Waste Rock Behavior at High Pressures: Dimensioning High Waste Rock Dumps - Multiscale Geomechanics: From Soil to Engineering Projects - page 86

making up less than 10% of the sample. The grain size distribution curves of the
waste rock analyzed are represented in this same figure. However, in the remolded
samples tested, the maximal grain size was limited to 20 cm (8”), to produce grain-
size distributions compatible with dimensions of triaxial and oedometric equipment.
With the purpose of analyzing the influence of grain size distribution on
compressibility and particle crushing in these waste rocks, the remolded samples
were prepared with two grain size distributions: homothetic to the one obtained
in situ with a maximum grain size of 20 cm; and one cut under 20 cm, by
eliminating particles of sizes larger than the latter dimension.
4.4. Characterization of mechanical behavior of the waste rock
The mechanical behavior of the three waste rock samples was characterized by
the use of oedometric drained tests and compression triaxial tests, in drained (CID)
and undrained (CIU) paths. All these tests were carried out on large samples,
compacted manually with an initial dry density of γ d = 18.15 kN/m 3 (initial void
ratio of about 0.46), with a water content of about 5%. This density represents the
average of the densities measured in situ , obtained by replacing the excavated
volumes with water once the materials were dumped by the trucks.
Oedometric tests were carried out on cylindrical samples 0.95 m in diameter and
1.0 m in height, while the triaxial compression tests were carried out on cylindrical
samples 1.0 m in diameter by 1.8 m in height. Each oedometric or triaxial sample
was prepared into four or eight layers, respectively, by compacting waste rock to the
initial density indicated. For each layer, the corresponding grain size distribution
was reconstituted, homogenized, and placed in a homogeneous way before
compaction.
4.4.1. Oedometric tests
Oedometric compressibility was analyzed for ROM and leached waste rock with
homothetic grain-size distributions. The maximum vertical stresses imposed in these
tests corresponded to σ v ' = 4, 8 and 12 MPa. Once the maximum vertical stress
planned for each test was reached, it was maintained for a period of 30 hours until
the vertical deformation became stable. Then, always under a constant vertical
stress, these materials were saturated, in counter-current, by imposing low hydraulic
gradients. The deformations induced by saturation were also followed for a period of
30 hours. After this last period, permeability tests with the constant head method
were carried out. Finally, the resulting grain size distribution was determined after
each test.