COFFER DAM

Contents

Definition

Uses

Selection of Coffer Dams

Types of Coffer Dams

Design features of coffer dams

Leakage Prevention

Economic Height

Definition

Cofferdams are temporary enclosures to keep out water and soil so as to permit dewatering and construction of the permanent facility (structure) in the dry.

A Coffer Dam 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.

Meaning of Coffer Dam : Coffer = Box

To take up the Foundation works in the Marine Region, it is necessary to obstruct the water flow by means of Coffer Dam.

PURPOSE TO USE COFFER DAM

STRUCTURE

To retain Soil & Water

Can be used as either Temporary or Permanent

Main purpose is to provide dry working area for workers

It is constructed to facilitate pile driving operations.

It is used to place grillage as well as the raft foundations

It is used ,when the foundations for piers and abutments of a bridge,

dams, locks, etc. are to be constructed.

Some times it is also provided to store water temporarily.

Cofferdams can be used in all the sectors as explained below.

Civil Engineering: Underground Car Parking, Foundation,

Basement Construction

Transport Engineering: Bridge Pier, Support Walls, Ramps,

Ground Water Retention, Tunnel Work etc.

Water Engineering: Weirs, Culverts, Flood Protection

Walls, Scour Protection Walls, Securing Embankment etc.

Port Construction: Dock Works, Jetty Works etc.

SELECTION OF TYPES OF COFFERDAMS

1. Low heads of water – earth fill cofferdams

2. Narrow Excavations – single wall sheet pile cofferdams otherwise double wall cofferdams or cellular sheet piling are suitable for wide excavations

3. The materials available at site of work for construction

4. The possibility of overtopping by floods, tides, etc. and sliding – rock filled crib cofferdams

5. The nature of bed on which the cofferdam is to rest –deep deposits of soft clay, A heavy earth filled crib or cellular cofferdam could not be suitable but single wall sheet pile would be required in these conditions.

6. The nature of velocity of flow,i.e. water flowing with slow current or with swift current

7. The chances of bed erosion due to reduction of water way caused by the construction of a cofferdam.

8. The facilities available for the transport of equipment and material required for the construction of a cofferdam.

Types of Coffer Dam

1.Earth fill cofferdams - mainly for low level water

2. Sand Bag Coffer Dam – to divert coarse of water

3.Rock fill cofferdams

4.Timber Crib or rock filled crib cofferdams -

Construction on land and than floated into place,

which is also known as Gravity Dam

5.Braced / Sheet Pile Coffer Dam- Consisting of Sheet

Piles, mainly used in shallow water depth

(i). Single wall coffer dams

(ii). Double wall coffer dams

(iii). Cellular cofferdams

6. Movable or suspended cofferdams

EARTH FILL COFFERDAMS

Suitable for rivers or streams having shallow depth of

water, about 120 to 150 cm, with low velocity of flow

It is the simplest type of cofferdam.

Free board – 60 to 90 cm

Top width of earth dyke – one meter

Suitable side slope should be provided

Provided suitable drain

Its depends on soil availability and site conditions.

Dry conditions – clay embankment provided

Wet condition – sand materials suitable

ROCK FILL COFFERDAMS

The construction of rock fill cofferdams is similar to

earth fill dams

It can be constructed with steeper slopes

Water depths – 3 m.

In case of high heads of water , this can be achieved

by a clay or concrete care wall

Impervious layer provided(concrete or caly corewall

or sheet pile ) for reducing entry of seepage

TIMBER CRIB OR ROCK FILLED CRIB

COFFERDAMS

It consists of timber cribs made from logs of wood

Constructed on land and floated into place

It consists of a unit or a cell open at the bottom and

having the framework of horizontal timber members.

Hollow space thus formed are then filled with rock or

gravel

Depth of water – 10 to 20 m

Used for wide excavation and rocky river bottoms

SHEET PILE COFFER DAM

1. Single wall cofferdams

-it used in places, where the area to be enclosed is very small

-depth of water is more = 4.5 to 6 m

- A single row of sheet piles is provided on the whole length of the cofferdam

- Guide piles, Wales and struts (wood)

- guide piles are first driven deep in the river bed and spacing between them 1.8 to 3.0 m

- The sand bags are stacked on the inside and outside faces of the sheet to increase the stability of cofferdam

Use of Guide Block to prevent

twisting of Sheet Piles

2. Double wall cofferdams

- when the area to be enclosed is large it is

necessary to provide double wall cofferdam

- The double wall cofferdams are of two types:

(i) Ohio – river type cofferdams

(ii) timber or steel sheeting cofferdams

3. Cellular Cofferdams

- The cellular cofferdams are mostly used for

dewatering large areas , where the depth of water may

be 19 to 21 m.

-Mostly used the constuction of marine structures like

Dams ,Locks etc.

- The two common shapes of the cellular cofferdam

are:

(i) Circular type

(ii) Diaphragm type

MOVABLE OR SUSPENDED

COFFERDAMS

Where there are numbers of repetition

work in under water foundations ,such

as in the piers of multi span river bridges

, it is economical to design the

cofferdams to be moved as a single unit

from one foundation to another.

DESIGN FEATURES OF COFFERDAMS

The design of a cofferdam depends on various factors such as.

(i).Hydrostatic head of Water

(ii).Dimensions of the area to be covered by the cofferdams

(iii).Subsoil conditions

(iv).Fluctuations or of outside water level

(v).Possibility of erosion

(vi).Presence of ice

(vii).Floating logs(the stem of a cut-down tree, etc.

A purely theoretically designed cofferdam may fail

for factors unaccounted in its design.

Therefore ,become necessary to combine practical

knowledge or experience with the theoretical aspects

in the design of a cofferdam.

For width and Depth of cofferdam :

for H < 3 m, W = W

H > 3, W = 3 + ½*(H-3)

Where, W = Width of cofferdam in

metres

H = Height of water above river bed

in metres

Construction Sequence of

Coffer Dam :

1.Driving of Guide Frame

2.Lowering of Sheet Pile

3.Driving of Sheet Pile

4.Horizontal Bracing (Waller & Strut arrangement) to

withstand the load coming on sheet piles from outside.

5.Excavation and Plug in Concrete to resist uplift pressure

6.Reinforcement and Concrete work of the Permanent

Structure

COFFER DAM

• Factors governing the Economic Height of Coffer Dam

• 1.Depth of Water

- Low depth => Earthen Dam

- High Depth => Sheet Pile Coffer Dam

• 2.Current and nature of flowering of Sheet Pile

- High Current => Sheet Pile Coffer Dam

(Higher Seepage Control Capacity)

- Low Current => Earthen or any other Dam

(Less Seepage Control Capacity)

• 3.Type and Period of Work

- Short Duration Work => Timber Dam

- Long Duration work => Sheet Pile Coffer

Dam

• 4.High and Low Tide Level of Reservoir

- Sheet Pile must have top level slightly higher

than that of the HTL. So that even in

extreme case water can not over top the

coffer dam, to achieve the dry working

through out the season.

• 5.Scour Level of Reservoir

- Scour level can be considered as the bottom

most layer of the loose soil of river Bed Level.

- Up to Scour level there are chances of erosion

due to the current of water. In any Pile or Well

foundation, the care to be taken that the

founding level must be below the scour water

level

• 6.Type of Material to be used

- Structural Sheet pile is costly but can be reused

at different projects.

SHEET PILE COFFER DAM

• Some Photographs

GAMMON INDIA LIMITED

CABLE STAY BRIDGESURAT

AP-02 (EXCAVATION WORK)

Bracing welding work in

progress

SHEET PILE COFFER DAM

• Some Photographs