Wells and Caissons are large-diameter foundations adopted in an underwater situation such as bridge foundations in rivers. Caissons are large width foundations which carry the load of the superstructure through a layer of weak soil to firm stratum at considerable depth.
Unlike pile foundations, which rarely have diameters greater than 2 meters, caissons can have a width ranging from 10-30 meters and can be founded at depths of 40m or more.
Caissons are used extensively in the river and marine underwater construction. in particular, they are used as foundations for bridges. Caissons are usually prefabricated above the ground level and above the water level and sunk to the bearing level as one single unit. when the process of fabrication and sinking proceed simultaneously they are referred to as well foundation.
Types of Caissons
1. Box Caisson
A caisson which is closed at the bottom but can be open or closed at the top.
2. Open Caisson or Well
A caisson which is open at the bottom as well as at the top.
3. Pneumatic Caisson
An open caisson with a chamber at the bottom for maintaining high air pressure.
Shape or Types of Well foundation
Different shapes of wells that are commonly used are as follows:
1. Circular Well – These are more commonly used shape is circular, as it has high structural strength and is convenient in sinking. the chance of tilting is also minimum in this type.
2. Double-D Wells – They are generally used for the piers and abutments of bridges which are too long to be accommodated on a circular well of 9m diameter.
3. Double Octagonal Wells – These are better than the double-D wells in many respects. The square corners are eliminated and bending stresses are considerably reduced. However, they offer greater resistance than double-D well against sinking on account of increased surface area. Moreover, the construction is more difficult.
4. Twin-Circular wells – These are two independent wells placed, very close to each other and having a common well cap. the wells are sunk simultaneously. These wells are suitable where the length of the pier is considerable, which cannot be accommodated on a double -D or double-octagonal well.
5. Rectangular Wells – They are generally used for bridge foundations having depth up to 7-8 m. for large foundations, double-rectangular wells are used. for piers and abutment of very large size, rectangular wells with multiple dredge holes are used.
A Well foundation can be constructed on the dry bed or after making a sand island. At a location where the depth of water is greater than 5m to 6m and the velocity of water is high. Well can be fabricated on the river bank and then floated to the final position and grounded. Once the well has touched the bed, sandbags are deposited around it to prevent scour. The well may sink into the river bed by 50 to 60 cm under ists on weight. Further sinking operation is similar to the sinking of a well in a dry bed. The well is sunk into the ground to the desired level by excavating through the dredge holes.
Component of Well Foundation