A foundation is the part constructed
under the pier/abutment and over the
underlying soil or rock.
The loads transmitted by the foundation
to the underling soil must not cause soil
shear failure or damaging settlement of
Foundation is the important structural
part of bridges
Foundation is the component part of
It receives the load from the piers and
abutments and transfers it to the soil.
Purpose of foundation is to distribute the
load of bridge over a large bearing area
and provide stability of bridge against
settlement and tilting.
It is provided sufficiently deep so that it
not affected by the scouring.
The type of foundation is designed when
subsoil data of bridge site is obtained.
The different types of foundations for
bridges are given following;
Spread Foundation and Raft foundation
Pile, caisson and well Foundation
Spread Foundation is generally constructed
It may be provided for small bridges when
bed is of rock or of soil having good bearing
It is suitable when flow in stream is less and
hence the scouring effect is less.
A typical spread foundation is shown in
It is constructed with stone or brick masonry.
Raft or mat foundation is suitable for
small bridges when bed contains
different types of soil.
It may be provided when bearing
capacity of soil is less.
Raft or mat foundation is shown in Figure.
In this the RCC slab is laid at throughout
the span of bridge.
o in which the depth of the
foundation is very large in
comparison to its width.
o Which are not constructed
by ordinary methods of
open pit excavations.
In cases where
o The strata of good bearing capacity is not available
near the ground
o The space is restricted to allow for spread footings
In these cases the foundation of the structure has to
be taken deep with the purpose of attaining a bearing
stratum which is suitable and which ensures stability
and durability of a structure.
The bearing stratum is not the only case. There may
be many other cases. For example, the foundation for
a bridge pier must be placed below the scour depth,
although suitable bearing stratum may exist at a
Most common forms of construction
pertaining to deep foundations are:
o Caisson Foundation
o Well Foundation
o Pile Foundation
The caisson means, “Box”. A caisson is a huge
box made up of reinforced concrete. Inside of
the box is kept hollow.
Caissons are water light structures, employed
where the foundation is under water
It is constructed for foundations of bridges, piers,
abutments in river, dock structure and shore
The caisson remains in its position and ultimately
becomes as integral parts of the permanent
Caisson foundation is suitable when depth of
water in the river is more and quality of bed
soil is good.
These are best for a depth of 20m.
Caisson may be of circular, rectangular or
any other shape.
The caisson is prepared at bank of river
and then transported to the required
pier position on the bridge site.
During sinking soil from the caisson is
removed by excavators or grab
buckets. When this is sunk up to the
desire depth then inside of caisson is
completely filled with concrete.
Caisson can be broadly classified into the
following three types:
o Box Caisson (Floating Caisson)
o Open Caisson
o Pneumatic Caisson
Box caisson is as like a box that is
opened on topside and other sides
Open caisson is as like well that is
opened on top and bottom ends.
Bottom edges are made sharp.
During digging and sinking the inside
material is taken out.
Open caissons are used in soft soil and
in high water level.
Pneumatic caisson is special type of caisson
that is suitable for heavy bridges.
There is a working chamber made at its
lower end and sealed at the top.
A pressure higher than water pressure is
maintained in the working chamber with
the help of compressed air.
Thus water not entered in the chamber
and excavation work is continued in dry
Workers excavate the soil from the edges of
the working chamber
Excavated material is removed manually or
a crane removes the soil with a grab
In early bridge construction, when they built
with the help of caissons, numerous workers
were killed or permanently injured by
caisson disease during its construction.
Afterward adopting proper measures the
disease was controlled.
Pneumatic Caisson (Cont.)
Large, open-ended compartments –
shell or box with cutting edge at the
Sunk into the ground by digging the soil
out of the center and loading the walls
Filled with concrete (and compacted
Well foundation is suitable when heavy
loads are applied on the bridge.
Well foundation can be construct in
granular soil and in cohesive soil.
Some year ago the well foundation was
the common type of deep foundation.
It is usually constructed of stone or brick
The well may be of many shapes but
mostly it is constructed in circular shape.
The inside of well is filled with granular
Well foundation can be constructed for
a depth more than 7m.
Piers are constructed on single well or
over several connected wells.