The experimental data in Fig. 13 show that the peak interface
friction coefficient decreases with an increase of the normal
stress. This relationship follows Archard’s theory (Archard
1957). The constants and power indices in the equations describing
these curves are different, and depend on the roughness
of the interface materials and the angularity of the sands.
With a relatively rough interface surface and angular particles,
such as the FRP/VBS interface, the constant and the power
index are both high, whereas with a relatively smooth interface
surface and round particles, such as the steel/GB interface, a
lower constant and power index are obtained. Based on Archard’s
findings, the power index ranged from 20.33 (perfectly
smooth surface) to 0 (extremely rough surface). Hence,
the test results from the present study fall within this range.