What is the assumption made about the back of the wall in Rankine's theory of earth pressure?
Try it first: pick an option, then read the explanation.
Correct answer: D. Vertical and smooth
Asked in SSC JE 22/01/2018 (Morning)
Detailed Explanation
Rankine's theory treats the wall as if it simply replaced a vertical section through a semi-infinite soil mass. On such a section there is no shear stress, so the wall must be smooth.
Why “Vertical and smooth” Is Correct
The back of the wall is assumed to be vertical and smooth. Coulomb's theory, by contrast, allows an inclined, rough wall back.
| 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. Plane and rough: A rough wall is allowed in Coulomb's theory, not Rankine's.
- B. Plane and smooth: ‘Plane’ alone allows an inclined back; Rankine requires vertical.
- C. Vertical and rough: Rankine ignores wall friction, so the wall is smooth, not rough.
Key Point
Rankine: vertical, smooth back. Coulomb: any inclination, rough back.
Key Concepts for Students
- Neglecting wall friction usually over-estimates active pressure slightly.
- For a cantilever wall, a vertical ‘virtual back’ through the heel is often used.
- Rankine's resultant is parallel to the backfill surface.
Practice more: this is question 19 in Earth Pressure and Retaining Wall MCQs – Part 2. Try the whole set and check every answer instantly.