Building Foundations

Level 1 Diploma in Carpentry and Joinery

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Foundations

3.1 3.2 3.3 3.4

Unit 101/501: Principles of building construction, information and communication

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ObjectivesTo be able to:

3.1 Identify types of foundations

3.2 Identify materials used in concrete foundations

3.3 State the information required to work out the quantity of materials used in a foundation

3.4 Calculate volume of concrete used in single strip foundation

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Purpose of Foundations

The purpose of a foundation is to ensure the structural stability of a building.

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Make-up of subsoil material

Different soil structures have different loading-bearing abilities.

For example, when you walk on a beach you will sink a little into the sand. But if you are walking on the road, the tarmac is able to support your weight completely. You will not sink!

The same principle applies when putting structures on different soil types: a structure built on “soft” (non-loading bearing) soil will require a different (stronger) foundation than a structure built on load-bearing (“hard”) ground.

So, different ground conditions will require construction workers to use different types of foundations – to match what the ground is able to support.

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It is important that the ground conditions are carefully assessed so that the correct foundation is laid during construction.

Failure to match foundation method to ground conditions will result in unacceptable settlement problems, including subsidence of the structure.

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Types of foundations

There are four main types of foundations commonly used within the construction industry:

• Strip foundation

• Raft foundation

• Pad foundation

• Piled foundation

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Strip foundations

Strip foundations are the most common and cheapest form of foundation to construct.

Strip foundations involve the removal of a strip of land to a set depth and width. Concrete is then poured into the trench (the strip) and tamped level. This supports the weight of the building as transmitted through the load-bearing walls.

Strip foundations can only be used on good and firm, load-bearing strata, and economically to a depth of no more than 1.5 to 2.0 metres.

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Raft foundations

Raft foundations are expensive. It is used where the load will be heavy (eg an office block) and, crucially, where the ground is soft or variable.

To stop yourself breaking through thin ice, you can lie flat on the ice, spreading your weight over a greater area.

Raft foundations use the same principle: the load of the building is spread across the whole ground level area.

The raft is a rigid slab of concrete, reinforced with steel.

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Pad foundations

Pad foundations are generally shallow foundations.

They are a form of spread foundation (so similar to raft) but built as square, rectangular or circular ‘pads’ which then support structural columns or framed structures.

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Piled foundations

Strip, raft and pad foundations are all shallow foundations.

Piled foundations are deep foundations: the piles transfer building loads to the earth much further down below ground level.

Piles are driven or drilled deep into the earth at the building site. This is expensive work but is necessary where soil quality is poor (eg water-logged soil). The piles sit on deep but solid earth that is able to support the building’s weight.

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Materials used in concrete foundations

Concrete for foundations is made up of:

• Coarse aggregate (gravel 5mm to 50mm size)

• Fine aggregate (natural sand/tiny pebbles, up to 5mm size)

• Cement

• Water

• Additive (plasticiser)

• Steel reinforcement (rebar)

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Calculating the quantity of materials used in concrete foundations

To work out the quantity of materials used in foundations, you need to know just TWO things:

1. The VOLUME of concrete required

2. The RATIO of materials required in the concrete mix

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VOLUME = Depth x Width x Length

So, in this example:

Depth = 2m (metres)

Width = 1.5m

Length = 6m

So, Volume = 2 x 1.5 x 6 = 18m3

m3 = metres “cubed”

Calculating the VOLUME OF CONCRETE required in a single strip foundation:

Worked example:

Depth 2m

Width 1.5m

Length 6m

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Calculating the QUANTITY OF MATERIALS required in a concrete foundation

Worked example:

The RATIO of materials required in the concrete mix might be:

4 parts coarse aggregate to 2 parts fine aggregate to 1 part cement to 1 part H20

4 to 2 to 1 to 1

Ratio of materials = 4:2:1:1

This is an example, NOT what the required rations will probably be