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Geotechnical News • December 2014
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WASTE GEOTECHNICS
models and to increase confidence in
their results. The laboratory tests are
integrated into the process for water
recovery estimates using the approach
shown in Figure 2.
The laboratory testing procedure
allows for the assessment of sedimen-
tation and evaporation from the tail-
ings and consists of columns that are
built using acrylic cylinders as shown
in Figure 3. The columns are nomi-
nally 15.24 cm in diameter, 40-cm
high and equipped with:
• Measuring tape to track settlement
of tailings in the column;
• Piezometers to track pore water
pressures;
• Tensiometers to track negative pore
water pressures (suction); and
• Ports to collect drainage and tail-
ings samples to determine mois-
ture content.
The tailings are placed in the columns
at the target solids content dictated by
mill processing, and water losses are
tracked for several days by measuring
the weight of the column and water
outflows. Weight measurements with
time determine the water losses from
sedimentation (or potential surface
runoff) and evaporation from the tail-
ings or actual evaporation (AE).
Relative humidity (RH) and room
temperature (under laboratory condi-
tions) are measured throughout the
test and the weight loss of a column
filled with water is used to determine
the rate of potential evaporation (PE)
during the test. These data serve as
the climatic input for validation of the
numerical models.
Typical laboratory test results are
shown in Figures 4a, b and c and
include PE, RH, temperature, water
losses in grams or as a percentage of
initial mass of water (%IMW) and
suction at different depths with time.
Calibration of models and
predictions of tailings drying
under field conditions
The results obtained from the labo-
ratory tests are used for calibration
of the numerical models required to
predict evaporation and infiltration
from the tailings. Calibration is car-
ried out by comparing the AE from
the tailings to modelled results using
material properties and climatic data
(PE, RH and temperature) measured in
the laboratory. The calibration process
generally considers:
• Determining the initial condition of
the tailings at the time evaporative
drying begins (in terms of their av-
erage density) based on the results
of the sedimentation stage;
• Adjusting unsaturated properties
such as soil water characteristic
curves and more importantly hy-
draulic conductivity functions; and
• Developing appropriate consid-
erations in the model to address
the effect of crusts at the surface
(crusts due to accumulation of salt
at the surface result in a decrease
of evaporation due to the increase
of osmotic suction as shown in
Figure 5a).
Computed results for AE are com-
pared to those measured in the labora-
tory, and calibration is completed once
computed and measured values agree
(as shown in Figure 5a).
The calibrated unsaturated models
are then used to predict the expected
response of tailings layers under aver-
age field climatic conditions, and the
results are combined with the deposi-
tion plan to determine water recovery
from the surface of the impoundment.
The numerical models to predict field
conditions are developed to obtain
Figure 4a. Potential evaporation
(PE), relative humidity (RH) and
temperature versus time.
Figure 4b. Total water losses from
tailings (sedimentation and evapora-
tion).
Figure 4c. Suction within the tailings
versus time.
Figure 5a. Computed and measured
evaporation for tailings prepared
with fresh water and seawater under
the same laboratory conditions.
Figure 5b. Expected rewetting losses
(m3/m2) as a function of tailings
drying time (days).