Application of a predictive model to estimate the tensile strength of unsaturated tailings

Bibiana Narvaez, Michel Aubertin, Faustin Saleh-Mbemba

In the proceedings of: GeoRegina 2014: 67th Canadian Geotechnical Conference

Session: Unsaturated Soils

ABSTRACT: Tailings produced by the milling process are usually disposed of within surface impoundments on mine sites. Tailings from hard rock mines are typically cohesionless, fine-grained materials. Upon hydraulic disposal, the initially saturated tailings may become unsaturated over time due to moisture loss by drainage and evaporation. The development of negative pore water pressures under unsaturated conditions tends to increase their tensile strength, but little is known about this behavior. This article first recalls a procedure applied to conduct bending tests to measure the tensile strength st of unsaturated tailings. The relationship observed between the value of st and the degree of saturation Sr is then presented using testing results obtained on three different tailings. These are compared with the st - Sr relationships given by the predictive equations of the Schubert (1975, 1984) model, which makes use of the water retention curve. Key results from this investigation are shown and discussed. RÉSUMÉ Les rejets du concentrateur sont généralement entreposés en surface dans des parcs à résidus miniers. Les résidus de mines en roche dure sont typiquement des matériaux fins sans cohésion. Suite à leur déposition hydraulique, les résidus miniers initialement saturés peuvent se désaturer avec le temps par drainage et évaporation. Les pressions interstitielles négatives qui se développent dans les résidus miniers à l'état non saturé contribuent à l'augmentation de leur résistance en traction; on connait toutefois mal ce type de comportement. Cet article rappelle une méthode d'essais de flexion utilisée pour déterminer la résistance en traction des résidus miniers non saturés. On présente aussi les relations entre la résistance en traction mesurée et le degré de saturation obtenues pour trois types de résidus. Ces résultats sont ensuite comparés aux valeurs calculées à partir du modèle prédictif de Schubert (1975, 1984), qui utilise la courbe de rétention d'eau. Les principaux résultats de cette étude sont présentés et discutés. 1 INTRODUCTION Mining operations generate a variety of waste materials, including the tailings produced by the milling process that are usually stored within surface impoundments. Tailings are initially saturated upon hydraulic deposition, but may become unsaturated over time due to drainage and evaporation. This decrease in water content may affect their shear strength. Freshly deposited tailings from hard rock mines, having a silty grain-size, are typically cohesionless (Vick, 1990; Aubertin et al., 2002; Bussière, 2007); their tensile strength is thus nil. However, as is the case with some granular soils (e.g. Lu et al., 2005; Goulding, 2006; Kim and Sture, 2008), such tailings may show an apparent cohesion capp induced by capillary and adhesion forces under unsaturated conditions. This apparent cohesion, which can vary with the water content, is related to the appearance of a tensile strength st due to negative water pressure (or suction). Many studies have been conducted to assess the tensile strength of cohesive (cemented or clayey) soils (e.g. Al-Layla et al., 2007; Ammeri, 2009; Amarasiri et al., 2010, 2011). Others have investigated the tensile behavior of unsaturated cohesionless soils under varying degree of saturation Sr (Kim, 2001; Kim and Hwang, 2003; Lu et al., 2005, 2007; Rodriguez, 2006; Kim and Sture, 2008). To the authors' knowledge, there has been no specific investigation on the tensile strength of unsaturated tailings produced by hard rock mines. The main objective of this project was to develop a relatively simple experimental protocol to determine the tensile strength st of fine-grained tailings as a function of the degree of saturation Sr, and then to use this testing procedure to assess the typical response of tailings from hard rock mines. For the experimental program briefly described here, the tailings specimens were prepared at the same initial water content (saturated state) and then naturally air-dried to the desired degree of saturation. The unsaturated specimens were then submitted to the bending test up to failure. The tests results are used to define a relationship between the tensile strength st and Sr. The experimental data are then compared with values obtained from the predictive equations of Schubert (1975, 1984), which are summarized here.

RÉSUMÉ: ication of a predictive model to estimate the

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Bibiana Narvaez; Michel Aubertin; Faustin Saleh-Mbemba (2014) Application of a predictive model to estimate the tensile strength of unsaturated tailings in GEO2014. Ottawa, Ontario: Canadian Geotechnical Society.

@article{GeoRegina14Paper123,author = Bibiana Narvaez; Michel Aubertin; Faustin Saleh-Mbemba,title = Application of a predictive model to estimate the tensile strength of unsaturated tailings,year = 2014}