Civil Engineering · Geotechnical
Soil Phase & Effective Stress
Void ratio / porosity conversion, and Terzaghi effective stress (total stress minus pore water pressure) at a given depth.
Standard method — stated scope limits apply
Void Ratio / Porosity
Effective Stress
Enter values and calculate.
About This Calculation
Soil is a three-phase material — solid particles, water, and air — and the relationships between their relative volumes and weights (void ratio, porosity, water content, degree of saturation, unit weight) are foundational to nearly every geotechnical calculation that follows, from settlement prediction to bearing capacity. Effective stress specifically — the stress actually carried by the soil skeleton, as distinct from pore water pressure — is the concept that explains why saturated soil behaves so differently from dry soil under the same total load.
Frequently Asked Questions
Why does effective stress matter more than total stress for soil behavior?
Terzaghi’s principle of effective stress states that soil strength and deformation are governed by the stress carried by the SOLID SKELETON, not by pore water pressure, which the water simply transmits without contributing shear resistance — this is why a saturated soil under sudden loading (before pore pressure can dissipate) can behave dramatically differently than the same soil once fully drained.
What is the difference between void ratio and porosity?
Void ratio is the ratio of void volume to SOLID volume; porosity is the ratio of void volume to TOTAL volume — they describe the same physical void space using different reference volumes, and while related by a simple formula, they are not numerically the same value and shouldn’t be used interchangeably in a calculation.
Why does degree of saturation affect soil unit weight so significantly?
As void spaces fill with water instead of air, the soil mass increases while its volume stays essentially constant (water is far denser than air) — a soil at 100% saturation genuinely weighs more per unit volume than the same soil skeleton at a lower saturation level, which is why unit weight calculations must specify which saturation condition (dry, saturated, or a specific moisture content) they apply to.
How is specific gravity of soil solids used in these phase relationships?
It relates the solid particles’ mass to an equal volume of water, letting the calculation convert between volume-based ratios (void ratio, porosity) and weight-based quantities (unit weight, water content) — without it, the phase diagram’s volume and weight sides couldn’t be connected into the single consistent set of relationships these calculations rely on.