What is the primary factor that governs the shear strength of sands?

Asked in SSC JE 06/06/2024 (Morning)

Detailed Explanation

For a clean sand there is no bonding between particles, so the cohesion term in τ = c + σ′ tan φ is zero. The shear strength is then

\[ \tau = \sigma'\tan\phi \]

Why “Friction angle” Is Correct

With cohesion gone, the only soil property in the equation is the angle of internal friction φ. That angle captures sliding friction between grains and their interlocking. It is the primary factor controlling sand strength.

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. Void ratio: Void ratio (density) affects φ — dense sand has a higher φ — but it acts through the friction angle rather than directly.
B. Cohesion: Clean sands have practically zero cohesion.
C. Pore pressure: Pore pressure reduces effective stress and so strength, but it is a loading condition, not a property that governs sand strength.

Key Point

Sand: τ = σ′ tan φ. Typical φ: loose sand 28–32°, dense sand 35–45°.

Key Concepts for Students

φ increases with density, angularity and good grading.
Dense sands dilate when sheared; loose sands contract.
Sand strength is usually measured by direct shear or drained triaxial tests, or estimated from SPT N.

Practice more: this is question 5 in Shear Strength of Soils MCQs – Part 1. Try the whole set and check every answer instantly.