Hilal Ahmad Dar
Roll Number: 120107094
Structural foundations are the substructure elements which
transmit the structural load to the earth in such a way that the
supporting soil is not over stressed and not undergo
deformations that would cause excessive settlement of the
The various types of structural foundations can be broadly
grouped into 2 categories, namely,
Well foundations are deep foundations.
They provide a solid and massive foundation for heavy loads
and are commonly used for transferring heavy loads to deep
strata in river or sea bed.
Wells provide a large section modulus with the minimum
cross-sectional area and hence effectively in taking large
vertical and horizontal loads even when the unsupported
length is large.
Concreting of well steining is done under dry and controlled
conditions and hence quality of work is assured.
Wells have a large cross sectional area.
Design of well foundation involves the following:
Determining depth of well foundation
Determining shape of well foundation
Determining size of well foundation
Determining type of well foundation
The two important requirements that influence the depth of a
well foundation are:
1. Minimum grip length below the scour depth
2. Base pressures to be within permissible limits
The grip length is taken as one third the maximum scour
depth. Thus as per IS:3955 (1967), the depth of the foundation
should not be less than 1.33 times the deepest scour below
The maximum base pressure should be less than the allowable
bearing pressure. This is important from the consideration of
safety of well.
The choice of a particular shape of well is dependent mainly
on the base dimensions of pier or abutment, the ease and cost
of construction, tilt and shift during sinking and the magnitude
of forces to be resisted.
Shapes of well foundation are:
1. Circular well
2. Double D well
3. Twin circular well
4. Double octagonal well
5. Rectangular well
Wells are of three types, namely, open wells, box wells and
Open caisson or well: The top and bottom of the caisson is
open during construction. It may have any shape in plan.
Box caisson: It is open at the top but closed at the bottom.
Pneumatic caisson: It has a working chamber at the bottom of
the caisson which is kept dry by forcing out water under
pressure, thus permitting excavation under dry conditions.
Well foundations are designed in such a way that the loads are
directly transmitted by bearing to the base of the well ignoring
the skin fraction developed by grip length of the well and
passive pressure offered by soil.
The size of the well is determined to a large extent taking into
consideration a safe bearing capacity of the soil at the
The size of the well has to be such that it can accommodate
the pier without much need of cantilevering out the cap.
The commonly adopted method for laying out a bridge is by
laying out the station point line at right angles to the center
line of the bridge on the high bank on one side of the proposed
bridge or anywhere between the abutments where level ground
may be available. Masonry pillars are constructed on this line
to serve as station points for checking location of piers
The curb is usually pitched at about 15 cm above the low
Excavation should be carried out upto the level at which the
well curb is proposed to be pitched and the center of the well
curb carefully marked.
The well curb should then be assembled on wooden blocks or
sand bags placed at suitable intervals so that while assembling
the curb, it does not sink.
Concreting is done after placing the reinforcement.
Well steining could be of masonry or R.C.C.
Initially, when the well has a very small grip length, the
chances of tilting are more.
The chances of tilting increase considerably, if the well is
made top heavy by raising the steining too high in the first
A better approach is to first sink the well curb alone without
raising any steining above it.
The steining should then be raised in heights of above 1.5 m at
a time, after allowing at least 24 hours for setting.
Once the well has attained a grip length of about 6 m or more,
the steining can be raised in installments of about 3 m.
The well is sunk by excavating the soil from within the dredge
A large size spade jham is used for excavation under water.
The jham tied to a rope moving over a pulley is lowered into
The blade is pushed into the soil by the driver.
The jham full of soil is then pulled up by men and emptied
The process is continued.
When a clay stratum is to be pierced, rail chisel is more
The following precautions are useful in avoiding tilts and shifts
The outer surface of the well curb and well steining must be
as regular and as smooth as possible.
The radius of the well curb should be kept about 20 to 40
mm larger than the outer radius of the well steining.
Cutting edge should be of uniform thickness and sharpness.
Dredging should be done uniformly and on all sides of the
well. For a twin well, dredging should be done in both
dredge holes uniformly.
Well foundations are very common and trusted for bridge
construction. They have a huge success rate and tend to have
large life span.
Well foundations are very time consuming. It may take years
together for well sinking (in worst cases).
It still remains a favourite and a good choice because of low
cost, life span and success rates.
Well foundations have varied shapes and types to
accommodate to various conditions.
Since the cost is also low, it is the ideal foundation to be used.