Module 13 · Lesson 13.2
Wet and dry of critical
State relative to the CSL, and what it predicts.
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.
- Liquefaction is the extreme wet-side case — loose saturated sand losing all effective stress.
This is educational material. It uses simplified examples to teach principles, and must not be relied on for real design or safety-critical decisions. Module overview and checkpoint