The Angles of Friction of soils?

Question

Why the angle of friction for dense sand differs from that of loose sand?

Answer 1

Granular materials under compression (e.g., vertical compression) have a greater resistance to shear (relative motion in a non-vertical plane) due to more and stronger interlocking between grains. Non-vertical shear motion requires some vertical motion to accommodate the protrusions and depressions of interlocked grains, and this is resisted by the compression. The result is a steeper angle of repose or of friction for compressed grains.