Correct Answer - Option 3 : maximum resistance to shear stresses just before the failure

**Concept**

Shear strength of the soil:

The shear strength of a soil is its maximum resistance to shear stresses just before the failure.

The shear strength (S) of soil at a point on a particular plane was expressed by coulomb as a linear function of the normal stress on the plane as,

S = C + σ tan ϕ

Where,

σ = Normal stress on the plane

C = Cohesion of the soil

ϕ = Angle of internal friction or angle of shearing resistance

The factor of safety of slope with respect to shear strength

It is the ratio of shear strength of the soil to the mobilized shear stress acting on the slope.

\({\rm{F}}.{\rm{O}}.{\rm{S}} = \frac{{{\rm{Shear\;strength\;of\;soil}}}}{{{\rm{Mobilized\;shear\;stress}}}}\)

If the Mobilized shear stress is greater than the shear strength of the soil, then the translational failure of slope occurs.

i.e F.O.S > 1, then the slope is safe

F.O.S = 1, then it is critical condition

and F.O.S < 1, then the failure occurs

__Important note:__

When F.O.S = 1, the slope reaches the critical condition, but it starts failing when the shear stress is greater than shear strength of soil, i.e F.O.S < 1