MEND Report 2.21.2
October 1993

EXECUTIVE SUMMARY

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.

Last Modified: 2003-11-26		Important Notices

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