Effect of frictional heat dissipation on the loss of soil strength

In the present paper through a shear test on a fully saturated granular medium, simulated by the discrete element method, the effect of the heat produced by friction on the internal pore water pressure is explored. It is found that the dissipated energy is enough to increase the pore pressure and reduce the soil strength. In adiabatic and impermeable conditions the heat builds up quickly inside the shear band, and the softening is more pronounced. It is found as well that for real geological materials, heat conduction is not enough to reduce the pore pressure, and the softening prevails. Nevertheless, it is observed that the hydraulic conduction may mitigate or completely eliminate the temperature growth inside the shear band. This result provides new understanding on the thermodynamic factors involved in the onset of catastrophic landslides.

Figure 1

(a) An array of MV particles enclosed by two horizontal lids to simulate a shear band. The vertical $z$ direction is shown along with the normal stress applied on the top lid (${\sigma }_v$). (b) The contact law for MV particles.

Figure 2

The thickness of the shear band during the second stage of the shearing stage. The shear strain $\gamma$ is calculated by multiplying the constant shear rate by the time. The third stage of the simulation, where the effect of water is included, starts at the critical state (marked by a line) where there is no further dilation due to mechanical shearing.

Figure 3

Shear stress over the upper lid ${\tau }_{\mathrm{wall}}$ of Fig. 1 for two different loads: 7 MPa (circles) and 3 MPa (squares) for samples (a) without water and (b) with water. The velocity of the upper lid is set to $0.1$ m/s.

Figure 4

Evolution of the porosity in time for the sample with water.

Figure 5

(a) Excess pressure $\Delta p$ and (b) water temperature $\theta$ for different ${\sigma }_v$ values.

Figure 6

Evolution of the excess pore pressure as a function of different shear rates $\dot{\gamma }$ and different numbers of particles along the $y$ direction.

Figure 7

Pore pressure $p$ for two different values of the heat conductivity coefficient.

Figure 8

(a) Excess pore pressure $p$ for different hydraulic conductivity coefficients. The arrow shows the direction in which $k_h$ grows. (b) Residual pressure $p_s$ for different values of $k_h$, showing an asymptotic power-law dependence for large conductivities.

Figure 9

Macroscopic friction coefficient ${\mu }^{*}$ vs $k_h$. The horizontal line represents the microscopic friction value of $\mu =0.5$.

Figure 10

Comparison between GA model and simulation for ${\sigma }_v=4.0$ MPa and $k_h={10}^{-8}$ cm${}^2$.

Figure 11

Velocity profile obtained from the simulation.