caissons new

CAISSONS

GROUP 06

CONTENTS What is caisson? Materials used for construct a caisson Design aspects of caisson Types of caisson Open caisson Pneumatic caisson Box caisson Comparison between the types of caissons Failures of caissons Applications of caisson Illustrative example

WHAT IS CAISSON ?

A caisson is a structure used in construction and engineering, particularly underwater projects. Its purpose is to provide a dry, protected environment for workers and construction materials.

It’s a form of deep foundation which are constructed above ground level, then sunk to the required level by excavating or dredging material from within the caisson as a single unit.

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MATERIALS USED FOR CONSTRUCT A CAISSONS

Depending upon the importance and magnitude of the job there are three types of materials which are used to construct a caisson. 1.Timber - Timber is much less used these days than steel and

reinforced Concrete & comparatively high cost.

2.Steel - It provides the necessary weight to aid in the sinking process, which is more continuous, and relatively faster when compared with Caissons built of reinforced Concrete.

3.Reinforced concrete - It utilizes concrete for the dual purpose of providing the necessary

strength and the dead weight for sinking. It is more economical. 3

DESIGN ASPECTS OF CAISSON

1. Shape & size

2. Design loads

3. Allowable bearing pressure

4. Skin friction & sinking effort

5. Concrete seal

6. Cutting edge

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1. Shape & size Caissons are constructed with practically straight and

vertical sides from top to bottom. The shape of a Caisson in plan may be Circular, Square, Rectangular, Octagonal, Twin-Circular, Twin-Rectangular, Twin-Hexagonal, Twin- Octagonal, or Double-D

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Sometimes, the choice of shape of a Caisson is influenced by its size and by the shape of the superstructures.

Twin-Circular, Twin-Rectangular, Twin-Hexagonal, Twin-Octogonal, and the Double-D types are used to support heavy loads from large bridge piers.

The size of a Caisson is governed by the following factors:

Size of Base Bearing Pressure Practical Minimum Size

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2. Design loads A Caisson must be designed to resist all kinds of

loads which may act at different times during service Dead Loads Live Loads Impact Loads Wind Loads Water Pressure Longitudinal Force Earth Pressure Centrifugal Force Seismic Forces Resultant Force

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3. Allowable bearing pressure Caissons are carried to a hard stratum, such as

compact sand, hard clay, gravel, or rock and never to a soft stratum or weathered rock. The Net allowable bearing pressure, Qnet, for a Caisson in cohesionless soil may be obtained from the following equation;

Where;

B = Smaller dimension of the Caisson, m

Df = Depth of Foundation below scour level, m

N = Standard penetration number (corrected)

Rγ and Rq = Correction Factors for Water Table

Qnet = 0.22N2BRγ + 0.67(100 + N2) Df · Rq

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The factor of safety is 3 and Qnet will be got in kN/m2. In the case of pure clays, undisturbed samples should be tested to determine the value of cohesion, C. The ultimate bearing capacity Qult is obtained from;

Where;

Qult = Ultimate bearing capacity, kN/m2

C = Unit Cohesion, kN/m2

Nc = Bearing capacity factor

The allowable bearing pressure of Caissons resting on Rock should not exceed that for the concrete seal. Since the seal is in water or in adverse working conditions, the allowable bearing pressure is usually limited to 3,500 kN/m2

Qult = C.Nc

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4. Skin friction & sinking effort

Skin Friction is the shearing resistance between the soil and the exterior surface of the caisson, encountered during the process of sinking.

Caissons are usually designed to have sufficient weight in each lift to overcome skin friction to facilitate the sinking process.

Occasionally, the use of water jets on the sides tends to reduce the skin friction. Even the injection of bentonite solution on to the exterior of the well has been found to reduce skin friction.

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Values of the skin friction vary within a wide range for each type of soil. Terzaghi and Peck (1948) give the following values

If it is desired to proportion a circular Caisson such that no ballast is required for sinking, the self-weight should be at least equal to the force due to Skin Friction.

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This leads us to the Equation

where ;

De and Di = External and Internal diameters of the Caisson

D = Depth of Penetration

γc = Unit weight of the Caisson Material

f = Unit Skin Friction.13

5. Concrete seal After the Caisson is placed in its final position a

thick concrete layer is placed at the bottom to plug it. This is known as ‘Concrete Seal’ or ‘Plug’, and forms the permanent base for the Caisson.

The thickness of the seal should be sufficient to withstand the upward hydrostatic pressure after dewatering is complete and before concreting of the Caisson shaft is done.

The seal may be designed as a thick plate subjected to uniform pressure due to maximum vertical loads from the Caisson.

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The thickness of the concrete seal, t, may be obtained from the following equations:

These are for simply supported conditions.

Here; Di = Internal diameter of caisson

α = Bi/Li

Li , Bi = Internal length and breadth of caisson

q = Net upward pressure on the seal

σc = Allowable flexural stress for concrete (≤ 3,500 kN/m2)

For Circular Caissonst = 0.59Di q

σc

For Rectangular Caissons

t = 0 866.Bi q σc ( 1+ 1.61α )

√

√

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6. Cutting edge The Cutting Edge protects the walls of the Caisson

against impact and obstacles encountered during penetration.

A cutting edge is usually made of angles and plates of structural steel or reinforced concrete and steel. Since sharp edges are easily damaged, blunt edges are more commonly used.

To avoid tearing off the cutting edge, the shell concrete must be anchored to the cutting edge.

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TYPES OF CAISSONS

Open caisson Pneumatic caisson Box caisson

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OPEN CAISSONS The top & bottom of the

caisson is open during construction.

They may have any shape in plan as round, oblong, oval,rectangular, etc.

Open Caissons are normally used on sandy soils or soft bearing stratum and where no firm bed is available at a higher depth. 18

Components of open caisson Topping

Covering provided over the

caisson is called as topping.

Sand is filled in between

topping and bottom plug.

Topping also acts as a part

of shuttering for laying the

well cap.

Bottom plug

The lower portion of caisson

is sealed by the concrete is

called as bottom plug 19

Steining

Steining is constructed in concrete or masonry work. Use of steining is to provide dead load during sinking operation

Well curbs

It is a transition member between the sharp cutting edge and the thick steining. It is thus tapering in shape. It is usually made of reinforced concrete as it is subjected to severe stresses during the sinking process

Well caps

R.C.C Slab covering provided over the top of well is termed as well cap

Sand filling

The portion between top and bottom plug is filled with sand so as to increase the self weight of the well and makes safe during earthquake. 20

Construction method of open caisson

The open-end caisson usually has a cutting edge. It is first fabricated at the site & the first segment of the shaft is built on it.

Then the soil inside the shaft is removed by grab buckets & the segment sunk vertically.

Another segment is added to the top & the process of sinking is continued by excavating the soil inside.

After the required depth is reached, concrete is placed under water on the open bottom as seal to a depth that will contain the hydrostatic uplift pressure.

Finally the concrete seal is completely cured, the water in the caisson can be pumped out.

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Advantages & disadvantages of open caisson

Advantages Disadvantages

The caisson can be constructed to greater depths.

The clearing and the inspection of the bottom of the caisson cannot be done

The construction cost is relatively low

Concrete seal placed in water will not be satisfactory

The rate of progress will be slowed down if boulders are met during construction

The help of divers may be required for excavation near haunches at the cutting edges

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PNEUMATIC CAISSONS This type of caisson is

open at the bottom and close at the top & it is used at the place where it is not possible to construct the well.

The working chamber at the bottom of the caisson is kept dry by forcing out water under air pressure.

Pneumatic Caissons are suitable in soft soils with danger of scour and erosion

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Components of pneumatic caisson Working Chamber

This is made of structural steel,

about 3 m high, with a strong roof,

and is absolutely air tight. It helps to

prevent entry of water and soil into

it.

Air Shaft

This is a vertical passage which

connects the working chamber with

an airlock. It is provide access to the

working chamber for workmen &

also used to transport the excavated

material to the ground surface. 25

Air Lock

This is a steel chamber provided at the upper end of the air shaft above the water level. Its function is to permit the workmen to go in or come out of the caisson without releasing the air pressure in the working chamber.

Miscellaneous Equipment

It is the equipment such as motors, pressure pumps, and compressors are usually located outside at bed level. Pressure in the working chamber is maintained through an air pipe connected to a compressor.

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Construction method of pneumatic caisson The cutting edge is carefully positioned & compressed

air is introduced into the working chamber to keep off mud and water.

After dewatering the working chamber keep it to dry. As workmen carry out the excavation in the dry, the

caisson gradually sinks. After the caisson has reached the desired depth, the

working chamber is filled with concrete. The air pressure in the chamber is kept constant till the

concrete has hardened up to the roof level. The shaft tubes are then dismantled, and finally, the shaft

itself is filled with lean concrete.27

Advantages & disadvantages of pneumatic caisson


Control over the work & preparation of foundation for the sinking of caisson are better since the work is done in dry

Extreme care is required for the proper working of the system

The caisson can be sunk vertically as careful supervision is possible

The depth of penetration below water is limited to about 35m (3.5 kg/cm3).

The bottom of the chamber can be sealed effectively with concrete as it can be placed dry.

A lot of inconvenience is caused to the workmen while working under compressed air pressure, and they may be afflicted with caisson disease.

Obstruction to sinking, such as boulders can be removed easily.

Construction cost is quite high.

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BOX CAISSONS In here the bottom is

closed. This type of caissons is

first cast on land & then towed to the site & then sunk on to a previously leveled foundation base. It is sunk by filling inside with sand, gavel, concrete or water.

The box type of casing is also called as floating caisson

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Components of box caisson Concrete cap

It is for received the loads from the superstructure

Rip rap

To prevent scour, rip rap is placed around the base

Sand or gravel

It is invariably used as the ballast inside the caisson to aid the sinking process. Concrete is seldom used to fill a box caisson. 30

Construction method of box caisson Preparation & paving construction Bottom rebar binding (rebar fabrication) Bottom formwork erection (formwork fabrication) Bottom concrete casting after concrete testing Bottom Formwork Removal and Curing Upper Rebar Cage Installation (Rebar Cage Binding) Upper Formwork Upper Concrete Casting Upper Formwork Removal Inspection and Mark Launching of a casted caisson (Heavy duty marine air

bags were used to move a caisson on yard premises) Placing of Caissons

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Advantages & disadvantages of box caisson


Since floating caissons are precast, good quality can be ensured

The foundation bed has to be leveled before installing the caisson

The installation of a floating caisson is quick and convenient

Deep excavation for seating the caissons at the required depth is very difficult below water level

Floating caissons are less expensive than other types; they may also be transported at a low cost by floating

Due care has to be taken to protect the foundation from scour

The bearing capacity of the base should be assessed in advance 32

COMPARISON BETWEEN THE TYPES OF CAISSONSOpen of monolith Pneumatic Box or floating

Relatively low cost High construction cost due to use of compressed air in working chamber

Construction cost is low

Can extend to great depths

Depth is limited to a maximum of 35 m below the outer side water level

Shallow depth of excavation

Bottom of caisson cannot be easily inspected ,cleaned and tested physically

Bottom of the caisson can be easily inspected, cleaned and tested as the working chamber is kept dry

Bottom of caisson can only be inspected by divers with difficulty

Concrete placed under water is of doubtful quality

Concrete placed in dry condition is good & reliable

Construction of caisson in control condition & of known quality

Relatively difficult to control plumbness

Easier to control plumbness than open caisson

Plumbness requires preparation of level seating surface. Provision for protecting against scour must be made.

Slow construction if boulders or logs are encountered

Obstructions like boulders or logs can be easily removed

Only less compacted material is removed & bearing layer is not well compacted

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FAILURES OF CAISSONSThere are two types of failures of caissons. They are;

1. Tilting

2. Shifting

The well should be sunk straight and vertical at the correct position. Sometimes the well tilts onto one side or it shifts away from the desired position.

The following precautions may be taken to avoid tilts and shifts: The outer surface of the well curb and steining should be smooth. The curb diameter should be kept 40 to 80 mm larger than the outer

diameter of the steining, and the well should be symmetrically placed.

The cutting edge should be uniformly thick and sharp. Dredging should be done uniformly on all sides and in all the

pockets.34

Remedial Measures for Rectification of Tilts and Shifts

Remedial measure Figure Description

Regulation of Excavation

The higher side is grabbed more be regulating the dredging. If it is not initial stage, the caisson may be dewatered if possible & open excavation may be carried out on the higher side.

Eccentric Loading

Eccentric placing of the kentledge may be resorted to provide greater sinking effort on the higher side. depth of sinking increases, with heavier kentledge

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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 caisson

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.

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Pushing the Well with Jacks

Tilt can be rectified by pushing the well by suitably arranging mechanical or hydraulic jacks. In actual practice, a combination of two or more of these approaches may be applied successfully

Strutting the caisson

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

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APPLICATIONS OF CAISSON

Foundation for bridge piers and abutments in lakes, rivers, and seas, breakwaters and other shore protection works

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large water-front structures such as pump houses, subjected to huge vertical and horizontal forces.

Occasionally caissons, especially Pneumatic Caissons, have been used as foundations for large and tall multi-storey buildings and other structures.

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Illustrative example

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the table given under the concrete seal

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SUMMARY

A Caisson is a type of foundation, built above ground level and sunk to the required depth as a single unit.

Caissons are broadly classified as Open Caissons, Pneumatic Caissons, and Floating or Box Caissons.

Caissons are mostly used as foundations for bridge piers and abutments, and water-front structures, as also for multi-storey buildings occasionally.

How to solve a small problem to find out the parameters of the caisson according to the design aspects.

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ANY QUESTIONS…….?

THANK YOU

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