Besides the mean grain size, interface roughness plays an
important role in interface behavior. Fig. 11 shows that the
peak interface friction coefficient increases proportionally with
an increase in the relative roughness, Rn. This result confirms
the observations of Uesugi (1987). When Rn is equal to 0, the
friction coefficient for OTW or GB can be approximately considered
as the ‘‘true interface friction coefficient’’ between the
quartz mineral and steel or FRP. However, the coefficient for
VBS at Rn equal to 0 includes the effect of the angularity of
the particles. It is reasonable for VBS to have a higher interface
friction coefficient than OTW or GB, since the particles
of VBS are angular, while those of OTW and GB are rounded.
As the relative roughness of the surface increases, the interface
friction coefficient approaches the internal friction coefficient
of the sand (SLC), which means that the failure will occur in
the sand mass instead of at the interface