EngiMetric
Geotechnical Calculation guide Understand a concept

Reading soil test results so the numbers mean something

Converting water content, unit weight and specific gravity into void ratio, saturation and the unit weights every other calculation expects.

The engineering question

A lab reports bulk unit weight, water content and specific gravity. What are the derived values, and why does every later calculation need them?

Worked example

The figures below were produced by running the Soil Voids Ratio Calculator with the same inputs — during the build, from its authoritative engine.

Results
Void ratio (e) 0.7007 –
Porosity (n) 41.2022 %
Degree of saturation 84.767 %
Dry unit weight 15.5738 kN/m³
Saturated unit weight 19.6157 kN/m³
Submerged unit weight 9.8057 kN/m³
Assessment Void ratio 0.70, porosity 41.2%, saturation 85%.
  1. Void ratio follows from e = (Gs·γw·(1+w)/γ) − 1.
  2. Porosity is e/(1+e) and degree of saturation is w·Gs/e.
  3. Dry, saturated and submerged unit weights then follow, and submerged weight is the one used below the water table.

Sanity check

A saturated soil reports 100 % saturation. If it does not, either the water content or the bulk unit weight has been measured on the wrong basis.

Tools for this problem

These are the EngiMetric calculators that compute the numbers in this topic. Each one shows its formula, its units and its limits rather than returning a bare result.

References & standards

  • BS 5930:2015 — sampling and testing of soils
  • Das — Principles of Geotechnical Engineering, phase relationships

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