for the subject offered in GTU, BCT, ACE module 5 cofferdam & diaphragm wall, for the 3rd sem & also for the 6th sem subject

1. MODULE : 3

2. Definition:
It’s Defined As A
Structure Which Is
Sunk Through Ground
Or Water To Exclude
Water And Semi-fluid
Materials During The
Process Of Excavation
Of Foundations And
Which Subsequently
Becomes An Integral
Part Of The Sub-
structure.

3. Function:
The Function Of Caissons Is
Essentially The Same As For
Piles, Viz To Transmit The
Applicable Combination
Of Permanent And
Transient Loads Applied At
The Top Of The Caisson
Through Weak Compressive
Soil Or Fill
Materials Onto Stiff Or
Dense Soil Strata Or Rock At
Lower Levels, In Such A
Manner As To Prevent
Excessive
Settlement, Horizontal
Displacement Or Rotation
Of The Supported Structure
At The Caisson Cap Level.

4. Uses Of Caissons:
To Reach The Hard Bearing Stratum For
Transferring The Load Coming On Supports For
Bridge Piers And Building Columns.
To Serve As An Impervious Core Wall Of The
Earth Dams, When Placed Adjacent To Each
Other.
To Provide An Access To A Deep Shaft Or A
Tunnel.
To Provide An Enclosure Below Water Level
For Installing Machinery,pump,etc.

5. Construction Materials:
Cast-iron R.C.C. Steel Timber
•Mostly Suitable
For Open-well
Type.
•New Segment Of
C.I. Are Bolted As
The Caisson Sinks.
•Unsuitable For
Pneumatic
Caisson.
•Risk Of Failure
Due To Tension
Developed By The
Compressed Air.
•More Costlier
Than Steel Or
R.C.C.
•Mostly Suitable
For Caisson Shoes.
•Due To Heavy
Weight It’s
Difficult For
Handling And
Floating The
Caisson At Early
Stage Of
Construction.
•Costlier Than
Steel Caisson.
•Most Suitable For
The Construction
Of Caisson.
•Usually In The
Form Of Double
Skin Of Steel
Plates.
•Hollow Space Is
Filled With
Cement Concrete.
•Mostly Used In
The Early Age Of
Caisson
Construction.
•Use Of Timber Is
Very Less
Nowadays.
•Timber Is Now
Practically Not
Adopted Due To
Its Bulk And Risk
Of Fire.

6. Shapes Of Caissons:
1. Basic Shapes:
m

7. 2. Combination Of Basic shapes:

8. Types Of Caissons:
Open Caissons Pneumatic Caissons
Well Type Floating (Box) Type
Single
Monoliths (Multiple)
Cylinders

9. Types Of Caissons

10. Open Caissons:
Floating (Box) Type:
A Box Caisson Is Open At
Top And Closed At
Bottom.
It Is Merely The
Variation Of The
Suspended Type
Cofferdam.
The Box Caissons May
Be Built Of Reinforced
Concrete, Steel Or Timber.

11. Construction Of Floating (Box) Caisson
a) A Level Bearing Surface Is Prepared To Receive The
Bottom Of The Box. This Can Be Done Either By
Dredging Or By Divers.
b) In Case The Caisson Is To Rest On Piles, The Group
Of Piles Is Brought To The Required Height.
c) The Caisson Is Constructed On Shore, Launched And
Floated To The Site Of Work.
d) The Box Is Then Filled With Suitable Materials To
Effect Further Sinking, If Necessary.
e) The Top Is Then Sealed And Further Foundation
Work Is Started Above The Water Surface.

12. Favourable Conditions For
Construction Of Floating (Box) Caisson
a) The Excavation For Preparing The Bed Of
Foundation Is Not Required Or Bed Of Foundation
Can Be Previously Prepared Or Pre-installed
Foundation Such As Group A Group Of Piles
Available.
b) The Velocity Of The Flow Of Water Is Slow So As To
Give Stability To The Caisson Against Scouring.
c) The Bed Materials Is Loose Enough And It Is
Possible To Dredge Out This Material And Form A
Levelled Bearing Surface.
d) The Depth Of Water Is About 6 To 8 M.

13. Methods of founding Box Caisson

14. Open Caissons:
Well Type:
A Well Caisson Is Open At
Top As Well As Bottom.
It’s Provided With A
Cutting Edge At The Bottom
To Facilitate Sinking.
The Shape Of Well Is
Generally Decided By The
Requirements Of The
Superstructure, Forces On
The Well, Cost Of Sinking,
Base Of The Pier, Chances
Of Tilting, Etc.

15. Basic Components Of Well Caissons:
1)Cutting Edge: The Function Of The Cutting Edge Is To
Facilitate Easy Penetration Or Sinking Into The Soil To The
Desired Depth. As It Has To Cut Through The Soil, It Should
Be As Sharp As Possible, And Strong Enough To Resist The
High Stresses To Which It Is Subjected During The Sinking
Process. Hence It Is Usually Consists Of An Angle Iron With
Or Without An Additional Plate Of Structural Steel.
2)Curb: The Well Curb Is A Transition Member Between
The Sharp Cutting Edge And The Thick Steining. It Is Thus
Tapering Its Shape. It Is Usually Made Of Reinforced
Concrete As It Is Subjected To Severe Stresses During The
Sinking Process.

16. Basic Components Of Well Caissons:
3)Steining: The Steining Forms The Bulk Of The Well
Foundation And May Be Constructed With Brick Or Stone
Masonry, Or With Plain Or Reinforced Concrete
Occasionally. The Thickness Of The Steining Is Made
Uniform Throughout Its Depth. It Is Considered Desirable
To Provide Vertical Reinforcements To Take Care Of Tensile
Stresses Which Might Occur When The Well Is Suspended
From Top During Any Stage Of Sinking.
4)Well Cap: The Well Cap Serves As A Bearing Pad To The
Superstructure, Which May Be A Pier Or An Abutment. It
Distributes The Super Structure Load Onto The Well
Steining Uniformly.

17. Basic Components Of Well Caissons:
5)Concrete Seal/Bottom Plug: After
The Well Foundation Is Sunk To The Desired
Depth So As To Rest On A Firm Stratum, A
Thick Layer Of Concrete Is Provided At The
Bottom Inside The Well, Generally Under
Water. This Layer Is Called Concrete Seal Or
Bottom Plug. It Has To Be Designed For An
Upward Load Equal To The Soil Pressure,
Including The Pore Water Pressure. It Is
Generally Made Bowl-shaped So As To Have
Inverted Arch Section. This Is Primarily Meant
To Distribute The Loads On To A Large Area Of
The Foundation, And Hence May Be Omitted
When The Well Is Made To Rest On Hard Rock.

18. Basic Components Of Well Caissons:
6)Top Plug: After The Well Foundation Is Sunk To The
Desire Depth, The Inside Of The Well Is Filled With Sand
Either Partly Or Fully, And A Top Layer Of Concrete Is
Placed. This Is Known As ‘Top Plug’. It Serves To Transmit
The Loads To The Base In A Uniform Manner.
7)Sand Filling: It Serves To Distribute The Load More
Uniformly To The Base Of The Well, To Reduce The Stresses
In The Steining , And To Increase The Stiffness Of The Well.
However, As This Adds To The Weight And The Load
Transmitted To The Foundation Stratum, The Engineer Has
To Consider The Desirability Or Otherwise Of Providing The
Sand Filling From The Point Of View Of Bearing Power And
Settlement.

19. Open Caissons:
Well Type:
1)Single Wells:
A Caisson With A Single Wall Is
Used Where Much Sinking Is Not
Required Or Where The Material
To Be Sunk Through Is Very Soft.
If Soft Material Covers The Site,
It Is Necessary To Remove As
Much Of It As Possible Before
Placing The Caisson.
Depending Upon The Nature Of
Superstructure And Structural
Requirements, Suitable Size And
Shape Of Well Are Determined.

20. Construction Of Single Well Caisson
 If Caisson Is Sunk Through
Water ,It Is Partially
Constructed On Land,
Floated And Located In Its
Correct Position.
 The First Section Of The
Single Well Consists Of A
Well Arch With Cutting
Edge And Small Height Of
Masonry, The Bolts Or
Rods Are Kept Projecting
For Necessary Grip As
Shown In Fig.

21. Construction Of Single Well Caisson
 The Soil Below The Cutting
Edge Is Removed. But
Material Is Not Allowed To
Be Removed. This Is
Essential For The Initial
Uniform Settlement Of The
Well.
 For Deep Well To Overcome
Increased Skin Friction And
The Loss In Weight Due To
Buoyancy, The Additional
Loading Known As Kentledge
Is Applied On The Well.

22. Open Caissons:
Well Type:
2) Multiple Wells(Monoliths):
Monoliths Are
Multiple Wells Which
Are Sunk Together.
Each Individual
Well Has Got The
Separate Cutting
Edge And Dredging In
Each Of The Well Can
Be Done Separately.
The Monoliths are
Bigger Dimensions.

23. Construction Of Monoliths
 A Suitable Site Is Selected On
The Upstream Side Of The River
And Dry Dock Is Constructed.
 The Dimensions Of Dry Dock
Should Be Kept Bigger Than
Those Of The Monoliths And It
Should Be Provided With A
Door Of Sufficient Width To
Take It Out.
 The Monolith Is Constructed
Up To Such Height That Some
Portion Of It Will Remain Above
Water Level When It Is Sunk.

24. Construction Of Monoliths
 The Gate Of Dry Dock Is Opened. The Monolith Is
Then Floated And Located In Its Correct Position.
 The Dredging Operation Is Done In A Sequence As
Shown In Fig. The Four Dredging Units Are Used
Simultaneously And The Middle Row Is Touched
Last.
 After Dredging To The Required Depth, The Whole
Monolith Is Gradually Sunk Uniformly Up To The
Required Level.
 The Bottom Is Provided With A Concrete Seal, The
Wells Are Filled-with The Concrete And Cap Is
Provided At The Top To Finish Up The Construction
Of Monolith.

25. Advantages Of Monoliths
All The Work Is Done Above Water Level And Hence,
There Is Reduction In Cost.
There Is No Theoretical Limit For The Depth Up To
Which A Monolith Can Be Sunk.
The Unskilled Labour Can Be Employed For The
Construction Of A Monolith.
Uses Of Monoliths
Used For Foundation Work Of Bridge Piers, Quay Walls
Etc.
Its Use Is Justified For Such Depths Of Water Which
Will Be Unsuitable For The Cofferdam Process Or Which
Will Prohibit The Use Of Pneumatic Caissons.

26. Disadvantages Of Monoliths
The Rate Of Sinking Cannot Be
Estimated Correctly When
Boulders Or Such Obstructions
Are Encountered.
It Is Difficult To Have A Through
Control Over The Sinking Of
Monolith. Hence, It Is Made
Large Enough To Accommodate A
Moderate Amount Of Tilting.
The Concrete Is Placed Under
Water And Hence, It Won't Be As
Strong As Concrete Under Dry
Conditions.

27. Open Caissons:
Well Type:
3)Cylinders:
An Open Cylinder Is
Convenient Form Of Foundation
For Bridge Pier.
It Is Used When Depth Of
Water Is More Than 12 M Or
When It Become Essential To Go
Deep To Avoid Exposure Of
Foundations Due To Scouring
Action Of Flowing Water.
The Cylinder Is Light Shell
Which Invariably Becomes The
Part Of The Permanent
Structure.

28. Construction Of Cylinders
The first section of cylinder is
brought to the site of work and it’s
kept in a vertical position before
sinking of cylinder start.
For this purpose, the staging is
invariably used as shown in figure.
Cylinders are usually taken in
lengths of 2 meters or 3 meters.
They are sunk by excavating the
inside material
In case of metal cylinders,
different sections are joined by
bolting or welding. The cylinder
itself will work as cutting edge
while for others, the sharp cutting
edges are to be provided.

29. Construction Of Cylinders
Cylinder Is Sunk Upto The
Desired Bearing Surface. It May
Also Rest On The Pile Cap As
Shown In Figure.
After Removing The Materials
From Inside Of The Cylinder, The
Space Is Generally Filled With
Concrete For The Full Height Of
The Cylinders
When Cylinders Are Used For
Bridges Of Greater Widths, Two
Cylinders Are Sunk A Slight
Distance Apart And Suitable
Bracing Is Provided.

30. Advantages Of Cylinders
For Smaller Cylinders As Diameter Is Small, The
Obstruction To Water Flow Is Less.
For Smaller Cylinders Excavation Is Less.
They Are Simple In Construction And Cheap In Cost.
They Do Not Required Expensive And Complicated
Lifting Devices.
It Has Minimum Perimeter For A Given Dredge Area
Hence The Ratio Of The Sinking Effort To Skin Friction Is
Maximum.
Sinking Of Well Is More Uniform As Compared To
Other Shapes Because The Perimeter Is Equidistant At
All Points From The Centre Of The Dredge Hole.

31. Disadvantages Of Cylinders
The Large Cylinders Causes More Obstruction To The
Waterway Than The Bridge Pier. It’s Due To The Fact That
The Diameter Of Well Is Much More In Direction Parallel
To The Span Of The Bridge.
It’s Difficult To Sink Them Vertically In Running Water
Having High Velocity Of Flow.
Smaller Cylinders Have Very Low Load Bearing Capacity
Due To Smaller Diameter.
There Are More Chances Of Tilting Of Cylinder.
Thickness Is Less Hence, There Are Not Suitable For High
Bridges
They Required Additional Loading For Sinking As Their
Self Weight Is Less.

32. Drilled Caissons:
The Term Drilled Caisson Is Used To Indicate Mainly A Compression
Member Subject To The Axial Load At The Top And The Reaction Of The
Bottom.
Parts:
Bell Shaft Cap
•This Is The
Lowest Part Of
The Caisson And
It May Be Of Any
Shape.
•The Ideal Shape
Of The Frustum
Of The Cone
With Side Slope
Of 60 Degree
With Horizontal.
•Such A Shape
Prevents The
Caving Of Soil.
•This Is A Short
Column
Connecting Bell
With Cap.
•The Surrounding
Soil Exerts
Confining
Pressure Which
Tend To Increase
The Compressive
Strength Of
Concrete.
•The Minimum
Diameter Of
Shaft Is 600mm.
•This Is The Top
Most Portion.
•Its Design Is
Made In Such A
Way That All The
Column Loads
Are Transferred
To The Shaft.
•The Light
Reinforcement Is
Provided
Between Cap
And Shaft For
Necessary
Bonding.

33. Pneumatic Caissons:
 This Type Of Caisson Is Open At
The Bottom And Close At The Top.
Pneumatic Caisson Is Specially
Used At The Place Where It Is Not
Possible To Construct The Well.
It Is Suitable For The Depth Of
Water More Than 12 M.
In The Construction Of
Pneumatic Caisson, The
Compressed Air Is Used To Remove
Water From The Working Chamber
And The Foundation Work
Is Carried Out In Dry Condition.
This Type Of Caisson Can Be
Made Of Timber, Concrete Or
Steel.

34. Components Of A Pneumatic Caissons
Air Shaft :
A Passage Connecting In Between The Working
Chamber And Air Lock Is Termed As 'Air Shaft' .
This Passage Or Air Shaft Is Used By The Workmen Or
Workers To Reach To The Working Chamber To Ground
Surface. If Caisson Is Too Large In Size, The Separate
Unit Of Air Shaft May Be Provided For Workers And
Material.
Air Shaft Is Made Up Of Steel Material. The Joint
Involved In Air Shaft Are Sealed By Rubber Gasket Air
Lock Is Provided On Each Air Shaft At Top. When
Sinking Process Is Going On, Air Shaft Is Extended
Above The Water Level.

35. Components Of A Pneumatic Caissons
Working Chamber:
Working Chamber Height Is
About 3 M And Is Totally Air
Tight And Made Up Of
Structural Steel.
To Prevent The Entry Of Air
And Water Into The Chamber,
The Air Inside The Chamber
Is Kept At A Pressure Just
More Than
Atmosphere Pressure. External Surface Of Chamber Is Kept Thick.
Chamber Is Leak Proof And Smooth To Reduce Skin Friction. To
Facilitate The Proper Processing Of Sinking, A Cutting Edge Is
Provided At The Bottom.

36. Components Of A Pneumatic Caissons
Air Lock:
A Chamber Made Of Steel Provided At The Upper End Of The Air Shaft
Above The Water Level Is Called As 'Air Lock'.
Air Lock Allows The Worker Or Workmen To Enter Or Exit From The
Caisson Without Releasing The Air Pressure In The Working Chamber.
The Air Locks Has Two Air Tight Doors, One Door Opens Into Shaft And
Another Door Opens To The Atmosphere. When Workmen Enters The
Airlocks Through The Outside Door, Then Pressure In The Chamber Is
Kept At Atmospheric Level.
Pressure Is Increased Gradually Till It Becomes Equal To The Working
Chamber. Under This Condition Workmen Is Allowed To Go Into The Air
Shaft. Complete Procedure Is Again Done When Workmen Comes Out Of
The Air Shaft To Air Lock.
By Opening A Valve In The Airlock, Fresh Air Is Circulated In The Shaft
Workers Or Workmen Are Allowed To Work Into The Working Chamber
Up To 2 Hrs.
The Maximum Limit Of Working Into The Chamber Is 2 Hrs.

37. Components Of A Pneumatic Caissons
Miscellaneous Equipments:
Different Types Of Miscellaneous Equipments Used
In Pneumatic Caisson Are As Follows:
Pumps
Motors
Air Compressors
These Equipments Are Normally Placed Above The
Bed Level.
Through Compressed Air Pipe, Can Be Applied To The
Working Chamber.

38. Advantages Of Pneumatic Caissons
 Quality Control Is Good Because Work Is Done In Dry
Conditions.
 Insitu Soil Tests Are Possible To Determine The Bearing
Capacity.
 There Is Direct And Easy Passage To Reach The Bottom
Of Caisson, Hence Any Obstruction Can Easily Be
Removed.
 Concrete Gain More Strength Due To Dry Conditions.
 For Major Projects, Greater Depths In Bed Rocks Can Be
Possible.
 There Is No Danger Of Settlement Of Adjoining
Structures Because Of No Lowering Of Ground Water
Table.

39. Disadvantages Of Pneumatic Caissons
 Following Are Some Of The Disadvantages Of Pneumatic
Caisson.
 Construction Of Pneumatic Caissons Is Much Expensive
Than Open Caissons.
 During Working The Various Constructional Activities, A
Proper Care Has To Be Taken, Otherwise It May Lead To
Fatal Accidents.
 Maximum Depth Below Water Table Is Limited To 30 M
To 40 M. Beyond 40 M Depth, Construction Is Not
Possible.
 There Is More Chances Of Caisson Diseases To
Workmen Working Under High Pressure.
 Labour Cost Is High.

40. Loads On Caissons:
Dead Loads: The Weight Of The Superstructure And The Self
Weight Of The Well Foundation Constitute The Dead Loads.
Live Loads:
Longitudinal Force: Longitudinal Force Occurs Due To Tractive
And Braking Forces. These Are Transmitted To The Substructure Mainly
Through Fixed Bearing And Through Friction Is Movable Bearings.
Earth Pressure: The Earth Pressure Is Calculated Based On One Of
The Classical Earth Pressure Theory Of Rankine And Coulomb. Passive
Earth Resistance Of The Soil Is Taken Into Account For The Stability Of
Foundation Below The Scour Level.
Centrifugal Forces: A Centrifugal Force Is Taken To Be Transmitted
Through The Bearings If The Superstructure Is Curved In Plan.
Temperature Stresses: Longitudinal Forces Are Induced Owing To
Temperature Changes. The Movements Due To Temperature Changes
Are Partially Restrained In Girder Bridges Because Of Friction.

41. Loads On Caissons:
Seismic Forces: These Are To Be Considered In Seismic Zones. The
Force Is Taken As αw, Where W Is Weight Of Component And α Is
Seismic Co-efficient. The Value Of Α Depends Upon The Zone And Is
Given In IS:1893-1975 “Indian Standard Criteria Of Earthquake
Resistant Design Of Structures”. It’s Value Ranges From 0.01 To 0.08.
Resultant Force: The Magnitude, Direction And The Point Of
Application Of All The Applicable Forces Are Found For The Worst
Possible Combination. The Resultant Can Be Imagined To Be Replaced
By An Equivalent Vertical Force W, And Lateral Forces, P & Q In The
Longitudinal And Transverse Direction Of The Pier, Respectively. The
Action Of Q Will Be More Critical In The Consideration Of The Lateral
Stability Of The Well.
Sinking Loads: These Loads Include The Force To Which A Caisson
Is Subjected During Sinking Operations.
These Chief Sinking Forces Are As Follows :

42. Loads On Caissons:
Sinking Loads: These Chief Sinking Forces Are As Follows :
Buoyancy Forces: Buoyancy Reduces The Effective Weight Of The Well.
In Masonry Or Concrete Steining, 15% Of The Weight Is Taken As Buoyancy
Force To Account For The Porousness. When The Well Is Founded On Course
Sand, Full Buoyancy Equal To The Weight Of The Displaced Volume Of Water
Is Considered. For Semi Previous Foundation, Appropriate Reduction May Be
Made Based On The Location Of Water Table.
Friction: This Force Will Depend On The Sink Friction Of The Type Of Soil
And It Should Be Limited To Such An Extent That The Caisson Sinks Of Its Own
Weight When Earth Is Removed From Within The Caisson.
Internal Air Pressure: This Forces To Be Taken Into Account In Case Of
Pneumatic Caissons Only.
Hogging And Sagging Stresses: These Forces Are Developed In Early
Stages Of Sinking Large Rectangular Caissons. The Excavations Within The
Caisson Should Be Carried Out Carefully To Minimize The Intensity Of These
Stresses.

43. Loads On Caissons:
Sinking Loads:
Water Pressures: This Forces Are Encountered Especially When The
Caisson Is Being Floated For Its Final Location In Deep Water.
Another Kind Of Water Pressure Is Encountered After Launching
Of Caisson Adjacent To Caisson, Which Is Due To The Water Current
Acting On The Part Of The Substructure Between The Water Level
And The Maximum Scour Level.
The Intensity Of Water Pressure On The Piers Parallel To The
Direction Of Flow Is Given By,
P= K.V2
Where=intensity Of Water Pressure (N/M2)
V=velocity Of The Water Current (M/S)
K=constant, Depends Upon The Shape Of The Well
(Max. 788 For Square Ended Piers, Min. 237 For Pier With Cut-
water And Ease Water)

44. Floating Of Caissons:
Sometimes, A Part Of The Bottom Portion Of The Caisson Including
Cutting Edge Is Constructed On The Shore And It Is Then Floated And
Placed In Position, This Process Is Called Floating Of Caisson.

45. Methods Of Floating Of Caissons:
1) Construction Of Dry Rock
A Dry Dock Is Constructed On The
Upstream Side Of The Flow. The
Required Portion Of The Caisson Is
Built In The Dry Dock.
The Dry Dock Is Then Flooded To
Float Out The Prepared Portion Of The
Caisson. The Caisson Should Be
Properly Anchored When It Is Floating
And Loading.
This Is Some Time Is Done By
Providing R.C.C. Pre-cast Block Near
The Final Position Of The Caisson. The
Blocks Are Dumped Into The River And
Wrapped By Cables Or Wire By Divers.

46. 2) Floating From Bank
The Caisson Can Be
Floating From The River
Bank. In Case Of River
Bank With Steep Slope ,
Girders Projection Inside
The River Are Supported
On Piles.
The Caisson Is Prepared
On The Shore, Rolled To
Other End And Then
Lowered Into The River By
Suitable. In Case Of River
Bank With A Gentle Slope,
The Slipway Is Prepared.

47. 2) Floating From Bank
The Caisson Is Built On
The Shore And Rolled On
Slipway. When It Reaches
The Other End Of Slipway,
It Starts Floating In Water.
The Caisson In Both The
Case Is Then Suitably
Carried And Placed In
Final Position.

48. 3) Turning Of Caissons
This Method Is Adopted
When Depth Of Water Is
High. The Caisson Is
Launched Upside Down
I.E. Cutting Edge Is Upward
And The Top Is At Bottom.
The Potion On Side Is
Kept Side Higher Than The
Remaining One. The Sand
Is Used To Maintain
Symmetry During
Launching.
The Stone Ballast And Water Are Then Added To Cause Tilting
Of Caisson. When Complete Turning Occurs, The Caisson Starts
Floating In The Water And Stone Ballast Falls Out Of The
Caisson.

49. 4) Use Of Compressed Air
The Completed Portion Of Caisson Is Provided
With A Steel Dome And The Compressed Air Is
Applied Through It.
The Compressed Air Keeps Out Water And
Facilitates The Floating Of Caisson.
When The Caisson Settles Down At Its Proper
Position, The Steel Dome Is Removed.

50. Cutting Edges Of Caissons:
The lowermost portion of a caisson is provided with a cutting edge
so as to facilitate the sinking of caisson.
Properties Of A Cutting Edge:
It should be strong enough to resist the stresses likely to develop
during the sinking operations of the caisson.
It should be capable of preventing sudden sinking of the caisson
when a soft stratum is met with.
In case of pneumatic caissons, it should be air tight i.e. capable of
preventing escape of air.
The edge should be wall protected by the steel point to avoid any
damage to the edge.
The outer edge should be vertical.
The inside angle of inclination depends on the type of the soil. For
hard soil ,it should be small.
The reinforcing bars should be provided between the cutting edge
and the wall to have proper bond to avoid reparation between them.

51. Types Of Cutting Edges:
1)Cutting Edges With Sharp Ends:
This type of cutting edge has a sharp edge. It is used for the open
steel cylinders.

52. Cutting Edges With Sharp Ends:
Advantages:
The Sinking Of Caisson Is Carried Out Without Doing
Excavation Under The Cutting Edge.
In Case Of Pneumatic Caissons, It Prevents The Escape
Of Air Effectively As The Thin Edge Readily Penetrates The
Soil And Seals The Joint.
Disadvantages:
If One Side Of Caisson Strikes A Soft Stratum, It Does
Not Prevent Enough Bearing Area For Support Of The
Other Part.
It Gets Twisted Or Damaged When Any Obstruction
Such As Boulder Is Met With During Sinking Of A Caisson.

53. Types Of Cutting Edges:
2)Cutting Edges With Blunt Ends:
it is suitable when a caisson is to be sunk through a rock and land
surface.

54. Cutting Edges With Blunt Ends:
Advantages:
The Bearing Area Is More.
It Does Not Easily Get Twisted Or Damaged.
Disadvantages:
It Is Not Much Effective In Preventing The Escape Of Air
Through The Joint Formed Between It And The Surface.
It Is Necessary To Do Excavation Under The Cutting Edge
To Cause The Sinking Of Caisson.

55. Sinking Of Caissons:
The Various Methods Adopted To Facilitate The Sinking Of Caissons Are:
A) Air And Water Jets:
In This Method, Jets Are Provided Near The Cutting Edge Level
To Reduce The Skin Friction.
The Jet May Be Embodied In The Body Of The Caissons Or They
May Be Operated Independently By Drivers Or Other Workmen.
Air Or Water Is Forced Through The Jets Which Ultimately
Facilitates The Sinking Of Caisson.

56. Sinking Of Caissons:
B) Blasting:
In This Method, Explosives Are Used To Remove Any Obstruction
Such As Rock, Boulder, Etc. And There By To Facilitate Sinking Of
Caisson.
The Blasting Also Helps In Reducing Skin Friction Of Hard Soils.
The Cutting Edge Is Cleared And Suitable Charge Of Explosive Is
Provided At A Depth Of About One To Two Metres Below The Curb
Level.
The Usual Explosive Used In Blasting Gelatine.
It Is Powerful Under Water And Convenient To Use. The Quantity
To Be Used Per Blast Is About 0.10 To 0.50kg.
It Is Necessary To Take Extreme Care To See That The Cutting Edge
Of Caisson Is Not Damaged The Blasting Operations.

57. Sinking Of Caissons:
C) Loading:
While Sinking Of Caisson, When A Hard
Surface Is Met With, The Weight Of The
Steining Proves To Be Insufficient For The
Further Sinking Of The Well.
In Such Cases, Loading Of The Well Is
Carried Out. The Old Rails Are Put Up At
The Top. These Rails Provide Good And
Smooth Large Bearing Surface On The
Masonry And Can Take Up Heavy Loads.
Wooden Or Steel Joists Are Then Placed On There Rails And 50mm
To 60mm Thick Planks Are Placed Over The Joists To Complete The
Loading Platform.
On This Platform, Bags Filled With Same Are Placed To Impart
Additional Weight To The Well. This Facilitates Sinking Of The Well.

58. Sinking Of Caissons:
D) Sand Island:
This Method Is Adopted
When The Subsoil
Condition Are Such That It
Is Not Possible To Keep
The Caisson Stable.
This Method Consists In
Sinking A Steel Cylinder
Around The Site Of Work
And Then Filling This
Cylinder With Sand Or
Other Dredged Material.
The Caisson Is Sunk
Through This Filling In The
Usual Manner.

59. Construction Of Sand Island:
In Order To Prevent Scour, A Mattress Is Laid On The River Bed.
The Mattress Is Usually Prepared From Loosely Woven Timber.
The Timber False Work Is Constructed Around The Position Of The
Sand Island.
The Steel Shell Is Constructed And Lowered. The Shell Is Generally
Prepared From Steel Plates Of 12 Mm Thickness And Angle Flanges
Are Used To Keep The Sections Together.
When The Steel Shell Reaches The Mattress Level, A Sharpened
Steel Pile Is Moved Inside The Steel Shell. The Circular Area Of
Mattress Which Is Thus Cut Is Then Removed From The Shell.
The Inside Shell Is Then Filled With Sand Or Other Dredged
Material.
The Construction Of Caisson Is Then Commenced. The Sinking Of
Caisson Is Effected Under Its Own Weight And By Dredging Through
The Sand Or Dredge Material.

60. Construction With Sand Island:

61. Rectification of tilts and shifts
Regulations Of Excavation: The
Higher Side Is Grabbed More Be Regulating
The Dredging. In The Initial Stages This May
Be All Right. Otherwise, The Well May Be
Dewatered If Possible, And Open Excavation
May Be Carried Out On The Higher Side.
Eccentric loading: Eccentric Placing Of
The Kentledge May Be Reported To Provide
Greater Sinking Effort On The Higher Side. A
Platform With Greater Projection On The
Higher Side May Be Constructed And Used
For This Purpose. As The Depth Of Sinking
Increases, Heavier Kentledge With Greater
Eccentricit Would Be Required To Rectify
Tilt.

62. Rectification of tilts and shifts
Water Jetting: If Water Jets Are Applied
On The Outer Face Of The Well On The
Higher Side, The Friction Is Reduced On That
Side, And The Tilt May Get Rectified.
Pulling The Well: In The Early Stages Of
Sinking, Pulling The Well To The Higher Side
By Placing One Or More Steel Ropes Round
The Well, With Vertical Sleepers Packed In
Between To Distribute Pressure Over Larger
Areas Of Well Steining, Is Effective.
Excavation Under The Cutting Edge:
If Hard Clay Is Encountered, Open
Excavation Is Done Under The Cutting Edge,
If Dewatering Is Possible; If Not, Drivers May
Be Employed To Loosen The Strata.

63. Rectification of tilts and shifts
Strutting The Well: The Well Is
Strutted On Its Tilted Side With Suitable
Logs Of Wood To Prevent Further Tilt. The
Well Steining Is Provided With Sleepers To
Distribute The Load From The Strut. The
Other End Of The Logs Rest Against A Firm
Base Having Driven Piles.
Insertion Of Wood Sleeper Under
The Cutting Edge: Wood Sleepers May
Be Inserted Temporarily Below The Cutting
Edge On The Lower Side To Avoid Further
Tilt.
Pushing The Well With Jacks: Tilt
Can Be Rectified By Pushing The Well By
Suitably Arranging Mechanical Or
Hydraulic Jacks.

64. Questions:
Q-1. Define Caisson. What Are The Functions Or Uses Of Caissons?
Q-2. Describe Box Caissons With Sketches. In Which Situations They Are Adopted?
Q-3. Describe The Construction Procedure Of Open Caissons With Sketches. In
Which Situations They Are Adopted?
Q-4. What Are Monoliths? Describe Its Construction Procedure With Sketch, Uses,
Advantages & Disadvantages.
Q-5. What Is Pneumatic Caisson? Where Is It Adopted? Describe Procedure For
Sinking, With Advantages & Disadvantages.
Q-6. Describe The Methods Employed For The Floating Caissons.
Q-7. What Is Meant By Tilting Of Caissons? What Are Its Reasons? How The Tilted
Caissons Can Be Brought In Correct Position?
Q-8. What Is Caisson Sickness Or Diseases? What Are Its Causes? What Are The
Precautions To Be Taken To Avoid Caisson Diseases?
Q-9. Explain With Sketch, Different Parts Of Well Foundation Along With Bridge Pier
In The River?

65. Scouring:It is the removal of material around piers, abutments,
spur dikes, and embankments caused by flow acceleration and
turbulence near bridge sub-structural elements and embankments.
Miscellaneous

66. Dredging: It Is An Excavation Activity Or Operation Usually
Carried Out At Least Partly Underwater, In Shallow Seas
Or Freshwater Areas With The Purpose Of Gathering Up
Bottom Sediments And Disposing Of Them At A Different Location.
Miscellaneous

67. Picture Gallery:

76. Each caisson has the inner and outer wall, the gap between the walls forms the circular
compartments filled with the air. This keep the caisson buoyant.
To sink the caisson, engineers fraught the compartments with the seawater.
Once the caisson located on the seabed, they filled with the concrete.

78. The Brooklyn Bridge Caisson
Cross section demonstrating work above and below the water’s surface.

86. by