Waste Rock Behavior at High Pressures: Dimensioning High Waste Rock Dumps - Multiscale Geomechanics: From Soil to Engineering Projects - page 98

The k values appear lower than the typical values for other granular materials,
particularly for ripraps. It is necessary to note that these tests were carried out
under controlled loading and information related to mining waste rock, dumped
with a low initial density, is hard to come by. However, the results of the
compressional triaxial tests agree with those of the oedometric tests.
4.4.4.5. Particle crushing
Just as for the oedometric tests, to evaluate the particle crushing on
compressional triaxial tests, the grain size distributions of the waste rocks tested
were systematically determined after each test.
The grading curves of ROM WR, before and after drained compressional triaxial
tests, are presented in Figure 4.12 for the different confining pressures applied.
These results are summarized, in terms of the evolution of d 10 and d 60 , in Table 4.6.
In this same table, the effective mean stresses reached at the end of the test are also
indicated, as are the stresses associated with each grain size distribution obtained.
As the final effective mean stress p' is increased, particle crushing is manifested
as the initial grain size of the sample evolves towards finer grain sizes, clearly
increasing the content of the sands, and, to a lesser proportion, the fines. In other
words, this evolution depends on the confining pressure and, significantly, on the
deviatoric stress and imposed axial deformation .
For the different initial grain size distribution tested, it was observed that particle
crushing was more significant in the coarser grain sizes, except for the leached waste
rock. Indeed, the latter materials with a finer initial graduation experienced a greater
amount of particle crushing than the remainder of the waste rock tested. However,
this effect is more progressive when angular to subangular particles are at stake (see
Figure 4.13). Furthermore, the larger sized particles tend to present more defects
and, therefore, a greater probability of being crushed when subjected to high
tensions (see Figure 4.14). It could be noted in Figure 4.13 that the compressional
triaxial path produces a greater amount of particle crushing than the oedometric
path, a result which is not surprising.
For larger confining pressures and the same stress path, the final grain size
curves, originating from both cut grain size distribution and homothetic grain size,
evolve to a single grain size curve. This result is important, since it indicates that
waste rock coming from rocks with similar mechanical characteristics but with
different initial grain sizes resulting from blasting, loading, segregation, and/or
process conditions, will evolve to a similar grain size when subjected to high
confining pressures. This result has already been observed in materials with friable
grains and sands [BAR 93] .