As mentioned, there are many different styles of foundations and footings that are used in modern construction. Each type is designed to meet a specific criterion, with each individual foundation being engineered to meet the relevant load bearing requirements of the project. Here we will look at the most common types of residential foundations, and how the groundworks pertaining to their creation are carried out. In the interest of wider knowledge, we will also look at some of the older style of foundations that were used in the past, that are still found under the ground of older buildings. Note that this section once again pertains to residential scenarios, though many of these methods are used/scaled up for larger works.

Strip foundations

Strip foundations, also known as strip footings, are the most abundant type of foundations in terms of standard residential construction. They are a form of shallow foundation that are appropriate for use provided the ground conditions are sound. The relative cost of the labour and materials required for the installation of this type of foundation is the lowest of all of the common types of residential foundations. Looking at the diagram, we can see the through section of a typical strip foundation. A trench is excavated to an appropriate depth, at which point the concrete can be poured. The typical width of a footing is 600mm, to accommodate a standard 300mm cavity masonry wall. The load is exerted at 45 degrees to the downwards force of the wall, and so a minimum footing thickness of 300mm is required to meet the load bearing requirements of the masonry. The 600mm wide trench can be effectively pulled with the use of a 600mm wide excavator bucket. In accordance with BS:8004 “guidelines for foundations” the minimum depth of the trench must be 1m down from the common ground level, though it's not uncommon to excavate further deeper than 1m in the pursuit of suitable ground. In areas with a low water table and a quality subsoil layer such as hard gravel or sand, the trench may well only need to reach this minimum depth of 1 metre. In areas with a high water table or poor soil buildup, the depth of the trench may reach as much as 2 metres in depth before decent ground is found. This is common in areas where wet clay is present.

Quality ground at the bottom of the trench can be distinguished from layers above it in a number of ways. When an appropriate depth is reached, a change in colour and consistency of the soil is often noticeable. The soil at the bottom of the trench when good ground is reached becomes harder to remove, as it is more compacted and stable. An experienced groundworker/plant operator will know by sight and feel when good ground is reached. Suitable ground can also be more accurately assessed through the use of an indentation test, with the parameters having been mapped to the appropriate load bearing capacity of the foundations

Trenches that are excavated beyond 1200mm must be shored and braced if any operatives are to enter the trench. In poor ground, trenches less deep than this may require bracing anyway to prevent the edges from collapsing.

For areas with poor soil build up, the width and depth of the actual foundation may be increased to improve the load dispersion across the soil. This is often done even if good ground is reached at the bottom of the trench.

For trenches excavated at the rear of existing properties as a part of renovation/extension works, the bottom of the new trench must accommodate the existing footing, so that it can be underpinned, in the event that the existing footing does not meet current structural requirements. Underpinning is a method of supporting existing footings with a new concrete pad/footing that is poured to current regulations. It's common to underpin existing footings even if they do meet structural requirements, to negate any future issues caused by the disturbance of the ground involved in the new foundation excavation process.

Depending on the load bearing requirements of the foundation, rebar may be installed to reinforce the concrete footing in accordance with the engineer’s specification. This rebar provides tensile strength to the concrete. This detail is becoming more common in recent times in order to add additional stability to the footings and is often specified as a part of the engineers’ calculations. The rebar is laid into the wet concrete as it is being poured, in a pattern that can be seen in the diagram. 

To achieve the correct height when pouring strip foundations, vertical rebar pegs are driven into the ground at regular intervals to denote the top of the footing. These pegs are driven with a hammer into the soil to a depth determined by a previously set up laser level. As the concrete is poured, a rake or tamp can be used to feel the top of the peg, serving as a guide to level the wet concrete to the correct height.

Trench fill foundations

Trench fill foundations are a variety of footings that are similar in preparation to strip foundations. This style is used in areas with poor ground or a high water table. The trench is dug in a similar fashion to a strip foundation, with a minimum depth of 1 metre also. This style sees concrete poured almost all the way to the top of the trench, typically around 150mm below the common ground level. This eliminates the requirement for erecting masonry below the ground level. The wide, deep concrete footing spreads the load of the masonry above it across the poor-quality soil more effectively and is typically reinforced with rebar.

That being said, the additional concrete can increase the overall cost, rendering this method a more expensive option than regular strip foundations. Due to the depth of the concrete slab, the width of the trench can be reduced below 600mm to around 450mm, decreasing the chance that the poor ground will cave in around the trench.

Rafts

Rafts are a form of shallow foundation that are used for the construction of smaller residential properties in less favourable ground conditions. This style deviates from the trench style of footing by instead utilising a wide slab of reinforced concrete. The total area of the building or extension is excavated to a depth suitable for the raft to be poured - not necessarily a full metre down from the common ground. The slab is continuous and supports the weight of the building above by evenly distributing the load over the total footprint of the slab. This is a good option for areas where the soil buildup is poor, or where digging conventional trenches is not possible. Reinforcing mesh and rebar are included in the slab to provide additional stability, preventing it from cracking. Towards the outside of the slab where the external masonry will be constructed, a deeper “trench” is excavated to provide additional concrete thickness. 

The exact depths and dimensions are subject to the specific scenario and are engineered accordingly. The diagram displays a common through section for demonstration purposes.

Piled ground beams

This is the most heavy-duty type of foundation used in standard residential construction and is most often implemented in areas with poor soil build up, as well as areas with high water tables. It is categorised as a deep footing. Beyond around 2m into the ground, trench excavation becomes impractical, costly, and dangerous for any operatives to enter. In these instances, concrete or steel piles are driven into the ground through the use of a pile driving machine. The machine hammers the piles through the poor layers of soil, until the base of the piles reaches either suitably hard soil, or bedrock. Once the pile is driven, not only is it supported by suitable ground, but also held captive by the compressive forces of the soil surrounding it. These forces, in combination with the depth of the base of the pile, creates a stable anchor in the ground that can bear the load of the construct above it. These piles are driven into the ground at regular intervals along the line of the build to provide stability to the structure. Once the piles are driven, a concrete footing/ground beam is established atop them to provide a suitable base for the masonry to be laid upon. The tops of the piles are typically tied into the ground beam via rebar. This style of footing is relatively common, especially in coastal areas with soft ground and high water tables. Piled foundations are installed by specialist companies, with the concrete footing being poured afterwards on top.

For lightweight residential applications, a minimum depth of 3-4 metres below the common ground level is observed for the piles.

The dimensions of the ground beam are typically the same as a regular trench fill footing (300x600)

Concrete pads

Concrete pads are a smaller type of shallow foundation that are often utilised in renovation scenarios, as well as new residential builds. In essence, they are localised strip foundations that are implemented to support specific components within a build. The depth of the hole must reach solid ground, and the specific width of the pad is often stated by the engineer as a part of the detail. In renovation works, these pads are commonly installed inside an existing property in order to support new load bearing steel posts.

Corbel brickwork footings

When carrying out works on existing properties, particularly those that are older, it's not uncommon to find traditional corbel brickwork footings. These footings are assembled via layered “corbel” brickwork and disperse the load of the masonry above evenly across the ground. In some instances, these bricks are laid on a shallow concrete footing, though sometimes they are laid directly on the earth. This style of foundation lies far from the structural requirements that are involved in modern construction, making it a necessity to underpin or make safe the existing footings when carrying out works in close proximity. That being said, there are plenty of houses around that are perfectly structurally sound, despite the fact that they are sat on shallow corbel brickwork footings. If subsidence or ground movements occur in an area, it is shallow foundations like these that are most likely to be disturbed.