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Pore Pressure Ratio Calculator

Calculates the dimensionless pore-pressure ratio ru = u / (γ·z) used in simplified slope-stability analysis, together with the total vertical (overburden) stress γ·z. Used by geotechnical engineers assessing slopes, dams and embankments where pore water reduces effective stress.

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How to use this calculator

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Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.

How to use this calculator

  1. Enter the pore pressure u (kPa) at the depth of interest.
  2. Enter the total unit weight γ (kN/m³) of the overlying material.
  3. Enter the depth z (m) below surface; read ru and the overburden stress γ·z.

How it works

The pore-pressure ratio normalises pore pressure against the total vertical (overburden) stress: ru = u / (γ·z), where γ·z is the total vertical stress at depth z. It is dimensionless and typically falls in the range ≈ 0.2–0.5 in slope work.

ru is the standard pore-pressure parameter in the Bishop & Morgenstern (1960) stability charts and captures how much pore water reduces effective stress along a potential failure surface. It differs from the plain pore-pressure calculation: here u is an input, not the output.

Worked example

u = 40 kPa at 5 m in soil of γ = 20 kN/m³. Total vertical stress γ·z = 20 × 5 = 100 kPa. Pore-pressure ratio ru = 40 / 100 = 0.400 — a typical mid-range value.

Common mistakes

  • Using effective unit weight instead of the total unit weight γ; ru is defined against total vertical stress.
  • Confusing ru with the pore pressure u itself — ru is dimensionless, u is in kPa.
  • Mixing units — keep u in kPa, γ in kN/m³ and z in m so γ·z comes out in kPa.

Frequently asked questions

What is the pore-pressure ratio ru?

A dimensionless number, ru = u / (γ·z), expressing pore pressure as a fraction of the total vertical (overburden) stress at that depth. A key input to simplified slope-stability methods.

What is a typical value of ru?

For slope-stability analysis ru is commonly about 0.2 to 0.5. Higher values mean more pore water and lower effective stress, reducing the factor of safety.

How is ru used in slope stability?

It feeds the Bishop & Morgenstern (1960) stability charts and effective-stress limit-equilibrium methods, where a higher ru directly lowers the factor of safety.

How is this different from the Pore Pressure Calculator?

The Pore Pressure Calculator computes u itself (u = γw·hw). This tool takes u as an input and returns the ratio ru relative to overburden stress.

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Cite this tool

Zerdly. (2026). Pore Pressure Ratio Calculator. Retrieved from https://www.zerdly.com/tools/pore-pressure-ratio-calculator/

Tip: Enter any known values to calculate the remaining results.

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