4. Defination:
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 preventwater 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.
7. Earthen Cofferdam
1) Constructed at the place where depth of
Water is low (3m) and velocity of flow is low.
2)These dams are built using the local
available material such as clay, fine sand or
even soil.
3)The height of the dam is kept 1m more than
that of max water level.
4)After the construction of earthencofferdam,
the water from the other site is pumped out
and construction is executed.
10. 1) This type of cofferdam is preferred when the depth of
the water is more than 6m and area of construction is
less.
2) Usually this is used in construction of bridges.
3) A single row of sheet piles is provided on the whole
lengthof the cofferdam.
4) The sand bags are stacked on the inside and outside
faces of the sheet to increase the stability of cofferdam.
SINGLE WALLED COFFERDAM
11.
12.
13. DOUBLE WALLED COFFERDAM
1)When the area to be enclosed is large it is necessary
to provide double wall cofferdam.
2)The double wall cofferdams are of two types:
(i) Ohio – river type cofferdams.
(ii) timber or steel sheeting cofferdams.
3)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 enclosed.
14.
15.
16. CELLULAR COFFERDAM
1)The cellular cofferdams are mostly used for
dewatering large areas , where the depth of water may
be 19 to 21 m.
2)Mostly used the constuction of marine structures
like Dams ,Locks etc.
3)The two common shapes of the
cellular cofferdam are:
(i) Circular type.
(ii) Diaphragm type.
17. CIRCULAR COFFERDAM
1) 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.
2) One cell is completely filled up it can be used
for the construction of other cells.
3) Each cell acts as a self-supporting
independent unit.
21. CONSTRUCTION SEQUENCE
For a typical cofferdam, such as for a bridge
pier, the construction procedure follow the listed
pattern.
1. Pre-dredge to remove soil or soft sediments
and level the area of the cofferdam.
24. CONSTRUCTION SEQUENCE
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.
34. Principal hazards and risks
1) Falls from height from dam walls into
excavation or into water.
2) Health hazards from contaminated
water,hazardous gases in confined spaces, noise
and vibration from piling operations.
3)Hazards from services such as cables in river bed,
temporary power cables in water etc.
35. CONCLUSION
Cofferdams are temporary structures and
used in cases where the plan area of
foundation is very large, depth of
water is less and for the soft soils, where
soils allow easy driving of sheet piles.
