The effect of tailings properties on their consolidation near waste rock inclusions

E.M. Jaouhar, M. Aubertin, M. James

Dans les comptes rendus d’articles de la conférence: GeoMontréal 2013: 66th Canadian Geotechnical Conference; 11th joint with IAH-CNC

Session: Mining Geotechnics I

ABSTRACT: The consolidation behavior of hydraulically-deposited mine tailings in an impoundment is significantly affected by the properties of the tailings. This long term process can be accelerated through the use of vertical drains. This paper presents the results of a numerical investigation on the use of linear drainage inclusions made of waste rock. Such waste rock inclusions (WRI) facilitate dissipation of the excess porewater pressures induced during deposition and thus accelerate consolidation. More specifically, the results of finite element simulations illustrate the effect of the saturated hydraulic conductivity ksat (magnitude and anisotropy) of the tailings on excess porewater pressure dissipation with time and distance during filling of the impoundment. These results are useful in determining the optimal WRI configuration in a tailings impoundment with respect to consolidation.

RÉSUMÉ: Les propriétés des résidus miniers affectent la consolidation lors de leur déposition dans le parc à résidus. Ce long processus peut être accéléré par l'utilisation de drains verticaux. Cet article présente drainantes faites de roches stériles dans un parc à résidus miniers. Ces inclusions linéaires facilitent la dissipation des pressions interstitielles en excès générées pendant la déposition des résidus, ce qui accélère leur consolidation. Plus précisément, les résultats des calculs par éléments finis illustrent leffet de la conductivité hydraulique saturée des résidus ksat (ampleur et anisotropie) sur la dissipation des surpressions interstitielles durant le remplissage du bassin, en fonction du temps et de distance. Ces résultats sont utiles pour déterminer la meilleure configuration des inclusions de roche stérile afin de maximiser leur efficacité. 1. INTRODUCTION There have been several failures of tailings impoundments over the last few decades (Azam and Li, 2010; WISE, 2011). The associated flows have resulted in environmental and economic damage, and even loss of life. The risk of failure can be minimized in various ways, including the use of a co-disposal technique consisting of placing waste rock in the impoundment prior to and during tailings deposition (Aubertin et al. 2002a; James and Aubertin, 2009, 2010; James et al. 2013). The waste rock is placed to create continuous (linear) inclusions (or isolated heaps) within the impoundment that provide reinforcement against static and dynamic loads and also accelerate the tailings consolidation. Some of the potential benefits of this method have been confirmed by laboratory testing using a seismic simulator (Pépin et al. 2012) and by numerical simulation of tailings impoundments, with and without inclusions, during filling (Jaouhar et al. 2011; L.-Bolduc and Aubertin, 2013) and under seismic loadings (James et Aubertin, 2012). This paper presents key results from a numerical modeling investigation on the effect of the hydraulic conductivity of the tailings on their consolidation in the presence of waste rock inclusions (WRI); the results shown here complement those presented by Jaouhar et al. (2011). 2. HORIZONTAL AND VERTICAL CONSOLIDATION SOLUTIONS Vertical drains are commonly used to accelerate the consolidation of fine-grained soils by providing reduced drainage paths. Various solutions have been proposed to calculate the consolidation rate around such drains (see review by Jaouhar, 2012). For instance, Leo (2004) solved the equal strain consolidation equations around cylindrical drains, considering coupled vertical and radial drainage with progressive surface loading. The solution developed by Leo (2004) was used to validate the numerical solutions obtained for the axisymmetric vertical drain problem. The numerical simulations for validation with the analytical solution were conducted with the finite element code SIGMA/W 2007 (Geo-Slope International Inc.). The numerical model used for validation consisted of a unit cell with a width of 2 m and a height of 10 m high with a 0.425 m wide vertical drain. The left and right sides of the model were fixed with respect to horizontal displacement and the bottom of the model was fixed with respect to vertical displacement. The presence of a smear or transition zone around the drain was ignored. The properties of the two materials (tailings and waste rock) are shown in Table 1; the hydraulic conductivity of the tailings was fixed at 1x10-8 m/s. A vertical loading (100 kPa) was applied to the top of the model to generate excess porewater pressure (EPWP).

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