Definition, uses, selection of coffer dams, types of coffer dams, design features of coffer dams, leakage prevention, economic height

1. 1
PREPARED BY : ASST. PROF. VATSAL D. PATEL
MAHATMA GANDHI INSTITUTE OF
TECHNICAL EDUCATION &
RESEARCH CENTRE, NAVSARI.

2. 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 designed tokeep water and /or soil out of
the excavation in which a bridge pier or other structure is built.
Meaning of Coffer Dam : Coffer = Box
Totake up the Foundation works in the Marine Region, it is necessary to
obstruct the water flow by means of Coffer Dam.

3. To retain Soil & Water
Can be used as either Temporary or Permanent
Main purpose is to provide dry working area for workers
It is constructed to facilitate pile driving operations.
It is used to place grillage as well as the raft foundations
It is used ,when the foundations for piers and abutments of a bridge,
dams, locks, etc. are to be constructed.
Some times it is also provided to store water temporarily.

4. 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, Ramps, Ground Water
Retention, Tunnel Work etc.
Water Engineering: Weirs, Culverts, Flood Protection Walls, Scour Protection
Walls, Securing Embankment etc
Port Construction: Dock Works, Jetty Works etc.

5. 1. Low heads of water – earth fill cofferdams
2. Narrow Excavations – single wall sheet pile cofferdams otherwise double
wall cofferdams or cellular sheet piling are suitable for wide excavations
3. The materials available at site of work for construction
4. The possibility of overtopping by floods, tides, etc. and sliding – rock filled crib
cofferdams.
5. The nature of bed on which the cofferdam is to rest – deep deposits of soft clay,
A heavy earth filled crib or cellular cofferdam could not be suitable but single
wall sheet pile would be required in these conditions.
6. The nature of velocity of flow, i.e. water flowing with slow current or with swift
current
7. The chances of bed erosion due to reduction of water way caused by the
construction of a cofferdam.
8. The facilities available for the transport of equipment and material required for
the construction of a cofferdam.

6. 1. Earth fill cofferdams - mainly for low level water
2. Sand Bag Coffer Dam – to divert coarse of water
3. Rock fill cofferdams
4. Timber Crib or rock filled crib cofferdams - Construction on land and than
floated into place, which is also known as Gravity Dam
5. 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
6. Movable or suspended cofferdams

7. 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 90cm
 Top width of earth dyke – onemeter
 Suitable side slope should be provided
 Provided suitabledrain
 Its depends on soil availability and siteconditions.
 Dry conditions – clay embankmentprovided
 Wet condition – sand materialssuitable

8. The construction of rock fill cofferdams is similar to earth fill dams
 It can be constructed with steeper slopes
 Water depths – 3m.
 In case of high heads of water , this can be achieved by a clay or concrete care
wall
 Impervious layer provided(concrete or clay core wall or sheet pile ) for reducing
entry of seepage

9. It consists of timber cribs made from logs ofwood
 Constructed on land and floated intoplace
 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 20m
 Used for wide excavation and rocky river bottoms

10. 1. Single wall cofferdams
-it used in places, where the area to be enclosed is very
small
-depth of water is more = 4.5 to 6 m
- 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

12. 2. Double wall cofferdams
- when the area to be enclosed is large it is necessary to provide double wall
cofferdam
- The double wall cofferdams are of two types:
(i) Ohio – river type cofferdams
(ii) timber or steel sheeting cofferdams

13. 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 constuction of marine structures like Dams ,Locks etc.
- The two common shapes of the cellular cofferdam are:
(i) Circular type
(ii) Diaphragm type

14. CIRCULAR TYPE

15. DIAPHRAGM TYPE

16. 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.

17. The design of a cofferdam depends on various factors such as:
(1)Hydrostatic head of Water
(2)Dimensions of the area to be covered by coffer dam
(3)Subsoil condition
(4)Fluctuations or of outside water level
(5)Possibility of erosion
(6)Presence of ice
(7)Floating logs (the stem of cut-down tree etc.)

18.  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 = W
H > 3, W = 3 + ½*(H-3)
Where, W = Width of cofferdam in
metres
H = Height of water above river bed in metres

19. 1.Driving of Guide Frame
2.Lowering of Sheet Pile
3. Driving of Sheet Pile
4.Horizontal Bracing (Waller & Strut arrangement) to withstand the load coming on
sheet piles from outside.
5.Excavation and Plug in Concrete to resist uplift pressure
6.Reinforcement and Concrete work of the Permanent Structure

20. Factors governing the Economic Height of Coffer Dam
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 Control
Capacity)
- Low Current => Earthen or any other Dam (Less Seepage Control
Capacity)
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.

21. 5. Scour Level of Reservoir
- Scour level can be considered as the bottom most layer of the loose soil of river
Bed Level.
- Up to Scour level there are chances of erosion due to the current of water. In
any Pile or Well foundation, the care to be taken that the founding level must be
below the scour water level
6. Type of Material to be used
- Structural Sheet pile is costly but can be reused at different projects.

24. (i) Diaphragm type

25. 25