Learn: Hydrostatic pore pressure
Below a static water table, water pressure increases with depth because of the weight of water above the point. Hydrostatic pore pressure is found from water unit weight and pressure head.
What the formula is saying
Multiplying γw by h gives pressure. Here h is the vertical distance below the relevant piezometric level, not the depth below ground unless the water level is at ground surface.
Read the symbols in plain language
- γw
- Water unit weight
Weight force per unit volume, including gravity. Mass density in kg/m³ cannot be entered directly in a kN/m³ field.
kN/m³Use kN/m³ as the base unit shown here. γw is kN/m³ and h is m, giving kN/m² = kPa. This is gauge pressure relative to the chosen water-pressure reference, not automatically absolute pressure.
- h
- Pressure head
Equivalent height of a water column that produces the pore pressure at the point; zero is the reference water surface.
mMetres measure length; 1 m = 1000 mm.
- u
- Result to find
Hydrostatic pore pressure. Water unit weight times head gives gauge pressure in kilopascals.
kPa
Sort out the units first
γw is kN/m³ and h is m, giving kN/m² = kPa. This is gauge pressure relative to the chosen water-pressure reference, not automatically absolute pressure.
Assumptions before calculating
Assume static water of uniform unit weight and a known piezometric level. This lesson uses nonnegative pressure head for the submerged hydrostatic case.
Let’s solve one together
Read the given values, follow each operation, then check what the result means.
Read the supplied values as one complete study case. Find u and explain the result in the stated output unit.
- γw · Water unit weight
- 9.81 kN/m³
- h · Pressure head
- 5 m
Identify head below the piezometric level
Use the vertical water-pressure head at the point, not simply its depth below the surface.
(5) = 5 mConvert water head to pore pressure
Water unit weight times head gives gauge pressure in kilopascals.
(9.81) × (5) = 49.05 kPa
Does this worked answer make sense?
At the water-pressure reference level, h = 0 and u = 0 gauge. Doubling depth below that level doubles hydrostatic pressure.
A second worked example — different values
A second case uses different data. Predict which way the answer will change, then calculate it without reusing the first answer.
- γw · Water unit weight
- 9.81 kN/m³
- h · Pressure head
- 3 m
Identify head below the piezometric level
Use the vertical water-pressure head at the point, not simply its depth below the surface.
(3) = 3 mConvert water head to pore pressure
Water unit weight times head gives gauge pressure in kilopascals.
(9.81) × (3) = 29.43 kPa
Now try your own values
Change a value or its unit. The same method will show your calculation, step by step.
Results update only when you calculate. The lesson example above stays unchanged.
Your turn — check your understanding
Solve this separate case yourself. Use only the values below; the two worked examples use different data. Give the requested result in the selected unit.
- γw · Water unit weight
- 10 kN/m³
- h · Pressure head
- 4.5 m
Find: Learn: Hydrostatic pore pressure
A hint, not the answer
Multiplying γw by h gives pressure. Here h is the vertical distance below the relevant piezometric level, not the depth below ground unless the water level is at ground surface.
γw is kN/m³ and h is m, giving kN/m² = kPa. This is gauge pressure relative to the chosen water-pressure reference, not automatically absolute pressure.
Show the full practice solution
Compare the steps with your work; revealing a solution does not mark the lesson complete.
Identify head below the piezometric level
Use the vertical water-pressure head at the point, not simply its depth below the surface.
(4.5) = 4.5 mConvert water head to pore pressure
Water unit weight times head gives gauge pressure in kilopascals.
(10) × (4.5) = 45 kPa
Avoid the common trap
Do not use soil unit weight instead of water unit weight. Do not measure h from an arbitrary ground datum or add atmospheric pressure when an effective-stress calculation needs gauge pore pressure.
When this method applies — and when it does not
Seepage, artesian conditions, excess pore pressure during loading and unsaturated suction require the appropriate head or a different model. Ground depth alone does not determine pore pressure.
For study and understanding, not approval of a real structure, site operation or design. Apply the correct standard, National Annex and professional review to actual engineering work.
One idea understood. Keep going.
This optional checkmark is saved only in this browser. It is your own progress note, not a certificate.
Sources & further reading
References open in a new tab and explain the underlying principles. The teaching text and examples here are SimpleFlick’s own; the source organisations have not endorsed this calculator.
Reading focus: Hydrostatic pore pressure. Read the relevant soil phase, seepage, earth-pressure, settlement or foundation topic. Effective stress, drainage and idealized geometry determine whether the relationship applies.
Lesson updated: · Both examples and the separate practice case are checked against an independent high-precision numerical implementation. This verifies arithmetic for the stated model, not engineering certification.