Approaches to Waste Rock Disposal - Mining and the Environment: From Ore to Metal - page 771

TABLE 19.1
Comparison of Waste Rock Storages - Top Down and Bottom Up

Characteristic Comparison

Density

Much higher densities are achieved by Bottom Up Construction
(BUC), due to the compaction achieved as each layer is spread.
If achieving high density is important, this can readily be
accomplished using compaction equipment. TDC dumping
does not provide any control over compaction.

Homogeneity

Top Down Construction (TDC) results in particle size segregation
with larger, bulky fragments rolling to the foot of the slope, and
fines staying near the dump crest. Little segregation occurs
during paddock dumping or subsequent spreading so that BUC
storages are generally much more homogeneous than
TDC storages.

Stability and Safety
During Construction

Angle of repose slopes formed in TDC dumps are marginally stable
or 'meta-stable', and commonly result in land-slips, posing a risk
to dumping equipment and personnel. Regrading operations may
be even more hazardous to operators. BUC enables stable slopes
to be achieved from start to finish.

Potential for Settlement
Profile of Waste Rock Storage Formed by Top Down
Construction Showing Development of Cracks

TDC dumps settle substantially over long periods of time as a
consequence of low initial density and gradual readjustment of
particles. Differing thicknesses in TDC dumps lead to differential
settlement causing cracks which provide percolation paths for
rainfall, leading to internal erosion, further settlement and potential
instability. Settlement and consequent cracking is much less likely
in BUC storages, due to higher densities achieved.

Cracks and Scarps due
to Differential Settlement

Permeability

TDC dumps typically have a highly permeable zone at the base
and a low permeability zone at the crest. Initially the TDC dump
will be resistant to infiltration by rainfall; however, as settlement
cracks develop, infiltration will be facilitated. BUC storages have
a relatively constant permeability throughout, except that a low
permeability 'skin' tends to be present at the top of each layer.

Lower Permeability Zone

Coarse High
Permeability
Zone