Rankine's theory for active earth pressure is based on the assumption that:
Try it first: pick an option, then read the explanation.
Correct answer: D. All options are correct
Asked in SSC JE 03/03/2017 (Morning)
Detailed Explanation
Rankine's theory rests on a set of simplifying assumptions about the soil, the wall and how failure happens.
Why “All options are correct” Is Correct
(a) The soil is homogeneous, dry and cohesionless. (b) Wall friction is neglected (the wall back is smooth). (c) Failure of the soil wedge takes place along a plane — the rupture plane, inclined at 45° + φ/2 to the horizontal in the active case. All three are assumptions, so ‘all options are correct’.
| 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. The retained material is homogeneous and cohesionless: True, but (b) and (c) are also assumptions.
- B. The frictional resistance between the retaining wall and the retained material is neglected: True, but (a) and (c) are also assumptions.
- C. The failure of the retained material takes place along a plane called the rupture plane: True, but (a) and (b) are also assumptions.
Key Point
Rankine: cohesionless, homogeneous, smooth vertical wall, plane rupture surface, plastic equilibrium.
Key Concepts for Students
- Active rupture plane: 45° + φ/2 to horizontal.
- Coulomb also assumes a plane rupture surface but includes wall friction.
- For large wall friction, curved (log-spiral) surfaces are more accurate for passive pressure.
Practice more: this is question 24 in Earth Pressure and Retaining Wall MCQs – Part 2. Try the whole set and check every answer instantly.