COFFERDAMS
Challenges occur during the construction of
bridges , dams or any other structure where the
foundation is in water or very wet soil conditions.
Construction in water poses many difficulties
especially in the places where the depth is
considerable.
Hence, our main objective is to create dry and
water free environment for working in such a
manner that the structural stability of the structure
is not compromised.
2
Definition
Cofferdams are temporary enclosures to
keep out water and soil so as to permit
dewatering and construction of the
permanent facility (structure) in a dry
condition.
Also designed to keep water and /or soil out
of the excavation in which a bridge pier or
other structure is built.
Meaning of Coffer Dam : Coffer = Box
Purpose of Cofferdam Structure
To retain Soil & Water
Can be used as either Temporary or
Permanent structure
Main purpose is to provide dry working area
It is constructed to facilitate pile driving
operations.
Purpose of Cofferdam Structure
It is used to place grillage, raft foundations
It is used ,when the foundations for piers and
abutments of a bridge, dams, docks are to be
constructed and also during maintenance.
Sometimes it is also provided to store water
temporarily.
Cofferdams can be used in all the sectors
as explained below.
Civil Engineering: Underground Car Parking,
Foundation, Basement Construction
Transport Engineering: Bridge Pier, Support
Walls, Ground Water Retention, Tunnel Work
etc.
Water Engineering: Culverts, Flood Protection
Walls, Scour Protection Walls, Securing
Embankment etc.
Port Construction: Dock Works and other off
shore related structures.
FACTORS TO CONSIDER IN THE SELECTION OF
TYPES OF COFFERDAMS
Low heads of water – especially earth fill
cofferdams
Narrow Excavations – single wall sheet pile
cofferdams - otherwise double wall
cofferdams or cellular sheet piling are
suitable for wide excavations
Materials available at site of work for
construction
Possibility of overtopping by floods, tides,
etc. and sliding – rock filled crib cofferdams
Nature of bed on which the cofferdam is to
rest.
Nature of velocity of flow,i.e. water flowing
with slow current or with swift current
Chances of bed erosion due to reduction of
water way caused by the construction of a
cofferdam.
Facilities available for the transport of
equipment and material required for the
construction of a cofferdam.
Types of Cofferdams
1.Earth fill cofferdams - mainly for low
level water
2. Sand bag cofferdam – to divert coarse of
water
3.Rock fill cofferdams
4.Timber Crib or Rock filled crib
cofferdams - Construction on land and then
floated into place, which is also known as
Gravity dam.
Types of Cofferdams
5.Braced / Sheet Pile Cofferdams- Consisting
of Sheet Piles, mainly used in shallow water
depth
(i). Single wall coffer dams
(ii). Double wall coffer dams
(iii). Cellular cofferdams
6. Movable or suspended cofferdams
Earth Fill Cofferdams
Suitable for rivers or streams having
shallow depth of water, about 120 to
150 cm, with low velocity of flow
It is the simplest type of cofferdam.
Free board – 60 to 90 cm
Top width of earth dyke – one meter
Suitable side slope should be
provided
Rock Fill Cofferdams
The construction of rock fill cofferdams is
similar to earth fill dams
It can be constructed with steeper slopes
Water depths – 3 m.
In case of high heads of water , this can
be achieved by a clay or concrete core
wall
Impervious layer provided(concrete or
clay corewall or sheet pile ) for reducing
entry of seepage
Timber Crib Or Rock Filled Crib Cofferdams
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 thus formed are then filled with
rock or gravel
Depth of water – 10 to 20 m
Used for wide excavation and rocky river
bottoms
Sheet Pile Cofferdam
1. Single wall cofferdams
It used in places, where the area to be enclosed
is very small and depth of water is more = 4.5 to
6m
A single row of sheet piles is provided on the
whole length of the cofferdam
Guide piles, Wales and struts (wood) - guide
piles are first driven deep in the river bed and
spacing between them 1.8 to 3.0 m
The sand bags are stacked on the inside and
outside faces of the sheet to increase the
stability of cofferdam
2. Double wall cofferdams
When the area to be enclosed is large it is
necessary to provide double wall cofferdam
Can be timber or steel sheeting cofferdams
The space between the two walls is filled
with granular material such as sand, broken
rock or gravel
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 ,docks etc.
Two common shapes of the cellular cofferdam are:
(i) Circular type
(ii) Diaphragm type
Movable Or Suspended Cofferdams
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.
