1. Caissons are hollow structures that are installed in place and then filled with concrete or other material. They are used as foundations under water. 2. There are three main types of caissons - open caissons which are open at the top and bottom, box caissons which are closed at the bottom, and pneumatic caissons which are closed at the top and use compressed air. 3. Cofferdams are temporary structures built around construction sites under water. They exclude surface and ground water to provide a dry work area. Common types include braced, earth-fill, timber crib, and double-walled sheet pile cofferdams.

1. MODULE 2
CHINNU MARIAM NINAN
ASSISTANT PROFESSOR, CE
TKMCE

2. CAISSONS
“The caisson is a member with hollow portion, which after installing in
place by any means is filled with concrete or other material .”
 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.
It is preferred in sandy soils.

3. Purpose:
For placing a foundation in correct position under water.
Uses:
• 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, then 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.

4. • Shape of caissons
• Caisson consists of basic shapes and combination of basic shape.
• Basic Shape: (I) Circular (II) Rectangular (III) Square (IV) Octagonal
• Combination of basic shapes: (I) Double Circular (II) Double
Rectangular (III) Double hexagonal (IV) Double-D (V) Double
Octagonal

6. Types of Caissons:
 Open caissons or wells.
 Box caissons.
 Pneumatic caissons.

7. • 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:
• Single Wall Open Caisson
• Cylindrical Open Caisson
• Open Caissons with dredging wells.

9. OPEN CAISSON OR WELLS:
The caissons are open at both top and bottom.
They are used on sandy or soft bearing stratum.
They are generally built of timber, metal, reinforced concrete or masonry.
They form the most common types of deep foundations for bridges in INDIA.
Shapes of WELLS:
• Circular
• Rectangular
• Double-D
• Rectangular with D-shaped ends
• Twin hexagonal and two octagonal.

10. WELL COMPONENTS AND THEIR
FUNCTIONS:
 Cutting Edge:
 Provides a comparatively sharp edge to cut the soil below
during sinking operation.
Curb:
1. It has a two-fold purpose.
2. During sinking it acts as an extension edge & also provides support
to the well stein & bottom plug,
3. After sinking it transfer the load to the soil around.

11.  Steining:
1. It is the main body of the well.
2. It also serves dual purpose.
3. It acts as a cofferdam during sinking & structural member
to transfer the load to the soil afterwards.
 Bottom Plug:
 To transfer load from the steining to the soil below.
 Sand Filling:
 It is supposed to afford some relief to the steining by
transferring directly a portion of load from well cap to the
bottom plug.

12. Top Plug:
 Serves as shuttering for laying the well cap.
Reinforcement:
 Provides requisite strength to the structure during sinking
& service.
Well Cap:
 To transfer the loads & moments from the pier to the well or
wells below.

13. 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.

14. PROBLEMS IN WELL SINKING:
1. Sand Blowing:
 The trouble of sand blowing takes place during the process of dewatering of the well passing
through sandy strata.
 The fall of sand in the caisson is so sudden and huge that it amount to a depth of about 3 to 15
m of sand.
2. Tilting of well:
 When a well sinks more on one side than the other then it is known to have tilted.
 The tilting is mainly due to unequal dredging and non uniform bearing power of soil.

15. Following method may be employed to bring tilted
well in position
 Control of dredging
 Eccentric Loading
 Water Jetting
 Pulling the well
 Pushing by Jacks
 Use of explosives
 Deposition of Earth on one side and excavation on other
 Providing temporary obstacle below the cutting edge

16. Shifting of Wells:
The shifts change span lengths and thereby induce eccentric loads on the well
steining and the foundation.
The magnitude of the ill effects depends upon the size of the well and depth to which
it is sunk.
If a simple tilt occurs at a certain depth and the sinking continued till designed
foundation depth is reached, the shift at the bottom could be greater than at top.

17. The following precautions should be taken to
avoid tilts and shifts during sinking operations:
The outer surface of the well curb and steining should be as regular and
smooth as possible.
The radius of curb should be kept 2 to 4 cm larger than the outside radius
of well steining.
The cutting edge of the curb should be uniform in thickness and
sharpness.
The dredging should be done uniformly.

18.  BOX CAISSONS:
• Box caisson is similar to open caisson, only difference is that it is closed at the
bottom.
• A box caisson is open at top and closed at bottom.
• It is merely a variation of the suspended type cofferdam.
• The box caissons may be built of reinforced concrete, steel or timber.
• 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.

19. PNEUMATIC CAISSONS:
 A pneumatic caisson is open at bottom
and closed at the top.
 This is useful at location where it is not
possible to adopt well.
 They are suitable when depth of water
is more than 12 m.
 The maximum depth of water up to
which pneumatic caisson can be used is
limited from the consideration of health
of the workers.
 In this method the compressed air is
used to remove water from the working
chamber and the foundation work is
carried out in dry conditions.
They can be made of timber, concrete or
steel.

21. COFFERDAMS
 Requirement of a cofferdam:
 It should be reasonably water tight.
 The total cost of the construction, maintenance and pumping is the minimum.
 It should be generally constructed at site of work.
 It should be so planned as to facilitate any dismantling and reuse of materials.
“Cofferdams are temporary structures
designed to exclude both surface water
and ground water from the excavation
area.”
They provide an impermeable structure
around the periphery of the construction
area resulting in relatively dry work space.

22. • A cofferdam is a temporary structure designed to keep water
and/or soil out of the excavation in which a bridge pier or other
structure is built.
• When construction must take place below the water level, a
cofferdam is built to give workers a dry work environment.
• Sheet piling is driven around the work site, seal concrete is
placed into the bottom to prevent water from seeping in from
underneath the sheet piling, and the water is pumped out.
• The word "cofferdam" comes from "coffer" meaning box, in
other words a dam in the shape of a box.

23. Types of cofferdam:
 Braced cofferdam
 Earth-type cofferdam
 Earth fill cofferdam
 Rock fill cofferdam
 Timber crib cofferdam
 Single wall cofferdam
 Double wall cofferdam
 Cellular cofferdam

24.  Selection of cofferdam:
 Extent of area to be protected by a cofferdam.
 The depth of water to be dealt with.
 Velocity of flowing water.
 Nature of bed on which cofferdam is to rest.
 Availability of construction materials in the vicinity of site of work.
 Transportation facilities available.

25. BRACED COFFERDAMS
• Formed from a single wall of
sheet piling.
• Driven into the ground to form a
box around the excavation site.
• The "box" is then braced on the
inside.
• Interior is dewatered.
• Primarily used for bridge piers in
shallow water.
(30 - 35 ft depth)

26. EARTH-TYPE COFFERDAMS
• It is the simplest type of
cofferdam.
• It consists of an earth bank
with a clay core or vertical
sheet piling enclosing the
excavation.
• It is used for low-level waters
with low velocity and easily
scoured by water rising over
the top.

27. TIMBER CRIB COFFERDAM
• Constructed on land and floated into
place.
• Lower portion of each cell is matched
with contour of river bed.
• It uses rock ballast and soil to decrease
seepage and sink into place, also known
as “Rock filled crib cofferdam”.
• It usually consists of 12’x12’ cells and is
used in rapid currents or on rocky river
beds.
• It must be properly designed to resist
lateral forces such as tipping /
overturning and sliding

28. DOUBLE WALLED SHEET PILE /
DOUBLE WALL COFFERDAM
• They are double wall cofferdams
comprising two parallel rows of
sheet piles driven into the ground
and connected together by a
system of tie rods at one or more
levels.
• The space between the walls is
generally filled with granular
material such as sand, gravel or
broken rock.

29. CELLULAR COFFERDAM
• Cellular cofferdams are used only
in those circumstances where the
excavation size precludes the use
of cross-excavation bracing.
• In this case, the cofferdam must
be stable by virtue of its own
resistance to lateral forces.

30. DESIGN CONSIDERATIONS
• Scouring or undermining by rapidly flowing water
• Stability against overturning or tilting
• Upward forces on outside edge due to tilting
• Stability against vertical shear
• Effects of forces resulting from:
• Ice, Wave, Water, Active Earth and
• Passive Earth Pressures

31. ADVANTAGES
• Allow excavation and construction of structures in otherwise
poor environment.
• Provides safe environment to work
• Contractors typically have design responsibility
• Steel sheet piles are easily installed and removed
• Materials can typically be reused on other projects

32. •Items needed for installation:
•Pile driving hammer
•Vibratory or Impact
•Crane of sufficient size
•Steel sheet piles are typically used
•H-piles and/or wide-flange beams for wales and
stringers
•Barges may be required

33. COMPONENTS
• SHEET PILING
• BRACING FRAME
• CONCRETE SEALS
• BEARING PILES

34. • The typical cofferdam, such as a bridge pier, consists of sheet piles set around
a bracing frame and driven into the soil sufficiently far to develop vertical and
lateral support and to cut off the flow of soil and, in some cases the flow of
water.
• The structure inside may be founded directly on rock or firm soil or may
require pile foundations. In the latter case, these generally extend well below
the cofferdam. In order to dewater the cofferdam, the bottom must be stable
and able to resist hydrostatic uplift. Placement of an underwater concrete
seal course is the fastest and most common method.
• An underwater concrete seal course may be placed prior to dewatering in
order to seal off the water, resist its pressure, and also to act as a slab to
brace against the inward movement of the sheet piles in order to mobilize
their resistance to uplift under the hydrostatic pressure.

35. (i) BRACING FRAME AND SHEET PILES (ii) UNDERWATER CONCRETE SEAL COURSE

36. CONSTRUCTION SEQUENCE
1. Pre-dredge to remove soil or soft sediments and level the
area of the cofferdam.
2. Drive temporary support piles.
3. Temporarily erect bracing frame on the support piles.
4. Set steel sheet piles, starting at all four corners and
meeting at the center of each side.
5. Drive sheet piles to grade.
6. Block between bracing frame and sheets, and provide ties
for sheet piles at the top as necessary.
7. Excavate inside the grade or slightly below grade, while
leaving the cofferdam full of water.
8. Drive bearing piles.
9. Place rock fill as a leveling and support course.
10. Place tremie concrete seal.
11. Check blocking between
bracing and sheets.
12. Dewater.
13. Construct new structure.
14. Flood the cofferdam.
15. Remove sheet piles.
16. Remove bracing.
17. Backfill.