1. The document discusses various design considerations and components for cofferdams, including overall stability, bottom failure prevention, protection from aggressive site conditions, and load transfer mechanisms. 2. It describes different types of cofferdams like circular and rectilinear designs and their components like sheet piling, bracing methods using waling, anchors, and struts. 3. Failure reasons like poor workmanship, inadequate sections, and lack of embedment are outlined. Design of anchors, struts, walling, and protection from scouring are covered.

1. DESIGN CONSIDERATIONS &
COMPONENTS OF COFFERDAM
Presented by
1304081- Swagata Dasgupta
1304085 - Samiha Rabbani
1304090 - Obaidul Islam
1304104 - Shams Razzak Rothe

2. Cofferdam
 Temporary Structure
 Excludes earth and water
2

3. 1. Overall Stability
Risk occurs in –
 Sloping ground
 Riverfront cofferdam
 Where the differential height is greater
 Where cohesive soils extend to considerable depths on sloping sites
Material Stresses

4. 2. Bottom Failure by Piping and Basal Heave
Checking necessary for-
 Risk of hydraulic failure by piping: for narrow cofferdams
 Risk of basal failure: in cofferdams in soft clay
Material Stresses

5. 3. Aggressive Site Conditions
a) Effect of Wave Forces on the Face of the Structure
Waves in deep water
Where waves are reflected
Where waves break on the structure
Waves caused by ship movements
Material Stresses

6. b) Risk of Over-topping of the Cofferdam Sheeting
To prevent-
The cofferdam should be tied
Adequate sluices with safe locations for operating locations
Material Stresses

7. c) Scour Protection
Providing cutwaters: upstream and downstream ends of a cofferdam
maybe shaped to reduce scour
Rock or concrete beds: to avoid erosion of the river or sea bed
Grout mattresses weighted by rockfill: for river cofferdams
Material Stresses

8. d) Protection from Vessel Impact
Fendering or strongpoints built into the cofferdam sheeting
Material Stresses

9. Circular Cofferdams
General Layout of the Cofferdam

10. Circular Cofferdams
General Layout of the Cofferdam
What can be accommodated in circular
cofferdams cost-effectively?
Square shaped plan structures
Circular plan shape storage wells

11. Rectilinear Cofferdams
General Layout of the Cofferdam
 The most economical arrangement uses
Maximum straight runs of piling
Minimum of return angles

12. Load Transfer
Soil
Sheeting
Waling
Struts
12

13. Principle Reasons for Failure
Poor
workmanship
Inadequate
strut section
Inadequate
embedment
Allowance of
surcharge
13

14. Economical use of anchors
 Depends on the strength of the subsoil
 Dense sands and gravels would be preferred
Opportunity to use anchors
 Ownership of land
 Permission to found anchors
Ground Anchors

15. Acting earth pressures on anchor installation
 Soil strengths
 Wall and soil stiffness
 Anchor spacing
 Anchor yield
 Prestress locked into the anchors
Ground Anchors

16. Five anchor types
 Type A
 Type B
 Type C
 Type D
 Type E
Ground Anchors

17. Four items for design of anchors
 Overall stability
 Depth of embedment
 Fixed anchor dimensions
 Group effects
Ground Anchors

18. Components of Braced Cofferdam
 Sheet pile
 Bracing
1. Waling
2. Passive Anchorage
3. Struts
Figure 4
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19. Sheet Piling
Used for outer walls of marine or land cofferdam
Selection of section:
• Criteria- i) Flexural strength
ii) Resistance to driving stress
• Sufficiency of pile sections
Two types of pile driver:
1. Drives piles in panels of six to eight piles
2. Drives one pile at a time
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20. Bracing
Waling
Necessity
Load Transfer:
Continuity
• Diaphragm Walls
• Secant Piles
Sheeting Waling Struts/ Anchors
20

21. Waling (cont.)
Connections
Special Considerations
• For inclined struts
• For anchors
• For diagonal struts
Irregular Alignment Problem
Load Factor: 1.4
Waling
Figure 5
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22. Size of Walling
22

23. Size of Walling (cont.)
Varies with diameter of cofferdams
‘d’ should not be less than ‘D/35’
Waling load = 1.5EI / 10^5 R^3
Permissible compressive stress < 5.2 N/mm^2
Check tension on walling beam
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24. Passive Anchors
 Design Principal
Net available passive pressure= passive pressure-active pressure
 Special Considerations
i) Surcharge ii) Friction
 Materials
i) Mild steel - 111N/mm2
ii) High yield steel -140 N/mm2

25. Passive Anchors (cont.)
Factor of Safety: 1.5-2
Protection:
 To avoid effect of fill settlement
 To avoid corrosion

26. Struts
 A strut is a component designed to resist
longitudinal compression.
Importance
 The most likely collapse
mechanism of a braced
cofferdam is the buckling of
it’s strutting.

27. Struts cont.
Collapse of waling or sheeting is unlikely
Reasons of Failure:
 Passive failure below formation level
 Extreme loading
 Poor workmanship

28. Positioning the strut
Struts should be supported by
welding steel location angles to
end plate prior to bolting.
Struts are square to the wallings
in plan to avoid eccentric loading.
As precaution check 10% of strut
width or depth in each direction.

29. Spacing
 The vertical spacing of the struts depend on both the strut capacity and the
flexural strength of the walling.

30. Spacing (cont.)
The minimum frame spacing must be sufficient to allow mechanical
excavation plant to pass under the frame prior to placing the next lowest
frame.

31. Final stages of construction
The placing of the lowest frame in a multi-frame cofferdam is the period
of greatest risk to the bracing.
The next highest frame will be highly loaded and the factor of safety
against passive failure will be at it’s lowest.
The bending stress and deformation of the walling will be at their highest
values.
All bracing components should be prefabricated to avoid cofferdam
being unpropped at the lowest level for lengthy period.

32. Walling details
 lifting and bracing steel
 joining sections of cages
 lateral spacing between reinforcement cages of adjacent panels
 details of starter steel for floor slabs
 detailing for ground anchors
 inclusion of reinforcement for waling
 min spacing and cover
 water bar in vertical panel joints

33. Sheet Pile Walls Across Dock Entrance

34. Sheet Pile Walls Across Dock Entrance
Several means-
Cellular cofferdam for new construction
Raking struts to the dock floor to exclude water from
the existing dock
Driving sheet piles to a circular arc in plan for the
existing entrance

35. Cost of temporary cofferdams
 Design curves
• Section modulus of pilling
• No of bracing
• Bending moments included in cantilever
• Maximum penetration depth
35

36. DOUBLE WALL COFFERDAM
 Gravity structure
 Consists of twin parallel
lines of sheeters
 Driven below dredge line
 Tied together by steel tie
 Filled with material
 Usual height to width
ratio 0.8
 Berm inside for drainage
and stability
36

37. Factors Affecting Stability
Strength of filler material
Strength of sheeters and ties
Soil at foundation level
37

38. Modes of Failure Considered in Design of Double
Wall Cofferdam
Tie rod design and water pressure:
 Sheet piles should be designed using at rest pressure of
filling
 Hydraulic filling is used to place sand back fills between the
sheeters
 Walls should be designed for most severe assumption of
water pressure-water level within the filling may rise at top
level of sheeters.
38

39. Sliding:
Resistance to sliding provided by
 Passive resistance of soil
 Shear strength of sheeters
 Frictional resistance of filler
material
39

40. Overall stability
a stability analysis using force polygon
establishing the efficiency of fill material against failure by
horizontal movement at the top
Ensuring a factor of safety of 1.5
40

41. Alternative method:
A minimum factor of safety( ratio of overturning and
resisting moment) 1.5
Otherwise trial required with
 increased width
 improved filling material
 deeper driving of sheeters
41

42. Measures to protect scouring on outside
face
Placing rock or precast concrete blocks against outer
face
Laying grouter mattresses on the river bed
Lateral deformation at the head of cofferdam is
eliminated by using strongpoints
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43. THE END