Pore Pressure Calculator
Calculates hydrostatic pore water pressure u = γw · hw from the depth below the water table, and optionally the effective vertical stress σv′ = σv − u by Terzaghi's principle. Used by geotechnical engineers for effective-stress, slope-stability, bearing-capacity and settlement analysis.
Enter Values
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How to use this calculator
- Enter the depth below the water table hw (m). The unit weight of water γw defaults to 9.81 kN/m³ (edit for salt/other water).
- Optionally enter the total vertical stress σv (kPa) to get the effective vertical stress.
- Read the pore water pressure u; if σv is supplied, read σv′ = σv − u.
How it works
Under static (hydrostatic) conditions the pore water pressure increases linearly with depth below the water table: u = γw · hw. With γw in kN/m³ and hw in metres the result is directly in kPa (1 kN/m² = 1 kPa).
Terzaghi's principle of effective stress states that the effective (grain-to-grain) stress equals the total stress minus the pore pressure: σv′ = σv − u. Effective stress governs strength, stiffness and settlement, so it is the quantity most analyses actually need.
Worked example
Effective stress at 5 m depth. At hw = 5 m below the water table, u = 9.81 × 5 = 49.05 kPa. If the total vertical stress at that depth is σv = 120 kPa, the effective vertical stress is σv′ = 120 − 49.05 = 70.95 kPa.
Common mistakes
- Using the ground surface depth instead of the depth below the water table for hw — pore pressure is zero at the water table, not at the surface.
- Adding pore pressure to total stress instead of subtracting it — effective stress is σv − u.
- Assuming hydrostatic conditions where seepage or rapid loading creates excess pore pressure (then u is higher than γw·hw).
Frequently asked questions
What is pore water pressure?
It is the pressure of water in the voids between soil grains. Under static conditions it is hydrostatic: u = γw · hw, the unit weight of water times the depth below the water table.
What value should I use for the unit weight of water?
About 9.81 kN/m³ for fresh water (9.807 at 4 °C). Seawater is roughly 10.0–10.1 kN/m³. This tool defaults to 9.81.
What is effective stress?
By Terzaghi's principle, σ′ = σ − u — the total stress minus the pore water pressure. Effective stress controls soil strength and compressibility.
Why does pore pressure matter?
Rising pore pressure lowers effective stress, which reduces shear strength — a key cause of slope failures, liquefaction and reduced bearing capacity.
Is 1 kN/m³ × 1 m equal to 1 kPa?
Yes — kN/m³ multiplied by metres gives kN/m², which is exactly 1 kPa, so the pressure comes out directly in kPa.
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Tip: Enter any known values to calculate the remaining results.
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