What Is Cellular cofferdams ? Types And How Its Constructed ?
The cellular cofferdam is made of steel sheetpilesandthistypeofcofferdamisprovedsuccessfulunwavering 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 cofferdamswith 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.Theheightoffillinginallcellsshouldbenearlykeptuniformso as to avoid the possibility of distortion of the cross-walls.
Cellular Cofferdams With Circular Cells
In case of cellular cofferdamswith circular cells, the seriesof 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 cofferdamswith circular cells
This typeofcofferdamrequiresmorematerialthanthe previous type , but it has the following advantage :
(i) Each cell may be filled up independently of other cells without anydanger of distortion of cells. Hence, the construction workfor cellsmaybe started simultaneouslyfrom 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, lessquantityof steel per running length of cofferdam isrequired 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 requiresa rockyriver bed and the soil which isfound most suitable for thistype of cofferdam is clay, mud and silt on a bed of rock.
Following points are worth noting in connection with the cellular cofferdams
1) Thistype of cofferdam hasgot 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) Eachcellwhencompletedformsaworkingplatformfor the adjacent cell.
3) The materialswhich are suitable asfilling materialsfor thistype of cofferdam are crushed stone, broken bricks, gravel and sand. The clayand other materialswhich require consolidation before theycan offer resistance to external pressure should not be used as the filling materials.
4) The cellular cofferdamscan 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) Thecurvatureofthesheetingtoformthecell isobtained through the abilityof interlocking clutchesof the steel sheet piles. These clutchespermit 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) Ifthereisanydangerofoverturningofcellduetoearth 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 failswith seriousconsequences. Hence, thistype of cofferdam isunsuitable for groundscontaining boulders or such obstructions.
9) It isfound that the cellular cofferdamsare relativelyhard to pull. It is very difficult to remove the first sheet pile. It is therefore necessaryto provide sufficient grease in the interlocks of the sheet piles before they are placed in position.
10) Forsmallbridgepiers,asinglecell issometimesmade large enough to surround the entire foundation of the pier. As such a cell cannot be filled up, the steel sheet pilesare to be supported by circular rings.
11) The detailed structural design of cellular cofferdams is rather complicated and the forceswhich 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.