Friction angle comparison across grain shape and density For which of the following materials is the angle of internal friction φ generally the highest under comparable confining pressures?

Introduction / Context:
Granular shear strength arises from friction and interlocking. Both particle shape (angular vs rounded) and packing state (dense vs loose) influence the peak friction angle, which is vital for slope stability and bearing capacity assessments in sands.

Given Data / Assumptions:

• Similar mineralogy and gradation except for shape/packing differences.
• Comparable effective stress levels.
• Peak strength considered (not critical state).

Concept / Approach:

Angular grains interlock more effectively than rounded grains, promoting dilatancy and higher mobilised friction. Dense states permit greater dilation upon shearing, increasing peak φ above the critical-state value. Thus, angular-dense sands generally have the highest φ; at the other extreme, round-loose sands display the lowest peak φ.

Step-by-Step Solution:

Verification / Alternative check:

Laboratory drained triaxial/direct shear tests routinely show higher peak φ for angular dense sands versus rounded or loose states.

Why Other Options Are Wrong:

Loose packing reduces dilatancy; rounded grains reduce interlock; cohesive clays do not mobilise high frictional resistance in the same manner as sands.

Common Pitfalls:

Confusing peak φ with constant-volume/critical-state φ; ignoring the role of gradation and crushability at high stresses.

Final Answer:

Angular-grained dense sand