---
title: "Depth of well and sinking process"
subject: "Foundation Engineering"
chapter: 9
key: "semester_5/foundation_engineering:well-depth-sinking"
source: https://www.bctengineer.com/q/semester_5/foundation_engineering/well-depth-sinking
site: BCT Engineering
---

# Depth of well and sinking process

Foundation Engineering · Chapter 9: Well Foundations

- Asked in: 2079 Bhadra, 4+4 marks; 2074 Asoj, 4+4 marks; 2075 Chaitra, 1 marks (asked 3 times)
- Video: [Well Sinking(Module 4, Part 46),Foundation Engineering, Geotechnical Engineering](https://www.youtube.com/watch?v=WF8r_mGvbYM) — Vidhya Focus-2020
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/foundation_engineering/well-depth-sinking
- Whole chapter: https://www.bctengineer.com/md/semester_5/foundation_engineering/chapter-9

## Question

How do you determine the depth of the well? Describe the sinking process of the well (in a site having water table above the ground level).

## Short answer

Depth is determined by maximum scour depth (2d for piers) plus grip length (≥1/3 of that), bearing capacity, and stability. Sinking above water level uses a sand island or floating caisson; a cutting edge is placed, steining cast, soil dredged evenly under own weight with kentledge, verticality checked, a bottom plug placed by tremie, then filling and capping.

## Answer

#### Depth of a well foundation
The depth is fixed by:
1. **Scour depth.** The founding level is the **maximum scour depth** plus a **grip length**. The normal scour depth by Lacey's formula is $d = 1.34\left(\dfrac{q^2}{f}\right)^{1/3}$ ($q$ = discharge per metre width, $f$ = silt factor). The maximum scour depth is taken as $2d$ at piers (or $1.27d$ to $1.5d$ for abutments), measured below HFL.
2. **Grip length:** at least $\tfrac{1}{3}$ of the maximum scour depth below the maximum scour level, for lateral stability.
3. **Bearing capacity:** a stratum that can carry the load safely (settlement within limits).
4. **Stability** against overturning and sliding (passive resistance of soil), and **the depth at which a firm stratum or rock is found.**
5. The depth must also avoid **liquefiable** or soft layers.

#### Sinking of a well (in a site with water above ground level)
1. **Construction site:** a **sand island** (a temporary sand-filled island enclosed by sandbags/bamboo) is built above the water level in shallow water, or a **floating caisson** (a steel shell) is used in deep water.
2. **Cutting edge and curb:** these are placed on the island and a first lift of steining (about 2 to 3 m) is cast over them.
3. **Dredging:** soil is removed from the dredge hole by a grab, an air-lift or divers, so the well sinks under its own weight. Steining is built in lifts as the well sinks, keeping its top about 1 m above water.
4. **Aids to sinking:** kentledge (extra weights), water jetting to reduce skin friction, bentonite slurry (lubrication), or pulling.
5. **Verticality control:** tilt and shift are corrected as sinking continues.
6. **Founding:** at the founding level the base is cleaned and inspected, and the **bottom plug** is placed by tremie under water.
7. **Finishing:** the well is dewatered, filled with sand, topped with a **top plug** and capped by the **well cap**.

## Check yourself

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

1. The founding level is fixed as:
   A. Normal scour depth only
   B. Max scour depth plus grip length
   C. HFL
   D. Half the scour depth
   Answer: B. Founding depth is max scour depth plus grip length, with a firm stratum. (Key point: Founding level)

2. Grip length is at least:
   A. Equal to scour depth
   B. 1/3 of max scour depth below max scour level
   C. 1/10 of scour depth
   D. 1.5 m
   Answer: B. IRC practice takes at least one third of the max scour depth. (Key point: Grip length)

3. In shallow water above ground level, the well is built on:
   A. A pile cap
   B. A sand island
   C. Dry ground
   D. A floating pier
   Answer: B. A sand island is built above the water level; a floating caisson is for deep water. (Key point: Construction site in water)

4. The well sinks mainly by:
   A. Driving with a hammer
   B. Pumping in concrete
   C. Lifting with cranes
   D. Dredging soil from the dredge hole so it sinks under its weight
   Answer: D. Grab or air-lift dredging, with kentledge and jetting as aids. (Key point: Sinking)

## Related in Foundation Engineering

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- [Circular footing on stiff clay, water table](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Acircular-footing-stiff-clay) — Foundation Engineering, chapter 6
- [Active thrust with surcharge and water table](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Awall-6m-surcharge-water-table) — Foundation Engineering, chapter 3
- [Strip footing FOS, three water table cases](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Astrip-footing-fos-water-table) — Foundation Engineering, chapter 6
- [Factors affecting bearing capacity; water table](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Abearing-capacity-factors-water-table) — Foundation Engineering, chapter 6, asked 5×
- [Active pressure, sloping backfill and water table](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Awall-sheet-pipe-sloping-backfill) — Foundation Engineering, chapter 3

## Related in other subjects

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