Definition of the Term
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
Load imposed by the building
Purpose of a Foundation
The 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
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 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.
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
Solving the problem
The 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.
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.
When 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
and the roots are
growing and shifting
Cut down trees are
no longer absorbing
the moisture, so the
can become water
logged and swell
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 stable
the ground floor
Note! No ground
occur if the
foundation is at
a depth of more
Depth at least 1.00m below ground level will produce
only very slight movement which will not affect foundation
• Pad foundation
• Strip Foundation
• Wide strip foundations
• Stepped strip foundation
• Narrow strip or trench fill foundation
• Short bored piles
• Raft foundations
For 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.
The width of the strip will depend on the load
to be carried and the strength of the ground.
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
The diagram below shows the dimensions which will satisfy
the current Building Regulations.
The depth D must be
150mm minimum and
must be equal to, or
greater than, the
The distance P must
be equal on either side
of the wall.
Wide strip foundations
If 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
uneconomic and so
Stepped strip foundation
On sloping sites it is usual to step the foundation to follow the
line of the ground and therefore reduce the amount of
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 = three
Height of step = two
Steps should be constructed
to gauge of brickwork
or blockwork to avoid
Narrow strip or trench fill
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
Great care is needed with
regard to planning of
services and drainage with
this method as mistakes will
be costly to correct.
Short bored piles
Short 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.
piles at 1.8m to
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.
used in poor soil
No thickened toe
on edge required
Reinforced concrete raft Thick Raft Foundation