Construction foundations - Expert guide to your foundation options

A building is only as stable as the foundation it’s built on. That is true for every house, skyscraper, or sandcastle. Without a solid, well-designed foundation that suits both the load applications required and the external conditions, a structure can and likely will fail. That’s why it is important to have a comprehensive view of the foundation requirements before breaking ground. Our Construction foundations - Top tips for a successful build is a good place to start for understanding all the information you need.

Once all the data and information about the ground, site, and foundation requirements are collected, an engineer has several options for the building’s foundation. We’ve compiled a list of the most popular foundation designs used in modern construction today, when they should be used, and which ones require steel reinforcements. However, this information should be used for educational purposes only and does not replace consulting a qualified engineer for guidance.

1. Strip foundations

Strip foundations or footings are one of the simplest foundation designs. A strip foundation is a concrete strip, cast either in the ground or above ground with a formwork of shutters. Shutters are a temporary mold, used to hold the liquid concrete until it sets. The width and depth of the strip foundation can be adjusted to take more load, carry a wider structural footprint, or stabilise the volatile ground. The general rule is the bigger the footing, the stronger the foundation.

When should strip foundations be used? Strip foundations are linear which makes them ideal for use with masonry walls, which are walls made of brick, stone, or concrete blocks – stackable material. They are relatively cheap, easy to build, and commonly used, as modern excavators are designed to dig in straight lines. They can be used for small to medium applications, such as garden/retaining walls or single-story buildings, as long as the ground is stable, and the load is light to moderately heavy.

Are steel reinforcements required? Strip foundations are typically used in lighter applications, such as a garden wall, so they are not usually reinforced. However, if the ground is prone to movement, or the load is more dynamic, adding steel reinforcement will greatly improve the foundation’s strength. It’s important to note that for steel reinforcement to be effective it must be designed and installed according to the design – you can’t simply add metal to the foundation and expect it to work.

2.Trench fill foundations

Trench foundations are linear concrete footings cast in situ, similar to strip foundations. The primary difference between strip and trench foundations is that trench foundations tend to be deep excavations in the ground that are filled with concrete, called mass-fill concrete. This makes them deeper and wider than strip footings, so they can carry greater loads. The deeper excavation also allows the concrete to bear on what should be firmer ground, while the increased vertical face gives a stronger lateral grip on the ground to resist the structure sliding down a gradient.

When should trench foundations be used? As trench footings are stronger than strip footings, they are more suited to larger retaining walls or structures with more than one floor, and they have a better grip on the ground because of their increased depth.

Do I need steel reinforcement in a trench foundation? As trench foundations are deeper footings, they are intrinsically stronger than shallow strip footings which makes them useful for medium-duty applications. However, with the addition of a well-designed rebar cage, a trench footing in the correct ground scenario can carry a medium to high load structure. This is typically only applicable if the ground is very stable at a shallow depth as the issue the structure faces is not that the foundation is strong enough, but that the ground will fail under load.

3.Isolated/Pad foundations

Isolated or pad foundations are single concrete foundations not connected to any other foundation. They are often square and can be almost any size or depth depending on the load and ground suitability. These foundations can be mass-fill concrete, or heavily reinforced depending on the required loadings.

When should pad foundations be used? Pad foundations are generally only used for structures with columns (these are typically steel members but can be masonry piers). They do have an advantage over linear foundations in that they allow services to pass between them rather than having to design a solution to allow the service to pass through the foundation. They also, usually, use less concrete than their linear counterpart.

4.Piled foundations

Piles are very long reinforced concrete nails or stilts that fix a structure’s foundations into the ground. They can be mini piles that are as short as 5m to 120m long with diameters as small as 200mm up to 7.5m. Piles are typically formed by a specialist subcontractor and are then tied into the reinforcement of the main foundation, such as a raft or trench footing. Piling is a big topic so we’re only providing a brief overview in this article.

When should piled foundations be used? Piling is used to solve two main issues: ground instability and gradient. When the ground is unstable at the surface it will undermine most strong foundations and the structure will not hold. Even the best of boots will not stop you from sinking in the mud! That’s why you must excavate until you find stable ground deeper down. This is a very expensive way to build, especially if the stable ground is 5-20m down. As such, piles can be used to form a type of stilts that bear a portion of the load down on the deep, stable ground below. Some piles will go so deep that they bear directly on bedrock.

In other cases, the structure may be being built on a slope and the ground is liable to give way if the entire weight of the building is on the upper layers of the sub-soil. The solution is to drive the pile nails into the deep ground to grip the earth and keep the structure in place.

How are piles formed?

Piling is carried out by specialist contractors who operate a piling rig. The rig is a very large, tracked machine weighing up to 520 tonnes. Before the piling rig lands on site, a piling mat is laid to prevent the rig from sinking while tracking around the site. This is made of a thick compacted engineered layer of aggregate such as 300mm of Type 1 is usually laid.

The piling rig begins by drilling a deep hole using a large drill bit. The drill either screws into the earth, pulling up material to be removed by an excavator, or it rams into the ground, compacting the soil like a nail. Once the hole reaches the required depth, it is filled with concrete, usually pumped through a nozzle at the end of the bit as it is withdrawn. Finally, a steel reinforcement cage is lowered into the hole and pressed down until it reaches the full depth.

Once the pile has cured, the top of the concrete is removed or cropped to expose the reinforcement. This allows the reinforcement for the foundation to be tied into the pile, securing everything together. The entire structure is then cast in concrete when the rest of the foundation is poured.

5.Raft Foundations

Raft foundations are specialist foundations that are becoming more popular as technology improves. A raft foundation can be used with piles and with built-in strip footings called thickenings. A raft foundation is a reinforced concrete slab that bears on the ground. The reason it is called a raft rather than a slab (as might be used for a shed base) is because the foundation is designed to float over the volatile ground below.

When should raft foundations be used?

When building on ground that is prone to movement such as clay, a normal concrete slab would likely crack and snap because of the moving base below it. However, a raft foundation is designed to have a void below the slab which can accommodate ground like clay expanding by giving it room to move as needed. This void is made by installing a polystyrene foam slab under the concrete that often has a hollow honeycomb-style shape. The clay rises toward the concrete the honeycomb can collapse and allow the clay to fill the void. This prevents the slab from having to resist the movement and prevents cracking.