Fines Content and Density Effects on Tailings Behaviour : A Laboratory Study on Geotechnical Properties

Abstract: Tailings are a rest product from the extraction of metals and minerals, and is therefore produced in large volumes by all mining companies. One common way to store tailings material is to deposit it as a hydraulic slurry on a tailings impoundment, where the tailings are held in place by tailings dams. Deposit and discharge of tailings, often conducted along the dams, causes a particle segregation which creates different fines contents (percentage of particles smaller than 0,063 mm in the tailings) in the impoundment at various distances from the discharge. Another effect from the discharge is that different densities are created in the deposited layers. Since some tailings dams are constructed on top of old deposited tailings, and if possible with tailings as a construction material, the fines content and density effects on the tailings behaviour are important factors for dam stability. In this thesis, tailings material with different fines contents and different densities have been studied with the purpose to see how the behaviour in strength, compressibility and permeability varies. After an initial case study of sampled tailings from a specific impoundment, the fines content for the three tested materials were determined to be 10, 50 respectively 90%. The behaviour in strength was tested in both triaxial and simple shear tests. Only drained strength was studied for three consolidation stresses in both apparatuses. The result from both tests showed that the strength is increasing with decreasing fines content, and thus evaluated friction angles increases with decreasing fines content. Evaluated friction angles from the simple shear test are though significantly smaller than those from the triaxial tests. Friction angles from triaxial tests are seen as most reliable, since the principal stresses are controlled during the whole test. The difference in friction angles from simple shear and triaxial test is however not a new discovery, it has been found by others before. The results from the triaxial tests indicates that a transitional fines content must exist somewhere between 10 – 50 %, where the behaviour in strength switches from sand dominated to silt dominated. Oedometer tests were conducted to study the compressibility of the three materials. The results showed that the compressibility increased with increasing fines content and with decreased density. In agreement with that conclusion, evaluated oedometer modulus from the normal consolidation curve tended to increase with increasing density and to a smaller extent increase with decreasing fines content. Determination of characteristics in permeability were done by evaluating the hydraulic conductivity from constant head tests. Results from this showed that the hydraulic conductivity increases with decreasing fines content. Furthermore, with increasing density the hydraulic conductivity decreases. Results of both the compressibility and the hydraulic conductivity are of course expected. To develop the relations in compressibility and permeability is considered as hard, since the behaviours are both dependent of fines content and density. However, the results indicate that with different combinations of fines contents and densities similar behaviour in compressibility and permeability can be obtained for different materials.