A project was initiated in July 1990 under the MEND (Mine Environment Neutral
Drainage) program to assess the performance of engineered covers. The project
was funded by Noranda Inc., Canada Centre for Mineral and Energy Technology
(CANMET) and Centre de Recherches Minérales (CRM) du Ministère de l’Énergie
et des Ressources du Québec.
The principal objective of the project was to design, construct and evaluate the
effectiveness of soil covers and a plastic or geomembrane cover in reducing acid
generation in reactive mine tailings. The evaluation consisted of performance
monitoring of field test plots at the decommissioned Waite Amulet tailings site and
laboratory experiments at Noranda Technology Centre (NTC), as well as studies
by McGill University and École Polytechnique de Montréal. In particular, the
McGill University Geotechnical Research Centre measured geotechnical properties
of the tailings such as grain size, compaction and drainage parameters, and
resistance of the soils and HDPE membrane to freeze-thaw. École Polytechnique
de Montréal was mandated to measure the hydraulic properties of the tailings and
to perform flow modelling to verify the hydraulic conditions in the covered and
uncovered tailings. The department of geological sciences of McGill University
investigated the possible effects of sulphide oxidation on the concentration of
sulphide gases such as COS, CS2, and SO2.
The soil cover consisted of a 60 cm thick compacted silty clay layer placed
between two sand layers, each 30 cm thick. A final 10 cm gravel crust blanketed
the cover system to minimize erosion. These thicknesses were selected to provide
maximum reductions in the predicted oxygen flux and a sufficient safety factor to
minimize the effects of adverse climatic conditions such as freezing and thawing.
The design of the cover was based on the results of a previous laboratory study
which concluded that this composite cover would be able to resist significant
moisture losses for a long time. The uppermost layer consisted of a fine sand which
minimized the evaporation of water from the underlying, nearly saturated clay. The
coarser bottom sand drained to residual saturation (minimum water content at high
suction) and prevented significant moisture drainage from the clay. At high
suctions both fine and coarse sands have low hydraulic conductivities or
permeabilities (even lower than the saturated hydraulic conductivity of the clay)
which would minimize both upward and downward water fluxes. The upper fine
sand also reduces run off, increases storage and allows more water to percolate into
the clay.
The geomembrane cover consisted of an 80 mil (2 mm thick) high density
polyethylene (HDPE) placed between the upper fine sand and the bottom coarse
sand.
A total of four test plots, consisting of two composite soil covers, one
geomembrane cover and a control (tailings without cover) were constructed at the
Waite Amulet site. Each test plot was instrumented to measure gaseous oxygen
concentrations, water content, suction, temperature and porewater quality at
various depths. In addition, a collection basin lysimeter, initially filled with
unoxidized tailings, was installed below each cover to measure both the quantity
and quality of percolated water.
Six columns were installed in the laboratory to simulate soil-covered and
uncovered tailings. The soil cover consisted of a 30 cm thick clay layer placed
between two sand layers, each 15 cm thick. The soils were similar to those used in
the construction of the field test plots. Unoxidized tailings used in the laboratory
experiments were collected from the deep saturated zone of the south end section
of the Waite Amulet tailings impoundment. The covered and uncovered tailings
were subjected to cyclic wetting and drying, at laboratory temperature. Gaseous
oxygen concentration, water content, temperature and drainage water quality were
monitored over time. The covered tailings did not produce any drainage water
during normal wetting or rain application because of the low hydraulic
conductivity of the compacted clay layer. Most of the added water reported as run
off. The covered tailings were periodically flushed (by by-passing the soil cover) in
order to obtain drainage water to assess the amount of acid produced from sulphide
oxidation. The uncovered tailings were also flushed.
Results of the laboratory, field and modelling studies indicated that the oxygen flux
into is reduced by 91 to 99% by the soil cover. Acid fluxes, obtained from covered
and uncovered tailings, indicated the same degree of cover effectiveness.
Monitoring of acid fluxes over time suggested that the rate of acid production
decreases with time. This may be explained by the reduced diffusion of gaseous
oxygen to active sulphide mineral sites due to the formation of inert solids.
Hydrologic modelling indicated that water percolation through the cover is about
4% of precipitation. Field lysimeter data gave 6% or 54 mm per year which
indicates a reduction of 80% in the total annual infiltration into the uncovered
tailings.
The effects of freeze-thaw on the integrity of the compacted clay layer in the
composite cover was also investigated. The results showed that most of the
negative effects occur during the first two freeze-thaw cycles. Laboratory hydraulic
conductivities increased by one to two orders of magnitude after the first two
freeze-thaw cycles and then remained steady afterwards. Field hydraulic
conductivity was measured in 1991 and 1992 the results of which indicated a value
of ~1.0 x 10-7 cm/s, similar to the initial design value. Based on these results and
those of the laboratory freeze-thaw studies, it is concluded that freezing and
thawing have not adversely affected the cover and that no future negative effects
need be anticipated.
The stability of the geomembrane cover was evaluated with respect to acid leach,
freeze-thaw and tensile stresses. A tensile resistance of ~1.5 kN was obtained for
both untreated and acid leached (pH of 3) specimens of 80 mil HDPE. A similar
tensile resistance was obtained for specimens subjected to three freeze-thaw cycles.
From these results, it is inferred that the long term stability of the HDPE cover is
not a major concern except for the possible effects of equipment, burrow animals
and sunlight.
It is recommended that the tailings in each test plot lysimeter be sampled and
examined for signs and extent of oxidation. This would involve detailed porewater
analysis and mineralogical investigation. The water balance of the two soil-covered
test plots should be confirmed by further field monitoring through the fall of 1993.
The results presented and discussed in this report and those of the recommended
additional monitoring should be integrated into a set of design and construction
protocols for soil covers for use by mining companies and consultants. A new
project should be initiated to investigate the effects of root penetration on the soil
covers.
