cofferdam

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
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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
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13. Principle Reasons for Failure
Poor
workmanship
Inadequate
strut section
Inadequate
embedment
Allowance of
surcharge
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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
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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
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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
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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
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37. Factors Affecting Stability
Strength of filler material
Strength of sheeters and ties
Soil at foundation level
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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.
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39. Sliding:
Resistance to sliding provided by
 Passive resistance of soil
 Shear strength of sheeters
 Frictional resistance of filler
material
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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
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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
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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