---
title: "Cofferdam types, uses, merits and demerits"
subject: "Foundation Engineering"
chapter: 5
key: "semester_5/foundation_engineering:cofferdam-types-merits"
source: https://www.bctengineer.com/q/semester_5/foundation_engineering/cofferdam-types-merits
site: BCT Engineering
---

# Cofferdam types, uses, merits and demerits

Foundation Engineering · Chapter 5: Flexible Retaining Structures and Coffer Dams

- Asked in: 2081 Bhadra, 4 marks; 2079 Bhadra, 2 marks; 2076 Chaitra, 2 marks; 2075 Chaitra, 1+3 marks; 2074 Chaitra, 4 marks; 2076 Asoj, 1 marks; 2072 Chaitra, 2+2 marks (asked 7 times)
- Video: [Cofferdams Uses, Types, Construction, and Removal | Marine Construction Series #4](https://www.youtube.com/watch?v=mi3zKuhtEG4) — Pile Buck
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/foundation_engineering/cofferdam-types-merits
- Whole chapter: https://www.bctengineer.com/md/semester_5/foundation_engineering/chapter-5

## Question

What is a cofferdam? Describe with neat sketches the different types of cofferdam, their uses and their relative merits and demerits.

## Short answer

A cofferdam is a temporary enclosure to exclude water, enabling dry construction. Earthen types suit shallow, slow water at low cost; single-wall braced piles offer economy for moderate depths but struts obstruct work; double-wall types are stronger for greater depth; cellular cofferdams are self-supporting, reusable structures for deep water, though more expensive.

## Answer

A **cofferdam** is a temporary watertight enclosure built in water-bearing ground (river, lake, sea or wet soil) from which the water is pumped out so that foundations, piers or other structures can be built in dry conditions.

#### Types, sketches and uses
**1. Earthen (and rockfill) embankment cofferdam**
```text
   ~~~ water ~~~\
                 \___________
                  /  clay core \
   ______________/______________\____
```
Used in shallow, slow rivers and low water depth (up to 3 m) for dams and bridge piers.

**2. Single-wall (braced) sheet pile cofferdam**
```text
   ~~~ water   |sheet|====strut====|sheet|  water ~~~
               |pile |             |pile |
               |_____|  dry area   |_____|
```
A ring of interlocking sheet piles braced by wales and struts; used for piers and small foundations in water up to about 6-10 m deep.

**3. Double-wall cofferdam**
```text
   | sheet pile |  sand/gravel fill  | sheet pile |
   |            |  (tied by rods)    |            |
```
Two parallel sheet-pile walls tied together with a gravel or sand fill between; used for deeper water (up to about 12 m).

**4. Cellular cofferdam** (circular, diaphragm or cloverleaf cells of steel sheet pile filled with sand/gravel)
```text
    ()  ()  ()   (a row of circular cells joined by arcs,
   (  )(  )(  )   filled with granular soil)
```
Used for large works, dams, locks and deep water (up to about 25 m); self-supporting without internal bracing.

**5. Crib (timber or concrete) and other special types:** boxes or cribs filled with stones; used in rocky riverbeds.

#### Relative merits and demerits
| Type | Merits | Demerits |
|---|---|---|
| Earth/rockfill | Cheap, simple, local material | Needs large area; leaks; unsuitable for deep or fast water |
| Single-wall braced | Economical, quick for small sites | Struts obstruct work; limited depth; leakage at joints |
| Double-wall | Stronger, good for greater depth, less leakage | Costly, requires much material and space |
| Cellular | No bracing, large clear working area, reusable, good in deep water | High cost, needs skilled work, needs rock/stiff base to avoid scour and slip, failure if a cell breaks |

## Check yourself

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

1. What is the purpose of a cofferdam?
   A. To store water permanently for irrigation
   B. To provide a temporary watertight enclosure pumped dry for construction
   C. To support a permanent retaining wall
   D. To drain groundwater for ever
   Answer: B. A cofferdam is a temporary watertight enclosure from which water is pumped out so work can be done dry. (Key point: Definition of cofferdam)

2. Where is an earthen embankment cofferdam suitable?
   A. Deep fast rivers about 25 m deep
   B. Shallow, slow rivers with low water depth up to about 3 m
   C. Rocky beds only
   D. Sites needing no clear space
   Answer: B. Earthen cofferdams are cheap but need space and leak, so they suit shallow slow water. (Key point: Earthen cofferdam use)

3. Which is a merit of the cellular cofferdam?
   A. Needs no stiff base
   B. Very cheap and unskilled
   C. Cheapest for shallow water
   D. Self-supporting with no bracing, giving a large clear working area
   Answer: D. Filled cells act as gravity wall, so no internal bracing is needed; but cost is high and a rock or stiff base is needed. (Key point: Cellular cofferdam merits)

4. What is a main demerit of a single-wall braced cofferdam?
   A. Struts obstruct work and depth is limited
   B. It cannot use sheet piles
   C. It needs fill between walls
   D. It is the most expensive type
   Answer: A. Braced single-wall cofferdams are economical but struts block the site and depth is limited (about 6-10 m). (Key point: Single-wall demerit)

## Related in Foundation Engineering

- [Merits and demerits of percussion drilling](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Apercussion-drilling) — Foundation Engineering, chapter 2

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