Box caisson:
This type of caisson is similar to open caisson except that it is closed at bottom. The caisson is cast and cured on land and when required, it is launched in water and towed to the site for sinking. The caisson is sunk by filling sand, gravel, or concrete in the empty space inside. The place where the caisson base is to rest must be levelled and as such box caissons are used in places where the strata of sufficient bearing capacity is available near the ground. In normal practice, the soft natural bottom soil of the river bed is dredged out to some depth and the trench thus formed is filled with sand to have a levelled base. The function of the sand layer is to uniformly distribute the superimposed loads over the soil below and thus avoid tilting of the caisson.