Introduction of Well Foundation
- Well foundations are primarily used to support heavy structures such as bridge piers and abutments.
- Well foundation is a type of deep foundation for a special situation. Well foundation is used below the water level for bridges, docks, wharves, etc.
- The Taj Mahal was built by a Mughal builder on the banks of the river Yamuna. Therefore, a special type of foundation was needed for the construction of the foundation of the Taj Mahal. Mughal builders supported a huge structure like the Taj Mahal on a submerged masonry cylindrical base submerged in water using a well foundation.
What Is Well Foundation?
- The use of a well foundation works much like an open caisson. In general, a well foundation offers many advantages over other types of deep foundations for the work of large foundations such as bridge piers and abutments.
- Prefabricated hollow boxes or cylinders for well foundation are placed in the ground at the desired depth during excavation. Water is then removed from it, and concrete is poured into it. So that a well foundation is formed. Well foundation is used in the construction of structures like bridge piers that need a foundation below rivers and water levels.
- Well Foundation is useful in places where other foundations are not able to be used for a bridge foundation, and water depth is more than 5 to 6 meters.
- The cylinders used for well foundation can be made from materials like wood, iron and reinforced concrete.
- For well foundation, the cylinder is lowered into the ground to the desired depth and then concreted at the bottom.
Shapes of Well Foundation
1. Single Circular Well Foundation
- This type of foundation requires only a dredger to sink the cylinder. A single well foundation can be used for a single line railway or a road bridge.
- Well well The most common shape of foundations is the single circular well. It is used with very high structural strength. In a single well foundation, the probabilities of tilting the cylinder are only minimum.
2. Double-D Well Foundation
- The design of double-D shaped wells is usually a modified design on twin rectangular and twin spherical wells. It is made using two dredgers in d shape, as shown in the figure.
- Double-D well as the foundation, provides high lateral stability. Double-D wells are used as the most economical and optional when the size of the pier or abutment is large.
3. Double-Octagonal Wells Foundation
- These wells can provide better stability than double-D wells. In this type of well foundation, the square corners are removed according to the shape.
- So that bending stress in the well foundation is considered to be significantly reduced. This type of well foundation provides more efficient resistance to sinking than double-D wells.
4. Twin Circular Well Foundation
- In this type of well foundation, two independent wells are installed close to each other. The wells are submerged simultaneously with the help of a dredger.
- Twin-circular wells are used when the length of the pier of the bridge is significantly greater. In addition, a twin circular well foundation is used when the pier of the bridge is not able to fit into a double-D or double-octagonal well.
5. Rectangular Well Foundation
- The rectangular well foundation is mainly used for bridge foundation up to a depth of 7m-8m. In the case of a larger foundation, double-rectangular wells can be used for stability.
- Loading stress on staining is very high due to loading in rectangular wells.
Component of Well Foundation
1. Cutting edge
- The cutting edge is made at the bottom of the cylindrical used for the well foundation. The cylinder can be easily inserted into the ground due to the cutting edge. The cutting edge is usually made from a steel angle. The cutting edge makes it easier to cut the clay.
- The thickness and angle of the cutting edge should be sharp enough. So that it can be easily sunk. The cutting edge should be made of sufficiently hard material. So that it should not break when entering the hard rock.
- The outer edge is kept vertical according to the design of the cutting edge, while the inner edge is kept slope as per 1H: 2V.
2. Well Curb
- The curb in the well wall is designed to give stability to the well wall. The curb is 1.0 to 1.5 m high in the wall of the well foundation from the cutting edge to the staining wall.
- The shape of the curb is triangular. So an effective plug can be made at the bottom of the well foundation.
- The staining shows the walls of the well foundation. Staining is a material used in the old days, such as brick or stone. But now for staining is mainly made using rigid material like concrete.
- The design of the staining is properly reinforced to resist stress. The thickness of the staining is a quarter (1/4) of the diameter for the railway bridge as required. Similarly, the thickness of staining for road bridge is kept to be one-tenth of the diameter (1/10).
- The basics for staining thickness are determined based on the following considerations:
- The thickness of the staining must be as required. Its thickness should not be more than required.
- Staining can be easily immersed in the well, and unnecessary resistance can be avoided.
- The thickness of the staining should not be thinner than required. In addition, the stress generated in staining should be within limits under the designed loads.
- Staining should be able to withstand the stress generated during bending and shifting in the well.
4. Bottom Plug
- The RCC cylinder in the well is finalized to the required depth according to the design. Then, the inside of the well is concreted to some depth according to the design, commonly called a bottom plug.
- The bottom plug increases the bearing resistance to staining and gives the well base long-term stability.
5. Sand Filling
- The sand filling is done in the middle of the well foundation. Because of this, well foundation achieves greater stability. And its weight also increases.
- Due to sand filling, well foundation reduces the stress caused by bending moment.
6. Top Plug
- The top plug is made 300 mm thick on the sand filling according to M-15 cement concrete grade.
7. Well Cap
- R.C.C. slab Laid on top of staining. A thickness of at least 15 cm is maintained for the cap.
- Well cap provides the necessary base for a pier or abutment. Transfers its load safely to the ground in the well foundation.
Advantages of Well Foundations
- Well foundations are able to withstand heavy loads and moments. well foundation is especially useful in the case of structures like bridge piers, towers and high chimneys.
- Well foundation can withstand the effects of scouring due to its large cross-sectional area and rigidity.
- There is no risk of vibration damage to nearby structures during the well foundation process
- Heavy structure load can be safely transferred through well foundation.
- High rise buildings can be constructed as per requirement in soil with low bearing capacity using well foundation.
Disadvantages of Well Foundation:
- It is very difficult for a well foundation to clean and inspect the soil.
- For well foundation, the efficiency of labour is reduced in deep depth.
- It may not be satisfactory if the well foundation is concreted under seawater.
- If there is a hard rock obstruction in the ground on the site, the progress of the work slows down.
- Heavy crane is used to install RCC well as well foundation which is very expensive.