Soil Mechanics / module 13
Critical-state soil behaviour
One framework that ties the course together: the critical state a soil reaches at large strain, the critical-state line, and how a soil's state relative to it predicts strength, volume change and pore-pressure response.
What you will understand
- State the critical state: continuous shearing at constant volume and constant stress
- Draw the critical-state line in q–p′ (q = Mp′) and relate M to φ′cs
- Distinguish soils wet of critical (contractive) from dry of critical (dilative)
- Predict the sign of undrained pore-pressure change and the presence of a peak from the state
- See how consolidation, stress history and strength are unified by one framework
Lessons
The critical state
The unique end-point of shearing, and the critical-state line.
Wet and dry of critical
State relative to the CSL, and what it predicts.
One framework
How consolidation, stress history and strength join up.
Module summary
- The critical state is the unique constant-volume, constant-stress condition a soil reaches at large strain.
- The critical-state line q = Mp′ (M from φ′cs) ties strength to void ratio.
- State relative to the CSL predicts behaviour: wet of critical contracts (positive u, no peak); dry dilates (negative u, peak).
- The framework unifies compression, stress history, strength and pore-pressure response — a way of thinking used with judgement.
Teaching basis and review status
These are educational models and exercises. Check their assumptions before using the results. The full technical content and every diagram have not yet been independently verified.
Concrete and steel calculations retain their stated first-generation Eurocode teaching basis. Use the standards and amendments specified for your project; do not mix generations without an appropriate design basis.
Read the lesson register and outstanding review notesCheck your understanding
Practice
A soil has φ′cs = 27°. What is its critical-state stress ratio M? (M = 6 sinφ′/(3 − sinφ′).)
Check yourself
A loose, saturated sand (well wet of critical) is sheared undrained by rapid loading. What is the danger?