What is Cofferdam?
The word ‘coffer’
means a casket, chest or trunk. A cofferdam is a temporary structure built to
enclose an area for excavation of foundation. Coffer dams are designed & placed
when the size of excavation is very large and sheeting and bracing system
becomes difficult or uneconomical. Coffer dams are generally required for
foundations of structures, such as bridge piers, docks, locks, and dams, which
are built in open water. These are also used for underlying foundations on open
land where there is a high ground water table. A coffer dam generally consists
of a relatively impervious wall built around the periphery of the proposed
excavation to prevent the flow of water into the excavation to prevent the flow
of water into the excavation so that the foundation may be laid in dry
condition.
Types of cofferdams
Following are the
different types of cofferdams commonly used in practice:
Earth cofferdam
These are the
simplest type of cofferdams well-adapted to depths of water upto 3 m. Earth
embankments are constructed around the area to be dewatered. The earth coffer
dams are built of local soils, preferably fine sand. These usually have a clay
core or a vertically driven sheet piling in the middle. The upstream slope of
the bank is covered with a rip rap. A successful coffer dam need not be
completely watertight. For reason of economy, it is not possible to make it
watertight and hence some seepage of water into the excavation is usually
tolerated. The water collected is pumped out of the excavation. The embankment
should be provided with a minimum free board of 1 m to prevent overtopping by
waves. Sand-bag coffer dams are used in an emergency.
Rockfill cofferdam
Rockfill coffer dams
made of rockfill are sometimes used to enclose the site to be dewatered. These
are permeable and are usually provided with an impervious membrane of soil to
reduce seepage. The crest and the upper part of the impervious membrane are
provided with rip rap to provide protection against wave action. Overtopping
doesn’t cause serious damage in case of rockfill coffer dams. The slopes of a
rockfill cofferdam can be made as steep as 1 horizontal to 1.5 vertical.
These are generally
used to enclose small foundation sites in water for bridges at a relatively
shallow depth. In this type of coffer dams, there is a single row of cantilever
sheet piles. The piles are sometimes heavily braced. Joints in the steet piles
are properly sealed. This type of coffer dams are suitable for moderate-flow
velocities of water and for depth upto 4 m. The depth of penetration below
ground surface is about 0.25h for coarse sand and gravels, 0.5h for dine sand
and 0.85h for silts, where h is the depth of water. Sometimes single-sheet
coffer dams are provided with earth fills on one or both sides to increase the
lateral stability. The figure of single sheet pile cofferdam is shown on the
right.
Double-wall Sheet piling cofferdam:-
These dam consists of
two straight, parallel vertical walls of sheet piling, tied to each other and
the space between walls filled with soil. The width between the parallel piles
is empirically set as (h/2 + 1.5m); where h is height of water. Double-wall
sheet piling coffer dams higher than 2.5m should be strutted. Sometimes, an inside
berm is provided to keep the phreatic line within the berm.
The fill material should have a high coefficient of friction and unit weight so
that it performs as a massive body to give the coffer dam stability against
sliding and overturning. Suitable measures should be adopted to reduce the
uplift on the coffer dam. This is generally done by driving the sheet piling on
the upstream as deep as possible.
The double-wall sheet piling coffer dam has the advantage of having less
leakage than that in a single-wall coffer dam. These coffer dams are suitable
upto a height of 10m.
Braced cofferdam:-
A braced coffer dam
is formed by driving two rows of vertical sheeting and bracing with wale and
struts. These are similar to sheeting and bracing system with one basic
difference that braced cuts are required for excavations in dry areas whereas
braced coffer dams are used to isolate a working area surrounded by water. The
braced coffer dams are susceptible to flood damage.
Braced cofferdams are sometimes used as land coffer dams to prevent ground from
entering the foundation pile pit on land and to support the soil so as to
prevent cave in. After the pit is dewatered, the structure is concreted. When
concreting has been completed above the water level, the coffer dam is removed.
Cellular cofferdam:-
This is constructed
by driving sheet piles of special shapes to form a series of cells. Te cells
are interconnected to form a watertight wall. These cells are filled with soil
to provide stabilizing force against lateral pressure. Basically, there are two
types of cellular coffer dams that are commonly used:
1). Diaphragm Type: This type of cellular cofferdam consists of circular arcs on the
inner and outer sides which are connected by straight diaphragm walls. The
connection between the curved parts and the diaphragms are made by means of a
specially fabricated Y-element. The
To avoid rupture of diaphragms due to unequal pressure on the two sides, it is essential to fill all the cells at approximately the same rate. One advantage of the diaphragm type is that the effective length of the cofferdam may be increased easily by lengthening the diaphragm.
To avoid rupture of diaphragms due to unequal pressure on the two sides, it is essential to fill all the cells at approximately the same rate. One advantage of the diaphragm type is that the effective length of the cofferdam may be increased easily by lengthening the diaphragm.
coffer dam is thus made from
inter-connected steel sheet piles. The cells are filled with coarse-grained
soils which increase the weight of the cofferdam and its stability. The leakage
through the coffer dam is also reduced.
2). Circular Type: It consists of a set of large diameter main circular cells
interconnected by arcs of smaller cells. The walls of the connecting cells are
perpendicular to the walls of the main circular cells of large diameter. The
segmental arcs are joined by special T-piles to the main cells.
The circular type cellular cofferdams are self-sustaining, and therefore
independent of the adjacent circular cells. Each cell can be filled
independently. The stability of such cells is much greater as compared with
that of the diaphragm type. However, the circular cells are more expensive than
the diaphragm type, as these require more sheet piles and greater skill in
setting and driving the piles. Because the diameter of circular cells is
limited by interlock tension, their ability to resist lateral pressure due to
high heads is limited.
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