The linear form of the extended Mohr–Coulomb shear strength equation uses a [Formula: see text] parameter to quantify the rate of increase in shear strength relative to matric suction. When the [Formula: see text] value is unknown, a [Formula: see text] equal to 15° is sometimes used in the slope stability study to assess the influence of matric suction on the stability of a slope. In many cases, however, a [Formula: see text] value of zero is used, signifying that the effect of matric suction is ignored. Experiment results have shown that the relationship between the shear strength of an unsaturated soil and matric suction is nonlinear. Several semi-empirical estimation equations have been proposed relating the unsaturated shear strength to the soil-water characteristic curve. In this paper, the results of a study using two-dimensional slope stability analysis along with an estimated nonlinear shear strength equations is presented. The effects of using an estimated nonlinear shear strength equation for the unsaturated soils are illustrated using three example problems. Several recommendations are made for engineering practice based on the results of the example problems. If the air-entry value (AEV) of a soil is smaller than 1 kPa, the effect of matric suction on the calculated factor of safety is trivial and the [Formula: see text] value can be assumed to be zero. If the AEV of a soil is between 1 and 20 kPa, the nonlinear equations of unsaturated shear strength should be adopted. For soils with an AEV value between 20 and 200 kPa, an assumed [Formula: see text] value of 15° provides a reasonable estimation of the effects of unsaturated shear strength in most cases. For soils with an AEV greater than 200 kPa, [Formula: see text] can generally be assumed to be equal to the effective angle of internal friction, [Formula: see text], in applications where geotechnical structures have matric suctions around 100 kPa.

The potential effectiveness for long-term closure of a desulphurized tailings cover placed on reactive tailings at the Detour Lake Mine was assessed. The single-layer cover was aimed at reducing oxygen ingress using high water saturation to limit oxygen diffusion and consumption of oxygen in the cover by residual sulphide minerals. The cover was 1.0 to 1.5 m thick and met desulphurization targets, but was coarser grained than the design. One-dimensional unsaturated flow and oxygen diffusion modelling was used to predict water content profiles, depth of oxygen penetration, and diffusive oxygen fluxes. Effective diffusion and reaction rate coefficients estimated from field data matched laboratory measurements. Depth to water table was identified as the most important factor for reducing tailings oxidation. The deeper (4 m) water tables measured at the cover edges led to a lower degree of saturation and higher oxygen flux. Finer grain size and higher air-entry values in the cover materials helped maintain saturation, and a capillary break (fine over coarse) cover reduced oxygen fluxes to <5% of fluxes to uncovered tailings. The as-built cover (coarse over fine) reduced oxygen influx by at most 50% and as low as <1% of uncovered tailings.

The dynamic response of tailings from a gold mine located in western Quebec was evaluated using cyclic laboratory testing. These tailings are classified as nonplastic silt and sand. Specimens of the tailings were prepared as slurries, consolidated to vertical effective stresses of 100–400 kPa, and subjected to cyclic direct simple shear testing with cyclic stress ratio, CSR, values between 0.075 and 0.15. The shear modulus reduction of the tailings under cyclic loading was found to be fairly similar to that described for clean sands in the literature. The cyclic resistance ratio, CRR (which reflects the liquefaction resistance), of the samples was not significantly affected by the effective consolidation stress (in the range considered here). Analysis of test results with the simplified method of liquefaction evaluation indicates that this method may be applicable to these tailings. However, other factors, such as the possible effects of layering and ageing of the tailings in situ, should also be considered in such an assessment.

End pit lakes are common at open-pit metal mines around the world and are part of mine closure plans. Those at oil sand mines are no different except that the end pit lakes will be considerably larger, with the area averaging about 4 km2 and reaching up to about 15 km2. In a major study on end pit lakes for oil sand mines, the Cumulative Environmental Management Association defines an oil sand end pit lake as ‘an engineered water body, located below grade in an oil sands post-mining pit’. It may contain oil sand by-product material and will receive surface and groundwater from surrounding reclaimed and undisturbed landscapes. End pit lakes will be permanent features in the final reclaimed landscape, discharging water to the downstream environment. As a permanent feature, the long-term environmental effect of such an oil sand deposit must be carefully designed and monitored. If the end pit lake contains an appreciable thickness of tailings, the consolidation of tailings may continue for many decades. The effects of groundwater leakage are analysed and modelled to show the potentially large amounts of seepage into the underlying stratigraphic units.