3. Experimental verification
To validate the proposed model, we compare with the results from high
velocity-dependent ring shear tests of a loess landslide at different
saturation (Pei et al. , 2017), as well as high velocity rotary
shear frictional tests of familiar fault geomaterials concerning in
quartz sandstone (Dieterich, 1978),
granite (Dieterich, 1978; Di Toroet al. , 2004) , novaculite (Di
Toro et al. , 2004; Di Toroet al. , 2011).
Figure 2
compares the predicted velocity
effect results with the experimental results of loess at different
saturation and fault geomaterials at different
lithologies in a wide velocity
range. The model well captures the velocity weakening effect at close
saturation and saturation of loess materials. The experiment shows that
for wet loess with saturation higher than 0.8 (0.83, 0.941 and 0.995),
its velocity effect is obvious, which is well revealed by the proposed
theoretical model (figure 2a). The dry loess,
i.e., its saturation is zero, there is no observed velocity-dependent
friction effects, and the proposed model can only predict its almost
friction-constant behaviors at slide velocity lower
10-2 m/s (Figure 2a). The
proposed model can also well predict the friction behavior of all
compared fault geomaterials involving granite, quartz sandstone, and
dense quartzite (Figure 2b). Generally,
granite is denser with less porous than quartz and novaculite, which
brings about different velocity effects for other fault geomaterials.
We also compare the results from Aharonov and Schol’s model (Aharonov
and Scholz, 2018), which employs the averaging
stress at the contact surface. This means that the porosity of the
geomaterial is zero, which does not exist in nature. However, the new
model considers the influence of temperature and velocity for
geomaterials with different porosity (Figure
2c). It also precisely emerges the three modes
and its zones, i.e., no thermal effects, thermal effects, and melting,
of contact temperature with increasing slide velocity (Figure
2d). These have entirely consistent with
Aharonov and Schol’s model (Aharonov and Scholz,
2018).
Therefore, the above results show the validity and correctness of the
proposed model. It also makes us understand that the contact temperature
gradually increases until it accumulates to a very high value during the
slow sliding process. The high temperature further causes the phase
transition of the geomaterials, in turn which results in a sharp
decrease in the friction coefficient (Figure
2d). The coefficient of friction decreases
with increasing saturation in loess, as the water in the pores is
subject to pore pressure, which results in a lower friction due to the
reduction of the normal force between the contacting asperities. In
addition, the liquid also has a lubricating effect. Fault geomaterials
with smaller pores have greater internal friction, which means that the
actual contact area of the contact surface is bigger thus increasing the
tangential force of the contact surface. Therefore, the coefficient of
friction decreases with increasing porosity (Figure
2c).