A Brief presentation on cofferdam- final - .pptx

A BRIEF
DISCUSSION ON
COFFERDAMS
Sakib Hasnat ID: 0423042329
Md. Noor Tushar ID: 0423042333
K.M. Nurul Huda ID: 0423042218
CE-6409 REINFORCED
EARTH
1

Content
1. Definition
2. Cofferdam photo gallery
3. Uses of cofferdam
4. Advantage of cofferdam
5. Type of cofferdam
6. Cofferdam components
7. Construction method
8. Selection of cofferdam
9. Conclusion
10. Reference
CE-6409 REINFORCED EARTH 2

Cofferdams
• “A cofferdam is a
temporary
structure design
to keep water
soil out of
the excavation
which a bridge
other structure
built.”
• - Standard Handbook of Heavy
Construction
CE-6409 REINFORCED EARTH
3

Phot
o Gallery
Source: Standard Handbook of Heavy Construction
4

Uses of Cofferdam
To facilitate pile driving operations.
To place grillage as well as raft
foundation.
To provide working platform for the
foundation of building.
To construct piers & abutments of
bridges, dams ,docs etc.
To enclose a space for the removal of
sunken vessels.
https://en.wikipedia.org/wiki/Cofferdam
5

Requirement
of a
cofferdam
• Should be reasonably
watertight.
• Should be easily
constructed at site
of work.
• Should be such that the
total cost
of construction, maintenan
ce and pumping is mini
mum.
• Should be stable
against
bursting, overturn
ing and sliding.
• Ground water or water
lying above ground level
6

Necessity of cofferdam
The coffer dams are required in the
following situation:
When it is required to construct a
structure in the river bed.
When structure is to be constructed on a
sea shore.
When deep excavation are carried out in
a course grained soil.
When excavation is carried out below
ground water table.
During deep excavation, when sides of
the trenches are likely to collapse.
7

Types of
Cofferdams
1. Gravity and Crib Types
• Earth Fill Cofferdam
• Rock fill Cofferdam
• Timber Crib Cofferdam
2. Sheeted Types
• Single Sheet Pile Cofferdams
• Braced Cofferdams
3. Double Skin Types
• Double Wall Sheet Piling Cofferdams
• Cellular Cofferdams

Lets
watch a
Video
clip

Types of Cofferdams
1. Gravity and Crib Types
• Lateral forces due to water are resisted by the shear forces
developed at the base of a gravity structure.
2. Sheeted Types
• Uses sheet piles, anchors and bracings to provide stability.
External earth and water pressures are supported by system of
bracings.
3. Double Skin Types
• Uses steel sheet piling to form cells that either are circular or
parallel walls, each containing fill materials. The strength of these
structures depend on the composite action of the fill, interlock
strength of the sheeting and underlying soil support.

Gravity and Crib Types
Earth Fill Cofferdam
• Simplest type
• Used for shallow depth (1.2~1.5m)
• Velocity and head of pressure is low
Design Considerations
• Top width at least 1 m
• Waterside slope 1.5:1
• Inner side slope 2:1
• Clay/Sand or Clay/Gravel
• Overboard height 0.6 m at least
• Drain for pumping out water
12

 Rock fill Cofferdam
• Water depth up to 3 m.
• Water flow is turbulent
• Suitable where stones are available
Design Considerations
• Clay used to fill cavities and form watertight
structure
• Slope kept at 1:1
• Riprap to protect against wave action
13

Timber Crib
Cofferdam
• Constructed on land and floated into
places
• Lower portion of each cell is
matched with each portion of
contour of the river bed
• Rock ballast or soil is used to
decrease seepage and sink into the
place
• Usually consists of 12’ x 12’ cells
and used in rapid currents or on
rocky river beds
• Must be properly designed to resist
lateral forced such as tipping or
overturning and sliding
http://www.cv.titech.ac.jp/~courses/atce2/Lesson4.pdf
14

Single Sheeted Types
Single Sheet Pile
Cofferdams
• Generally used to enclose small
foundation sites in water for
bridges at shallow depth
• Single row of cantilever sheet
piles
• Can be heavily braced, sheet
pile joints are properly sealed
• Suitable for moderate flow
velocities
• Depth of penetration below
ground about 0.25h for course
soil and 0.50h for fine sand and
0.85h for silts.
15

Braced
Cofferdams
• Driving two rows of vertical
sheeting
• Bracing with wales and struts
• Isolate a working area surrounded
by water
• Prevent ground water from
entering the foundation pit
• Bags of sand stacked on the inside
and outside of the cofferdam
• After the cofferdam is constructed,
the water inside is pumped out
and construction work continues
16

Braced Cofferdams
17

Double Skin Types
 Double Wall Sheet Piling
Cofferdams
• Two straight, parallel vertical walls
• Space between filled with soil
• Width is empirically set h/2 + 1.50m
• Heights greater than 2.5 m should be strutted
• Sometimes provided with an inside berm
Fill material should have
• High friction coefficient and unit weight
• Reduce the uplift on the cofferdam
• Less leakage than a single wall cofferdam
18

Double Skin Types
Cellular Cofferdams
(Circular and Diaphragm Type)
Circular Types
• Each cell may be filled completely to the top
before starting the construction of the next cell
without causing any distortion
• One cell is completely filled up so it can be
used for the construction of other cells
• Each cell acts as a self supporting
independent unit
• Circular cells are more expensive than the
diaphragm type, since it requires more sheet
piles and greater skills.
• Interlock tension limits the resistance to large
lateral pressure due to high heads
19

Double Skin Types
Cellular Cofferdams (Circular and Diaphragm Type)
Circular Types
20

Double Skin Types
Cellular Cofferdams
(Circular and Diaphragm Type)
Diaphragm Types
• A series of diaphragm of steel sheet
piles are connected
• Straight diaphragm walls are
connected by sheet piles arranged in
the form of arches
• Radius of the connecting arches
generally equal to the distance between
the straight diaphragm walls
• Since the connecting diaphragms are
straight walls, the adjacent cells are
needed to be filled up at approx. the
same rate
• Otherwise, unbalanced pressure from
the fill will distort the diaphragm
21

Modes of Failure of Cells
(a) Shear Failure (b) Sheet Interlock Failure (c) Tilting on Base

(d) Bearing Capacity Failure (e) Global Instability

(f) Rotation About Toe (g) Sliding on Base

Cofferdams
Construction
Sequence
• For a typical cofferdam, such as
for a bridge pier, the construction
procedures follow the listed
pattern-
• Pre-dredge to remove soil or
soft sediments.
25

Cofferdams
Construction
Sequence
• Set steel sheet piles, starting at all four corners and meeting at the center of each side.
• Drive sheet piles to grade.
• Block between bracing frame and sheets and provide ties for
sheet piles at the top as necessary.

Cofferdams
Construction
Sequence
• Excavate inside the grade or slightly below grade, while leaving
the cofferdam full of water.

Cofferdams
Construction
Sequence
• Drive bearing piles.
• Place rock fill as a levelling
and support

Cofferdams
Construction
Sequence
• Place Tremie Concrete
Seal.

Cofferdams Construction Sequence
• Tremie concrete seal.

Cofferdams Construction Sequence

Cofferdams
Construction
Sequence
• Construct new structure.
• Flood cofferdam.

Cofferdams
Construction
Sequence
• Remove sheet piles.
• Remove bracing.
• Backfill

Selection of Type of Cofferdams
The depth of water to be dealt with e g. the
earth fill cofferdams are suitable for low
heads of water in still or sluggishly moving
water.
The area to be protected by a cofferdam.
e.g. single wall sheet pile cofferdams are
suitable for narrow excavations and
restricted site areas and double wall
cofferdams cellular sheet piling for wide
excavations.

Selection of Type of Cofferdams
• The possibility of overtopping by floods tides,
etc. e.g. Rock filled crib cofferdams are suitable to
give stability to the crib against Over topping.
• The nature of bed on which the cofferdam is to
rest. e.g. A heavy earth filled crib or cellular
cofferdam could not be carried by deep deposits of soft
clay and single wall sheet piling would be required in
these conditions.
• The nature of velocity of flow, i.e. water flowing with
slow current e.g. The rock filled crib cofferdams are
employed, when the current of water is swift, similarly
earth fill cofferdams are suitable for slow current.

Design Features
of Cofferdams
The design of a
cofferdam depends
on various factors
such as-
• Hydrostatic head of water
• Dimensions of the area to
be covered by the
cofferdam
• Subsoil Conditions
• Fluctuations of outside
water level
• Possibility of erosion
• Floating logs, etc.

Design Features of Cofferdams
• The various thumb-rules for width and
depth of cofferdam are as follows:
• For H < 3 m, W = H
• H > 3, W= 3 + ½ (H-3)
Where,
• W= Width of cofferdam in metres
• H= Height of water above river bed in metre.

Conclusion
• As it is, sometimes not possible to
determine accurately the forces due
to these factors, the worst condition
should be assumed in the design of
the cofferdam to prevent its failure.
• A purely theoretically designed
cofferdam may fail for
factors unaccounted in its design.
• Necessary to combine practical
knowledge or experience with
the theoretical aspect in the design
of a cofferdam.

References
• Cofferdam
• https://en.wikipedia.org/wiki/Cofferdam
• Cofferdam
• http://www.cv.titech.ac.jp/~courses/atce2/Lesson4.pdf
• Cofferdams
• http://courses.washington.edu/cm510/cofferdams.pdf
• How The Cofferdam Process Works
• https://www.damitdams.com/how-a-cofferdam-works/
• Types Of Cofferdams
• https://theconstructor.org/water-resources/types-of-
cofferdams-construction-details/13807/

THANK YOU …............

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