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Steel Sheet Pile Cofferdam Calculator

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

Also known as cofferdam design · water exclusion · sheet pile cofferdam

Quick answer

With the default inputs: Cofferdam length 20 m · Cofferdam width 12 m · Sheet pile section Larssen IV etc.Sheet piles 160 bars

Use your own dimensions ↓

Inputs

Water depth + embedment + top freeboard

Result

Sheet piles
160bars
Cofferdam perimeter64.0m
Total pile length1920m
Unit weight76.1kg/m
Total weight146112kg
Tonnes146.1t
How this is worked out

A steel sheet pile cofferdam drives Larssen piles around the perimeter, interlocked to cut off water. Effective width is about 0.4 m, count = perimeter ÷ 0.4 rounded up, with corner piles to close the ring. Pile length = water depth + embedment + freeboard; embedment must satisfy base heave, seepage and overall stability, not just reach the bed. Weight follows the section (III 60, IV 76.1, V 105, VI 120 kg/m). After closing, dewatering pumps are sized separately from seepage and are not included.

Next up: Scaffold Tie Calculator · Scaffolding Calculator

Formula

PerimeterP = 2 × (length + width)
Pile countn = ⌈P ÷ 0.4⌉
Total lengthL = n × pile length
Total weightW = L × unit weight (kg/m)
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.

How to work it out

  1. Perimeter = 2 × (length + width); sheets run continuously around it, count = ⌈perimeter ÷ unit width⌉.
  2. Larssen sheets are about 0.4 m wide; pick the section by stiffness: type III 60 kg/m, type IV 76.1 kg/m.
  3. Total weight = count × length × unit weight, for rental or purchase take-off.
  4. Internal bracing (waler + struts / corner braces) must be checked; the water and earth pressure is carried by the bracing system.
  5. Sheet length must satisfy bottom seepage stability (against piping and heave), not just reach the pump-down level.

Where it goes wrong

Material and cost

Steel sheet piles are rented on a tonne or metre basis and are the economical choice for temporary cofferdams and shallow pits; they are pulled and recovered after use, far cheaper than one-shot retention (bored piles, diaphragm wall) for temporary works.

On site

Basis

FAQ

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.

Cite this page

Use this format in reports, method statements or coursework. The link is permanent.

Building Calculator. Steel Sheet Pile Cofferdam Calculator. 2026-10-25. https://jianzhujisuan.com/en/cofferdam

Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.