coffer dam and its type
TRANSCRIPT
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
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
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
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
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.