Caisson

Caisson

• Introduction• Uses of caisson• Material used for caisson• Type of caisson• Load on caisson• Design feature of caissons• Floating of caisson• Cutting edge• Sinking of caisson• Problem in well sinking

Caissons

• The term caisson is derived from French word caisse means BOX.

• Caisson is a watertight structure round or rectangular, which is sunkthrough ground or water to exclude water and semi-fluid materialduring the process of excavation of foundation and whichsubsequently becomes an integral part of the substructure.

Uses of caissons

• To reach the hard bearing strata for transferring the load ofsuperstructure.

• For the excavation for foundation of bridges, piers, abutments in riverand lakes.

• To serve as an impervious core wall of earth dam, when placedadjacent to each other.

• To provide access to a deep shaft or tunnel

• To provide an enclosure below water level for installing machinerypump.

Material used for caissons

• Steel

• Timber

• Reinforced cement concrete

• Cast iron

Steel

• It is most suitable material for the construction of a caisson.

• It have form of a double skin of steel plating and hollow space which can filled with cement concrete.

Reinforced cement concrete

• It is suitable for caisson shoes.

• It become costly in the early stage of construction as more weightcreates problem is handling and floating of caisson.

• It become economical to construct a steel caisson with concretelining.

Cast iron

• It is suitable for open well type of caisson.

• New segment of C.I are bolted as the caisson sinks.

• It is not suitable for pneumatic caisson due to risk to tension developed by the compressed air.

• The cost of C.I. caisson is more as compared to steel and RCC.

Types of caissons

• Box caisson (Open at top and closed at bottom)

• Open caissons (Open at top and bottom)

• Pneumatic caisson(Closed at top and open at bottom)

Box caisson

• It is strong water tight vessel open attop and closed at bottom and made oftimber, steel or RCC.

• It is build on land , cured and then floator launched to pier site where it isplaced in position.

• Mainly it is used for shallow depth andfor light weight.

Conditions for Construction of box caisson

• Excavation for foundation is not required. The bed of foundation canpreviously prepared.

• The velocity of flow of water is slow so as to give stability to thecaisson against scouring.

• The bed material is loose enough and it is possible to dredge out thismaterial and from a levelled bearing surface.

• The depth of water is about 6 to 8 m.

Open Caissons Or Wells

• Open caisson is a box of timber,reinforced concrete, steel or masonrywhich is open at the top and atbottom.

• It used for building and bridgefoundation .

• Open caisson are also called wellfoundation.

• They are used on sandy or soft bearingstratum liable to scour and where nofirm bed available for large depthbelow the surface.

Types Of Open Caisson

• Single well

• Multiple well /monoliths

• cylinder

Shapes Of Wells

• Single circular

• Rectangular

• Twin circular

• Dumb well

• Double-D

• Twin-hexagonal

• Twin-octagonal

Shapes Of Wells

Factor For Choice Of Shape Of Well

• The dimension of base of the pier of abutment.

• The case of sinking.

• The cost of sinking and shuttering.

• The vertical and horizontal forces acting on the well.

• The considerations of tilt and shift during sinking.

Component Of Well

• Cutting edge

• Curb

• Steining

• Bottom plug

• Top plug

• Well cap

• Sand filling

• Cutting edge

Provides sharp edge to cut the soil below during sinking operation.

• Curb

During sinking it acts as extension of cutting edge and provide support the well. It is made of RCC.

• Steining

It is main body of the well. Made of RCC or masonry with minimum thickness of 45 cm.

• Bottom plug

Made of concrete and designed for an upward load equal to the soilpressure minus self weight of the bottom plug and sand filling.

• Top plug

It supports the well cap.

• Well cap

It transfers the load from pier to the well.

• Sand filling

It transfers a portion of load from well cap to the bottom plug.

Well Sinking Operations

• For dry ground open excavation up to half a meter above subsoil water levelis carried out and the well curb is laid.

• Well Steining built in short height about 2 m.

• Well is sunk by excavating material from inside the curb where excavationmay carried out by worker or dredger.

• “Kentledge” is applied on well by iron rail or sand bag to counter skinfrication and loss in the weight of well due to buoyancy.

• Pumping out the water from inside the well when well has gone deepenough.

• A concrete plug is provide at the bottom which made bowl shaped.

• Internal space is filled with sand and top of well provide concrete with RCCcap.

Multiple well/ Monoliths

• The monoliths consists of multiple wells,which are sunk together.

• The individual well of the monoliths hasgot separate cutting edge and thedredging can be done in each of the wellseparately.

• It having bigger dimension so they cannot built on the site in the water.

• Therefore, they are constructed in dryplace, floated and placed in final position.

Construction

• A suitable site is selected on the upstream side of the proposed bridge in a riverand a dry dock is constructed.

• Dimension of the dry dock is bigger then monolith and it should providedsufficient width of door.

• The monolith is constructed in the dry dock to such a height that some portion ofit remain above water level.

• The gate of the dry dock is opened the monolith is then floated and located in itscorrect position.

• Dredging operation done in sequence as four dredging well used simultaneouslyand the middle row is touched last.

• After dredging to the required depth the whole monolith is gradually sunk up torequired level.

• The bottom is provided with concrete seal and then filled with concrete and capis provided at top.

Pneumatic caisson

Pneumatic caisson

• A pneumatic caisson is open at bottom and closed at top.

• It is useful at location where well is not possible.

• It suitable for water depth of 12 m to 35 m.

• Compressed air is used to remove water from working chamber at thebottom and foundation work is carried out in dry condition.

• The pressure of air in chamber is higher than water at that depth forsubsoil water may not enter in working chamber.

• The maximum pressure is limited up to 0.35 N/mm2 .

Construction

• The caisson is sunk in water till no problem is encounter. When watercreates problem the air lock is placed inside the well.

• There are two air locks are provided in caisson one for sending meninside and second for removing the excavated material with help ofmuck bucket and hosting rope.

• After the lock is placed in position direct entry of air is sealed water ispumped out from the bottom and compressed air is prevent the entryof water from bottom.

• The work of foundation is then started by sending the labour in toworking chamber.

Construction conti..

• The height of chamber is kept about 2m and lighting arrangement isprovided.

• The excavation material may be taken out by a muck hoisted by arope or cable.

• The explosive or reduction of air pressure for a short duration may beapplied.

• When a caisson has reached the desired level a concrete seal isprovided at bottom .

• The air lock and other equipment removed and filling caisson withsuitable material.

Pneumatic caisson sickness

• Dizziness

• Double vision

• Headache

• Trouble to speaking

• Pain in lag

• Etc.

Precautions to avoid sickness

• Work only for one shift in a day

• Duration of shift should not exceed 12 hours.

• Maintain temp. 250 C.

• Persons employed on the work should be physically fit.

• The main lock should be well-ventilated.

• Medical chamber must be provided just near the side of work

• Etc.. As per your view

Loads On Caisson

• Vertical load

• Lateral load

• Sinking load

• Seismic forces

• Longitude force

• Wind force

• Centrifugal force in case of well on a curve

Floating of caisson

• Construction of dry dock

• Floating from bank

• Turning of caisson

• Use of compressed air

Construction Of Dry Dock

• A dry dock is constructed on the upstream side of the bridge site.

• After construction of caisson in dock it may flooded to float out.

• The caisson should be properly anchored when it is floating and landing.

Floating from bank

Turning of caissons

• This method is used when depth of water is high.

• The caisson is launched upside down cutting edge is upward and top is at bottom.

• The portion on one side is kept slightly higher than the remaining one.

• Sand is used to maintain symmetry during launching.stone ballast and water are then added to cause tilting of the caisson.

• When complete turning occurs, the caisson starts floating in the water and stone ballast falls out of the caisson.

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.

Cutting Edge• It should be sharp angle for knifing into the soil to resist the various

stresses induced by boulders blow, blasting, etc.

• It should be well protected by the steel point to avoid any damage tothe edge

• The outer edge should be vertical

• In case of pneumatic caisson it should be air tight.

• If water jet is to be used for sinking purpose the nozzle should beaccommodated in the cutting edge to reduce skin friction.

• In concrete caisson the lower portion of the cutting edge is rigidlyfastened with 12 mm steel plate with help of steel straps.

Types of cutting edges

1. Cutting edge with sharp ends

2. Cutting edge with blunt ends

Cutting edge with sharp ends

• Cutting edge shown infig (a) and fig. (b) areused for opencylinder/ caisson.

• Fig. (c ) is used forpneumatic caisson.

Cutting edge with blunt ends

• a is suitable for Avg. condition ofsinking

• b is suitable when a caisson is to besunk through rock

• C is suitable when it is to be builtfrom a land surface.

Factors to selection of a cutting edge

• Method of sinking to be adopted.

• Construction method of caisson

• Nature of soil

• Nature of ultimate soil stratum on which the cutting edge is to rest while concrete is being placed for the bottom plug.

Sinking of caisson

• Various method for sinking of caisson are here below;• Air and water jets

• Blasting

• Loading

• Sand island

Air and Water jets

• Jet are provided near the cutting edge level to reduce the skin friction

• It may be embodied in the body of the caisson or it may be operated independently by driver or other workmen.

• The air or water is forced through the jets which ultimately facilitates the sinking of caisson.

Blasting

• Explosive are used to remove any obstruction like rock, boulder etc .

• It is reduce skin friction of hard soils.

• for blasting gelignite is used which is a absorbing powder and blastinggelatin.

• It is powerful under water and convenient to use.

• It is necessary to take extreme care to see that the cutting edge ofcaisson is not damage during the blasting operation.

Loading

Sand island

• It is used when subsoil condition aresuch that it is not possible to kept thecaisson stable.

• In this method sinking a steel cylinderaround the site of work and then fillingthis cylinder with sand or other dredgedmaterial.

• The caisson is sunk through this filling inthe usual manner.

• It is also known as artificial islandmethod.

Problem in well sinking

• Sand blowing

• Tilting of wells

• Shifting of wells

Sand blowing

• Sand blowing is takes place during the process off dewatering of the well passing through sandy strata.

• The ground around the well starts breaking up to wide cracks are seen in it.

• The fall of sand in the caisson is sudden and huge amount to a depth of about 3 to 15 m of sand.

• This may result in a fatal accident and the equipment and worker may get buried during sand blowing.

Shifting of wells

• The shifts change spanlength and thereby inducedloads on the well stainingand the foundation.

• The magnitude of the illeffect depend upon the sizeof well and depth of whichit is sunk.

Tilting of wells

• Control of dredging

• Eccentric loading

• Water jetting

• Pulling the well

• Pushing by jacks

• Use of explosive

• Deposition of earth on one side and excavation on other

• Providing temporary obstacle below the cutting edge