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Content

• Definition
• Uses
• Construction material
• Types of caissons
• Loads on caisson
• Design features of Caissons

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• Floating of Caissons,
• Cutting Edges
• Sinking of Caisson
• Tilting of Caisson
• Shifting of Caisson
• Caisson Diseases

Introduction

• Definition
• Caisson is a structure, which is sunk through ground or water for the purpose of excavating and placing the foundation at the prescribed depth and subsequently it becomes an integral part of the foundation

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A diagram of an open caisson, devised by Jacques dated 1846

Introduction

• It’s a hollow box or cylinder.
• It is sunk into the ground to some desired depth and then filled with concrete thus forming a foundation
• Most often used in the construction of bridge piers & other structures that require foundation beneath rivers & other bodies of water.

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A diagram of an open caisson, devised by Jacques dated 1846

Use of Caissons

1. To reach the hard bearing strata for transferring the load of superstructure.
2. For the excavation for foundations of bridges, piers, abutments in rivers and lakes.
3. To provide an access to a deep shaft or a tunnel.
4. To provide an enclosure below water level for installing machinery, pump etc.

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Construction Materials

1. Steel
2. Timber
3. Cast- iron
4. Reinforcement Cement Concrete ( R.C.C)

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Types of Caissons

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Box Caisson

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Box Caisson

• A box caisson is open at top and closed at the bottom.
• Used where bearing stratum is available at shallow depth and where loads are not heavy
• Box caisson is built on land and then it is launched and floated to pier site, where it is sunk in position.
• The launched caisson is then sunk by filling it with suitable material usually sand or gravel.
• The top of the caisson is then sealed

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Open Caissons (Wells)

• It is open both at the top and bottom
• An open caisson is rectangular or square in plan and is further sub-divided into smaller section from inside forming open wells.
• Made of Timber, metal, reinforced concrete or masonry
• Used for bridge pier, abutments, etc.
• Curb-stress induced by boulders, blows, blasting

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Shape of Well

• The common types of well shapes are :
• Single circular
• Rectangular
• Twin circular
• Dumb well
• Double – D
• Twin – Hexagonal
• Twin - Octagonal

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Well Component And Their Function

1. Cutting Edge
2. Curb
3. Steining
4. Bottom Plug
5. Top Plug
6. Well Cap
7. Sand Filling

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• Cutting Edge : it provides a comparatively sharp edge to cut the soil below during sinking operation. It usually consist of a mild steel equal angle of side 150 mm.

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• Curb : It has two fold purpose. During sinking it acts as an extension of cutting edge and also provide support to the well steining and bottom plug while after sinking it transfer the load to the soil below. It is made up of RCC of grade M 20.

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• Steining : It is the main body of well. It act as a structural member to transfer the load to the soil.

the steining may consist of brick masonry or reinforced concrete.

• Bottom Plug : The bottom plug of concrete has to be designed for an upward load equal to the soil pressure minus self weight of the bottom plug and sand filling.
• Top Plug
• Well Cap
• Sand Filling

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Monoliths (Multiple wells)

• It consist of multiple wells, which are sunk together
• The individual well of the monolith has got separate cutting edge and the dredging can be done in each of the well separately
• As it bigger in size the cannot be built on the site in the water
• So, they construct in dry place, floated and placed in final position

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Procedure of construction

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18 m

Procedure of construction

1. Suitable site for casting of monoliths is selected on the upstream side of the proposed bridge in river and dry dock is constructed near the river bank
2. Dimension of the dry dock > those of monolith &

a door of sufficient width

3. Monolith constructed up to such height that some portion of it will remain above the water level, when it sunk
4. The gate of dry dock is opened, the monolith is then floated and located in its correct position

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Procedure of construction

5. The dredging operation now starts, in fig shows the sequence of dredging, four dredging well are used simultaneously and the middle row is touched at last
6. After dredging to the required depth, the whole monolith is gradually sunk uniformly up to required level
7. The bottom is provided with a concrete seal, and the wells are filled with concrete and cap is provided at the top to finish up the construction of monoliths

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Pneumatic Caisson

• Its open (during the construction) at the bottom and the closed at the top
• Only adopted if the D _W >12 m.
• Max. Dw up to 35 m
• because limiting pressure under which a man can work us about 0.35 N/mm²
• Entire process of sinking of well is under controlled conditions

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Pneumatic Caisson – Sinking procedure

1. The caisson is sunk exactly in the same manner, as used for well sinking
2. When the presence of water creates problems, the air lock is placed inside the well.
3. After properly placing the air locks in position, so that direct air entry is sealed, water is pumped out form the bottom and air pressure is gradually increased so that fresh water does not enter in the working chamber.
4. The labors are then sent down to the working chamber, through the air lock
5. Excavation is carried out in the working chamber

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Pneumatic Caisson – Sinking procedure

6. Excavated material sent up through the muck bucket lifted up by a hoisting rope operated by winch drum, through the air lock
7. Caisson reached at the desired level, concrete seal is made
8. Air lock are also removed well filled with water or sand
9. Well cap is then formed out at its top usual.

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Pneumatic caisson- Advantages

1. The work can be done in dry condition
2. Comparatively The verticality is easier to check and control
3. Concrete is placed in dry so good and reliable
4. Excavating by blasting may be easily done

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Pneumatic caisson- Dis-Advantages

1. Expensive compare to other caisson
2. Slow rate of work
3. Skill Labor is required
4. Proper health is required for labor
5. Depth of penetration below the water level is limited to 35 m
6. Chances of Fatal Accidents during construction.

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Loads on Caisson

1. Vertical Load
2. Lateral Load
3. Sinking Load
1. Friction
2. Internal air pressure
3. Hogging and Sagging stresses
4. Water pressure
4. Longitudinal Force (Variation in temperature)
5. Seismic Force, Centrifugal Force, Wind Force etc….

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Design Features of Caissons

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Design Features of Caissons

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Design Features of Caissons

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Design Features of Caissons

• Open Caisson and Monoliths
• Heavy and thick wall may be desirable
• Rigidity is essential
• Ideal shape is Circular
• Straight wall are preferable if soil is stiff
• Heavy monoliths sunk through only soft material
• Control of verticality

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Design Features of Caissons

• Pneumatic Caisson
• Provide single working chamber
• Working chamber height is 2.3 to 2.4 m
• Air pressure should be require in a proper quantity
• Lifting unit have a sufficient capacity to lift our excavated material
• Sufficient light and safety provided in working chamber for labors

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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
• Following methods are used for floating of Caisson
• Construction of Dry dock
• Floating from Bank
• Turning of Caisson
• Use of compressed air

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Floating of Caissons

1. Construction of Dry dock
1. Portion of the caisson is built in this dry dock
2. float out the prepared portion of the caisson
3. The caisson should be properly anchored when it is floating and landing
4. This is sometimes done by providing R.C.C pre cast blocks near the final position of the caisson
5. The blocks are dumped in to the river and wrapped by cables

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Floating of Caissons

2. Floating from Bank

• The caisson can be floated from the river bank
• In case of river bank with a steep slop, girders projecting inside the river are supported on piles as shown in fig.
• caisson is prepared on the shore, rolled to other end and then lowered into the river by suitable equipment

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Floating of Caissons

2. Floating from Bank

• In case of river bank with a gentle slope, slipway is prepared as shown in fig.
• The caisson built on the shore and rolled on the spillway, it starts floating in the water
• The caisson in both the case is then suitably carried and placed in final position

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Cutting edges

• Lowest part of a Caisson for sinking can be easy
• Properties of cutting edges
• Sharp angle for knifing 🡪 angle to the vertical of 30˚ or slope 1:2
• In concrete caissons, the lower portion of the cutting edge is wrapped with 12 mm steel plates
• In pneumatic caissons, a sharp vertical edge is provided along the out side face of the caisson to facilitate the rate of the sinking and prevent air leakage
• For water jet, nozzle should be provided in the cutting edges

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Cutting edges

• Types of Cutting edges

1. Cutting edges with sharp ends

• Cutting edges with blunt ends

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Steel Cylinder

Sunk through the sand

Soft soil, pneumatic Caisson

Cutting edges

• Types of Cutting edges

2. Cutting edges with blunt ends

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Average condition

In Rock

Sinking of Caisson

1. Air and water jets
2. Blasting
3. Loading
4. Sand island

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Sinking of Caisson

3. Loading

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Sinking of Caisson

4. Sand island

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Tilting of Caisson

• Sometimes a caisson sinks more on one side than the other, this known as tilting of caissons
• It is mainly due to Unequal Dredging and Non-uniform bearing power of soil
• Following methods may be employed to bring a tilted well in position
• Control of Dredging
• Eccentric Loading
• Water jetting
• Pulling the well

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Tilting of Caisson

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Tilting of Caisson

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Shifting of well

• Change span lengths and thereby induce eccentric load on the well steining and the foundation
• The magnitude of the ill effects depends upon the size of the well and depth of which it is sunk
• If a simple tilt occurs at certain depth and the sinking continued till designed foundation depth is reached the shift at the bottom could be greater than at top as shown in fig.

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Caisson Diseases

• When the workers returned to the atmospheric pressure after working under compressed air, they suffered a certain type disease, this sickness is known as caisson sickness.
• Main symptoms of disease were dizziness, double vision, headache, trouble to speaking, pain in legs, etc.
• The disease resulted in loss of consciousness, paralysis or some time even death is possible

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Caisson Diseases- Causes

• The atmosphere consist one part of Oxygen to fourth part of Nitrogen by volume
• Now person is subjected to compressed air, Nitrogen is also absorbed along with the oxygen by the tissue-fluids of the person
• The Oxygen dissolved in the blood, while Nitrogen remains in suspension, as it is not soluble in blood.
• When the pressure is reduced, the Nitrogen will form bubbles within the tissue-fluids and these bubbles are blocked in joints and it cause bends or violent pains in joints
• If they are developed in spinal cord or heart, it will result in paralysis or death respective

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Caisson Diseases- Precautions

1. Shift and working period
2. Interval between working period
3. Temperature in the working chamber
4. Physical examination of workers
5. Rate of Compression and decompression
6. Man locks
7. Passages
8. Air compressing plant

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9. medical Chamber
10. Notice of work
11. Miscellaneous

Thank You

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