---
title: "Components of well foundation with sketch"
subject: "Foundation Engineering"
chapter: 9
key: "semester_5/foundation_engineering:well-components-sketch"
source: https://www.bctengineer.com/q/semester_5/foundation_engineering/well-components-sketch
site: BCT Engineering
---

# Components of well foundation with sketch

Foundation Engineering · Chapter 9: Well Foundations

- Asked in: 2082 Bhadra, 4 marks; 2081 Baisakh, 4 marks; 2078 Bhadra, 4 marks; 2078 Kartik, 1+4 marks; 2076 Chaitra, 6 marks; 2076 Asoj, 1+4 marks; 2075 Asoj, 5 marks; 2072 Chaitra, 4 marks; 2080 Baisakh, 4 marks (asked 9 times)
- Video: [Well Foundation | Components and Their Functions | Pile Foundation vs Well Foundation](https://www.youtube.com/watch?v=lNfQZyXjifE) — STRUCTURE-TECH
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/foundation_engineering/well-components-sketch
- Whole chapter: https://www.bctengineer.com/md/semester_5/foundation_engineering/chapter-9

## Question

Draw a well foundation showing its different components (with the function of each component) and describe a well foundation.

## Short answer

A well foundation is a hollow caisson sunk by dredging. Main components: cutting edge (cuts soil), curb (inclined ring), steining (walls), bottom plug (seals base), sand filling (adds weight), top plug and well cap (distribute pier load).

## Answer

A **well (caisson) foundation** is a hollow, box-like masonry or concrete structure, circular, rectangular or D-shaped in plan. It is sunk through the soil by dredging from inside until it reaches the founding level. It is then plugged and filled, and it carries the bridge pier above. It is widely used for bridges over rivers where scour is deep.

```text
        Pier
     ____|____
    |  Well   |  <- Well cap
    |  cap    |
  --|---------|-- GL / LWL
    |  Top    |
    |  plug   |
    | Sand/   |  <- Steining (walls)
    | filling |
    |         |
    |Interm.  |
    |plug     |
    |  Bottom |
    |  plug   |
    |_Curb____|  <- Cutting edge
     \       /
```

#### Components and functions
1. **Cutting edge:** steel shoe at the bottom that cuts the soil as the well sinks; it protects the curb.
2. **Well curb:** reinforced concrete wedge-shaped ring above the cutting edge (with an inner slope of 30° to 37°); it helps sinking and transfers the load to the soil.
3. **Steining:** the walls of the well, of masonry or reinforced concrete; they carry the load to the base and give the weight needed for sinking; usually 0.5 to 1.5 m thick.
4. **Bottom plug:** concrete plug placed under water (by tremie) at the bottom of the well to seal the base and transfer the load to the soil; its thickness is fixed by design (commonly not less than about half the internal diameter).
5. **Sand filling:** clean sand placed above the bottom plug to give weight and stability and to transfer the load.
6. **Top plug:** a layer of concrete (about 300 mm) at the top of the sand filling, to support the well cap.
7. **Well cap:** a reinforced concrete slab on top of the steining that distributes the pier load uniformly to the well walls.
8. **Intermediate plug (if required):** a concrete plug placed between the bottom plug and the top plug when the sand filling is not enough, to seal and stiffen the well.
9. **Dredge hole:** the central opening through which soil is removed by grabs during sinking.

#### Description
Well foundations resist large vertical and horizontal loads (by their weight and by the passive resistance of the soil on the sides, with a **grip length** below the maximum scour level). They are suitable for sinking through soft soils, boulders or sand, and are common in Nepal for large river bridges.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. The cutting edge is:
   A. A concrete slab at the top
   B. The sand filling
   C. The dredge hole
   D. A steel shoe at the bottom that cuts soil as the well sinks
   Answer: D. It cuts the soil and protects the curb. (Key point: Cutting edge)

2. The function of steining is to:
   A. Seal the base under water
   B. Form the walls, carry load to the base and give weight for sinking
   C. Distribute pier load at top
   D. Remove soil
   Answer: B. Steining is the masonry or RCC wall giving weight and load transfer. (Key point: Steining)

3. The bottom plug is placed:
   A. In the dry by hand
   B. Under water by tremie to seal the base
   C. At the top above the cap
   D. Before the cutting edge
   Answer: B. Tremie concrete seals the base and transfers load to the soil. (Key point: Bottom plug)

4. The well cap:
   A. Cuts the soil
   B. Is a RC slab distributing pier load uniformly to the steining
   C. Is a sand layer
   D. Is the grip length
   Answer: B. It sits on top of the steining and spreads the pier load. (Key point: Well cap)

## Related in Foundation Engineering

- [When well foundation is suitable](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Awell-suitability-advantages) — Foundation Engineering, chapter 9, asked 2×
- [Forces acting on well foundation](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Awell-forces) — Foundation Engineering, chapter 9, asked 2×
- [Remedies for tilt and shift of well](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Awell-tilt-shift-remedies) — Foundation Engineering, chapter 9, asked 9×
- [Depth of well and sinking process](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Awell-depth-sinking) — Foundation Engineering, chapter 9, asked 3×
- [Methods of lateral stability analysis of well](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Awell-lateral-stability-methods) — Foundation Engineering, chapter 9, asked 2×
- [Precautions against tilt and shift of well](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Awell-tilt-shift-precautions) — Foundation Engineering, chapter 9

## Related in other subjects

- [Well foundation: excavation, sinking, brick, PCC curb](https://www.bctengineer.com/question.md?key=semester_7%2Festimating_and_costing%3Awell-foundation-d-four-items) — Estimating and Costing, chapter 6, asked 2×
- [Well foundation: sinking, brick, sand filling, PCC](https://www.bctengineer.com/question.md?key=semester_7%2Festimating_and_costing%3Awell-foundation-g-sand-filling) — Estimating and Costing, chapter 6, asked 2×
- [Well foundation, 5 m internal diameter](https://www.bctengineer.com/question.md?key=semester_7%2Festimating_and_costing%3Awell-foundation-5m-k) — Estimating and Costing, chapter 6, asked 2×
- [Well foundation, 4.5 m internal diameter](https://www.bctengineer.com/question.md?key=semester_7%2Festimating_and_costing%3Awell-foundation-4-5m-p) — Estimating and Costing, chapter 6, asked 2×