In the case of box caisson, the top portion is open and the bottom portion is closed. It may be made from reinforcement cement concrete, steel or timber. Box caissons are preferred under the following conditions.
1. When the depth of water is not more than 6 to 8 m.
2. When bed material consists of soft and loose material which can be easily dredged out to form a levelled bearing surface.
3. When the velocity of flow is not so large that it affects the stability of the caisson against the scour.
4. When the excavation work for preparing the bed of the foundation is not required or bed of the foundation may be prepared with a group of piles.
Construction Procedure of Box Caissons
The following procedure is followed in the construction of the box caissons.
First of all, a level bearing surface is made to receive the bottom of the box. The surface can be made by dredging all the loose and soft soil.
In case the caisson is to be rest on piles, all the piles head must be kept at the same level before placing box caisson over the piles. Sometimes, piles cap may be used to group all the piles together which act as a one structural member.
Then the empty space in the box caisson is filled by using sand, gravel or concrete. Then the top portion of the caisson is sealed and further work is started above the water surface.
The box caissons are used in the construction of quay walls