Which of the following is NOT an assumption of Rankine's theory?
Try it first: pick an option, then read the explanation.
Correct answer: B. Soil is cohesive
Asked in SSC JE 15/11/2022 (Shift-I)
Detailed Explanation
Rankine derived his earth pressure coefficients for an idealised soil and wall. The original theory was for a cohesionless soil; Bell later extended it to cohesive soils.
Why “Soil is cohesive” Is Correct
Options (a), (c) and (d) are all among Rankine's assumptions. The original theory assumes the soil is cohesionless, so ‘soil is cohesive’ is not an assumption of the theory.
| 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
- A. Semi-infinite mass of soil bounded by a horizontal surface: Semi-infinite soil with a plane (here horizontal) surface is an assumption.
- C. Soil is homogeneous and dry: Homogeneous, dry soil is assumed.
- D. Vertical boundary formed by the vertical back of a smooth wall surface: A smooth, vertical back of wall is assumed (no wall friction).
Key Point
Rankine: dry, cohesionless, homogeneous, semi-infinite; wall vertical and smooth.
Key Concepts for Students
- Bell's extension: σh = Ka σv − 2c√Ka (active).
- Ignoring wall friction makes Rankine's active pressure slightly conservative.
- Coulomb's theory allows wall friction and an inclined wall back.
Practice more: this is question 8 in Earth Pressure and Retaining Wall MCQs – Part 1. Try the whole set and check every answer instantly.