cofferdam

COFFERDAM

PRESENTED BY

JIAD

CIVIL - S5

COFFERDAMS

“A cofferdam is a temporary structure

designed to keep water and/or soil out of the excavation in which a bridge pier or other structure is built”.

REQUIREMENTS Cofferdam should be reasonably watertight.

Absolute water tightness is not desired in a cofferdam.

Designed in maximum water level.

Ground water or water lying above ground level should be excluded.

Materials – earth , timber , steel & concrete.

Constructed at site of work.

Depend- depth , soil condition , difference in water level, availability of materials.

USES• Pile driving operation.

• Place grillage & raft foundation.

• Construct new structures.

• Enclose space for removal.

• Constructing without disturbances.

FACTORS FOR CHOICE OF TYPE OF

COFFERDAMSArea to be protected by a cofferdam.

Depth of water to be dealt.

Possibility of over topping.

Nature of velocity of flow.

Availability of materials at site of work.

Easy transportation.

TYPES OF COFFERDAMS DIKES

SINGLE WALL COFFRDAM

DOBLE WALL COFFERDAMS

CELLULAR COFFERDAMS

ROCK-FILLED CRIB COFFERDAMS

CONCRETE COFFERDAMS

DIKESEmbankment of some materials.

• EARTH

• ROCK

• SAND-BAGS

EARTH DIKES

• Shallow depth of water 1200 - 1500 mm.

• Low velocity of flow

• Free board-1m.

• Suitable side slope.

• Top width of bank.

• Material-mixture of clay &sand.

• Puddle – best material .

• Rip rap – protected against wind & wave actions.

• Piece of rock wt. 70 – 700 N.

• After all, construction work of foundation is started.

• Not used in over topping situation.

ROCK DIKES

• Depth about 3 m.

• Availability of rock.

• Not impervious.

• Impervious layers of earth are laid on side.

• Voids are filled by earth.

• Sometimes, core wall or steel sheet pile are used.

• Core wall – clay or concrete.

• Prevent over topping.

SAND – BAG DIKES

• Mixture of sand & clay in a bag.

PRECAUTIONS

• Use empty cement bags.

• Voids of sand bag may exceed – 40%.

• Small depth.

• Joints filled with puddle.

SINGLE WALL COFFERDAM

• Preferred small area.

• Guide piles, wales , struts – wood.

• Sheet piles are of wood or steel.

• Wooden piles – depth about 10 m.

• More depth - steel piles( 25 m ).

CONSRUCTION

o Soft material - removed.

o Guide piles are driven – 3m.

o Sheet piles are driven with

guide frame.

o Struts & Wales (strength).

oWater pumped out .

DOUBLE WALL COFFERDAMS

• In large area.

• Ohio river type & wood or steel sheeting cofferdam.

OHIO RIVER TYPE COFFERDAM

• Ohio river in USA.

• Used in hard & soft bed (no erosion).

• Unsuitable in deep water & swift flow.

Construction• Wales fixed at 1.50m vertically.

• Wale joints, double vertical planks are provided.

• Tie rods threaded & cross braces fixed.

• Fixes sheet piles.

• Berms (inside & outside)

• Removing safely.

WOOD OR STEEL SHEETING COFFERDAMS

• Depth ( 6-10 m)

• For small & ordinary type

• No possibility of driven of guide piles steel (depth) sheet piles are used.

• Tie rods connects guide piles.

oConstruction•Driving of guide piles ( 2-3 m) .•Wales fixed guide piles.•Driven sheet piles.•Space filled by puddles.•Berms provided for safe.

CELLULAR COFFERDAM

• Large areas.

• Steel sheet piles.

• Diaphragm cells & Circular cells.

• Radius of arcs are equal to distance b/w cross walls.

• Filling height equal (avoid distortion)

Cellular cofferdam – diaphragm cells.

Cellular cofferdam-circular cells.

• Radius of arc 2500 mm.

• 30-45 angle .

Advantages

Fast construction work of cells.

Self-supporting unit.

Less quantity of steel per running length (circular cells).

Little false work is required.

ROCK FILLED CRIB COFFERDAM

• Crib is a box / a Cell open at bottom(timber).

• Rock / gravel / earth (overturning & sliding).

• Depth 10-20 m.

• Limited space.

• Cheap material (timber).

• Swift water flow.

Construction

1.Made as long for safe handling , wide for

structural stability (depth & current of flow).

2.Bottom on land.

3.Floated & placed in position.

4.Space filled by rock , earth , gravel.

5. Providing sheet piles.

6. Suitable earth filling.

7. Pumping.

CONCRETE COFFERDAM

• Frame work of precast R.C.C. piles & sheets.

• Driven as steel sheet piles.

• Disadvantage (COSTLY).

• Avoid vibration (pile foundation).

PREVENTION OF LEAKAGE IN COFFERDAM

Leakage through sheet piling & under flow.

Fissures / cracks of rock – Pumping cement grout by pipes (100mm dia ).

Clay , sand , ashes etc. – dumped around cofferdam.

Single wall cofferdam – Placing V- shaped wooden on outside & Filling puddles.

Leakage in sheet piling – box(sawdust & ashes) hold near joints , leads to sealing.

Interlocks of steel sheet piles – grease.

Double wall cofferdam –

• Driving piles through filling.

• Filling the holes with quick swelling material.

• Forcing clay cylinders for better compactness.

ADVANTAGES OF COFFERDAM

Poor environment.

Safe environment.

Design responsibility.

Easy work.

Re-usability of materials.

PUDDLE FOR COFFERDAM

o Clay & sand / clay & gravel.

o Clay - cohesive & impermeable. Test for cohesive – 40 mm dia 250 mm length

cylinder water(sufficient) .Suspended from one end(wet) ,not break-cohesion.

Test for impermeability – Plastic mass spread ,water stored above . Water held for some time –impermeable.

o Layers of 80 mm , well rammed (water should added).

FACTORS AFFECTING DESIGN OF A COFFERDAM

Hydrostatic head of water

Dimensions of area.

Sub-soil condition.

Fluctuations of outside water level.

Possibility of erosion.

Floating logs.

Presence of ice.

CONCLUSION

Purely theoretically cofferdam will fail reasonably.

More than theory practical knowledge & experience are necessary for good construction of cofferdams.

……THANK YOU