Experimental Characterization of the Shrinkage and Water Retention Behaviour of Tailings from Hard Rock Mines

Mining of hard rock ore deposits produces large amounts of tailings. The safe disposal and management of these tailings require an extensive characterization that should include their drying and desaturation behaviour. Desiccation tests have been performed to characterize the shrinkage response of low plasticity tailings having an initially loose state. The testing procedure developed for this purpose is briefly described here. The main shrinkage tests results are then presented. The experimental data are compared with those obtained from water retention tests performed in a pressure plate with volume change measurements. These two types of results are combined to define unsaturated (drying and shrinkage) relationships in six complementary planes that include the volumetric shrinkage curve and the water retention curve. Specific material characteristics are then determined, including the shrinkage limit w _S, final void ratio e _f, and air entry value ψ _a. Additional tests were also performed to define critical parameters at the initiation of cracking in terms of suction ψ, water content w, and degree of saturation S _r. The original results presented here indicate that the onset of desaturation is closely linked with the end of volumetric straining and with crack initiation. Results also show that the shrinkage limit w _S is a function of the specimen initial water content w ₀. Other related characteristics are also presented and discussed.     

Predicting Hysteresis of the Water Retention Curve from Basic Properties of Granular Soils

The water retention curve (WRC), which represents the relationship between volumetric water content (θ) and suction (ψ), is required to analyze the hydro-geotechnical response of unsaturated soils. The laboratory (or field) determination of the WRC can however be time consuming and difficult to conduct. A practical alternative, particularly useful at the preliminary stages of a project, is to estimate the WRC using a predictive model based on basic geotechnical properties that are easy to obtain. One common limitation of such predictive models is due to hysteresis effects, which are not taken into account by most of these models. The authors present in this paper an extended version of the Modified Kovács (MK) predictive model that incorporates hysteresis of the WRC along different paths, including the main wetting and drying curves and the wetting and drying scanning curves for granular soils. The model formulation is presented, and predictions are compared to experimental data obtained on different granular soils. The results show a good agreement for the main and scanning curves.     

Practical pedotransfer functions for estimating the saturated hydraulic conductivity

The saturated hydraulic conductivity k is one of the most important and widely used geotechnical parameters, commonly involved in a diversity of applications. The value of k depends on many factors, which can be divided into three classes: properties of the fluid, pore size distribution, and characteristics of the solid surfaces. Because the latter two are not necessarily constant within a given deposit, the hydraulic conductivity may vary significantly in space. Engineers and scientists need indications about how changing factors may affect the actual k value. In this paper, the authors propose some simple expressions, based on pedologic properties, to estimate the value of k. Using experimental results of their own and taken from the literature, it is shown that the proposed pedotransfer functions can be used for quickly estimating the k value for granular and plastic/cohesive soils. Such expressions can be employed, with a useful chart format, for the preliminary design phase of a project, and also for estimating the range of k values to be anticipated within a given deposit.     

Influence of frozen conditions on the oxygen diffusion coefficient in unsaturated porous materials

The efficiency of soil covers used as oxygen barriers to control the generation of acid drainage from sulfidic mine wastes can be evaluated in terms of the diffusive oxygen flux reaching the underlying wastes. Oxygen diffusion has been extensively investigated over the last few decades for unsaturated porous materials that are not frozen. However, little attention has been paid to materials that are fully or partially frozen, and thus, the diffusion of oxygen through soil covers during the winter freezing period has been generally neglected. This paper presents a laboratory method developed to evaluate the effective diffusion coefficient of oxygen (D_e) in frozen, inert materials. The method is a modified version of the conventional double-chamber cell in which the temperature and unfrozen volumetric water content of the sample are measured in addition to the more commonly monitored change in oxygen concentration. Several tests were conducted on non-reactive materials: that is, a sand at multiple degrees of saturation (S_r?=?20, 30, 39, and 42%), a silt (S_r?=?47%), and a mixture of the two (S_r?=?90%). Experimental data were interpreted using the POLLUTE code. Values of D_e for frozen materials were slightly lower than those obtained at ambient laboratory temperatures. In addition to the development of an empirical method for determining D_e, a preliminary model based on the model proposed by Aachib et al. (Water Air Soil Pollut 156:163–193, 2004) was created for the prediction of D_e in frozen materials by defining the involved parameters as temperature-dependent. The results indicate that predicated values of D_e are slightly higher than experimental values, suggesting that there remains room for improvement in the model.

     

A laboratory study of covers made of low-sulphide tailings to prevent acid mine drainage

Covers with capillary barrier effects (CCBE) are considered to be one of the most effective ways to control acid mine drainage (AMD) production from mine wastes. The use of low-sulphide tailings in CCBE has been proposed recently for cases where other types of material may be unavailable near the mining site. This paper presents leaching column test results showing that CCBEs with a moisture-retaining layer made of slightly reactive tailings, with three different sulphide contents, can effectively limit the production of AMD from the acid-generating tailings placed underneath. With these layered covers, the leachate pH was maintained near neutrality throughout the testing period. When compared to uncovered tailings, the efficiency of the cover systems for reducing the amount of contaminants in the percolated water was determined to be greater than 99% for zinc, copper and iron. This study shows that the use of low-sulphide tailings can improve the ability of a CCBE to limit gas diffusion by consuming a fraction of the migrating oxygen.