Soil Mechanics / Critical-state soil behaviour

Wet and dry of critical

State relative to the CSL, and what it predicts.

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Stress-path and critical-state illustrations simplify stress anisotropy, total vs excess pressure, loading history, drainage, structured soils, strain softening and cyclic behaviour. Full specialist review remains necessary; selected corrections do not certify the models.

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Why this matters

The real power of the framework is that a soil's state relative to the critical-state line — before you shear it — predicts how it will behave: whether it contracts or dilates, whether it shows a peak, and which way its pore pressure moves when sheared undrained. One picture, many predictions.

Compare a soil's current void ratio with the critical void ratio at its mean stress:

Wet of critical (looser than critical, higher void ratio — typically normally or lightly overconsolidated): the soil wants to contract towards the CSL as it shears. Drained, it densifies; undrained, it generates positive excess pore pressure (the effective stress path bends left, Module 9). It shows no peak — strength rises to the critical value.
Dry of critical (denser than critical, lower void ratio — typically heavily overconsolidated): the soil wants to dilate towards the CSL. Drained, it expands; undrained, it generates negative excess pore pressure (the effective path bends right). It shows a peak above the critical strength, then softens to it.

'Wet' and 'dry' refer to being above or below the critical void ratio — a loose soil carries more water at saturation, a dense one less.

Check yourself

A heavily overconsolidated (dense, dry-of-critical) clay is sheared undrained. What happens to its pore pressure, and does it show a peak?

Practice

A soil has a critical-state friction angle φ′cs = 33°. What is its critical-state stress ratio M? (M = 6 sinφ′/(3 − sinφ′).)

Summary

A soil's state relative to the CSL predicts its behaviour before shearing.
Wet of critical (loose/NC): contracts, positive excess pore pressure undrained, no peak.
Dry of critical (dense/OC): dilates, negative excess pore pressure undrained, a peak then softening.
Contractive saturated soil can lose strength through pore-pressure generation; cyclic mobility can also affect denser sand. Assess the material, stress state and loading rather than density alone.