Vid Luleå tekniska universitet sker forskning kring anrikningssandoch gruvdammar. Vid bedömning av gruvdammars stabilitet är ConePenetration Test (CPT) en vanligt förekommande metod. I ett pågåendeprojekt tillverkas nu en kalibreringskammare där CPT kan utföras i jordmed fullt kontrollerade förhållanden. Forskningsresultatet kommer geoss bättre utvärderingsmetoder vid analys av CPT-resultat.

The mining industry has experienced rapid growth, leading to the accumulation of substantial mine waste, commonly referred to as tailings. Tailings are typically stored in tailings storage facilities, conventionally consisting of an impoundment surrounded by tailings dams. The construction of tailings dams can involve various methods, with the upstream method being commonly used in the industry. It is crucial to comprehend the long-term mechanical and geochemical behavior of deposited tailings to ensure the safety of upstream constructed tailings dams. The mineral composition, particle size distribution, and particle shape all affect the susceptibility to particle breakage or physical alternation. Therefore, there is an interest in understanding how grain size and grain shape relate to mineral composition and potential particle breakage to ensure the understanding of the long-term mechanical behavior. This study focuses on characterizing deposited tailings from various depths and investigates the impact of increased vertical stress on tailings, particularly examining the potential for crushing effects. The findings highlight the importance of considering these factors for a comprehensive understanding of tailings behavior and their implications for the long-term safety of tailings dams.

Characterization of tailings deposits is associated with significant challenges in tailings dam design and dam safety assessments. One of the challenges is static liquefaction in the deposited tailings and how this has to be addressed in stability assessments. Concerns have led to design concepts that deposited tailings considered susceptible to liquefaction, will liquefy. Facillities should therefore be designed for such post-liquefaction scenario, i.e. fluidized tailings. Existing upstream-constructed tailings dams are seldom built for a design case with post-liquefaction strength in the tailings. Consequently, mining companies must take dam safety-enhancing measures and rapidly conduct major changes in their tailings dam design.

At the same time, there are questions and uncertainties in evaluating static liquefaction potential in tailings that must be addressed. The uncertainties are, among others, related to differences between natural sands and tailings. There is a need to address the applicability of today’s evaluation methods on tailings and this paper will describe how a research project at Luleå University of Technology, funded by Boliden, will investigate and address the uncertainties in the evaluation. Cone Penetration Testing (CPT) in a calibration chamber will be undertaken in the project. A new calibration chamber for CPT is developed, where silty-sandy tailings material will be tested in a controlled laboratory environment. Combined with other laboratory test results, the CPT calibration chamber results will be used to derive new and update existing correlations between CPT data and tailings properties. The research will focus on the mechanical properties of tailings and its influence on the soil behavior around the penetrating cone. This paper will highlight uncertainties in state interpretation from CPT in tailings, present the design work of the calibration chamber and the expectations for the outcome of the CPT calibration chamber research.

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.

Managing tailings deposition in cold climate requires specific measures not to create permafrost. The risk of generating permafrost due to tailings deposition exists even in regions where permafrost would naturally not occur. Material being frozen during winter might not fully thaw in the following summer due to added height of the tailings on the surface. Such embedded layers of permafrost should be avoided especially close to tailing dams. Main reasons are to prevent impermeable layers in tailings facilities, and to reduce the risk of having implications if such layers thaw during warmer summers causing increase in pore water pressure, reduced effective stress, and increased water content.

This paper presents a numerical study on the effects of tailings deposition in cold regions in relation to the potential formation of permafrost. Various deposition rates, schedules and tailings properties were evaluated. One-dimensional heat conduction analyses were performed with a temperature scenario representing a mine district in northern Sweden. Results show, that the thickness of permafrost layers increase with increased deposition rate and with increased water content. It was also shown that wet and loose tailings must be deposited in short periods during summer to avoid permafrost generation. In the case of dry and dense tailings more time is available for deposition in order not to cause aggradation of permafrost in the deposit.

These findings can help mining operation to set up deposition schedules for tailings facilities in cold climate. For known tailings properties, results can be used to identify periods of the year when, and how much, tailings can be deposited in critical areas of a deposit in order to avoid permafrost formation.

Mining operations produce huge volumes of waste products. Tailings, the fine grained waste material, is often managed in tailings management facilities (TMFs) surrounded by dam structures, i.e. tailings dams. Stability of tailings dams, amongst other things, is an in-creasing concern as tailings dams continue to fail. There is not just one single reason why dam failures occur. Dam stability is, however, one of the keystones required for good tailings management and good tailings dam safety. Dam stability can be divided into two main parts: a) stability analysis and b) surveillance and monitoring. The first is carried out at the initial design (normally by consultants) and is thereafter updated during operation of the TMF. A commonly used method for the analysis is the limit equilibrium method (LE). Here a factor of safety (FS) is calculated and in Sweden this is normally 1,5. In order to verify the behavior of the dam surveillance and monitoring is used. Typically pore pressures, horizontal and vertical movements and seepage are monitored in order to find changes in the trend of readings or to identify unexpected behavior. There is, however, no way of linking the readings to the stability analysis as the LE analyses are based on analysis of the conditions at failure. Thus it is not possible to describe the behavior of the dam before failure and the monitoring cannot be used to “give” warning signals before failure. This paper describes a case study where advanced numerical modelling, have been used to determine deformations in the dam structure, which have been verified by inclinometer readings. It has been possible to verify the actual stability for the dam as it has been possible to link in-situ readings to the model. The concept described is not only applicable to tailings dams, but can also be used for any type of dam.

This paper presents a case study of how the finite element methodcan be utilized to analyze stability of upstream tailings dams. Upstream tailings dams are usually raised gradually and the increased load normallyinfluencesthe stability in an unfavorableway;the load generatesexcess pore water pressures and reduced stability. In this study, an upstream tailings dam in Northern Sweden wasnumericallysimulated with the finite element software PLAXIS 2D in order to assess the stability of the dam. Upstream tailings dams are sensitive to high raising rates since initiated excess pore water pressures might not have time to dissipate. Stability analysis of a tailings damis an application that is very suitable to carry out using finite element software; once a finite element model of thecomplex geometry of adam has been established, it is easy to stepwiseadd new soil volumes, associated with each new raising, to the model.In this case study, it was found that strengthening actions were needed in order to maintain a stable structure. Rockfill berms weregradually added onthe downstream slope of the model to obtaina factor of safety above a recommended value. The volumes of rockfill needed for the berms wereminimized by numerical optimization to reduce costs. The stability betweenthe years2024 and2034 was analyzed; with an annual deposition cycle. The performednumerical studyresulted in a future plan for placement of rockfill berms to establishsufficient stability ofthe tailings dam. It was found that the volume of rockfill in the berms needed, varied during the years studied. Numerical modeling, as presented in this paper, is a useful tool for the dam owner to plan and design for future raisings of a tailings dam

This paper presents a case study where numerical modeling, with the finite element method, has been utilized to assess future stability of a tailings dam in Northern Sweden. The finite element software PLAXIS was utilized to simulate future dike raisings for the years 2024 to 2034. The simulations were conducted by computing each dike raising, the subsequent consolidation of the soil and the stability of the dam during the process. The factors of safety directly after each dike raising resulted in values continuously below a recommended value of 1.5. To increase the dam stability, rockfill berms were stepwise added on the downstream slope of the dam. An optimization technique was applied to place as small volumes of rockfill as possible in the berms at the most suitable locations on the downstream slope. By adding various volumes of rockfill in the berms each year, sufficient stability of the dam was obtained in the simulations. The excess pore water pressures increased annually in the simulations. It was shown that the excess pore water pressures did not totally dissipate before the following dike was constructed. The highest excess pore water pressures were located deep in the impoundment and did not have large effects on the dam stability.