Question

The failure surface in a dense sand is very thin, while that for a loose sand starts thin but becomes wider with time. Use your knowledge of volume change vs. strain to explain why this would occur.

Answer 1

from the graph, we can clearly see the failure surface in a dense sand is very thin, while that for a loose sand starts thin but becomes wider with time.

Because, in loose sand initial void ratio is high, but due to continue shearing at several stages volume is decreasing , after that void ratio starting to decrease and the volume change is high (after exceeding of strain value ) the failure surface starting to become wider with time ( as axial strain is high).

Where, for dense sand after applying of shear stress, due to low void ratio, change of volume is so less , that's why the failure surface for dense sand is very thin .