Well foundations notes

Course Content
• Definition, uses, construction material, types
of caissons, loads on caisson, design features
of caissons, floating of caissons, cutting edges,
sinking of caisson, tilting of caisson. Caisson
diseases.

Definition of Caissons
• Caisson is a water tight structure made of
wood, steel, R.C.C i.e. reinforced cement
constructed in connection with excavation for
the foundation of bridges, piers in rivers, dock
structures etc.

Uses of Caissons
• Caissons are more suitable for the deep foundation under water where the foundation
should be extended up to or below the river bed so as to obtain the proper stability.
• Caissons as type of well foundation is constructed in connection with excavation for
foundation of piers and abutments in rivers and lake, bridges, break water dock
structures for the point of view of shore protection, lamp house etc.
• When the construction of well foundation to be done under water. The construction
Of caisson are more preferable.
• When depth of water in river, lake, or sea etc. are more, than caisson structure is
used.
• Caisson are used as foundation for bridges piers, and abutments in rivers, seas, lakes,
break waters and other shore construction works.
• It is also used for pump house which are subjected to huge vertical as well as
horizontal forces.
• It is also occasionally used for large and multi-storey building and other structures.

Construction Material
• The Material used for the construction of caisson are
as follows:
• Wood
• Steel
• Reinforced Concrete

Shapes of Caissons
Caisson are constructed into two basic shapes and
combination of basic shape.
Shapes of Caissons:
(I) Basic Shape
(II) Combination of Basic Shape
Basic Shape:
(I) Circular
(II) Rectangular
(III) Square
(IV) Octagonal

Combination of Basic Shapes
• Double Circular
• Double Rectangular
• Double hexagonal
• Double-D
• Double Octagonal

Types of Caissons
• The Caissons is used for the purpose of placing a
foundation in correct position under water.
• Types of Caissons
• Open Caisson
• Box Caisson
• Pneumatic Caisson

Open Caissons
• It is a box type of structure which is open at the top
and at the bottom. Open Caisson are normally
used on sandy soils or soft bearing stratum and where
no firm bed is available at a higher depth.
• According to the shape of caissons, open caissons can
be further classified into three types as;
• Open Caisson
• Single Wall Open Caisson
• Cylindrical Open Caisson
• Open Caissons with dredging wells.

Construction of Open Caissons:
• The sinking process of open caisson can be done in the
following conditions:
• Dry
• Dewatered Construction
• Artificial Island
• In case of an artificial island called as sand island
method, the island can be made by raising the ground
surface above water level temporarily for obtaining
relatively dry area for the sinking process. The size of
sand island should be sufficient so that it can provide
more working space all around the caisson.

Construction of Open Caissons
• If dry Conditions is not possible on site, then caisson is built in barges or
slipways. Then it is towed to its towed to its final position by floating.
Guide piles are normally used for sinking the first few lifts. Note that
sinking of Caissons are directly done in open water.
• Excavation of soil by dredging the well sinks the caisson by its own weight
and the excavation process is done by dredging with the help of grab
buckets. The soil besides the cutting edge can be removed by man power.
Water Jets are used on exterior walls side so-as to run the sinking process
more easily.
• When the caisson is constructed to the required stage, then concrete seal
cap is provided caisson is dewatered by the method of pumping.

Advantages and Disadvantages of
Open Caissons
• Advantages of Open Caissons:
• Following are the advantages of Open Caissons:
• This type of Caisson can be extended up to large
depths.
• Cost of Construction is relatively less on bed level or
lower side.

Open Caissons
• Disadvantages of Open Caissons:
• Following are the disadvantages of open Caissons;
• Since the placing of concrete is done for concrete seal
under water, it may not be satisfactory.
• If any obstruction of boulders or logs are encountered,
then progress of work becomes slow.
• Through cleaning and inspection at the bottom of
caisson is very difficult and hence not possible.
• The help of divers may be required for excavation near
haunches at the cutting edges.

Box Caissons
• Box caisson is similar to open caisson, only
difference is that it is closed at the bottom.
• Box caisson is cast and cured properly
on ground and then it is launched in water
by filling sand or gravel or concrete in the empty
spaces.

Pneumatic Caissons
• This type of caisson is open at the bottom and close at
the top. Pneumatic caisson is specially used at the place
where it is not possible to construct the well.
• It is suitable for the depth of water more than 12 m.
• In the construction of Pneumatic Caisson,
the compressed air is used to remove water
from the working chamber and the foundation work
is carried out in dry condition.
• This type of caisson can be made of timber, concrete or
steel.

Components of a Pneumatic Caissons
Following are the various components of a pneumatic
caissons:
Air Shaft
A passage connecting in between the working chamber and
air lock is termed as 'air shaft'
This passage or air shaft is used by the workmen or workers
to reach to the working chamber to ground surface.
If caisson is too large in size, the separate unit of air shaft may
be provided for workers and material.
Air Shaft is made up of steel material. The joint involved in
air shaft are sealed by rubber gasket air lock is provided on
each air shaft at top. When sinking process is going on, air
shaft is extended above the water level.

Components ofa Pneumatic Caissons
• Working Chamber:
• Working Chamber height is about 3 m and is
totally air tight and made up of structural steel.
• To prevent the entry of air and water into the
chamber, the air inside the chamber is kept at a
pressure just more than atmosphere pressure.
External surface of chamber is kept thick.
Chamber is leak proof and smooth to reduce skin
friction. To facilitate the proper processing of
sinking, a cutting edge is provided at the bottom.

• Air Lock: A chamber made of steel provided at the upper end of the
air shaft above the water level is called as 'Air Lock'. Air lock
allows the worker or workmen to enter or exit from the caisson
without releasing the air pressure in the working chamber.
• The air locks has two air tight doors, one door opens into shaft and
another door opens to the atmosphere. When workmen enters the
airlocks through the outside door, then pressure in the chamber is
kept at atmospheric level. Pressure is increased gradually till it
becomes equal to the working chamber. Under this condition
workmen is allowed to go into the air shaft. Complete procedure is
again done when workmen comes out of the air shaft to air lock.
• By opening a valve in the airlock, fresh air is circulated in the shaft
workers or workmen are allowed to work into the working chamber
up to 2 hrs.
• The maximum limit of working into the chamber is 2 hrs.

• Miscellaneous Equipments
• Different types of miscellaneous equipments used
in pneumatic caisson are as follows:
• Pumps
• Motors
• Air Compressors
• These equipments are normally placed above the
bed level.
• Through compressed air pipe, can be applied to
the working chamber.

Advantages and Dis-advantages of
Pneumatic Caissons
• Following are the various advantages of pneumatic
caissons:
• Quality control is good because work is done in
dry conditions.
• Insitu soil tests are possible to determine the bearing
capacity.
• There is direct and easy passage to reach the bottom of
caisson, hence any obstruction can easily be removed.
• Concrete gain more strength due to dry conditions.
• For major projects, greater depths in bed rocks can be
possible.
• There is no danger of settlement of adjoining structures
because of no lowering of ground water table.

Disadvantages of Pneumatic Caisson
• Following are some of the disadvantages of pneumatic
caisson.
• Construction of pneumatic caissons is much expensive
than open caissons.
• During working the various constructional activities, a
proper care has to be taken, otherwise it may lead to
fatal accidents.
• Maximum depth below water table is limited to 30 m to
40 m. Beyond 40 m depth, construction is not possible.
• There is more chances of caisson diseases to workmen
working under high pressure.
• Labor cost is high.

Difference between Open Caisson and
Pneumatic Caissons

Loads on Caissons
Caisson is a well acted upon by the following loads:
Dead load of super-structure
Load of bearing pier
Various horizontal forces
Caissons is liable to acted upon by the following various horizontal
forces:
Wind force
Earth Pressure
Seismic load
Centrifugal forces
Water current forces
Bracking and tractive efforts ofthe moving vehicles
Force offered by the bearing against movement because of variation of
temperature.

Floating of Caissons
• When Caisson are casted away from the site or off-
site, then these caisson are transported to the required
location by means of floating, then it is called as
floating of caissons In short, method of transportation
the casted caisson off-site to the desired location by
floating is called as floating of caissons.
• Floating Caissons are commonly termed as box
caisson. A box caisson is cast and cured on land and
when required, it is launched in water and towed to
the site by floating for sinking operation.

Floating Caisson
• After Casting the box Caission, it is taken to the site by floating
in water, then it is called as'floating' Caission.
• Sinking Process can be made-faster by increasing the self weight
of caisson, self weight is increased by adding sand or gravel
inside the caisson.
• Floating caisson are not provided by cutting edge as compared to
the other caisson. This type of caisson is simple rest on a hard
or level strata. In this way, load carrying capacity is a function
the resistance at the base since there is no skin friction on sides.
• On the top of Caission, a concrete cap is provided to carry the
loads uniformly from the super structure. To prevent the scouring
at the base of floating caisson, rip-rap is provided around the base
for better understanding.

Construction of Floating Caissons
Following are the various steps involved in the construction process of
floating caissions:
Floating Caission are cast and cured on land.
After casting and proper curing, it is floated to the desired location.
Caissons are filled with gravel or sand so as to facilitate proper sinking
process.
The required base location is excavated and levelled.
Then Caission is sunk to required location and depth.
Rip-Rap is provided all around the base of caisson which prevents the
scouring.
Concrete cap is casted at the top of caisson so as to carry the loads
uniformly from superstructure.
Floating caissons are constructed in reinforced cement concrete or steel.
When it is to floated and placed in rough waters, internal strutting and
diaphragm walls are used.

floating Caissons
• Advantage of floating Caissons;
• Since concrete are pre-casted, good quality
control is possible
• Its installation is quick and more convenient
• These types of caissons are less expensive as
compared to other types of caissons

floating Caissons
• The foundation of floating caisson:
• The foundation bed require levelled surface
before installation of a floating caisson
• Rip-rap should be provided to protect the
caisson from scouring
• Floating caisson or box caisson is only
advantageous when hard strata is available.
• Its load carrying capacity is less as compared
to other caisson of equal and same size.

Components of Well Foundation
• Cutting Edge:
• Sharp edge which is provided at the lower end of the well or open
and pneumatic caisson for accelerating sinking operation is called
cutting edge. It is made up steel or it is made in R.C.C. Its angle to
vertical is 30 0 and normally slope of 1 horizontal to 2 vertical given
better result.
• In concrete caissons, the lower part of the cutting edge is rigidly
fastened with 12 mm steel plates with the help of steel strap.
• The cutting edge should be sharp so as to penetrate into the soil and
it should resist the various stresses caused by blows, boulders,
blasting, etc.
• A sharp vertical edge is generally provided to the outside face of the
caisson. Edge facilities the rate of sinking.

• Topping
• Covering provided over the well or caisson is called as topping.
• Sand is filled in between topping and bottom plug. Topping also
acts as a part of shuttering for laying the well cap.
• Bottom Plug:
• The lower portion of well is sealed by the concrete is called as
bottom plug.
• Stening
• Stening is constructed in concrete or masonary work.
• Use of stening is to provide dead load during sinking operation.

• Well Curbs
• It is made of concrete or brick. Cutting edge of well or
caisson is attached to well curb. During sinking
operation well curb impart to the well-stening and
facilities the formation of bottom
• Well Cap
• R.C.C Slab covering provided over the top of well is
termed as well cap.
• Sand filling
• The portion between top and bottom plug is filled with
sand so as to increase the self weight of the well and
makes safe during earthquake.

Sinking of Caissons
• Sinking of Caisson in dry river bed:
• Construction of caisson curb or well curb:
• Well curb or caisson curb is built in case of a dry river
bed, so as to place at the correct position after
excavating the bed for about 15cm for seating. If the
depth of water is upto 5m, then sand island is
constructed. For even distribution of load, wooden
sleeps can be placed below cutting edge. When the
shuttering of caisson curb is done, then reinforcement
for the curb is placed in position. Concreting of curb is
done in one stroke and it should be done without gap so
as to obtain monolithic concreting structure.

Sinking of Caissons
• Construction of Caisson steining or well
stening:
• For a height of 1.5 m, the caisson steining is
constructed at a time when the caisson reaches
at a depth of 6 m below ground stening can be
raised with a height of 3 m at a time. Sinking
of Caisson is commenced after concrete is set
for 24 hrs.

Sinking of Caissons
• Sinking process:
• When the curb is cast, then sinking operation is started.
• The first stage of stening is ready after curing. In inner material if
comes as a obstruction can be excavated manually or
mechanically. If hard rock comes in the way, then blasting may
be done.
• For proper sinking operation, additional loading termed as
kentledge is used if required. Sand bags can be used as kentledge
which can be placed on a suitable platform on the top surface of
caisson.
• When caisson reaches at a depth of 10 m, dewatering is done by
pumping. Jetting of water is also helpful in sinking operation.
Proper care has been done by adopting the proper measures and
techniques so as to avoid shifts and tilts of caisson during sinking

Tilting of Caissons
• At the time of sinking process caisson should sink
exact vertically downward, straight and at the
corner position without any tilting of caisson.
• If the caisson tilt any one side from its position
while sinking operation, then it is called as tilting
of caisson. During sinking operation, it may also
shift way from the required position. Hence it is
much essential to take the suitable precautions so
as to avoid tilting and shifting of caisson.

Precautions to be taken to avoid tilts
and shift
• The cutting edge of caisson must be thick and
sharp pointed
• The external surface of steining and caisson curb
should be smooth.
• Dredging should be done uniformly on all sides
and in all pockets of caisson.
• Caisson should be symmetrically placed.
• The diameter (D) of the curb must be placed from
40 mm to 80 mm or larger or more than external
diameter of steining.

Remedial Measures to Rectify Tilt
and Shift
• Following are the remedial measures to be carefully
implemented to avoid tilting of caisson during sinking
process:
• Water Jetting:
• This is the one of the method used to prevent tilting. In
this method, water jet is forcedly applied on tilt.
• Application of water jetting on higher side reduces skin
friction. Thus the tilting is rectified.
• This method is not more effective but gives the better
result if used with the combination of other methods.

and Shift

and Shift
• Eccentric loading:
• The caisson is normally given the additional
loading called kentledge in order to have
necessary sinking effort. In this method, eccentric
loading or kentledge is applied in higher side so
as to have greater sinking effort.
• For proper application of eccentric loading a
platform with projection on higher side can be
placed over the top of caisson
• The eccentric load is kept on projected part of
platform. Thus tilt can be rectified.

and Shift
• Excavation under cutting edge;
• During sinking process, filled caisson will not
set or straighten due to unbroken stiff strata on
its higher side. In such situation, dewatering is
preferably done to loosen stiff strata. If
dewatering is not possible or unsafe, then
drivers are sent to loosen the stiff strata.
• Sometimes if possible and safe, an open
excavation is done under the cutting edge.

Excavation under cutting edge;

and Shift
• Regulation of Excavation;
• Sinking of caisson on higher side due to
excess excavation is more. This is all right in
the early stages, otherwise dewatering of
caisson or well is needed and open excavation
may be done on higher side.

and Shift
• Providing temporary obstacles below the
cutting edge:
• Rectification of tilt can be done by inserting
the wooden sleeper temporarily as an obstacles
below the cutting edge on the lower side so as
to prevent further tilt of the well or caisson.
• Later on, wooden sleeper can be removed, for
better understanding.

Providing temporary obstacles
below the cutting edge:

and Shift
• Pushing the caissons or well with jack:
• Mechanical jack or hydraulic jack can be used
to rectify the tilt of well or caisson. Well or
caisson can be pushed by jack to bring it a
vertical position.

Pushing the caissons or well with
jack:

and Shift
• Pulling the well or caisson: This method is
most suitable and effective in preliminary or
early stages of sinking operation. Steel ropes
or cables are used pull the caisson or well.
Pulling of caisson or well is done on higher
side of well or caisson

and Shift
• Strutting the caissons or well:
• Method of strutting the caisson or well is used to
prevent any further and possible rise in tilting of
the caisson or well.
• The caisson or well is supported on the tilting side
by giving inclined support of a strong wooden
member. This inclined wooden member is called
as a strut.
• The caisson or well steining is provided so as to
distribute the uniform pressure or load from strut.

Strutting the caissons or well

Caisson Diseases
• In case of sinking process of pneumatic caisson, workers or
workmen have to work in working chamber under
compressed air. If the compressed air pressure is less than
0.35 N/mm 2 to 0.4 N/ mm 2, then workmen may suffer
from the following pains:
• Workmen may suffer from giddiness
• There is pains in ears of workmen
• There is breaking of ear drums of workmen
• There is bursting of blood vessels in the nose or ears of
workmen
• The above mentioned pains are not that serious or fatal, but
workmen is actually suffering during decompression and
effect causing depression is called caisson disease.

Caisson Diseases
• Following are the caisson disease caused by decompression:
• Severe pains in joints leading to bends
• It may cause paralytic death
• Excessive oxygen get absorbed in the blood and tissues
during decompression is more trouble some to workmen.
Absorbed oxygen gas is thrown out of blood in the form of
bubbles which can block in vessels and may cause bursting
of vessels.
• If bubbles are developed in joints it causes bends
• If the bubbles are developed in spinal cord, it causes
paralysis and if the bubble are developed in heart, it causes
heart attack.
• Caisson diseases can be controlled by recompression
followed by slow decompression.

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