Summary: | The potash industry in Saskatchewan produces potash
tailings as a waste material. These tailings are
transported in a brine slurry to the tailings facility where
they are spigotted onto the tailings pile. The current
maximum height of the potash tailings piles in Saskatchewan
ranges from 50 to 60 meters, in which only the bottom 3 to 5
meters of tailings is saturated. In the management of these
waste piles, concerns regarding the environmental impact of
the tailings pile on the existing groundwater, an
tmderstand ing of a brine balance within the pile, and the
stability of the pile arise. In order to address these
concerns, an understanding of the flow of brine within the
saturated-unsaturated potash tailings is critical. Analysis
of the saturated-unsaturated flow syntem within the potash
tailings requires that two basic relationships be
established. These are the fluid retention and the
permeability-suction curves.
In order to examine the validity of the fluid retention
curves evaluated in the laboratory and the calculated
permeability-suction curves to the analysis of flow through
the unsaturated potash tailings, a series of field
infiltration tests were carried out on a tailings pile.
These tests included single ring infiltrometer, shallow pit,
open caisson, and trench type infiltration tests. The
results of the single ring infiltrometer, shallow pit, and
open caisson type infiltration tests Were used to establish
the field saturated permeability of the tailings using
analytical techniques. Monitoring of the trench
infiltration test site for negative fluid pressure and fluid
content within the tailings using tensiometers and neutron
probes was used to establish the fluid retention curves for
the tailings in the field. These field curves were then
compared with laboratory evaluation of the fluid retention
curve using the Tempe pressure cell method.
The responses of the field instrumentation during the
trench infiltration tests were also compared to the results
of the computer simulation of infiltration which utilized
the laboratory fluid retention curves and permeability
functions as input data. In addition, sensitivity analyses
of the various parameters characterizing the tailings
material properties were performed also.
In general, measurement of the field saturated
permeability using the infiltration tests gives quite
consistent results and indicates a significant degree of
anisotropy in the permeability of the tailings. A good
correlation is found between the fluid retention curve
measured in the field using a gravimetric determination of
the fluid content and the curves that were obtained in the
laboratory. The field curves that were defined by using the
neutron probe measurements indicate that fluid contents are
too high in comparison to the laboratory curves. Numerical
simulation of brine infiltration using laboratory data is in
good agreement with field measurements both in terms of
infiltration rate and pressure renponse. Sensitivity
analyses illustrate that both the infiltration rate and the
propagation of the wetting front are strongly dependent on
the horizontal and vertical permeability, the degree of
anisotropy and the fluid storage capacity of the tailings.
In conclusion, the characterization of the potash
tailings by a fluid retention curve determined in the
laboratory and a calculated permeability-suction
relationship is found to be adequate.
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