A double-walled cofferdam is the same as a single-walled cofferdam, except for the construction of a double wall in place of a single wall.
A double-walled cofferdam is provided where the area is to be enclosed is large and also the depth of water is large. It is very important to choose the perfect cofferdam for a particular situation. A double-walled cofferdam is very essential in case of large depth of water say about 10 to 11 m.
In the case of a double-walled cofferdam, a pair of walls is constructed along the length of the cofferdam, and a suitable gap is kept between the two walls. The space between the two walls is generally filled with a thick mixture of sand or clay and gravel
The filling is to be done in thin layers and it should be well consolidated when it is in a damp state
Sometimes, a clay berm is provided on the outside of the cofferdam in order to provide an additional safeguard against leakage.
The width of the cofferdam depends on various factors such as structural stability, degree of imperviousness desired etc. So, the width of the cofferdam should be decided after considering all the influencing factors. The following thumb rule should be followed in this respect.
B = The width of the cofferdam
h = Depth of river water
Up to depth of river water 3 m ———— B = h
For depth more than 3 m ———— B = 3 + 1/3(h-3)