Table of Contents
In this article, we will discuss everything about well foundations.
1. Everything About Well Foundation
Well foundation is a type of deep foundation in the shape of a hollow prismatic box that is built above the ground level and then sunk to the required depth as a single unit.
It is a watertight chamber used for laying foundations underwater for monuments, bridges, and aqueducts.
Well, foundations provide a solid and massive foundation for heavy loads and are useful in situations where the loads have to be transferred to a soil stratum deep below, for example, in the case of bridge foundations.
2. Types of Well or Cassions
The cassions are of the following three types:
i. Open Cassions or Well
It is a type of cassion which is open both at the top and bottom during the construction. It is provided with a cutting edge at the bottom to facilitate sinking.
When the cassions have reached the desired location, a fairly thick concrete seal is provided.
Advantages
a. It is feasible to extend to large depths.
b. The cost of construction is relatively low.
Disadvantages
a. The bottom of the cassions cannot be inspected and thoroughly cleaned.
b. Concrete seal placed in water will not be satisfactory.
c. The rate of progress will be slowed down if boulders are met during construction.
ii. Pneumatic Cassions
Pneumatic cassions are open at the bottom and closed at the top.
It has its lower end designed as a working chamber in which compressed air is forced to prevent the entry of water, and thus permit excavation in dry conditions.
Advantages
a. The cassions can be sunk vertically as careful supervision is possible and control over work is better.
b. The bottom of the chamber can be sealed effectively with concrete as it can be placed dry.
c. Obstruction to sinking, such as boulders can be removed easily.
Disadvantages
a. Construction cost is high.
b. The depth of penetration is limited to 30 m to 40 m below the water table.
c. A lot of inconveniences is caused to the workman while working under compressed air pressure.
iii. Box Cassions
Inbox cassions, the bottom is closed and the top is open.
It is also called floating cassions.
This type of cassion is the first cast on load and then towed to the site where it is sunk onto a previously leveled foundation base.
Advantages
a. Cost of construction is relatively low.
b. It can be used where the construction of another type of cassions is not possible.
Disadvantages
a. The foundation bed has to be leveled before installing the cassion.
b. The base of the cassions must be protected against scour.
c. The load carrying capacity is smaller than that of other types of comparable size.
3. Components of a Well Foundation
i. Well Cap
The well cap is an RCC slab laid at the top of the well steining and is usually cast monolithically with the steining. It transmits the load of the superstructure to the steining.
ii. Steining
The well steining it the body of the well which transfers the load to the subsoil. It also acts as a cofferdam during the sinking and provides the weight for sinking.
iii. Curb
The lower wedge-shaped portion of the well steining is called the well curb. The curb facilitates the process of sinking.
iv. Cutting Edge
The lowermost portion of the well curb is the cutting edge. It cuts into the soil during the sinking.
v. Bottom Plug
After the well is sunk to the required depth, the base of the well is plugged with concrete. This is called a bottom plug. It transmits the load to the subsoil.
vi. Dredge edge
The well is sunk by excavating soil from within the well. The hole formed due to the excavation of soil is called a dredge hole. It is filled with sand.
v. Top Plug
A concrete plug covering the sand filling is usually constructed at the top. It is called the top plug. It provides contact between well cap and sand filling and helps in transferring the load through the sand filling.
4. Shapes of Well
Wells of different shapes are in use. Some of the commonly used shapes are given below.
The choice of a particular shape of well is dependent mainly on the:
a. Dimension of the pier or abutment.
b. Ease and cost of construction, tilt, and shift during the sinking.
c. The magnitude of forces to be registered.
The most commonly adopted shape however is circular, which provides high structural strength and uniform sinking.
Some common shapes of wells are:
i. Single Circular Well
ii. Twin Circular Well
iii. Double D Well
iv. Rectangular Well
v. Hexagonal Well
vi. Octagonal Well
vii. Twin Hexagonal
viii. Twin Octagonal
ix. Dumb Well
5. Selection of Shape of Well
Any shape that fulfills the following requirement can be adopted.
i. The dredge hole should be large enough to permit dredging.
ii. The steining thickness should be sufficient to transmit the load and provide adequate strength to sustain the forces.
iii. It should accommodate the base of the structure and not cause undue obstruction to the flow of water.
iv. The overall size should be sufficient to transmit the loads to the soil.
v. It should allow for the permissible tilt and shift of the well.
6. Depth of Well foundation
The two important requirements that influence the depth of well foundation are:
i. Minimum grip length below scour depth.
ii. Base pressures to be within the permissible limits.
The normal depth of scour may be calculated by Lacey’s formula;
7. Advantages of Well foundation
The following are the advantages of well foundations over other types of deep foundations such as pile foundations:
a. The effect of scour can be better withstood by a well foundation because of its large cross-sectional area and rigidity.
b. A well foundation can withstand large lateral loads and moments that occur in the case of bridge piers, abutments, tall chimneys, and towers.
c. There is no danger of damage to adjacent structures since the sinking of the well does not cause any vibrations.
d. Well foundation has been found economical for large structures when a suitable bearing stratum is available only at large depths.
Hope you got an idea of everything about well foundation.
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