What Is Cellular cofferdams ? Types And How Its Constructed ?

 What Is Cellular cofferdams ? Types And  How Its Constructed ?

cofferdam construction

The cellular cofferdam is made of steel sheet  piles  and  this  type  of  cofferdam  is  proved  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.

(i) Each cell may be filled up independently of other cells without any  danger of distortion of cells. 
Hence, the construction work  for cells  may  be started simultaneously  from several points. The arcs are installed after the completion of cells.

(ii)  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

1) This  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.

2) Each  cell  when  completed  forms  a  working  platform  for the adjacent cell.

3)  The materials  which are suitable as  filling materials  for this  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.

5)  The  curvature  of  the  sheeting  to  form  the  cell is  obtained through the ability  of interlocking clutches  of the steel sheet piles. These clutches  permit the deviation from one sheet pile to the other.

6)  The driving of steel sheet piles should be smooth. Otherwise, it will break the interlocking joint and the cell may fail.

7) If  there  is  any  danger  of  overturning  of  cell  due  to  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  with serious  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.

10) For  small  bridge  piers,  a  single  cell is  sometimes  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.