Deposition of mine tailings in a cold climate requires precautions as temporary sub-zero temperatures can imply considerable consequences to the storage due to creation of permafrost. The risk of creating man-made permafrost lenses due to tailings deposition exists even in regions with no natural permafrost, as material being frozen during winter might not fully thaw by the following summer. When such frozen layers thaw during later longer warmer periods, excess pore water pressure and large settlements might develop. Such implications close to the dam structure have to be avoided and therefore the risk of generating permafrost should be reduced. This paper describes a geothermal model for one-dimensional heat conduction analysis. The model is able to simulate the temperature profile in tailings where the surface elevation is constantly increased due to deposition. At the tailings surface, the boundary condition is the air temperature changing over time during the year. Air temperatures, tailings deposition schedule and tailings properties are given as input to the model and can easily be changed and applied to specific facilities. The model can be used for tailings facilities in cold regions, where the effects of tailings deposition on the temperature regime are of interest. Findings can improve tailings management by explaining man-made permafrost generation. The model can also aid in setting up appropriate deposition schedules and to prevent generation of permafrost layers.

In an upstream tailings dam, loose layers might occur at different depths due to melting of frozen layers deposited during freezing temperature in Sweden. Reduced shear strength of such layers in a tailings dam might cause stability problems. Due to slow consolidation process, it is unknown, whether self-weight of a high tailings dam could have influence on strength and stiffness of soft tailings located at different depths. For numerical modelling, appropriate strength and stiffness properties of soft tailings are needed. For this purpose, loose layers in an upstream tailings dam were identified based on results of cone penetration tests.Consolidated Drained (CD) triaxial tests were conducted on undisturbed soft tailings collected from different depths of the dam. The results indicated that depth did not have considerable influence on strength and stiffness of tailings. Hardening Soil Model (HSM), at high confining pressures and axial strains underestimated stiffness of soft tailings under CD triaxial state. This study shows that: (i) proper care is needed in evaluating strength and stiffness parameters for soft tailings, and (ii) use of the HSM is likely to predict more deformations which could give an early warning before an actual failure of a tailings dam.Keywords: Soft tailings; Stress-strain behavior; Triaxial tests, Hardening Soil Model, strength and stiffness. IntroductionTailings dams may be constructed using three methods such as: upstream, downstream and centerline [1]. Tailings dams raised using upstream construction method are relatively economical as compared to ones constructed with other two methods [1]. It is generally understood that soil becomes stronger in deep layers as compared to the surface. This implies that lower layers in an embankment dam might be stronger than those at the top. This is because the strength is dependent on the confining pressure coming from the layers above. Materials StudiedThe undisturbed samples of tailings were collected from various locations of Aitik tailings dam. Void ratio and bulk density of tailings obtained from various depths are shown in Table 1. Samples were collected from those layers of the dam which were described as loose according to CPT results. Particle size distribution curves conducted by [6] showed that the tailings particles at shallow depth were more angular than those obtained from deeper depths.All samples were fully saturated having water content from 15 to 44%. The average specific gravity and bulk density of the studied tailings were 2.83 and 1.73-1.98 t/m 3 , respectively.Place Table 1 here.All tests were subject to axial strain rate of 5 × 10 −3 mm/min. The axial strains were measured indirectly by volume changes in lower chamber, and radial strains were calculated from changes in specimen height and volume. Similarly, volumetric strains were calculated from changes in back volume of specimen by measuring amount of water coming into or going out of specimen and initial sample dimensions. The axial deformations in the test s...

Tailings storage facilities (TSFs) will, after closure of the mine, have to be stable in a long-term perspective (e.g. 1,000 years or more). In many cases, due to the characteristics of the tailings, a high phreatic surface is required to keep the tailings saturated in order to prevent, or minimise, the process of oxidation. Due to this the slope stability of the embankment, or the land form slope, is critical as any material exposed to a hydraulic gradient is exposed to a load. So, the question is: Is the embankment, or landfill slope, that is exposed to a hydraulic gradient safe in the long term with respect to the actual design and material properties? In order to answer that question, an understanding of the structure, its stability and level of actual safety during operation is necessary. This paper will therefore discuss slope stability for embankments during operation and the long-term perspective and how the factor of safety (FS) can be verified. Practice today for dam stability is that a certain FS is required, i.e. a safety margin (in Sweden FS>1.5), and for that condition we design the embankment. The design includes the geometry of the structure, material properties, water management/water levels and requirements for compatibility between different materials, as well as for construction and operation. The FS can, however, not be physically measured on, or in, the actual embankment. What can be measured is seepage, pore pressure and movement (vertical and horizontal displacements). But how can the readings be used to verify the actual FS? In order to illustrate this, an example from a TSF in northern Sweden is presented where readings have been taken through numerical modelling (PLAXIS), comprehensive geotechnical investigations, lab testing and inclinometers. In order to predict how an embankment, or landform slope, will behave in the long-term phase and what the actual FS will be, the authors believe it is necessary to understand the behaviour of the structure during operation. The method used for the example illustrated in this paper shows a method to gain an understanding for a structure, which is absolutely crucial for understanding the actual FS and for the possibility to predict the level of safety in the long term.

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Seasonal freezing and thawing can have significant effects on tailings management. Tailings delivery, depositional schemes and water treatment are examples of activities that must be dealt with extra concern in sub-zero temperatures. Changes in mechanical properties, drainage possibilities or embedded frozen tailings layers are effects that can arise in poorly managed facilities. To avoid such consequences, a good understanding of the seasonal effects on the tailings deposit is needed. To get a better understanding of the geothermal regime in tailings, this paper presents a case study with geothermal modelling performed for the Laiva tailings facility in Finland, where major seasonal freezing and thawing periods are present. Ground temperatures and frost lines were predicted via one-dimensional modelling using air temperatures and snow cover depths from adjacent weather stations, and basic soil properties from the facility. Simulated results were compared to data obtained from thermal instruments in the field. The snow cover and its estimated thermal properties were shown to have large influence on the results. The model was able to accurately predict the thermal regime measured in the field. Strong agreement was shown, both in terms of ground temperatures and frost front positions. The methodology presented is useful for tailings management schemes in cold regions. Keywords: Geothermal Monitoring, Geothermal Modelling, Tailings, Soil Temperature, Frost Tubes, Snow Cover.