The difference between the interface friction coefficients for round and angular
materials becomes smaller with an increase in the relative
roughness due to a decrease in the particle size. This comparison
indicates that the angularity of small particles plays a less
important role in interface behavior than that of large particles.
Therefore, the lower bound of the interface friction coefficient
is the true interface friction coefficient, as discussed earlier,
and the upper bound is the internal friction coefficient of sand.
The plot also shows that the correlation between the interface
friction coefficient and the relative roughness is almost unique
for a given granular material, regardless of the kind of interface
material, which corroborates the finding by Esashi et al.
(1966) using three construction materials (steel, wood, and
concrete). This statement may be only true when the normal
stress is relatively low and no plowing of particles into interface
surfaces takes place.