Rankine's theory considers the stress in a soil mass when it reaches the state of ______.
Try it first: pick an option, then read the explanation.
Asked in SSC JE (previous year paper)
Detailed Explanation
Rankine (1857) studied a semi-infinite soil mass that is stretched or compressed horizontally until it is just at the point of shear failure everywhere. In that condition the soil is said to be in plastic equilibrium: still in equilibrium, but every element is at failure.
Why “plastic equilibrium” Is Correct
When stretched, the soil reaches the active state of plastic equilibrium; when compressed, the passive state. Rankine's earth pressure formulas come from these two states, so the answer is plastic equilibrium.
| Assumption of Rankine's earth pressure theory |
|---|
| Soil is semi-infinite, homogeneous, isotropic, dry and cohesionless (the basic theory) |
| Ground surface is a plane (horizontal or inclined) |
| Back of the wall is vertical and smooth (no wall friction) |
| Soil is in a state of plastic equilibrium (active or passive) |
| Failure of the retained soil takes place along a plane (rupture plane) |
Why the Other Options Are Wrong
- B. local state of equilibrium: ‘Local equilibrium’ is not the condition Rankine used; his whole mass is at failure.
- C. limited equilibrium: ‘Limited equilibrium’ is not the standard term.
- D. general state of equilibrium: ‘General state of equilibrium’ means not at failure, which would give at-rest pressure instead.
Key Point
Rankine = plastic equilibrium (active or passive), whole mass at failure.
Key Concepts for Students
- Elastic equilibrium corresponds to the at-rest condition.
- Coulomb used a wedge (limit equilibrium) approach instead.
- Rankine's failure planes are inclined at 45° ± φ/2.
Practice more: this is question 5 in Earth Pressure and Retaining Wall MCQs – Part 1. Try the whole set and check every answer instantly.