# Steel Sheet Pile Cofferdam Calculator

Perimeter to pile count to total weight, by Larssen section.

- Page: https://jianzhujisuan.com/en/cofferdam
- Category: Temporary Works (https://jianzhujisuan.com/en/c/temporary-works)
- Chinese version: https://jianzhujisuan.com/zh/cofferdam.md
- Data checked: 2026-10-25

## Quick result

With the default inputs (Cofferdam length 20 m, Cofferdam width 12 m, Sheet pile section Larssen IV (76.1 kg/m), Pile length 12 m): **Sheet piles 160 bars**

## Inputs

| Parameter | Type | Default | Unit | Note |
|---|---|---|---|---|
| Cofferdam length | number | 20 | m |  |
| Cofferdam width | number | 12 | m |  |
| Sheet pile section | select | Larssen IV (76.1 kg/m) | — |  |
| Pile length | number | 12 | m | Water depth + embedment + top freeboard |

## Formula

- **Perimeter**: P = 2 × (length + width)
- **Pile count**: n = ⌈P ÷ 0.4⌉
- **Total length**: L = n × pile length
- **Total weight**: W = L × unit weight (kg/m)

## Worked example

A 20×12 m cofferdam in Larssen IV at 12 m long: 64 m perimeter, 160 piles, 1920 m total, 1920×76.1 ≈ 146.1 t.

## Questions

### Larssen III or IV?

It follows water depth and load. Section III is light (60 kg/m) with a small modulus, for shallow, lightly loaded cofferdams; IV is stiffer (76.1 kg/m) and the workhorse for deep water and deep pits. Deeper water and higher earth pressure push you to larger section modulus. V and VI suit long spans, deep water or single-anchor deep support. Selection is really 'does the modulus carry the design moment'.

### What to watch when dewatering a cofferdam?

Dewatering is the most dangerous step - lower the level slowly in stages and watch as you go. Dropping it too fast spikes the head difference and can cause piping under the toes or whole-body instability. Lower part way, hold, watch seepage and deflection, then continue once stable. Seepage grows as the level falls, so size pumps with reserve and a standby, and guard against power loss at night and in rain.

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