Coffer dam and its type

coffer dam and its type

1. COFFER DAM

2. Contents Definition Uses Selection of Coffer Dams Types of Coffer Dams Design features of coffer dams Leakage Prevention Economic Height

3. 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.

4. 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.

5. 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.

6. 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

7. 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.

8. 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

9. EARTH FILL COFFERDAMS

10. 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

11. ROCK FILL COFFERDAMS

12. 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

13. TIMBER CRIB OR ROCK FILLED CRIB COFFERDAMS

14. 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

15. 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

16.   Use of Guide Block to prevent twisting of Sheet Piles

17. 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

18. 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

19. 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.

20. 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.

21. 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

22. 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

23. 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)

24. 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.

25. 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.

26. SHEET PILE COFFER DAM Some Photographs

27. GAMMON INDIA LIMITED CABLE STAY BRIDGESURAT AP-02 (EXCAVATION WORK)

28. Bracing welding work in progress

29. SHEET PILE COFFER DAM Some Photographs

30. YOU THANK