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Types of Coffer
of coffer dams
DEFINITION OF COFFER
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.
A Coffer Dam is a temporary structure which is built
in a river, lake etc. to remove water from an area and
make it possible to carry on the construction work
under reasonably dry conditions.
Meaning of Coffer Dam : Coffer = Box
Cofferdams are usually required for projects such as
dams, docks and construction of bridge piers and
REQUIREMENTS OF COFFEDAM
• Cofferdam should be reasonably watertight.
•Ground water or water lying above ground level should be
• Materials – earth , timber , steel & concrete.
• Constructed at site of work.
• Depend- depth , soil condition , difference in water level,
availability of materials.
• The design and layout of a cofferdam should be such that
the total cost of construction, maintenance and pumping
• It should be stable against bursting, overturning and
sliding, under the floods and waves.
USES OF COFFERDAMS
• To facilitate pile driving operations.
• To place grillage & raft foundations.
• To construct foundations of piers & abutments of
bridges, dams, docks etc.
• To provide a working platform for the foundations
when water is met with.
SELECTION OF TYPE OF COFFERDAMS
1) The area to be protected by a cofferdam (small area or
2) The depth of water (shallow depth or deep depth).
3) The possibility of overtopping by floods, tides etc.
4) The nature of bed on which the cofferdam is to rest
(pervious or impervious layers).
5) Velocity of flowing water.
6) The possibility of scour due to reduction of waterway
caused by the construction of cofferdam.
7) The availability of construction materials.
8) Transportation facilities available.
TYPES OF COFFER DAM
1.Earth fill cofferdams - mainly for low level water
2.Rock fill cofferdams
3.Timber Crib or rock filled crib cofferdams - Construction on
land and than floated into place, which is also known as Gravity Dam
4.Braced / Sheet Pile Coffer Dam- Consisting of Sheet Piles,
mainly used in shallow water depth
(i). Single wall coffer dams
(ii). Double wall coffer dams
(iii). Cellular cofferdams
It is constructed in places where the depth of water is not much, say
1.3 to 1.8 m.
The earth bank is carried about one meter above the water level.
The top width of the bank should not be less than 1 mtr. and the side
slopes in a vary from 1 : 1 to 1 : 2.
The earth embankment should be built from a mixture of clay and
sand or clay and gravel.
In order to prevent the embankment from scouring due to the action
of water, side slopes of the bank on water side should be pitched with
If the depth of water to be retained by the embankment
of cofferdam is of order of 3 m .
Stone or rubble is used for the embankment.
This construction is adopted only if the stone is easily
available in the nearby areas.
The stones are assembled in the required shape of the
embankment and the voids are partially filled with earth
On the waterside, the rock fill is to be provided with an
impervious layer of earth.
It consists of timber cribs made from logs of wood
Constructed on land and floated into place
It consists of a unit or a cell open at the bottom and
having the framework of horizontal timber members.
Hollow space formed are then filled with rock or
Depth of water – 10 to 20 m
Used for wide excavation and rocky river bottoms
TIMBER CRIB OR ROCK FILLED CRIB
1. SINGLE WALLED COFFER
Area to be enclosed is small and the depth of water is
more 4.5 to 6 m.
Timber piles known as guide piles are first driven deep
into the firm ground below the river bed.
Depending upon the velocity of the current of the water in
the river, the c/c spacing of the piles may vary between 1.8
to 4.5 m.
Longitudinal runners called Wales are then bolted to the
Steel or wooden sheet piles are then driven into the river
bed along the Wales and are secured to the Wales by bolts.
The sheets on the two faces are braced by trussed
arrangement of struts. This helps in increasing the stability
of walls against the water pressure.
Half-filled bags of sand stacked on the inside and the
outside faces of the sheets help in increasing the stability of
After the cofferdam is constructed, the water in the enclosed
area is pumped out and the construction work is taken up.
2. DOUBLE WALLED COFFERDAMS
Double-walled cofferdam is provided to enclose a larger area.
Its construction is essentially the same as that of a single-walled
cofferdam except that in place of one wall, a pair of walls with a
gap in between is used all along the boundary of the space to be
This type of cofferdam can be used in depth of water up to 12 m.
- The double wall cofferdams are of two types:
(I) Ohio – river type cofferdams
(ii) Timber or steel sheeting cofferdams
OHIO RIVER TYPE COFFERDAM
Ohio river in USA.
Used in hard & soft bed (no erosion).
Unsuitable in deep water & swift flow.
Wales fixed at 1.50m vertically.
Wale joints, double vertical planks are provided.
Tie rods threaded & cross braces fixed.
Fixes sheet piles.
Beams (inside & outside)
Timber Sheet Pile
Timber sheet pile were extensively
used for cofferdam when depth of
water is about 6 to 10 m.
It can be gradually replaced with
It can only be used for small
depth since no deep piling in
3. CELLULAR COFFERDAMS
- The cellular cofferdams are mostly used for dewatering
large areas , where the depth of water may be 19 to 21 m.
- Mostly used the construction of marine structures like
Dams, Locks etc.
- The two common shapes of the cellular cofferdam are:
(i) Circular type
(ii) Diaphragm type
Circular type cellular cofferdam:
The circular type of cellular cofferdam has the advantage that
each cell may be filled completely to the top before starting
the construction of the next cell without causing any
distortion to the shell of the cofferdam.
Thus, when one cell is completely filled up with crushed
stone, broken bricks, gravel and sand. It can be used for
placing crane or other equipment required for the construction of
In addition, each cell acts as a self-supporting independent unit
and in case one of the cells collapses due to scour or interlock
damage or some other reason, it does not produce any adverse
effect on the neighboring cells. It is found that the interlock
stresses reach their maximum permissible value when the
diameter of cell is about 21 meter.
Hence in case, from design consideration it is necessary to have
effective width of the cofferdam more than 21 meter,
diaphragm type of cofferdam must be used.
Diaphragm type cellular cofferdam:
• This Consists of a series of arcs of steel sheet piles connected as
shown in the image.
• The straight diaphragm wails are connected to each other by steel
piles arranged in the form of arches on either sides.
• The radius of the connecting arcs is generally made equal to
the distance between the straight diaphragm walls.
• After the cells are driven to the required depth, they are filled with earth,
sand, gravel or other filling material. In this type of cofferdam, as the
diaphragm which separates the two cells is a straight wall, it is
necessary to fill adjacent cells at approximately the same rate.
• If this is not done, the unbalanced pressure from the fill will distort the
diaphragm (cross-walls) which may result in the failure of the
• In this respect, the circular type cofferdam has the advantage over the
diaphragm type cofferdam because in the former, it is not necessary to fill
the adjacent cells at the same time.
These are actually small concrete dams and they have been used
economically on many jobs.
The framework usually consist of pre cast R.C.C piles and sheets.
The pre cast R.C.C sheet piles are provided with suitable edges
and they are driven in a similar manner to steel sheet piles.
The main disadvantage of this cofferdam is that it is costly. But
when it is to be incorporated as a part of permanent structure it
proves to be economical.
MOVABLE OR SUSPENDED
Where there are numbers of repetition work in
under water foundations ,such as in the piers
of multi span river bridges , it is economical to
design the cofferdams to be moved as a single
unit from one foundation to another.
Water may leak through the underground flow of water or through the
piling sheet of the cofferdam. The following measures can be adopted
to prevent leakages in cofferdam.
1.Water entering the dewatered area through fissures or cracks in the
rocks can be stopped by grouting the fissures by cement grout.
2. Clay or mixture of clay and sand can be dumped in form of beams
both on inside and outside faces of the cofferdam.
3. In case of single-walled cofferdams, sheeting should be connected
through V-shaped notches and filled with clay puddle.
4. Cracks or fine joints or nail holes in the sheet piles should be
closed using bitumen or cement mortar.
LEAKAGE PREVENTION IN COFFERDAMS
5. If a lot of leakages is taking place and no measures stated above
have been to control it, the water face of the cofferdam may be
completely covered with canvass coated with tar or tarpaulin.
6. In case of double-walled cofferdams leakage is generally because of
insufficient compaction of the filling material. In this case, measures to
control seepage through filling material should be adopted.
7. Another method may be a box filled with the mixture of ashes and
sawdust held near the leaking joints. Leaking water carries these
contents into the joints and thus helps check the leakage to some
8. Sufficient grease applied at interlocks of sheet piles also helps check
leakage of water.
DESIGN FEATURES OF COFFERDAMS
The design of a cofferdam depends on various factors such as.
(i) Hydrostatic head of Water
(ii) Dimensions of the area to be covered by the cofferdams
(iii) Subsoil conditions
(iv) Fluctuations or of outside water level
(v) Possibility of erosion
(vi) Presence of ice
(vii) Floating logs(the stem of a cut-down tree, etc.
A purely theoretically designed cofferdam may fail for
factors unaccounted in its design.
Therefore ,become necessary to combine practical
knowledge or experience with the theoretical aspects in the
design of a cofferdam.
For width and Depth of cofferdam :
for H < 3 m, W = H
H > 3, W = 3 + 1/3*(H - 3)
Where, W = Width of cofferdam in metres
H = Height of water above river bed in metres
• 1.Depth of Water
- Low depth => Earthen Dam
- High Depth => Sheet Pile Coffer Dam
• 2.Current and nature of flowering of Sheet Pile
- High Current => Sheet Pile Coffer Dam (Higher Seepage
- Low Current => Earthen or any other Dam (Less Seepage
Factors governing the Economic
Height of Coffer Dam 51
The maximum height of cofferdam for which its total cost is
minimums known as the most economic height of cofferdam.
• 3.Type and Period of Work
- Short Duration Work => Timber Dam
- Long Duration work => Sheet Pile Coffer Dam
• 4.High and Low Tide Level of Reservoir
- Sheet Pile must have top level slightly higher than that
of the HTL. So that even in extreme case water can not
over top the coffer dam, to achieve the dry working
through out the season.