“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”.
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
Materials – earth , timber , steel & concrete.
Constructed at site of work.
Depend- depth , soil condition , difference in water level,
availability of materials.
• Pile driving operation.
• Place grillage & raft foundation.
• Construct new structures.
• Enclose space for removal.
• Constructing without disturbances.
FACTORS FOR CHOICE OF
TYPE OF COFFERDAMS
Area 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.
Embankment of some materials.
• 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.
• 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.
• 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 ).
o Soft material - removed.
o Guide piles are driven – 3m.
o Sheet piles are driven with
o Struts & Wales (strength).
o Water 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.
• 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
• 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.
•Driving of guide piles ( 2-3 m) .
•Wales fixed guide piles.
•Driven sheet piles.
•Space filled by puddles.
•Berms provided for safe.
• 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 .
Fast construction work of cells.
Less quantity of steel per running length (circular cells).
Little false work is required.
ROCK FILLED CRIB
• 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.
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.
• Frame work of precast R.C.C. piles & sheets.
• Driven as steel sheet piles.
• Disadvantage (COSTLY).
• Avoid vibration (pile foundation).
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.
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).
DESIGN OF A COFFERDAM
Hydrostatic head of water
Dimensions of area.
Fluctuations of outside water level.
Possibility of erosion.
Presence of ice.
Purely theoretically cofferdam will fail reasonably.
More than theory practical knowledge & experience are
necessary for good construction of cofferdams.