1. COFFERDAM
Coffer – large strong box for valuables; sunken panel or box
1. A temporary watertight enclosure that is pumped dry to expose the bottom of
a body of water so that construction, as of piers, may be undertaken.
2. A watertight chamber attached to the side of a ship to facilitate repairs below
the water line.
3. A water tight enclosure pumped dry to permit work below the water line eg
for the construction of underwater foundations for bridges, piers (Oxford
Dictionary)
What is a Cofferdam?
A cofferdam is a type of watertight construction designed to facilitate
construction projects in areas which are normally submerged, such as
bridges and piers.
A cofferdam is installed in the work area and water is pumped out to expose the
bed of the body of water so that workers can construct structural supports, enact
repairs, or perform other types of work in a dry environment. In some regions of
the world, a cofferdam is better known as a caisson.
Working inside a cofferdam can be hazardous if it is installed improperly or not
safely pressurized, but advances in engineering have led to increased safety for
workers using this unique work environment.
A variety of materials can be used to construct a cofferdam, which is truly a feat
of engineering. Although a cofferdam is a temporary structure, it must reliably
hold water back from the work area and also withstand very high pressures in
order to be safe, and the construction of cofferdams is often used as a project for
engineers learning their craft.
The most basic type of cofferdam uses sheet metal, which is pounded into the
bed of the body of water to create a watertight wall. Next, pumps are used to pull
water out of the enclosure so that it will be dry.

2. Some cofferdams are built from wood or concrete, while others use a double
walled mechanism, with filler made from aggregate materials in between the two
walls.
The walls of a cofferdam can extend all the way to the surface of the water,
leaving it open at the top, or it can be built as an enclosed structure.
In very deep water, enclosed and pressurized cofferdams are used for worker
safety, while in shallower bodies of water, an open cofferdam can be used.
Workers access a closed cofferdam through hatches and tubes, and care is
taken to make sure that the air supply is consistent and the pressure is kept at a
normal level.
Shipwrights and repair yards also use a form of portable cofferdam, which can be
attached to the side of a ship to enact repairs below the waterline. At sea, this
can be a useful way to quickly address potential problems until the ship is taken
into dry dock for more long-term repair. Minor repairs can be undertaken with a
portable cofferdam in a shipyard to avoid the expense of hauling the ship into dry
dock for the work to be completed.
COFFERDAM AND DEWATERING
2.04.01--Description: Work under this item shall consist of the design and
construction of cofferdams as and where shown and specifically designated as
such on the plans; necessary dewatering, adjustments, repair or reconstruction;
and the removal of temporary cofferdams and related facilities.
2.04.03--Construction Methods:
1--Cofferdams : Cofferdams shall be carried to adequate depths and heights,
shall comply with Section 1.10, and shall be safe and watertight as necessary for
the proper performance of the work which must be done inside them. Cofferdams
shall be constructed so that the work can be safely carried to an elevation 600
mm lower than the elevation shown on the plans for the bottom of the structure
footing, or, if a granular fill foundation is shown on the plans, to an elevation 600
mm lower than the bottom of the granular fill foundation. The interior dimensions
of the cofferdams shall be sufficient for the unobstructed and satisfactory
ompletion of all necessary substructure work, such as pile driving, form building,
inspection and pumping. Cofferdams which become tilted or displaced prior to
the completion of all work to be done within them, shall be righted, reset, or
enlarged as may be necessary to provide the clearance for the unobstructed
performance of all necessary work, and such corrections and adjustments of
cofferdams shall be at the sole expense of the Contractor. Cofferdams shall be
completely dewatered as required to complete the work entirely in the dry, except
as specified below.

3. When conditions are encountered that render it impractical to dewater the
cofferdam, the Engineer may require the placing of underwater concrete of such
dimensions as will be necessary to allow the Contractor to complete the
substructure in the dry. The placement of underwater concrete shall comply with
6.01.03-10.
Cofferdams must be constructed to protect uncured masonry and concrete
against damage from a sudden rising of the water and prevent damage to
structure foundations by erosion. No part of the cofferdam which extends into the
substructure may be left in place without written permission from the Engineer.
At least 30 calendar days prior to the start of constructing or installing a
cofferdam, the Contractor shall submit to the Engineer, for his information,
detailed plans and computations of its proposal prepared by a professional
Engineer licensed in the State. The furnishing of such plans and methods shall
not serve to relieve the Contractor of its responsibility for the safety of the work
and the successful completion of the Project. The Contractor's proposal must
meet all requirements established in regulatory permits for the Project and must
also conform to the requirements of Section 1.10.
2--Dewatering: Pumping from the interior of any cofferdam shall be done in such
a manner as to preclude the possibility of water moving through uncured
masonry or concrete. During the placement of concrete or masonry, and for at
least 24 hours thereafter, any pumping shall be done from a suitable sump
located outside the horizontal limits and below the elevation of the work being
placed or as directed by the Engineer.
The pumped water must be discharged in accordance with the requirements of
Section 1.10.
Pumping to dewater a cofferdam shall not start until any underwater concrete has
sufficiently set to withstand the hydrostatic pressure created by pumping.
3--Removal of Cofferdams : Unless the Engineer directs otherwise, the
Contractor shall remove all parts of the cofferdam after completion of the
required work. This shall be done in such a way as not to disturb or otherwise
damage any permanent construction.
Sheet piling used in constructing the cofferdam may be left in place with the
approval of the Engineer, provided the piling is cut off at elevations approved in
advance by the Engineer, and the cut off portions are removed from the site.
2.04.04--Method of Measurement: Work under this item will be measured for
payment by the number of meters of cofferdam designated numerically on the
plans.

4. 2.04.05--Basis of Payment: Payment for this work will be made at the Contract
unit price per meter for "Cofferdam and Dewatering," measured as described
above, which price shall include all costs of design, materials, equipment, labor,
work, and any related environmental controls used in dewatering operations,
which are required for the construction of cofferdams shown in the plans; of any
repair, correction, adjustment or reconstruction of such cofferdams required by
the plans; removal of obstructions; pumping and dewatering; removal of such
cofferdams and related environmental controls used in dewatering operations.
If the Engineer requires the Contractor to construct an additional cofferdam not
shown on the plans, or to enlarge a cofferdam beyond the dimensions of same
as designated on the plans, or if the Engineer accepts the
Contractor's proposal to do so as being essential for the purposes of the
Contract, the Department will revise the Contract to indicate those changes and
to designate the revised dimensions of cofferdam deemed necessary by the
Engineer. If the total number of meters of any given cofferdam as designated in
the revised Contract is greater than the number of meters designated on the
original Contract plans, the Department will pay the Contractor for the revised
number of such meters at the Contract unit price, subject to the provisions of
Articles 1.04.02 and 1.04.03.
To the extent that the Engineer allows the addition or enlargement of a cofferdam
for the convenience or other benefit of the Contractor, but does not deem that
addition or enlargement essential for the performance of the Contract work, the
Department will make no additional payment for the cofferdam or portion of the
cofferdam which the Engineer does not so deem essential. The Department shall
not in any event pay the Contractor for fewer meters of a cofferdam than were
designated on the original Contract plans unless the Department eliminates that
cofferdam in its entirety from the Contract.
Even if, however, the Contractor's plan for an additional cofferdam or
enlargement of a cofferdam deemed essential by the Engineer includes a
previously-existing structure, in no case will a previously-existing natural or built
structure, such as an abutment or an embankment, be measured for payment in
calculating the revised number of meters of cofferdam on the Project.
Any common cofferdam wall required for staged construction will be measured
for payment only once. In no case will a given length or portion of cofferdam be
measured for payment purposes more than once. Pay Item Pay Unit Cofferdam
and Dewatering m

5. DIAPHRAGM WALLS
TECHNOLOGY OVERVIEW
TABLE OF CONTENTS
1. INTRODUCTION
2. METHODS
3. APPLICATIONS
4. CONCLUSION
INTRODUCTION
Diaphragm walls are underground structural elements commonly used for
retention systems and permanent foundation walls. They can also be used as
deep groundwater barriers.
Diaphragm walls are constructed using the slurry trench technique, which was
developed in Europe and has been used in the United States since the 1940's.
The technique involves excavating a narrow trench that is kept full of an
engineered fluid or slurry. The slurry exerts hydraulic pressure against the trench
walls and acts as shoring to prevent collapse. Slurry trench excavations can be
constructed in all types of soil, even below the ground water table.
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METHODS
Cast-in-place diaphragm walls are usually excavated under bentonite slurry.
Various types of excavation equipment can be used depending on project
conditions, including hydraulic excavators and kelly-mounted or cable-hung clam
buckets. Depths in excess 150 feet are possible. (The Hydrofraise, a highly
specialized excavation tool, can reach depths of 500 feet.)
Diaphragm wall construction begins with the trench being excavated in
discontinuous sections or "panels". Stop-end pipes are placed vertically at each
end of the primary panel to form joints for adjacent secondary panels. Panels are
usually 8 to 20 feet long, with widths varying from 2 to 5 feet.
Once the excavation of a panel is complete, a steel reinforcement cage is placed
in the center of the panel. Concrete is poured in one continuous operation
through one or more tremie pipes that extend to the bottom of the trench. The
tremie pipes are extracted as the concrete rises; however, the discharge end of
the tremie pipe always remains embedded in the fresh concrete.
The slurry that is displaced by the concrete is saved and reused for subsequent
panel excavations. As the concrete sets, the end pipes are withdrawn. Similarly,

6. secondary panels are constructed between the primary panels to create a
continuous wall. The finished wall may be cantilever or require anchors or props
for lateral support.
A variation of the technique is the precast diaphragm wall. With this method, a
continuous trench, or longer panel is excavated under self-hardening cement-
bentonite slurry. The slurry is retarded to remain fluid during construction. After a
sufficent length of excavation is complete, a crane lifts the precast concrete wall
section into the trench. The cement bentonite slurry sets to form the final
composite wall. Alternately, the trench is excavated under bentonite slurry, which
is then displaced with cement bentonite slurry.
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APPLICATIONS
Diaphragm walls are commonly used in congested areas. They can be installed
in close proximity to existing structures with minimal loss of support to existing
foundations. In addition, construction dewatering is not required, so there is no
associated subsidence.
The cut and cover method is used to construct tunnels. Two parallel diaphragm
walls are installed and the area between the walls is excavated. Floor and roof
slabs are poured and area above the roof is backfilled.
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CONCLUSION
The diaphragm wall technique is a tried-and-true construction method, which
provides unequaled support of existing foundations during adjacent construction
operations. The method has many civil applications.