1146
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
of the beach having a liquidity index below 1 and the averag
for the r
elow the maximum liquid limit; as was the
case for Dam
beach.
4. This limitation on drying during active deposition on
strength gain was investigated. Only at the outer
position did liquidity indices show that substantial shear
strength developed. Liquidity indices for the remainder
of the beach indicated that the material was prone to fail
under shear. This observation was made for all test
depositions where the rate of rise for the outer sections
was 2.5 m•year
-1
. The baseline conditions on Dam 2 did
not exhibit this strength gain at the beach head due to
the 4.6 m•year
-1
rate of rise thus the requirement of a
waste rock impoundment.
e
4 ACKNOWLEDGEMENTS
emainder of the beach being 5 %. This is attributed to
the high rate of rise prior to the test deposition. Analysis at the
head of the beach within the first two test depositions indicated
that 51 % of the samples had a liquidity index below 1 with 75
% of the samples b
1 at this position. This drying front also appeared
to extend roughly 1 m below the first deposition yet insufficient
strength gain appeared to have occurred during the shorter
sampling window of the final deposition.
3 CONCLUSION
This paper reported on extensive test work carried out over
an 11 month period on two back-to-back platinum tailings
facilities. The following conclusions are drawn from the
analysis presented:
1. The beach acted as a natural thickener with the tailings
slurry settling within 65 hours from a water content of
on average 95 % to 41 %. This water is available for
recovery.
2. Following sedimentation water is lost through
evaporative drying. The gravimetric water content
decreased at a higher rate during summer and a lower
rate during winter. The k – value or ratio between water
content loss and reference evapotranspiration per day
was not constant for all depositions. The k – value was
lower during lower evaporative conditions. This
suggests that seepage and bleed water may contribute a
larger portion of moisture loss during lower evaporative
periods. A longer study is required to quantify this
relationship.
3. After the drying stage a steady state developed. This
was marked by water contents varying from sample date
to sample date within a narrow distribution. No apparent
trend with time was observed. This narrow distribution
was found to reflect the seepage of water into upper
layers during deposition and capillary rise during
drying. Closer to the edge of the dam this distribution
was defined by the materials field capacity as sufficient
moisture was available to prevent further suctions
developing. Only after long dormant periods was this
source of moisture observed to deplete. Closer to the
pool the water contents were observed to be saturated
with the closer phreatic surface preventing any suctions
developing. This observation was more pronounced on
Dam 1 presumably as the phreatic surface became more
depressed along the longer
The author wishes to acknowledge Anglo American Platinum
Mogalakwena Mine, Fraser Alexander Tailings, SRK
Consulting and the South African Weather Bureau for making
this study possible.
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