural engineer and a building control officer. The foundations of a property will have to meet all the necessary rules and regulations, and if the ground is not deemed to be fit, building work cannot commence. However, some building work may be able to get underway if a raft foundation is used. A raft foundation spreads the pressure of a building across a wider area which is usually a lot less demanding for the ground that it is built on.

Typically, the complete footprint of the area of the building to be supported will be excavated to a suitable depth with a raft foundation. This depth will vary depending on many factors. A suitable sub-base material like mot type 1 is then laid in layers and compacted using a mechanical plate.

The raft refers to the reinforced concrete that is laid over the sub base material, which is typically 250 mm – 450 mm thick and formed using concrete reinforced with steel fabric mesh. The weight of this will be heavy, but as it is spread out across a wide area, the ground will not subside or sink.

It must be emphasised that raft foundations are rarely used, and as a result not every building contractor will be able to carry out this type of work. However, if your property does require a raft foundation, speak to a construction company and ask them for construction advice and a quote depending on how serious you are about the work.

Building near Trees

Trees are often a detrimental problem to a lot of buildings throughout the UK. In fact, insurance claims for subsidence damage caused by trees totals up to be well over £400 million. This basically translates into a lot of damage that could have potentially been avoided if more attention was paid to a few tell tale signs. The official information that relates to building work near trees is highlighted in BS 5837: 1991. Any construction work that is carried out on your property will have to abide by these rules, as not complying with them will affect your planning permission application. However to give you a basic idea of what the BS 5837: 1991 document entails, some of its main points will be analysed in this article.

Trees can cause damage to properties either directly or indirectly. An example of direct damage would be when the root of a tree lifts or distorts the structure of the property. This is quite rare, and damage caused to buildings by trees usually occurs indirectly. Indirect damage tends to be caused by trees drawing water away from the ground that a property is built on. This can lead to subsidence and ground movements, which in turn can have a negative impact on the foundations and the structure of a building.

Ground and foundation movement that is indirectly caused by trees can occur in four main ways. These are…

  • Normal seasonal movements. Often associated with evaporation-transpiration
  • Enhanced seasonal movements. (Associated with trees)
  • Long term subsidence. Often associated with a long term water deficit caused by trees
  • Long term heave. Often caused when a long term water deficit suddenly becomes replenished

As these bullet-points suggest, different seasons and levels of ground moisture can have a negative impact on buildings that stand in close proximity to trees. The size of the tree will obviously need to be accounted for, but even the different varieties of tree can have an effect. Some demand more water and as a result are more likely to affect the structure of a property. For further information that relates to tree size and the different requirements for each species, consult the NHBC guidelines and regulations.

To bypass many of the problems that are associated with indirect tree damage, most new build foundations will be created at a depth that is not influenced by vegetation. This is sometimes done by using a piled foundation, which gives a property extra strength and support where needed, but this is not usually the preferred method. For small buildings, like extensions, or garages, or where piling costs would make the project non viable. A deep trench excavated to below the depth of the root system and filled with concrete is often the foundation of choice. If possible it is best to remove and trees when creating a new build. However it is not usual for ground swelling to occur as a by-product of this. To counteract this issue, Trees should be removed over a period of time. A structural Engineer and Tree surgeon will be able to give professional advice- If you require additional help send an e-mail via our specialist subjects section on the home page.

By reading this article we can now see that trees have a major impact on all areas of construction. Properties can be affected either directly or indirectly, and as a result careful consideration will need to be taken into account when applying for planning permission. This article is only intended to give a brief insight into the impact that trees can have, but for more detailed information look in our download section.