Building Methods &

Construction Technology

Types of Foundation

Definition of the Term ‘Foundation’

A foundation is that part of a building or structure which is in direct contact with the ground and which transfers the loads imposed upon it to the subsoil beneath.

Natural Foundation

Artificial Foundation

wall

Load imposed by the building

Purpose of a FoundationThe purpose of a foundation is to spread the load, from the structure above, over an adequate bearing area of the subsoil, and to provide a stable, level base on which to build.

Foundations are required to prevent undue settlement and consequent fracture of the building due to unequal loads and stresses at various points and to distribute these loads evenly over an area.

Spread of Loads on Foundations

The manufactured material which is used to carry the weight of the building is usually concrete. Concrete is stronger than the natural soil which has to support it, so that the forces acting through the foundation may have to be distributed over a wider area of subsoil to support the weight.

It is usually assumed that this spreading of the forces through the foundation happens within an angle of 45˚ from the vertical.

Load

Wall

Foundation

45˚ spread of compressive forces in the natural foundation

Spread of Loads on Foundations

The amount of excavation and concrete required would result in a very heavy and

expensive structure.

Solving the problemThe solution is to make the foundation wider without making it thicker. This will ensure the load is spread over an adequate area of ground to support it safely. In this case, there is a tendency for the foundation to bend across its width so a material which is strong in tension and compression must be used to reinforce the foundation. The material most commonly used is steel reinforcement, and can be either steel bars or steel mesh which is encased within the concrete.

Settlement Almost all of the settlement of foundations in soils is due to water between the particles being forced out, enabling the solid particles to pack closer together.After a long dry spell of weather, cracks sometimes appear in buildings which have stood for many years, indicating that the foundation has moved.In this case, the cause is likely to be that the water in the subsoil has dried out and the soil itself has shrunk.

HeaveWhen water freezes, it expands; in very cold weather, the water in the subsoil may freeze and cause the soil to heave’, i.e. move upwards.

When this occurs beneath a foundation, the pressure exerted could be sufficient to lift the foundation.

The tree is absorbing moisture and the roots are growing and shifting foundations causing settlement.

Cut down trees are no longer absorbing the moisture, so the surrounding sub-soil can become water logged and swell causing heave

To avoid settlement or heave due to the effects of frost or heat the foundation must be taken down to a greater depth. For some soils this may need to be 600mm deep, and for clay the depth must be a minimum of 1 metre.

Wall remains stableunder mostconditions

Roof protectsthe ground floorfrom elements

Note! No groundmovement shouldoccur if thefoundation is ata depth of morethan 1.80m

Depth at least 1.00m below ground level will produceonly very slight movement which will not affect foundation

At least1.00m

Foundation Types

• Pad foundation• Strip Foundation• Wide strip foundations• Stepped strip foundation• Narrow strip or trench fill foundation• Short bored piles• Raft foundations

Pad foundationFor single loads which are transmitted down a brick pier, concrete column or steel stanchion, the most common foundation is a square or rectangular block of concrete of uniform thickness known as a ‘pad’ foundation.

Strip FoundationThe width of the strip will depend on the load to be carried and the strength of the ground.

Strip foundation

In order to reduce failure through bending and shear, the foundation concrete has to be a minimum 150mm thick, and the projection from the face of the wall to the edge of the foundation concrete has to be at least equivalent to the concrete thickness. The diagram below shows the dimensions which will satisfy the current Building Regulations.

Strip Foundation

The depth D must be 150mm minimum and must be equal to, or greater than, the projection P.The distance P must be equal on either side of the wall.

Wide strip foundationsIf the use of traditional strip foundations is likely to overstress the bearing strata (the layer of ground on which the foundation sits), a wide strip foundation can be employed. This might necessitate the foundation thickness being increased to overcome the bending and shear stress.In some cases, the thickness of concrete could become uneconomic and so reinforcement is used.

Longitudinalreinforcement

Transverse reinforcement

Stepped strip foundationOn sloping sites it is usual to step the foundation to follow the line of the ground and therefore reduce the amount of excavation required.When constructing stepped foundations, the construction starts at the lowest level of the site.

Building regulations relating to stepped foundations states: The minimum overlap L should be equal to twice the height of the step S or the foundation thickness T or a width of 300mm whichever is the greatest.

Stepped strip foundation

Height of step = threecourses 225mm

Height of step = twocourses 150mm

Steps should be constructedto gauge of brickworkor blockwork to avoidunnecessary cutting

Narrow strip or trench fill foundation

Mass concrete poured into the excavations to finish two courses below ground level.

This method can be used if there is little space for the bricklayer to work in the footings.

Great care is needed with regard to planning of services and drainage with this method as mistakes will be costly to correct.

Short bored pilesShort bored piles are formed by boring circular holes 300mm diameter to a depth of about 3m by means of an auger.The holes are then filled with concrete and reinforced with steel bars which are left projecting from the top.The piles are placed at the corners of the building and at intermediate positions along the walls.The piles support reinforced concrete ‘ring beams’ which are cast in place in the ground on top of the piles. It is on to these ring beams that the brickwork is built.

Ring Beam

Short bored piles at 1.8m to 2.4m centres

Raft foundations

Raft foundations are often used on poor subsoils for lightly loaded buildings and are designed to be capable of accommodating small settlements of the subsoil.A raft foundation covers the whole of the floor and wall area of the structure. The reinforced floor slab is generally thickened out substantially under all the walls, and the loads from the walls are subsequently transferred into the whole of the slab.

Raft foundations

Thickened edge Raft Foundation used in poor soil conditions

No thickened toeon edge requiredfor lightweightbuildings

Reinforced concrete raft Thick Raft Foundation