Which of the following sentences is correct in the case of Bishop's simplified method of slices?

Asked in SSC JE 23/09/2019 (Morning)

Detailed Explanation

In the method of slices for slope stability, the sliding mass above a trial slip circle is divided into vertical slices. Each slice has forces on its base, its weight, and forces from its neighbours on its two sides (interslice normal and shear forces). Because there are too many unknowns, every method makes an assumption about these side forces.

Why “It disregards the effect of the forces acting on the sides of the individual slices” Is Correct

Bishop's simplified method (1955) assumes the interslice shear forces are zero — the resultant side forces on each slice are horizontal — so their effect is disregarded in the equations. It then uses vertical force equilibrium of each slice and overall moment equilibrium about the centre of the circle. Option (b) describes this simplifying assumption and is the accepted answer.

Why the Other Options Are Wrong

• A. It satisfies only vertical force equilibrium: Bishop's method uses vertical force equilibrium of the slices and overall moment equilibrium, not vertical forces only.
• C. It satisfies only horizontal force equilibrium: Horizontal force equilibrium is not satisfied in Bishop's simplified method.
• D. It satisfies only moment force equilibrium: It does satisfy overall moment equilibrium, but not ‘only’ that: it also uses vertical force equilibrium of each slice.

Key Concepts for Students

• FOS = resisting moment ÷ driving moment for a circular slip surface.
• Bishop's method needs iteration because FOS appears on both sides.
• It gives higher (more realistic) FOS than the ordinary method of slices.

Practice more: this is question 28 in Shear Strength of Soils MCQs – Part 2. Try the whole set and check every answer instantly.