Advance construction & Equipment

Coffer Dam
Coffer dam
• Introduction
• Use of coffer dam
• Selection of type of coffer dam
• Types of cofferdam
• Design feature of cofferdam
• Leakage prevention in cofferdam
• Economic height of cofferdam
• Piping in single wall cofferdam
What is cofferdam ?
• A cofferdam 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.
• It is required for project such as dams, docks and construction of
bridge piers and abutment.
Requirement of cofferdam
• It should be watertight.
• It should be generally constructed at site of work.
• The Design and layout of coffer dam must be economical in
construction , maintenance and pumping is minimum.
• Stable against bursting, overturning and sliding under the floods and
waves.
• Material used for construction are earth, timber, steel and concrete.
• It should be so planned as to facilitate any dismantling and reuse of
material.
Necessity of cofferdam

• Construction of a structure in river bed , sea shore, or inside lake.

• When deep excavation are carried out in a course grained soil.
• Excavation is carried out below ground water table.
• Chances of collapse of side trenches during deep excavation.
• Public/government property is situated close to the excavation.
• Chances of seepage of water in trenches form surrounding area.
Use of cofferdams

• To facilitate pile driving operation.

• To place grillage and raft foundation.
• To construction foundation of piers and abutment of bridge, dam,
locks.
• Enclose a space for the removal of sunken vessels.
• Provide a working platform for the foundation of building when water
is met with.
• To provide space for carrying out the foundation work without
disturbing or damaging the adjoining structure such as building,
sewer, pipelines.
Selection of type of cofferdams
• The area to be protected by a coffer dam
• The depth of water to dealt / execution.
• The possibility of overturning by floods tides etc.
• Nature of bed on which the coffer dam is to rest
• Velocity of flowing water
• Possibility of scouring due to reduction of waterway
• Availability of material on site.
• Transportation facility.
Types of cofferdam
• Earthfill cofferdam
• Rockfill cofferdam
• Rockfilled crib cofferdam
• Single walled cofferdam
• Double walled cofferdam
• Cellular cofferdam
• Concrete cofferdam
• Suspended cofferdam
Earthfill dam Cellular cofferdam

Rockfill Crib cofferdam Rockfill dam

Single walled cofferdam Concrete cofferdam

Double walled cofferdam Movable cofferdam

Earth-fill cofferdam

• It is constructed in places where the depth of

water is less about 1.2 to 1.5 m and velocity of
flow is low.
• Top of embankment should be 1 m above the
water level.
• Side slop vary from 1:1.5 to 1:2
• It is built up from a mixture of clay and sand or
clay and gravel.
• If clay is not easily obtain at site the banks may
constructed with central clay wall slopes of sand
on either side.
• To prevent the embankment from scouring water
side be pitched with rubble bounders.
Rockfill cofferdam

• Rocks have more stability in steeper slopes than earth

fill dam.
• They can constructed for 3 m. water depth with stone
or rubble for embankment.
• An impervious layer of earth is laid on the outer face
of the cofferdam which makes it impervious.
• For better construction the core walls or steel sheet
pile extending up to impervious layer may be
provided.
• The core wall may be of clay or concrete.
• A constructed rockfill cofferdam can withstand the
overtopping of water without any serious damage.
Rockfilled crib cofferdam

• A rock filled crib cofferdam consists of timber

cribs made from logs of wood.
• A crib is a framework of wooden horizontal
and cross beams laid in alternate courses.
• The cribs are open at the bottom and are filled
with rock or gravel or earth.
• It gives stability to the crib against overturning
or sliding.
Idea conditions favorable for this type of cofferdams

• The current of water is swift.

• The stream is having hard and uneven strata at bottom.
• Working space is limited.
• Depth of water is more.
• There is danger of overturning of structure.
• Timber is cheap as compare to sheet pile.
Construction procedure
• Dimension of crib is decided on depth of water and current of flow.
• Crib is built on land up to such height that when it is placed in water
some of its portion will projected above the water level.
• Crib is taken on site and it is floated and placed in position
• Pocket of crib are filled with rock which give stability and sheet pile
are driven on the outside face to give water tightness.
• The pumping is started and work of foundation is carried out.
• While dismasting the coffer dam the filling is removed and cribs are
dislocated.
Single walled cofferdam
• It is used in places where the area to be enclosed is
very small and the depth of water is more, say 4.5 to
6 m.
• Timber pile known as guide piles are first driven
deep into the firm ground below the river bed.
• A single row of sheet piles is provided on the whole
length of the cofferdam.
• The centre to centre spacing of the piles may vary
between 1.8 to 4 m. which is Depending upon the
velocity of the current of the water in the river.
• horizontal runners called wales are then bolted to
the guide piles at suitable distance apart.
• The sheet piles are then driven into the river bed
along the wales and are secured to the wales by
bolts.
• The sheet pile are supported and strengthen from
inside by wales and strut.
• Sand bag are stacked inside which increase stability
of cofferdam.
Double walled cofferdam

• It is used when area to be enclosed

is large to give stability to the
structure of the cofferdam.
• There are two type of the double
wall cofferdam
• Ohio river type cofferdam
• Timber or steel sheeting cofferdam
Ohio-River Type Cofferdam

• This type of cofferdam was frequently used

on the river ohio in USA hence it derives it
name as ohio river type cofferdam.
• It is cheap and can be built rapidly.
• It is suitable for hard bed where there is
problem of erosion.
• It can be also used on soft ground if
properly protected against erosion.
• It is not suitable for deep water and flow
of water with swift current.
Construction
• A dam is constructed by prefabricating a continues row of timber
frames having vertical post of 100 x 100 mm size and wales size is
200 x 150 mm or 200 x 200 mm.
• Cross bracing of size 50 x 50 mm is provide inner side and 20 mm and
25 mm dia. steel tie roads are provided outer side.
• The framework is assemble on barrage.
• The framework is lowered into a river.
• After placing some section sheet pile are fixed in to wales.
• When purpose of cofferdam is over it is removed by reverse process.
Timber or steel sheeting cofferdam
• It useful when depth of water is about 6
to 10m.
• For small and ordinary cofferdam
timber sheet pile are used.
• For large or it is not possible to drive
guide pile due to more depth steel
sheet pile are used.
• Various part of cofferdam are;
• Vertical guide pile and sheet pile
• Horizontal wales and caps
• Tie road on outer side
• Struts in side
Construction
• Guide pile are driven at interval of 2 to 3 m.
• Wales are fixed between two guide pile.
• The sheet pile are then driven along the inside edge of wales.
• The soft material from bottom of cofferdam is dredged out.
• Space between sheet piles is filled with puddle which have mixture of sand
or clay with gravel.
• A berm of clay may be provided on the outside of the cofferdam for
leakage .
• The width of wall should be equal to the depth of water upto 3 m.
• For greater depth
H>3 m width = 3+1/3(H-3)
Concrete cofferdam
• They are small concrete dam.
• Pre-cast R.C.C pile and sheets are used for framework which are
provided with suitable edge.
• They are driven in a similar manner to steel sheet pile.
• It costly but when it is to be incorporated as part of permanent
structure it become economical.
• It useful to avoid vibration from the process of pile driving.
Suspended cofferdam
• A cofferdam is design in such a way
that a single unit used several time.
• The cofferdam as such is lifted floated
and placed in another position as
soon its purpose is served.
• Such cofferdam are also known as
movable cofferdam.
• It consist of a hollow steel cylinder
which can be used to prepare a
temporary enclosed space for
construction work.
Cellular cofferdam
• The cellular cofferdam is made of
steel sheet pile.
• It is mostly used for de-watering
large area in place where the depth
of water may be 18 to 21 m.
• There is two type of cellular
cofferdam;
1. Circular type of cellular cofferdam
2. Diaphragm type of cellular cofferdam
Circular Type Of Cellular Cofferdam
• It consists of a set of large diameter main
circular cells interconnected by arcs of
smaller cells.
• Each cell may be filled independently to
other cells without any danger of
distortion of cells.
• Each cell is self supporting unit.
• Steel required in running length is less then
the steel required in diaphragm cell.
Diaphragm Type Cellular Cofferdam
• In Diaphragm cellular cofferdam,
series of arcs are connected to
straight cross walls.
• Distance between wall is
generally equal hence diameter of
radius is same due to this tension
in arcs and cross wall remain
equal.
• Cells are driven at desire depth
then they are filled with earth,
sand or gravel.
Design Feature Of Cofferdam
• Factors for design of a cofferdam
• Hydrostatic head of water
• Dimension of the area to be covered by the cofferdam
• Velocity of flow to be excluded
• Subsoil condition
• Fluctuation of outside water level
• Possibility of erosion
• Floating logs
Design Feature Of Cofferdam
• Force acting on cofferdam
• Self weight of cofferdam
• Water pressure
• Earth pressure from outside of soil
• Uplift pressure
• Scouring action
• Ice pressure
Leakage in cofferdam
• The water enters the cofferdam in two ways;
• By leakage of sheet pile
• By flow from base
Leakage Prevention In Cofferdam
• cement grouting.
• Dump material in out and inside
• bitumen or cement mortar.
• For serious damage tarpaulin is use which is canvas coated with tar.
• Sufficient grease applied at interlocks of sheet piles.
• In single walled cofferdam placing a V-shaped wooden trough outside
each joint and filling it with puddle.
• 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.
Economic Height Of Cofferdam
• The maximum height of cofferdam for which it total cost is minimum
is known as most economical height of cofferdam.
• The height of cofferdam can determined based on the height of
average normal flood during every year.
• The cost of damage caused by the flood higher then estimated height
is estimated.
• Cofferdam are constructed to excluded normal flood water and
provisional damaged caused by higher flood.
• It is advisable to increase the height of dam when cost of damage
caused by higher flood is high.
Piping In Single Wall Cofferdam
• Piping or boiling in the base result from heavy upward seepage of ground
water into the cofferdam.
• It is checked in cases where the sheet pile are not driven in to impermeable
strata.
• Piping begins when the hydraulic gradient become critical gradient.
• The soil at exit is removed by the percolating water.
• When the soil near the exit has been removed, more concentration of flow
lines in the remaining soil mass results increase of exit gradient.
• The process of backward erosion towards the upstream and pipe is formed.
• large amount of water from outside rushes through the channel and the
cofferdam fails.
Piping prevention
• Reducing the exit gradient by increasing the length of seepage path.
• The length of seepage path may be increased by driving sheet piles to
a sufficient depth.
• Reducing the head of water causing seepage.
• This can be done by pumping from well point or below the level of
the bottom of the sheet pile.
• Piping is occur in loose fine sand which have permeable.
• In silty or sandy clay piping is not occur due to low permeability.
• In gravel also piping is not occur due to high permeability as hydraulic
head is reduced.