Ordinary method of slices satisfies which equilibrium condition?
- AForce equilibrium only
- BMoment equilibrium onlyCorrect
- CBoth force and moment equilibrium
- DNone of these
Solution
Concept
Ordinary Method of Slices
Formula
FS = Σ [c' × l + (W × cos(α) - u × l) × tan(φ')] / Σ (W × sin(α))
Explanation
The Ordinary Method of Slices (also known as the Fellenius or Swedish circle method) calculates slope stability by taking the sum of moments about the center of a circular failure surface. It satisfies overall moment equilibrium but ignores interslice forces, thereby failing to satisfy force equilibrium equations.
Shortcut
To remember this, think "Ordinary = One" or "OM = Moment": the Ordinary Method satisfies only One type of equilibrium, which is Moment equilibrium (it ignores force equilibrium between slices).
Common mistake
Many students mistakenly select Option C (Both force and moment equilibrium) assuming that slope stability methods satisfy complete equilibrium, confusing the Ordinary Method with methods like Spencer's or Morgenstern-Price.
Correct answer: B) Moment equilibrium only