Caisson or Well foundation – Types and Components

Well foundation

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.

Caisson or Well foundation - Types and Components

Types of Caissons

1. Box Caisson

A caisson which is closed at the bottom but can be open or closed at the top.

Caisson or Well foundation - Types and Components

2. Open Caisson or Well

A caisson which is open at the bottom as well as at the top.

Caisson or Well foundation - Types and Components

3. Pneumatic Caisson

An open caisson with a chamber at the bottom for maintaining high air pressure.

Caisson or Well foundation - Types and Components

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.

Caisson or Well foundation - Types and Components

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.

Caisson or Well foundation - Types and Components

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.

Caisson or Well foundation - Types and Components

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.

Caisson or Well foundation - Types and Components

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.

Caisson or Well foundation - Types and Components

Component of Well Foundation

1. Cutting Edge – It provides a comparatively sharp edge to cut the soil below during sinking operation. It usually consists of mild steel equal angle of side 150 mm.

2. Curb – It has a two-fold purpose. During the sinking, it acts as an extension of cutting edge and also provides support to the well steining and bottom plug while after sinking it transfers the load to the solid below. it is made up of reinforced concrete using controlled concrete of grade M200.

3. Steining – It is the main body of well. It also serves a dual purpose. It acts as a cofferdam during the sinking and a structural member to transfer the load to the soil below afterwards. The steining may consist of brick masonry or reinforced concrete. The thickness of staining should not be less than that given by equation.

                                    t  = K( H/100 + D/10)

t = minimum concrete steining thickness
H = Well depth below the bed
D = External diameter of well
K = A constant 

   For soft clay 1.25 for hard clay and 1.3 for hard soil with the boulder. The brickwork steining should be about 10cm more than that the corresponding concrete steining.

4. Bottom Plug – After the well has been sunk to the final position, the bottom plug is formed by concreting. The bottom plug functions as the base of the well.

5. Sand Filling – It utility is doubtful. It is supposed to afford some relief to the steining by transferring directly a portion of the load from well cap to the bottom plug.

6. Top Plug – The opinion is divided about the top plug. It at least functions as a shuttering for laying well cap.

7. Reinforcement – It gives strength to the structure during sinking and service.

8. Well Cap – It is needed to transfer the loads and moments from the piers to the well or wells below. The shape of the well cap is similar to that of well with a cantilevering of about 15 cm. Whenever 2 or 3 wells of small diameter are needed to support the substructure the well cap should be extended to cover the wells. The well cap is designed  as a slab resting over the well or wells with partial fixity at the edge of the wells
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