FOUNDATIONS

Foundations are essential for any building and are required by law to meet certain standards. The foundations of the building take the weight and distribute it to the soil below stopping the building from sinking down into looser uncompressed soil.

Building foundations also provide an anchor into the ground to increase the strength and durability of the building particularly against weather and seismic activity.

There are two main types of foundations, deep and shallow. These two main types are suited for different applications. For example housing normally uses shallow foundations, with deep foundations used in larger structures or where the top layers of soil are too weak to support the load.

Shallow foundations

There are three main types of shallow foundations, spread footing foundations, slab-on-grade foundations, and mat-slab foundations.

Spread Footing foundations are commonly found on residential properties and consist of pads (or feet) of concrete placed below the frost line the ground. The building then sits on top of the pads, which distribute the building's weight over a wider area, in order to stop the property sinking in to the ground.

Slab-On-Grade foundations consist of a concrete slab on which the building sits. The ground for the foundation is dug into a mould into which concrete is poured, leaving no gap between the earth and concrete.

This method is favoured in warmer climates as it provides a large surface area, which in colder climates can lose heat. Slab-On-Grade is also a cheaper and easier method of foundation building and is suited more to areas with expansive clay soil.

The disadvantage to Slab-On-Grade construction is that once the foundations are in place it is very difficult to change. This makes installing new pipework for water, gas etc. particularly difficult. When used in Slab-On-Grade foundations copper piping will need to be lagged to prevent reacting with the concrete and electrical cabling will need to be insulated to protect against moisture.

Mat-Slab foundations are a cross between spread footing and slab-on-grade, using much larger pads to support the loads of a property when compared to normal spread footing foundations.

Deep foundations

Deep foundations are used for larger structures or where a shallow foundation may not be suitable. These consist of material placed at depth to support the structure with timber, concrete and steel being commonly used. These are generally installed using two methods drilling and driving.

In Driven or Piled foundations pre formed 'piles' are driven into the earth using heavy machinery such as a pile driver. These piles can be pre-formed lengths of concrete in various shapes, the trunks of large trees and lengths of steel tube.

The material is often pre-stressed to make sure it doesn't compress under load. Driving piles has an advantage over drilling piles as it compresses the earth down around the pile allowing it to bear more weight.

Above: Concrete delivery - small amounts for residential jobs such as extensions can be finally delivered by wheelbarrow.

In Drilled foundations a bore hole is drilled into the earth. A sheath is then put down the hole and concrete poured in. The sheath can then either be removed or left in place. Drilled foundations have the benefit of being able to be as wide as the drill diameter allowing much larger piles to be used then would be physically possible with a pile driver.

Micro pile foundations are similar to the larger structures but tend to be used where space is at a premium or where a building will only require temporary foundations (for example an on site construction office).

Materials

When considering choosing the materials and eco friendliness of the various methods of foundation building all have advantages and disadvantages:

Timber

Advantages

• can be from sustainable forests
• requires much less energy to turn into a construction material
• is easier to handle

Disadvantages

• will rot if placed at ground water level
• unable to support heavy loads

Concrete

Advantages

• recycled concrete can be used to form parts of the foundations
• relatively inexpensive
• when used in piles can be inspected before being used (driven) or cast into any shape required for a pile on site (drilled)
• strong material and can be reinforced with steel to provide additional strength

Disadvantages

• poor insulator when used in Slab-On-Ground
• requires a lot of energy (and in turn generates a lot of carbon) to produce and work with

Steel

Advantages

• can be used on a sloping surface
• can be cut to length on site
• can support heavy loads

Disadvantages

• steel production is a very heat intensive process generating a lot of carbon
• steel is not a sustainable resource
• steel will be required to be transported greater distance than either timber or concrete
• steel rusts
• steel is relatively expensive

Foundations are a vital part of a building's strength and the choice of foundation should not be taken lightly. On even a small self-build project, or any build job, professional advice from a structural engineer will always be required.

Extensions and any load increasing work will also require the existing foundations to be checked by a professional engineer.

Many other factors have to be taken into account when deciding on foundations. Location of trees is very important as each species has a different root structure and water uptake, which dries out the land and can lead to future problems. Underground waterways, mines, etc need to be checked. The soil will have to be analysed professionally.

When looking for an environmentally friendly foundation it is still important to consider the strength and suitability of a foundation and how it will interact with the soil types around it. Where possible foundations should be insulated to prevent heat from escaping from them. A good quality and well thought out foundation is key to a building's long life and safety.