Cofferdams

1. Cofferdams

2. Topics
• Coffer Dams: Definition, uses, selection of
coffer dams, types of coffer dams, design
features of coffer dams; leakage prevention,
economic height.

3. Cofferdams

4. Cofferdams
• “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.”
- Standard Handbook of Heavy Construction

5. Cofferdams
• 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.

6. Cofferdams

7. Cofferdams
• Because cofferdams are typically constructed under
adverse conditions in a marine environment, and
because significant deformations of elements may
occur at various stages of construction, it is difficult
to maintain close tolerances.
• Ample provisions must be made for deviations in
dimensions so that the finished structure may be
constructed according to plan.

8. Cofferdams

9. Cofferdams
• The loads imposed on the cofferdam
structure by construction equipment and
operations must be considered, both during
installation of the cofferdam and during
construction of the structure itself.
• Removal of the cofferdam must be planned
and executed with the same degree of care as
its installation, on a stage-by-stage basis.

10. Cofferdams

11. Cofferdams
• In cofferdam construction, safety is a
paramount concern, since workers will be
exposed to the hazard of flooding and
collapse.

12. Safety Requires
• good design
• proper construction
• verification that the structure is being constructed
as planned
• monitoring the behaviour of the cofferdam and
surrounding area
• provision of adequate access
• light and ventilation, and
• attention to safe practices on the part of all
workers and supervisors.

13. Cofferdams

14. Cofferdams
• Types of Cofferdam
• Braced
• Earth-Type
• Timber Crib
• Double-Walled Sheet Pile
• Cellular

15. Circular Cofferdam

16. Timber Crib

17. Double-Walled Sheet Pile

18. Braced Cofferdam

19. Cofferdams
• Advantages of Cofferdam
• Allow excavation and construction of structures in
otherwise poor environment
• Provides safe environment to work
• Contractors typically have design responsibility
• Steel sheet piles are easily installed and removed
• Materials can typically be reused on other projects

20. Cofferdams

21. Cofferdams
Items needed for Installation
• Pile driving hammer
• Vibratory or Impact
• Crane of sufficient size
• Steel sheet piles are typically used
• H-piles and/or wide-flange beams for Wales
and stringers
• Barges may be required

22. Cofferdams
Barges
Wales, Struts, Pile Arrangements

23. Cofferdams Types Of Imposed Loads
• Hydrostatic Pressure
• The maximum probable height outside the cofferdam
during construction and the water height inside the
cofferdam during various stages of construction need
to be considered.

24. These result in the net design pressure
shown below

25. Cofferdams Types Of Imposed Loads
• Forces due to Soil Loads
• The soils impose forces, both locally on the wall of
the cofferdam and globally upon the structure as a
whole. These forces are additive to the hydrostatic
forces.
• Lateral force on sheet-pile walls, causes bending in
the sheets

26. Cofferdams Types Of Imposed Loads

27. Current Forces on Structure
• Current Forces on Structure
• With a typical cofferdam, the current force
consists not only the force acting on the
normal projection of the cofferdam but also on
the drag force acting along the sides.

28. Wave Forces
• Wave Forces
• Waves acting on a cofferdam are usually the result of
local winds acting over a restricted fetch and hence
are of short wavelength and limited to height.
• Waves can also be produced by passing boats and
ships, especially in a restricted waterway

29. Wave Forces

30. Ice forces
• Ice forces
• These are of two types:
• The force exerted by the expansion of a
closed-in solidly frozen-over area of water
surface (static ice force) and the forces
exerted by the moving ice on breakup
(dynamic ice force).

31. Ice forces

32. Cofferdams Types Of Imposed
Loads
• Seismic Loads
• These have not been normally considered in
design of temporary structures in the past. For
very large, important, and deep cofferdams in
highly seismically active areas, seismic evaluation
should be performed.
• Accidental loads
• These are the loads usually caused by
construction equipment working alongside the
cofferdam and impacting on it under the action of
waves

33. Cofferdams Types Of Imposed Loads
• Scour
• Scour of the river bottom or seafloor along the
cofferdam may take place owing to river currents, tidal
currents, or wave-induced currents. Some of the most
serious and disastrous cases have occurred when these
currents have acted concurrently.
• A very practical method of preventing scour is to
deposit a blanket of crushed rock or heavy gravel
around the cofferdam, either before or immediately
after the cofferdam sheet piles are set. A more
sophisticated method is to lay a mattress of filter
fabric, covering it with rock to hold it in place.

34. Cofferdam Components
Cofferdam Components
• Sheet piling
• Bracing frame
• Concrete seal
• Bearing piles

35. Sheet Piling

36. Bracing frame

37. Cofferdam Components

38. Cofferdams
• The typical cofferdam, such as a bridge pier,
consists of sheet piles set around a bracing
frame and driven into the soil sufficiently far
to develop vertical and lateral support and to
cut off the flow of soil and, in some cases the
flow of water.

39. Cofferdams

40. Cofferdams
• The structure inside may be founded
directly on rock or firm soil or may require
pile foundations. In the latter case, these
generally extend well below the cofferdam.
• In order to dewater the cofferdam, the bottom
must be stable and able to resist hydrostatic
uplift. Placement of an underwater concrete
seal course is the fastest and most common
method.

41. Cofferdams

42. Cofferdams
• An underwater concrete seal course may
then be placed prior to dewatering in order
to seal off the water, resist its pressure, and
also to act as a slab to brace against the
inward movement of the sheet piles in order
to mobilize their resistance to uplift under the
hydrostatic pressure.

43. Cofferdams

44. Cofferdams Construction Sequence
• For a typical cofferdam, such as for a bridge
pier, the construction procedure follow the
listed pattern.
• Pre-dredge to remove soil or soft sediments

45. Cofferdams Construction Sequence
• Drive temporary support piles
• Temporarily erect bracing frame on the support
piles.

46. Cofferdams Construction Sequence
• Set steel sheet piles, starting at all four corners and
meeting at the center of each side.
• Drive sheet piles to grade.
• Block between bracing frame and sheets, and provide
ties for sheet piles at the top as necessary.

47. Cofferdams Construction Sequence
• Excavate inside the grade or slightly below grade,
while leaving the cofferdam full of water.

48. Cofferdams Construction Sequence
• Drive bearing piles.
• Place rock fill as a levelling and support

49. Cofferdams Construction Sequence
• Place Tremie Concrete Seal.

50. Cofferdams Construction Sequence
• Tremie concrete seal.

51. Cofferdams Construction Sequence
• Tremie concrete seal.

52. Cofferdams Construction Sequence
Check blocking between bracing and sheets.
• De-watering.
• Construct new structure

53. Cofferdams

54. Cofferdams Construction Sequence
• Construct new structure.
• Flood cofferdam.

55. Cofferdams Construction Sequence
• Remove sheet piles.
• Remove bracing.
• Backfill

56. Cofferdams
• Cofferdams
• Braced Cofferdam Construction
• Install Wale and Strut System for
Framework /Template

57. Cofferdams

58. Cofferdams
• Braced Cofferdam Construction
• Install Wale and Strut System for
Framework /Template

59. Cofferdams

60. Cofferdams

61. Design Features of Cofferdams
• The design of a cofferdam depends on various
factors such as
• Hydrostatic head of water
• Dimensions of the area to be covered by the
cofferdam
• Subsoil Conditions
• Fluctuations of outside water level
• Possibility of erosion
• Floating logs, etc.

62. Design Features of Cofferdams
• As it is, sometimes not possible to determine
accurately the forces due to these factors, the
worst condition should be assumed in the design
of the cofferdam to prevent its failure.
• It is thus, clear that a purely theoretically
designed cofferdam may fail for factors
unaccounted in its design.
• It has, therefore, become necessary to combine
practical knowledge or experience with the
theoretical aspect in the design of a cofferdam.

63. Design Features of Cofferdams
• The various thumb-rules for width and
depth of cofferdam are as follows:
• 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 metre.

64. Prevention of Leakage in Cofferdams
• The water enters the cofferdam in the two ways,
namely, by leakage through the sheet piling and by
under flow.
• The measures employed to prevent the leakage in a
cofferdam are as follows:
• If the entry of water is through the fissures or cracks of
the rock, it can be stopped by pumping cement grout
through pipes of about 10 cm in diameter.
• The material such as clay, sand, ashes, etc. may be
damped all round the cofferdam to reduce the quantity
of entering water.

65. Prevention of Leakage in Cofferdams

66. Prevention of Leakage in Cofferdams
• Leakage in case of a single wall cofferdam may be
prevented by placing a V-shaped wooden trough
outside each joint and filling it with puddle.
• The interlocks of steel sheet piles may be provided with
sufficient quantity of grease before driving. Fine
materials carried by water will stick and it will make
the joints effective in preventing leakage of water.
• If the leakage is of serious nature, it is reduced by
providing tarpaulin which is canvas coated tar. The
canvas is placed over the area of leakage and it
should be seen that it is not disturbed from its
position by the force.

67. Prevention of Leakage in Cofferdams

68. References
• Cofferdam
• https://en.wikipedia.org/wiki/Cofferdam
• Cofferdam
• http://www.cv.titech.ac.jp/~courses/atce2/Lesson4.pdf
• Cofferdams
• http://courses.washington.edu/cm510/cofferdams.pdf
• How The Cofferdam Process Works
• https://www.damitdams.com/how-a-cofferdam-works/
• Types Of Cofferdams
• https://theconstructor.org/water-resources/types-of-
cofferdams-construction-details/13807/

69. Thanks…