What Is Cellular cofferdams ? Types And How Its Constructed ?
|cofferdam construction |
The cellular cofferdam
is made of steel sheet piles
successful unwavering large areas.
It may consist of diaphragm cells or circular cell or modified circular cells or cloverleaf cells,
Cellular Cofferdams With Diaphragm Cells
In case of cellular cofferdams
with diaphragm cells, the series of arcs are connected to straight cross-walls as shown in Fig
|Cellular cofferdam with diaphragm cells |
The radius of arcs is generally made equal to the distance between the cross-walls. This will make tensions in arcs and cross-walls same.The height of filling in all cells should be nearly kept uniform so as to avoid the possibility of distortion of the cross-walls.
Cellular Cofferdams With Circular Cells
In case of cellular cofferdams with circular cells, the series of complete circles are connected by short arcs as shown in fig. The radius of the arc is generally 2500 mm and it makes an angle of 30° to 45° at the point of contact with the circular cell.
cellular cofferdams with circular cells
This type of cofferdam requires more material than the previous type , but it has the following advantage :
(i) Each cell may be filled up independently
of other cells without any danger of distortion of cells.
Hence, the construction work
be started simultaneously
from several points. The arcs are installed after the completion of cells.
Each circular cell is a self-supporting unit.
(iii) For the construction of cellular cofferdam with circular cells, less quantity of steel
per running length of cofferdam is
required as compared to that of the cellular cofferdam with diaphragm cell.
In case of cofferdams with modified circular cells,
the arrangement of circular cells is made as shown in fig.
|Modified cellular cell |
In case of cofferdams with cloverleaf cells
, the large circular cells are subdivided by straight diaphragm as shown in fig.
|cloverleaf cells cofferdam |
The cellular cofferdams are suitable for heights of 10 metres to 15 metres
. The cells are generally 10 metres to 15 metres in diameter
and they are placed at a centre to centre distance of about 12 metres to 18 metres.
|Section of cofferdam |
The cellular type of cofferdam requires
a rocky river bed
and the soil which is
found most suitable for this
type of cofferdam is clay, mud and silt on a bed of rock.
Following points are worth noting in connection with the cellular cofferdams
type of cofferdam has
got the advantage that little false work
is required in its construction. Only top and bottom templates are required to drive the piles in proper position.
a working platform for the adjacent cell.
3) The materials
which are suitable as filling materials
type of cofferdam are crushed stone, broken bricks, gravel and sand. The clay
and other materials
which require consolidation before they
can offer resistance to external pressure should not be used as the filling materials.
4) The cellular cofferdams
can also be used on an irregular river bed.
The lengths of steel sheet piles will have to be adjusted according to the profile of the river bed.
obtained through the ability
of interlocking clutches
of the steel sheet piles. These clutches
permit the deviation from one sheet pile to the other.
The driving of steel sheet piles should be smooth.
Otherwise, it will break the interlocking joint and the cell may fail.
earth pressure, the inside and outside berms should be provided.
8) It should be remembered that if one sheet or interlocking joint fails,
the whole cell fails
consequences. Hence, this
type of cofferdam is
unsuitable for grounds
containing boulders or such obstructions.
9) It is
found that the cellular cofferdams
are relatively hard to pull
. It is very difficult to remove the first sheet pile. It is therefore necessary
to provide sufficient grease
in the interlocks of the sheet piles before they are placed in position.
made large enough to surround the entire foundation of the pier.
As such a cell cannot be filled up, the steel sheet piles
are to be supported by circular rings.
11) The detailed structural design of cellular cofferdams is rather complicated and the forces
which are taken into account in the design are weight of filling, weight of sheet piles, pressure due to water, resistance of soil to sliding, etc.
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