Waste Rock Behavior at High Pressures: Dimensioning High Waste Rock Dumps - Multiscale Geomechanics: From Soil to Engineering Projects - page 92

4.4.4. Triaxial test results
The results and interpretation of the compressional triaxial tests [BAR 07a] are
presented in Figures 4.6 to 4.9, in the [q - ε 1 ], [q - p'] plots with the strength
envelope suggested for each waste rock analyzed, as well as the evolution of the
void ratio with the axial deformation and effective mean stress, [e - ε 1 ], [e -p']
plots, respectively. For the particular case of the ROM waste rock (see Figure 4.6),
the important strength reduction observed for the confining pressure of σ ' 3 =2 MPa at
an axial deformation of 12% is probably related to a localization of deformations
(kinematic discontinuity), a situation that can also be observed in the [q - ε 1 ] and
[e - ε 1 ] plots of this same figure.
4.4.4.1. Drained tests
Given the relatively loose initial state of the waste rock, these materials present a
contractive behavior (tendency towards volume decrease) for confining pressures
above 0.2 MPa, which is accentuated for higher pressures. Indeed, as has been
highlighted in the [e - p'] plots, the compressibility of these materials increases
significantly for confining pressures higher than 1 MPa for the ROM as well as the
leached waste rock. This increase in compressibility is associated with the
phenomenon of particle crushing, as evidenced by the more widely spread grain size
distributions obtained after tests (see Figure 4.12 and Table 4.6).
For the three waste rocks tested at low confining pressures, the results
of compressional triaxial tests (CID) show that the strength envelope, presented
in the [q - p'] plots of Figures 4.6 to 4.9, is characterized by a high internal friction
angle and a null cohesion. As the confining pressure increases, the slope of
this envelope decreases progressively ([q - p'] plots), which gives rise to its
curvature.
This behavior, common to granular materials under high stresses, is mainly due
to a greater compressibility of the granular matrix associated with the phenomenon
of particle crushing, which implies a decrease in the internal friction angle. Indeed,
this is accompanied by an evolution of the initial grain size to a finer grain size
distribution.
The tendency towards volumetric dilation at lower pressures disappears when the
confining pressures are higher than 0.2 MPa.