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Types
Of Foundations
understandconstruction.com
In this article we will discuss the common types
of foundations in buildings.
Broadly speaking, all foundations are divided
into two categories: shallow foundations and deep foundations.
The words shallow and deep refer to the depth of
soil in which the foundation is made.
Shallow foundations can be made in depths of as
little as 3ft (1m), while deep foundations can be made at depths of 60 - 200ft
(20 - 65m).
Shallow foundations are used for small, light
buildings, while deep ones are for large, heavy buildings.
SHALLOW FOUNDATIONS
Shallow foundations are also called spread
footings or open footings.
The 'open' refers to the fact that the
foundations are made by first excavating all the earth till the bottom of the
footing, and then constructing the footing.
During the early stages of work, the entire
footing is visible to the eye, and is therefore called an open foundation.
The idea is that each footing takes the
concentrated load of the column and spreads it out over a large area, so that
the actual weight on the soil does not exceed the safe bearing capacity of the soil.
There are several kinds of shallow footings:
individual footings, strip footings and raft foundations.
In cold climates, shallow foundations must be
protected from freezing. This is because water in the soil around the
foundation can freeze and expand, thereby damaging the foundation.
These foundations should be built below
the frost line, which is the level in the ground above which
freezing occurs.
If they cannot be built below the frost line,
they should be protected by insulation: normally a little heat from the
building will permeate into the soil and prevent freezing.
INDIVIDUAL FOOTINGS
 |
| Individual footings awaiting concreting of the footing column. |
Usually, each column will have its own footing.
The footing is just a square or rectangular pad of concrete on which the column
sits.
 |
| Individual footings connected by a plinth beam. Note that the footings have been cast on top of beds of plain cement concrete (PCC), which has been done to create a level, firm base for the footing. |
In practice, the designer will look at many
other factors before preparing a construction design for the footing.
Individual footings are usually connected by
a plinth beam, a horizontal beam that is built at ground or below
ground level.
STRIP FOOTINGS
Strip footings are commonly found in
load-bearing masonry construction, and act as a long strip that supports the
weight of an entire wall.
These are used where the building loads are
carried by entire walls rather than isolated columns, such as in older
buildings made of masonry.
Raft Foundations, also called Mat Foundations,
are most often used when basements are to be constructed.
In a raft, the entire basement floor slab acts
as the foundation; the weight of the building is spread evenly over the entire
footprint of the building.
It is called a raft because the building is like
a vessel that 'floats' in a sea of soil.
Mat Foundations are used where the soil is weak,
and therefore building loads have to be spread over a large area, or where
columns are closely spaced, which means that if individual footings were used,
they would touch each other.
DEEP FOUNDATIONS
A pile is basically a long cylinder of a strong
material such as concrete that is pushed into the ground so that structures can
be supported on top of it.
Pile foundations are used in
the following situations:
1. When there is a layer of weak
soil at the surface. This layer cannot support the weight of the building, so
the loads of the building have to bypass this layer and be transferred to the
layer of stronger soil or rock that is below the weak layer.
2. When a building has very
heavy, concentrated loads, such as in a high rise structure.
Pile foundations are capable of taking higher loads than spread footings.
There are two types of pile foundations, each of
which works in its own way.
End Bearing Piles
In end bearing piles, the bottom end of
the pile rests on a layer of especially strong soil or rock.
The load of the building is transferred through
the pile onto the strong layer. In a sense, this pile acts like a column.
The key principle is that the bottom end rests
on the surface which is the intersection of a weak and strong layer. The load
therefore bypasses the weak layer and is safely transferred to the strong
layer.
Friction Piles
Friction piles work on a different principle.
The pile transfers the load of the building to the soil across the full height
of the pile, by friction.
In other words, the entire surface of the pile,
which is cylindrical in shape, works to transfer the forces to the soil.
To visualise how this works, imagine you are
pushing a solid metal rod of say 4mm diameter into a tub of frozen ice cream.
Once you have pushed it in, it is strong enough
to support some load. The greater the embedment depth in the
ice cream, the more load it can support.
This is very similar to how a friction pile
works. In a friction pile, the amount of load a pile can support is directly
proportionate to its length.
In practice, however, each pile resists load
by a combination of end bearing and friction.
This is
a site that explains the art and science of building construction in great
clarity and detail. Our goal is to make you understand concepts in
building construction.
Written by architects and engineers, the content on the site is actually a result of accumulated years of work experience at building construction sites and design offices. This expert knowledge of building construction is not available in textbooks!
We also take great pains to ensure that our quality of writing is of a high standard. We aim to take complicated situations and make them simple and clear, as well as to provide content that is interesting to industry experts and newcomers alike. Do let us know where we succeed - and where we fail - in this task.
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Written by architects and engineers, the content on the site is actually a result of accumulated years of work experience at building construction sites and design offices. This expert knowledge of building construction is not available in textbooks!
We also take great pains to ensure that our quality of writing is of a high standard. We aim to take complicated situations and make them simple and clear, as well as to provide content that is interesting to industry experts and newcomers alike. Do let us know where we succeed - and where we fail - in this task.
We are a free site - we cover our costs by advertising, so do feel free to click on ads that interest you!
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