Civil Engineering

1. Contents
1 INTRODUCTION
2 TYPES OF COFFERDAMS
2.1 Braced Cofferdams
2.2 Earth-Type Cofferdams
2.3 Timber Crib Cofferdam
2.4 Double-walled cofferdam
2.5 Cellular cofferdam
3 COFFERDAM DESING CONSIDERATIONS
4 FORCES ACTION ON COFFERDAM
5 EQUIPMENT AND MATERIAL REQUIRED FOR INSTALLATION
5.1 Pile driving hammer
5.2 Cranes with clamshell buckets

2. 5.3 Concrete pump truck
5.4 Pumps for dewatering
5.5 Barge
6 COFFERDAMS COMPONENTS
7 GENERAL CONSTRUCTION METHOD
8 REMOVAL OF COFFERDAM
9 APPLICATION OF COFFERDAM
9.1 Safety requirements
9.2 Advantages of cofferdams
9.3 Disadvantages of cofferdams
10 REFERENCES

3. ABSTRACT
 Coffer dams are temporary enclosures to keep out water and soil so
as to permit dewatering and construction of the permanent facility in
the dry environment.
 The word "cofferdam" comes from "coffer" meaning box, in other
words a dam in the shape of a box.
 Generally a cofferdam involves the interaction of structure, soil and
water. In the construction of cofferdams maintaining close tolerances
is difficult since cofferdams are usually constructed offshore and
sometimes under severe weather conditions.
 Under these circumstances, significant deformations of cofferdam
elements may happen during the course of construction and
therefore it may be necessary to deviate from the design dimensions
in order to complete the project according to plan.

4.  In our report we will be focusing on the construction process,
equipment used for construction and the various types of coffer
dams.
 And also we will be discussing a case study which describes the
construction of a cofferdam that was built to facilitate the
construction of sub weir and rehabilitation of Taunsa Barrage,
which is situated on the huge river of the Indus valley known as
the Indus River, in the province of Punjab, India.
 In this case study, the equipments used in the cofferdam
construction are dump trucks, loaders, scrapers, vibratory pile
driver, backhoe and dewatering pumps.
 The means and methods utilized to construct the coffer dam and
their productivity analysis are clearly shown.
 To facilitate productivity calculations the project was divided
into four phrases.
 Taunsa barrage was constructed in the year 1958 and currently
supplying water for four main canals - two on the right of the
bank and two on the left of the bank.

5. 1. INTRODUCTION
 A cofferdam is a temporary construction method used in order to do
construction in wet excavations.
 It is installed in the work area and water is pumped out to expose
the bed of the body of water so that workers can construct
structural supports, perform repairs and any other types of work
using construction equipment.
 A coffer dam is also called as caisson in some parts of world.
 Working inside a coffer dam can be dangerous if it is not installed
properly or not safely pressurized. Various materials are used for its
construction and its design must be compatible with weather
conditions, waves, currents, construction equipment, construction
methods, internal permanent structures and ground conditions.
 There are various types of cofferdams such as braced, earth type,
timber crib, double walled sheet pile and cellular which are
discussed below

6.  Generally, major loads imposed on cofferdams are hydrostatic
forces of water and dynamic forces due to current and waves
and heavy equipment is used for its construction such as pile
drivers, cranes with clamshell buckets, concrete pumps trucks
as well as pumps for dewatering are used in the construction
process.
 The effective management of equipment on site as well as
workers is an important step in cost control and maintaining
efficient productivity.
“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.”
- Standard Handbook of Heavy Construction

7. 2. TYPES OF COFFERDAMS
The construction process for each type is different based on whether
it is used on land or in water, as illustrated in figure 1. In general
there are five types of coffer dam and they are as follow (Nemati,
2007):
 Braced
 Earth-Type
 Timber Crib
 Double-Walled Sheet Pile
 Cellular

8. TYPES OF COFFERDAMS
2.1 Braced Cofferdams
 Braced cofferdam is formed from a single wall of sheet piling.
 It is constructed by driving sheet piles into the ground to form a
box around the excavation site and then this “box” is braced on the
inside of it.
 Interior is dewatering using pumps.
 They are primarily used for bridge piers in shallow water around 30-
35ft depth.

9.  2.2 Earth-Type Cofferdams
It is simplest type of cofferdam, consists of an earth bank with a
clay core or vertical sheet piling enclosing the excavation. Used for
low-level waters with low velocity and can be easily scoured by
water rising over the top.
 2.3 Timber Crib Cofferdam
It is one of the kinds of cellular-type cofferdam. It is first
constructed on land and then floated into required place. The lower
portion of each cell matched with contour of river bed. It uses rock
ballast and soil to decrease seepage and sink into place. It is also
known as “Gravity Dam”. In general it consists of 12’ x 12’ cells. It
is used in rapid currents or on Rocky River beds. It should be
properly designed to resist two lateral forces i.e
tipping/overturning and sliding.

11. Figure 1 Types of cofferdams. For use on land: (a) cross-braced
sheet piles; (b) cast-in-place concrete cylinder; (c) anchored sheet
piles; (d) braced vertical piles with horizontal sheeting. For use in
water: (e) cross-braced sheet piles; (f) earth dam; (g) tied sheet
piles; (h) anchored sheet piles with earth berm; (i) steel sheet-pile
cellular cofferdam; (j) rock-filled crib.
 2.4 Double-Walled Cofferdam
In this type of cofferdam, two-parallel rows of steel sheet piles are
driven into the ground and tied together with anchors and wales
then filled with soil. There are three principle types:
- Box: Consists of straight flush walls
- Semicircular cells connected by diaphragms
- Circular cells connected with tie-rods or diaphragms

12.  2.5 Cellular Cofferdam
There are two main types of cellular cofferdam they are
circular and segmental. It can be used ona temporary or
permanent basic. In this type of cofferdam force are
resisted by the mass of the cofferdam. (Nemati, 2007)

13. 3. COFFERDAM DESIGN CONSIDERATIONS
 The following are some of the design considerations
which should be checked before the construction and
during the design of cofferdam.
 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

14.  An important consideration in the design of cofferdams is the
hydraulic analysis of seepage conditions and erosion of the
bottom when in streams or rivers.
 Significant deformations of elements may occur at different
stages of construction because of the typical construction of
coffer dam under adverse conditions in a marine environment,
thus it is difficult to maintain close tolerances.
 Provisions must be made for deviations in dimensions so that the
finished structure may be constructed according to plan.
 Deconstruction of the cofferdam must be planned and executed
with the same degree of care as its installation, on a stage-by-
stage basis. The effect on permanent structure due to the
removal of coffer dam must be considered.
 Due to this reason, sheet piles extending below the permanent
structure are often cut off and left in place, because their
removal may affect the foundation soils adjacent to the
structure. (Nemati, 2007)

15.  Where the cofferdam structure can be built on a layer of
impervious soil, the area within the cofferdam can be
completely sealed off. Where the soils are pervious, the
flow of water into the cofferdam cannot be completely
stopped economically, and the water must be pumped out
periodically and sometimes continuously.
 A dewatered area can be completely surrounded by a
cofferdam structure or by a combination of natural earth
slopes and cofferdam structure. The type of construction is
dependent upon the depth, soil conditions, fluctuations in
the water level, availability of materials,
working conditions desired inside the cofferdam, and
whether the structure is located on land or in
water. (Washington, 2013)

16. 4. FORCES ACTING ON COFFERDAM
A cofferdam involves the interaction of the structure, soil, and
water. The loads imposed include the following:
1. Hydrostatic pressure
2. Forces due to soil loads
3. Current forces on structure
4. Wave forces
5. Ice forces
6. Seismic loads
7. Accidental loads
8. Mooring forces
9. Scour

17.  The loads imposed on the cofferdam structure by
construction equipment and operations must also be
considered during installation of the cofferdam a well as
during construction of the structure itself. (Nemati, 2007)
 Hydrostatic pressure
 Two factors must be considered they are the maximum
probable height outside the cofferdam during
construction and the water height inside the cofferdam
during various stages of construction. The hydrostatic
pressure for partially dewatered cofferdam is shown in
figure 3

18. Figure -
2
Figure -
3
W = unity weight of water
h1 = outside water height
f1 = outside hydrostatic
force
f2 = inside hydrostatic force
If h1 = 2h2 then f1 = 4f2
and f3 = ¾ f1

19.  Forces due to Soil Loads
 The soils impose forces acts locally on the wall of the
cofferdam and globally upon the structure as a whole.
Local forces are main component of the lateral force on
sheet-pile walls, causing bending in the sheets, bending
in the wales, and axial compression in the struts. These
forces are added to the hydrostatic forces. Active
pressure and passive pressure due to soil load is shown
in the figure 4.

20. Figure - 4
Figure - 5

21. Current Forces on Structure
 In a cofferdam, the current force consist not only the force acting on the normal
projection of the cofferdam but also on the drag force acting along the sides. With
flat sheet piles, the latter may be relatively small, whereas with z-piles it may be
substantial, since the current will be forming eddies behind each indentation of
profile, as shown in figure 5.
Ice forces
 These are of two types, that is the force exerted by the expansion of a closed-in
solidly frozen-over area of water surface which is called as static ice force and the
forces exerted by the moving ice on breakup which is called as dynamic ice force.
(Nemati, 2007)
Seismic Loads
 In most of the projects, they are not considered in design of temporary structures.
But for very large, important, and deep cofferdams in highly seismically active
areas, seismic evaluation should be performed.

22. Accidental loads
 Accidental loads are the loads usually caused by construction
equipment working alongside the cofferdam and impacting on it
under the action of waves.
Mooring forces
 They are derived from two separate actions. The first is the impact
of the barge and tugboats as they moor to the cofferdam or the
waves are produced as they move the barges while moored. The
other force is the wind pressure on the total sail area of the barge.
Gale force wind is a common occurrence along most coasts and on
large lakes. The combination of high wind and waves will cause
major damage to the cofferdam and equipment if no preparation is
made to accommodate those events. (Washington, 2013)

23. Scour
 Scour of the river bottom or seafloor along the cofferdam may take
place due to river currents, tidal currents, or wave-induced
currents. Some of the most serious and disastrous cases have
occurred when these currents have acted concurrently .A very
practical method of preventing scour is to deposit a blanket of
crushed rock or heavy gravel around the cofferdam, either before
or immediately after the cofferdam sheet piles are set. A more
sophisticated method is to lay a mattress of filter fabric, covering it
with rock to hold it in place. (Nemati, 2007)

24. 5. EQUIPMENTS AND MATERIAL
REQUIRED FOR INSTALLATION
 Equipment’s:
 Pile driving hammer
 Vibratory or Impact
 Crane of sufficient size- clamshells and draglines
 Concrete pumps trucks
 Dewatering pumps
 Barges may be required
 Dozer, loader, backhoe, trucks be may required
 Materials:
 Steel sheet piles are typically used
 H-piles and/or wide-flange beams for wales and stringers

25.  5.1 Pile driving hammer
 A pile driver is a mechanical device used to drive piles into soil to provide
foundation support for buildings or other structures. The following are different
types of pile driving hammers:
 Diesel hammer.
 Hydraulic hammer.
 Hydraulic press-in.
 Vibratory pile driver.
 In the above 4 types of pile driving hammer, vibratory pile driver is most commonly
used for construction of cofferdam.

26.  5.1 Pile driving hammer
 Vibratory pile hammers contain a system of counter-rotating
eccentric weights which are powered by hydraulic motors and
designed in such a way those horizontal vibrations cancel out, while
vertical vibrations are transmitted into the pile.
 The pile driving machine is lifted and positioned over the pile by
means of an excavator or crane, and is fastened to the pile by a
clamp as shown in figure 6iaction breaks friction resistance
between pile surface and the soil, thus the force of gravity acting
on hammer causes the pile to sink.
 Vibrating hammer typically weighs from 2 to 20 tons and they are
used to drive bearing as well as sheet piles.

27.  Productivity Factors:
 The type of piling being driven
 The subsurface soil conditions
 Power requirement
 Type and size of the hammer
 Length of the crane boom
 The length of leads required (if they are used)
 Type and size of the crane used

28. Figure – 6 pile hammers
Figure – 7 crane with pile hammer

29.  Productivity:
 Operational efficiency for a pile driver typically range from 30 to
40 min per hour because of time required to move the crane and
equipment to the location of each pile and set up everything for
driving.
 Cycle time depends on cross-section of pile, its length and
subsurface soil conditions.
 Productivity = Estimating driving rate * operating factor
 The time required to drive piles (A) = (no of piles *length of the
pile) / (productivity*working hours per day)
 Total time = A + setup time + demolition time
(Schaufelberger, 1998)

30.  5.2 Cranes with clamshell buckets
 A crane-shovel which is equipped with crane boom, clamshell
bucket and accessories, fairlead assembly, and necessary cables is
known as clamshell. Clamshell is capable of operating at , above,
and below ground level and it can handle material ranging from soft
to medium stiff soils . It is used to lift material vertically during
construction of coffer dam. The clamshell is capable of excavating
to great depths but lacks the positive digging action and precise
lateral control as that of backhoe and shovel.

31. Figure -8 clamshell

32.  Productivity:
 Clamshell has three capacities heaped capacity, plate line
capacity/struck capacity and water-level capacity.
 Productivity factors:
 Class of materials.
 Height of lift.
 Angle of swing.
 Bucket size.
 Boom length.
 Job and management conditions.
 Disposal methods.
 Size of hauling units.
 Operator skills.
 Equipment conditions.

33.  General output model:
 Hourly output (cy/hr or m3/hr) = P = (3600 *Q * f * k* f1 * f2 * t)/CT
 Where
k = bucket fill factor.
P = productivity in cy/hr or m3/hr.
Q = bucket capacity in loose cy or m3.
f = earth volume change conversion factor.
f1 = swing-depth factor (Table 18.4).

34.  f2 = job and management conditions.
 t = operating time factor.
 CT = cycle time in seconds (Table 18.2).
 Productivity = Volume per cycle * Cycles per hour
 Productivity (cy/hr or m3/hr) = (3600 * Bucket capacity * Bucket fill factor * Job effiecency )/CT
 Where, CT= cycle time in sec, Bucket capacity in cy or m3
 5.3 Concrete pump truck
 The concrete pump is a machine which is used for transferring liquid concrete by pumping.
 There are two types of concrete pumps.
 The first type of concrete pump is attached to a truck. It is known as a trailer-mounted boom concrete pump because it uses
a remote-controlled articulating robotic arm to place concrete with pinpoint accuracy. Boom pumps are used on most of the
larger construction projects as they are capable of pumping at very high volumes and because of the labor saving nature of
the placing boom. They are a revolutionary alternative to truck-mounted concrete pumps. They are used in the construction
process of a cofferdam.

35. The second main type of concrete pump is either mounted on a truck and
known as a truck-mounted concrete pump and it is commonly referred to as a
line pump or trailer-mounted concrete pump. They are commonly used for
placing concrete applications such as swimming pools , sidewalks, and single
family home concrete slabs and most ground slabs.
Concrete pump process depends on the receipt of mixed concrete from the
concrete batch plant to the concrete pump to pump the concrete into the
formwork in the site. This operation has
several factors affecting the productivity that must be taken into
consideration from the hauling unit (Truck mixer) and the concrete pump, such
as:
 Workability of the concrete.
 Capacity of the pump.
 Speed of the pumping
 Crew skills.
 Type of formwork.
 Layout of the site.
 Height at which concrete has to be pumped.

36. Figure – 9 trailer – mounted boom concrete
pump

37. Figure –
10

38. To calculate Concrete pump production we have to image the process of pumping concrete to the final
place could be column, slab, wall formwork or any shape that need to fill concrete I assume the
process it will be as;
 RT 1 = Required time to pump maneuver for the first position.
 RT2 = Required time to change the position of the pump * No. of Positions Change.
 Where; No. of Positions Change depends on the site size and the boom length and the operation skills
are determining the no. of position change.
 RT3 = Required time to Truck mixer maneuver for the first position
 RT4 = Required time to pump concrete from the truck
 Where, CF4 = Truck mixer volume / (Speed of Pump * Type of formwork factor)
 RT5 = Required time to switch the mixing trucks * No. of Mixer Trucks
 Where; No. of Mixer Trucks = Total Concrete required / Truck mixer volume Concrete pump Production (CCY / hr.) =
Required Volume of Concrete / Total Time
 Where; Total Time = RT 1 + RT 2 + RT 3 + RT 4 + RT 5

39.  5.4 Pumps for dewatering

 Dewatering pumps are used to pump water from the interior of
the cofferdam and it should be done in such a way as to preclude
the possibility of water moving through uncured masonary or
concrete.

 Pumping is done by placing sump outside the horizontal limits and
below the elevation of the work being placed or as directed by
the engineer. Pumping to dewater a cofferdam should not start
untill any underwater concrete has sufficiently set to withstand
the hydrostatic pressure generated by the pump.

40. Figure -11 dewatering
pumps
Figure –
12
Figure – 13 steel sheet
piling

41. Figure – 14 H-piles and /or wind flange
beams
5.5 Barge:
A barge is a flat-bottomed boat, built mainly for river and canal transport of
heavy goods. Some barges are not self-propelled and need to be towed or
pushed by towboats. If the cofferdam is constructed far away from land then
the goods and equipment are required to be transported to the site of
cofferdam construction. In barge is used to transport the required heavy goods
and equipment to the site.

42. Properties of Steel Sheet Piling:
The following are the properties of steel sheet piles:
Moderately watertight
High shear and bending strength
High interlock strength
Easy to install/remove, Reusable
Can be cantilevered but typically require additional structural member i.e.
wales and cross bracing.

43. Steel Sheet Pile Interlocks
There is no industry Standard for steel sheet pile interlocks. Interlocks
should fulfill the following requirements:
Figure -15 traditional sheet pile
shapes

44.  Provide relative water or earth-tight connections

 Permit reasonable free sliding to connect sheets during
installation

 Provide minimum guaranteed pull strength

 Allow minimum swing between locks in order to form a
circle


45. Figure -16 typical types of
interlocks

46. The following are 4 types of components in braced type cofferdam:
 Sheet piling
 Bracing frame
 Concrete seal
 Bearing piles
 The typical cofferdam, such as a bridge pier, consists of sheet piles
set around a bracing frame and driven into the soil to sufficient
depth to develop vertical and lateral support and to cut off the
flow of soil or water. (Nemati, 2007)
 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. To dewater the cofferdam
bottom must be stable and able to resist hydrostatic uplift.
Placement of an underwater concrete seal course is the fastest and
most common method to withstand uplift
6. COFFERDAM COMPONENTS

48. Step-1
Pre- dredge to remove soil or soft sediments and level the area of the
cofferdam
Step-2
Drive temporary support piles and temporarily erect bracing frame on
the support piles .
SHEET PILE COFFERDAM CONSTRUCTION SEQUENCE:
For a typical cofferdam, such as for a bridge pier, the construction procedure follows
the listed pattern.

49. Drive sheet piles to grade and ties are provide for sheet piles at the top as
necessary .
Excavate slightly below grade , while leaving the cofferdam full of water
and drive bearing piles . Place rockfill as a leveling and support course .

50. Step -
5
Figure -
tremie
Figure – concrete pouring
using
Figure – tremie
concrete

51. Step -6
Check blocking between bracing and sheets and
dewater
Remove sheet piles and bracing, as well as backfilling and construct new
structure

52. 7. GENERAL CONSTRUCTION
METHOD
As we know, cofferdam is a kind of water tight construction which is
designed to facilitate construction projects where the particular area
is normally submerged. For the construction of a coffer cam we have
various kinds of materials and equipment’s which enable us to
perform the work at a faster rate. (htt)
Cofferdams are rarely installed as easily as they are planned and
designed. You must expect and anticipate problems that will require
redesign and innovative solutions. However, it is rewarding to solve
the demanding construction and knowing it will help successfully
complete the project.
The construction of a coffer dam completely relies on following the
exact process and sequence involved. And also the builder and
designer should possess proper understanding of the project. In
general, the cofferdams are limited to 60 foot long sheet piles
because if these sheets are made longer than 60 foot then it would
cause difficulties in transporting, handling, threading and
manufacturing .

53. The first step in construction is to place the wale system after the access
is worked out. The wales are placed over a barge and floated to the
position. Along with this to grip the wale system in place, guide piles and
support frames are installed. When the barge floods partially and towed
from under the suspended whale frame, then using cranes the wale frame
is lowered to elevation. After that the wales are used as a guide to thread
and drive the sheet piling. Normally at least two layers of wales are placed
where the top and bottom layers will be perform as a stabilizing template
to control the sheet piles. Generally in marine environment we will be
observing some waves, current, and wind. So to guide the sheet piles a
supporting template is used as it is almost impossible to maintain the
vertical and horizontal alignment which is necessary to close the
cofferdam and prevent the interlocks from splitting open. But if the sheet
piles are not kept plumb, then the interlocks will split apart in tension or
the closing pair can bind up due to compressive friction and refuse to be
driven.
During cofferdam installation a driving template is used. Usually the wale
system is used as a driving template. The template wales should be
marked with the proper location of every sheet pile pair interlock that
touches the wale. Special care should be taken to ensure that the first pair

54.  is set plumb in the proper location because it will be acting as a guide for
the rest of the sheet piles.
 In final closure, it should never be made at a corner as this corner works
in both directions. If either sheet wall line is out of plumb, the sheet
interlock will probably split open. The other reason to be careful in initial
alignment is that this will largely define the direction the piles will take as
they continue to penetrate the ground. If the interlock is started off tight
and out of line, it will likely split apart as it is being driven. This will
damage the pile and may require very expensive and time consuming
repair procedures.
 When the sheet piles are fully in place and driven to the top of the upper
template, the template wales can be lowered, if needed. The pairs of
sheet piles should be advanced in about five foot increments. With the
sheets carefully driven and the wale in position, often the sheets are
welded or bolted to the top wale to provide cofferdam stability during
excavation operations. A crane and a clam bucket usually perform the
excavation, although in some instances a backhoe can be effective.

55.  Excavation should be carried out along the sheet piles first, keeping a low
hump in the middle. This allows the clam bucket to rest against the sheets
and stays upright so it can stuff the bucket. If a depression is created in
the middle of the excavation, the bucket will roll on its side and it will not
be able to excavate the wedge of soil adjacent to the sheet piles. When
the excavation is nearly complete, a steel beam spud is placed between
the wales and the sheet pile alcoves.
 After the above process, tremie concreting is carried out so as to
minimize the flowing concrete contact with the water. The method is to
induce the fresh concrete under the previously placed concrete and pillow
it up and out. The tremie placement is a continuous operation until
completed, going 24 hours a day without interruption. Tremie pours
usually involve large volumes of concrete, often several thousand cubic
yards of concrete. When the concrete has cured enough to gain enough
strength to withstand the dewatering forces (about two or three days),
dewatering can begin. (Washington, 2013)

56.  DEWATERING:
 In dewatering process, the pumping out of water from the interior of a
cofferdam is carried out in such a manner that it prevents the possibility
of water moving through uncured concrete. A proper sump is placed
outside below the elevation of the work which is placed and the pumped
water should be properly discharged according to the regulations. The
most important aspect during dewatering is that the underwater concrete
should set so that it can withstand hydrostatic pressure created by
pumping. After the cofferdam is dewatered, the clean up process can
begin. The surface will be rough and undulating. There will be layers of
mud, debris, and dead fish that must be cleaned up. Once the cleanup is
done, the top of the tremie concrete will have about six inches of
laitance. The laitance is a weak layer of nearly pure cement that has been
washed to the surface of the concrete by the dynamics of the concrete
tremie placement. While the cleanup and laitance removal is progressing,
the cofferdam will continue to leak and require substantial pumping. The
leakage water will be contaminated by the mud and debris in the
cofferdam until all remedial work and cleanup is completed. All water
removed from the cofferdam during this stage probably will have to be
processed before returning the water to the river, lake, or bay.

57.  At this point, a safety precaution is inserted.
No gas-powered machinery should ever be
allowed inside a cofferdam. The danger for
explosion and carbon monoxide poisoning is
too great. Even the use of diesel powered
equipment in the cofferdam should be kept to
an absolute minimum. Whenever it is
possible, engines outside the cofferdam
should power all machinery. These actions
will both reduce congestion in the cofferdam
and provide for safer working conditions. (htt)
(Washington, 2013)

58. 8. REMOVAL OF COFFERDAM
 The contractor must remove all the parts of the
cofferdam after the completion of required work.
This shall be done in such a way as not to disturb
or damage the permanent structure. Sheet piling
used in the construction of cofferdam may be left
in place with the approval of the Engineer,
provided the pilling is cut off at elevations
approved in advance by the Engineer and the cut
off portions are removed from the site.
(Washington, 2013) (htt)

60. 9. APPLICATION OF COFFER DAM
 A nautical application of the term cofferdam is a watertight
structure used for making repairs below the waterline of a vessel.
The name also is applied to void tanks which protect the buoyancy
of a vessel. Cofferdam are constructed to permit dewatering an
area and facilitate the construction of foundations, bridge piers,
dams, dry docks, and like structures in the open air.
 The following are some of its major applications:
 Hydroelectric Dam Construction – Cofferdams are used to divert
water away from the shoreline of a river to allow for the
foundations of a dam to be constructed. In this application,
generally one half of the river width is enclosed by the cofferdam
at a time to maintain overall flow.
 Bridge Construction – Cofferdams are used to divert water away
from bridge foundation positions, either on the shore or within the
waterway

61.  Ship repair – Sometimes cofferdams are used to generate a “dry dock”
condition for a ship in order for repairs to proceed. This generally
occurs when the ship cannot be moved to an actual dry dock, and it
can also be more cost effective in some cases.
 Oil Rig and Dam Construction – This is the primary reason why coffer
dams exist. They are quick to build and use welded steel and other
metals; they provide a temporary and dry platform to work freely.
 Sunken Vessel Recovery:
 Cofferdams can be used to expose a sunken vessel in shallow waters
to allow for recovery and repair if appropriate.
 Ship Recovery
 A very rarer use of a Coffer dams is to help in recovery missions for
ships that have sunk in shallow water. They can be built quickly and
aid removal in certain circumstances. In

62.  the past coffer dams have helped recover
ships such as the USS Maine, a ship which
sunk in 1898 played an important part in
Spanish-American history. By using a
coffer dam to pull up this ship from the
sea bed it helped give researchers an
insight into the history of this boat.
 light and ventilation, and
 Attention to safe practices on the part of
all workers and supervisors.

63.  9.1 SAFETY REQUIREMENTS
 In cofferdam construction, safety is a paramount concern, since workers will be
exposed to the hazard of flooding and collapse. (Nemati, 2007)
 good design
 proper construction
 verification that the structure is being constructed as planned
 monitoring the behavior of the cofferdam and surrounding area
 provision of adequate access
 light and ventilation, and
 Attention to safe practices on the part of all workers and supervisors.

64.  9.2 ADVANTAGES OF COFFERDAMS
 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

65.  9.3 DISADVANTAGES OF COFFERDAMS
 Special equipment required
 Relatively expensive
 Typically very time consuming & tedious
 If rushed, sheets can be driven out of locks or out
of plumb
 When in flowing water “log jams” may occur
creating added stress on structure