Cofferdam Hydrostatic Pressure Calculator

Example Data Table

Replace example values with measured site water levels.

This guidance summarizes how hydrostatic actions influence cofferdam walls and bracing. Use it to document assumptions, review results, and support temporary works records.

1) Overview of hydrostatic loading in cofferdams

Cofferdams resist lateral water pressure on sheet piles, soldier piles, or diaphragm walls. Pressure increases linearly with depth, so the maximum demand occurs at the base. A clear pressure profile helps crews plan bracing installation, dewatering sequences, and safe access controls.

2) Differential water level and controlling head

Net loading is governed by the difference between outside and inside water levels. The calculator uses h = h_ext − h_int. If the inside is dewatered, h approaches the full external depth. If internal water rises during pump downtime, net pressure reduces, but monitoring remains essential.

3) Unit weight selection and practical ranges

Freshwater is commonly taken as 9.81 kN/m³ (or 62.4 lb/ft³), which the form provides as a default. Saline water is slightly higher, and slurry or sediment-laden water can be higher again. When density is known, the tool can compute unit weight from density and gravity for consistent reporting.

4) Base pressure and pressure diagram checks

With a triangular distribution, the base pressure is p_b = γh. Example: γ = 9.81 kN/m³ and h = 4.0 m gives p_b ≈ 39.24 kPa. Base pressure supports checks of seals, connections, and localized wall stresses near the lowest elevation.

5) Resultant force for brace and wale demand

The global lateral force per unit wall length is F = ½γh². For h = 4.0 m and γ = 9.81 kN/m³, F ≈ 78.48 kN/m. This load informs brace forces, tie capacity, and wale reactions, based on your selected bracing spacing and staging.

6) Bending moment and critical elevations

The resultant acts at one-third of the head above the base, so base moment is M = γh³/6. For h = 4.0 m and γ = 9.81 kN/m³, M ≈ 104.64 kN·m per m. Comparing moments across excavation stages can indicate when additional bracing or struts are required.

7) Safety factors and traceable design outputs

Temporary works often apply safety factors to address measurement uncertainty, construction tolerances, and short-term surcharges. This calculator multiplies pressure, force, and moment by your factor to provide a transparent “design” set. Record water gauge readings, pump logs, and inspection notes to support permits, audits, and shift handovers.

8) Field inputs, monitoring, and limitations

Measure water levels from a consistent datum at set intervals: shift start, after rainfall, and after pump changes. Confirm whether internal water is free-standing, artesian, or tidally influenced. If seepage, uplift, or boiling is suspected, seek geotechnical review because base stability is not captured by wall pressure alone.

FAQs

1) What does the calculator assume about pressure distribution?

It assumes a triangular hydrostatic distribution from the water surface to the base, based on unit weight and differential head. This is appropriate for still water without significant velocity effects.

2) Why do I enter both external and internal water depths?

The wall load depends on the difference between outside and inside levels. Internal water reduces net pressure. Entering both values lets the tool compute the controlling differential head directly.

3) How should I choose the factor of safety?

Use project temporary works requirements, approvals, and risk profile. Higher factors may be used where water level measurement is uncertain or where consequences of movement are severe.

4) Does this tool replace a full cofferdam design?

No. It provides hydrostatic load estimates. Full design may need soil pressures, seepage, uplift, surcharge, structural checks, and construction staging review by qualified engineers.

5) What unit weight should I use for freshwater?

Common defaults are 9.81 kN/m³ in metric and 62.4 lb/ft³ in imperial. Adjust if density differs due to salinity, temperature, or suspended solids.

6) Where is the resultant force applied on the wall?

For a triangular hydrostatic load, the resultant acts at one-third of the differential head above the base (or two-thirds below the surface). The calculator reports both positions.

7) When should I re-run calculations during construction?

Re-run after rainfall, tidal changes, pump adjustments, excavation depth changes, or any observed leakage. Updated inputs help maintain safe working margins and reliable records.