Well foundations are those types of cassion foundation that falls under the category of deep foundation. The word cassion is native to French, meaning a chest or box. So cassion foundations are boxlike foundation that may be round, rectangular, hexagonal or square. As we know, the foundations whose depth exceeds the width are generally known as deep foundation. Deep foundations are opted when there is a need for resistance to heavy loading by the structure. The cassion foundations are of three types. These are as follows:
- Box Cassions
Box cassions are open at top and closed at the bottom and they are made up of timber, reinforced concrete or steel. They are built on land, then launched and floated to pier site where they are sunk in position. The suitability of these foundations is confined only to those areas where the bearing stratum is available at shallow depth, and where loads are not very heavy.
- Open Cassions ( wells )
Open cassion foundations are boxes of timber, metal, reinforced concrete or masonry which is open at top and bottom. They are used in constructing the foundations of bridges and buildings. As they are similar to wells, they are termed so. It is the most widely used foundation type in India for construction of bridges and water house abutments.
- Pneumatic Cassions
These are bit different from the above mentioned types because the bottom of foundation is designed as working chamber in which compressed air is forced to prevent the entry of water and thus permit excavation in dry.
Shapes of well foundation
The shapes of well foundation vary with the requirements but the most commonly used shapes are mentioned below:
- Single Circular well foundation
- Twin Circular well foundation
- Dumb-well foundation
- Double- D well foundation
- Twin Hexagonal well foundation
- Twin Octagonal well foundation
- Rectangular well foundation
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Components of a well foundation
The designing of a well starts with the proper understanding of its components. The following components of the well have to be considered in the designing of a foundation :
1.Well Cap – It is a RCC slab having recommended minimum thickness of 0.75 m. Its function is to transmit the forces from the piers to the body of well. It is important that the dimension of well cap should be sufficient to accommodate the pier.
2.Steining – Steining is a wall that is built over a wedge shaped portion called well curb. The wall should be designed such that it can be sunk under its own weight. Moreover, the thickness should be enough to overcome the skin friction during sinking.
3.Well Curb – As mentioned earlier, the well curb supports the steining. Well curb is made up of RCC with steel cutting edge with a minimum reinforcement of 72 kg per cubic meter.
4.Cutting Edge – The cutting edge is that part of the well which helps in penetrating the strata of soil. Thus in case of hard strata such as gravels, the flat cutting edge is used whereas in case of soft strata the sharp edged cutting edge is used.
5.Bottom Plug – The bottom plug transmits the load to soil below and resists uplift forces. The thickness of the plug varies from 0.5 – 1 times the inner diameter of the well.
6.Sand Filling – Sand filling is the part that lies in between the top and bottom plug. Its main function is to provide stability to well and reduce the tensile stress due to the bending moment along with the distribution of load from the superstructure.
7.Top Plug – Top plug is situated below the well cap. This helps transferring the load through the granular material into the steining.
A typical representation of the well foundation is show below:
Selection of well foundation type
The well foundation is a shell sunk by dredging inside of it and which finally becomes a part of the permanent structure but the choice of the shape of the well is particularly depends upon the following criteria:
- Dimension of the base of the pier or abutment
- The care and cost of the sinking
- The consideration of tilt and shift during sinking
- The vertical and horizontal forces to which the well is subjected
But there are limitations which forbid the availability of usage of all type for a single work. A circular well foundation has minimum perimeter for a given dredge area and hence the ration of sinking is maximum. Also, since the perimeter is equidistant at all points from the centre of dredged hole, the sinking are more uniform than for other shapes. However, the disadvantage of a circular well is that, in circular well the diameter in the direction parallel to the span of the bridge is much more than the minimum size required to accommodate the bridge pier and hence the circular well causes more obstruction to the waterway than the bridge pier does.
The disadvantage of single circular well can be avoided in case of a Double-D well foundation shape which conforms to the shape of the bridge pier in plan. The dredge area is also smaller for a Double-D. Hence, for a large pier Double-D is more economical and desirable than a single circular well foundation.
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Twin circular well aim at combining the advantages of a circular well and Double-D, but the only snag is that the two wells sunk close to each other and have a tendency to close in or move apart. However, in abutments and wing walls where the tilt and shift in position are not important, a battery of small diameter well is adopted with advantage.
The double Hexagon and Octagon provides efficient grabbing to all parts of the curb, but suffer from the disadvantage that owing to sharp corners. They can dig and are therefore more likely to tilt, moreover the sharp corners produces greater scour.