Shear resistance of soil can be attributed to:

Asked in SSC JE (previous year paper)

Detailed Explanation

When soil is sheared, two kinds of resistance act along the potential failure plane: cohesion (attraction and bonding between fine particles, independent of normal stress) and friction (sliding and interlocking resistance between grains, which grows with the normal stress).

Why “cohesion and friction” Is Correct

Coulomb combined these two components in τ = c + σ tan φ. Clays get most of their strength from cohesion and sands from friction, but in general soil shear resistance is attributed to cohesion and friction.

Failure envelopes for the three kinds of soil. Pure clay under undrained loading (φ = 0): horizontal line at τ = c. Clean sand (c = 0): straight line through the origin at angle φ. Most real soils (c–φ soils): line with intercept c and slope φ.

Why the Other Options Are Wrong

A. only friction: Friction alone describes only cohesionless soils.
B. cohesion and plasticity: Plasticity is a behaviour (index property), not a component of shear resistance.
D. cohesion and pore pressure: Pore pressure reduces shear resistance by lowering effective stress; it is not a source of strength.

Key Point

Shear strength = cohesion + friction (c + σ tan φ).

Key Concepts for Students

Structural resistance from interlocking is usually included within φ.
In effective-stress terms: τ = c′ + (σ − u) tan φ′.
Cohesion in saturated clay under undrained loading is called undrained shear strength cu.

Practice more: this is question 11 in Shear Strength of Soils MCQs – Part 1. Try the whole set and check every answer instantly.