Waste Rock Behavior at High Pressures: Dimensioning High Waste Rock Dumps - Multiscale Geomechanics: From Soil to Engineering Projects - page 84

The testing chamber of this triaxial apparatus was adapted to receive an
oedometric mould, 0.95 m in internal diameter, permitting the application of vertical
pressures greater than 12 MPa and the assembly of samples of 1.0 m high. It is also
possible to perform permeability tests under a constant head method with this
mould.
4.3. Mining rock waste
Determination of the in situ grain size distribution (GSD) for mining waste rock
is a difficult task for many practical reasons, given:
- the size of the grains, the heterogeneity of deposited materials;
- the segregation that occurs during dumping;
- the increase in the contents of finer particles due to particle degradation
induced by the high pressures;
- the increase in finer particle content by migration due to natural or forced
infiltrations.
During construction of the dump, the dumping process produces the segregation
of these materials along the advancing slope according to the size of the particles;
fines accumulate close to the surface, while the larger particles roll towards the foot
of the layer. Thus, segregation leads to a stratification of the particles along the
profile of the dump and to the formation of heterogeneous layers, parallel to the
advancing face . In complicated situations, when challenges to the stability or
environment are serious enough, the most appropriate solution in most cases
requires the construction procedure to be controlled as tightly as possible, with
dumping in layers of predetermined thicknesses and, on occasion, selecting the type
of waste rock to build different sectors of the dump. Modeling this heterogeneity for
posterior analyses is generally rather difficult.
4.3.1. In situ grain size distribution
To characterize the waste rock, it is recommended to perform in situ macro grain
size distribution tests at different levels of the dump. The large-sized blocks and
particles are separated, sorted by size, measured by hand, and finally weighed at the
site. Materials that are smaller than 8 cm (3”) should be analyzed by laboratory tests.
A complementary approach is to estimate the grain size distribution of the materials
exposed on the slopes from digital image-processing. With the latter technique,
however, it is not possible to determine the content of the fine particles.