Underwater Construction
Underwater construction is a method on how to place the concrete underwater. During the construction of bridges, dams or any structure where the foundation is most likely to lie underwater, the underwater construction is opted. Construction in water poses many difficulties especially in the places where the depth is considerable. During underwater construction, the main objective is to create dry and water free environment for working so that the stability of the structure is balanced.
Underwater Construction techniques:
- Caissons
- Cofferdams
Caissons
A caisson is retaining water tight structure used to work for the construction of a concrete dam, on the foundation of
a bridge pier or for the repair of ships. They are sunk through water during the process of excavation of foundations to exclude water which eventually becomes an essential part of the substructure.
Types of Caissons
- Box caissons
- Open caissons
- Suction caissons
- Pneumatic caissons
Box Caissons
Box caissons are the prefabricated concrete boxes of various shapes with water tight floor and walls, which are set down on the prepared bases. Once they are in place, they are filled with concrete to become a part of the permanent structure.
Open Caissons
Open caissons are box type structure similar to box caissons except that they don’t have a bottom face and mainly consists of vertical walls. These caissons are sunk by self-weight, concrete, water ballast placed on top, by hydraulic jacks depending on site.
Suction caissons
Suction caissons are defined as an upturned bucket that is embedded in the marine sediment. The embedment is achieved either by pushing or by creating negative pressure. This type is usually used in offshore construction.
Pneumatic caissons
Pneumatic caissons are the ones which are closed on top and open at the bottom .The compressed air is used to exclude water from the caissons chamber. The construction of this type of caisson is similar to the others expect that, the working chamber and the shaft are made air tight.
Cofferdams
Cofferdams are temporary watertight enclosure pumped dry below the water line to execute the building operation to be performed on dry surface.
Types of cofferdams
- Earthen cofferdam
- Rock-fill cofferdam
- Cellular cofferdam
- Single walled cofferdam
- Doubled cofferdam
- Crib cofferdam
Components of cofferdams
- Sheet piling
- Bracing frame
- Concrete seal
The typical cofferdam consists of sheet piles set around a bracing frame and are driven in the soil sufficiently to cut off the flow of soil and to develop lateral and vertical support and in some cases to cut the flow of water.
Comparison of Caissons and Cofferdams
Caissons and cofferdams are selected depending on site conditions. Caissons are permanent structure used for small area where the water height is more than 12m whereas Cofferdams are temporary structures which are used for large area with water height up to 12m.
Underwater Concreting
This is a process where the prepared concrete is poured below the water surface using suitable methods.
Placement methods:
- Tremie method – A Tremie is a water tight pipe which is supported on a working platform above water level.
- Pump method – It’s a method where the concrete is pumped directly into its final position including both horizontal and vertical delivery of concrete.
- Toggle bags – Toggle bags are ideal for small amount of concrete placement .They are filled with wet concrete and is squeezed out by a diver.
- Bagwork – In this method, bags are filled with rich mix concrete, which are made of open weave material. They are either diver- handled bags or placed using a crane. This method is used for temporary works or short term solutions.