This document defines and describes caissons and cofferdams. It explains that caissons are watertight structures sunk into the ground or water to exclude water during foundation excavation. They are made of materials like steel, timber, or concrete. Different types include box, open, and pneumatic caissons. Workers in pneumatic caissons can experience decompression sickness. Cofferdams are temporary structures that hold back water to allow dry construction. Common types are earth fill, rock fill, timber, steel, plastic, and concrete walls. Design considerations include stability against forces from water, ice, and earth pressures.

1. CONTENT
 Definition of cassion
 Use of caisson
 Material use for caisson
 Type of caisson
 Shape of caisson
 Caisson sickness(disease)
 Precaution against Caisson disease
•Introduction of coffer dam
•Requirement of coffer dam
•Types of Cofferdam
•Design Consideration
•Advantage of cofferdam

2. DEFINITION OF CASSION
 Caisson is defined as watertight structure round or rectangular ,
which is sunk through ground or water to exclude water and semi
fluid material during the process of excavation of foundation and
become a part of the substructure.

3. USE OF CASSION
 To reach the hard bearing strata for transferring the load of
superstructure.
 For excavation for foundation of bridges , piers , abutments in
rivers and lakes.
 To provide an access to a deep shaft and tunnel.

4. MATERIAL USE FOR CASSION
 Steel
 Timber
 Reinforced cement concrete
 Cast iron

5. TYPE OF CASSION
 Box caisson
 Open caisson(open at top and bottom)
1. Single
2. Multiple
3. Cylinder
 Pneumatic caisson(closed at top and open at bottom)

6. SHAPE OF CAISSONS

7. OPEN CAISSON

8. BOX CAISSON

9. PNEUMATIC CASSION
 Open at bottom and closed at top.
 It is useful where it is not possible to adopt well.
 Suitable when depth of water more than 12m and maximum
depth of water up to 35m.
 Important feature of a pneumatic caisson is that
compressed air is used to remove water from working
chamber at bottom and foundation work Is carried out in
dry condition.

10. PNEUMATIC CASSION SICKNESS
 The worker working under compressed air inside the
working chamber , suffer a certain type of disease
when they return to atmospheric pressure . This
disease know as caisson sickness or caisson disease.
 The main symptoms of the disease are
I. Dizziness
II. Double vision
III. Headache
IV. Trouble to speaking
V. Pain in leg etc…

11. PENUMATIC CASSION

12. The following precaution should be
taken to avoid the caisson sickness.
 No person should work for more than one shift in a day.
 The duration of shift should not exceed 12 hours.
 The rate of compression and decompression should be properly
control.
 person with strong hearts , relatively low blood pressure and
good circulation should be employed on the work.
 The medical chamber must be provided just near the side work.
 Use of alcoholic drinks should be prohibited.

13. The following precaution should be
taken to avoid the caisson sickness.
 The worker should be made fully conversant
with use of the bells , signals , telephones etc…
 All the passenger , shafts etc. of the pneumatic
caisson should be maintained clear and kept
properly lighted.

14. What is Cofferdam?
 Cofferdams are temporary enclosures to keep out water
and soil so as to permit dewatering and construction of the
permanent facility (structure) in the dry.

15. REQUIREMENT OF COFFER DAM
 It should be a water tight.
 It should be generally constructed at site of work.
 The design and layout of coffer dam should be such a that total cost of construction ,
maintenance and pumping is minimum.
 It should be stable against bursting , overturning , under the flood and wave.

16. Types of Cofferdam
• The followings are the different types of cofferdam:
 Earth fill embankment
 Rock fill embankment
 Timber sheet pile
 Steel sheet pile
 Plastic sheet pile
 Concrete walled

17. Earth fill Embankment
 Its is suitable for narrow and sluggish river.
 Suitable for low and moderate head of water.
 Problem of erosion in sand embankment is common, solution can be the provision of the
drainage ditches.

18. Rock fill Embankment
 Suitable for steeper slope.
 It can be either dumped or hand packed.
 For hand packed rock-fill problem of seepage may occur. It can be removed by dumping
the fly-ash on outer side.
 A fair degree of water tightness is gradually occur.

20. Timber Sheet Pile
 Timber sheet pile were extensively used for cofferdam work in 19 century.
 It can be gradually replaced with steel pile.
 It can only be used for small depth since no deep piling in gravely strata possible.

21. Steel Sheet Pile
 Widely used specially for deepest type because of its structural strength.
 Very good in water tightness due to its interlocking section.
 Deep penetration provide resistance to inward movement and also provide sufficient cut-
off to prevent piping in permeable ground.

23. Plastic Sheet Pile
 A smooth, flat external wall surface improving asthetics through complete elimination of
traditional ridges.
 Less complicated formwork requirements, reducing installation time.
 Stiffer section of uniform configuration maximizing installation speed.
 Double interlocking.

25. Concrete Walled Cofferdam
 Where high heads and water velocities must be resisted, elaborate forms of cofferdam is
necessary.
 It is very costly so all other opinion must be screened before its selection.
 These can be used in different forms, like circular or semi circular concrete shells.

26. Cofferdam Design Consideration
 Scouring or undermining by rapidly flowing water.
 Stability against overturning or tilting.
 Upward forces on outside edges due to tilting.
 Stability against vertical shear.
 Effects of forces resulting from: ice, water, wave, active earth, passive earth pressures.

27. Advantages of Cofferdam
 Allow excavation and construction of structures in otherwise poor environment.
 Provides safe environment to work.
 Sheet piles are easily installed and removed.
 Materials typically can be reused on other projects.

28. Items Needed for Installation
 Pile driving hammer.
 Vibratory or impact hammer.
 Crane of sufficient size.
 Steel sheet piles are typically used etc….