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.

Tailings is a fine-grained granular mine waste, typically with particle sizes in the range of sands to silts. Conventionally, tailings are hydraulically deposited into impoundments surrounded by tailings dams. The safety against dam failure must be ensured, as a failure can result in catastrophic consequences. In recent times, catastrophic tailings dam failures of upstream constructed dams have reported static (or flow) liquefaction in tailings as a failure mechanism. Static liquefaction can betriggered in saturated and loose silty sandy soils, generating a nearly complete strength loss in the soil. Undoubtedly, static liquefaction is a critical failure mode to investigate in the design of tailings dams, especially for cases where the dam stability relies on strength of the deposited tailings.

In engineering practice, the primary focus for failure modes involving static liquefaction is to identify if the tailings are saturated and loose. If so, the tailings are considered to have liquefaction potential and it is common to assume that static liquefaction will occur independently of any triggering event. Thus, low strength values corresponding to a case of “liquefied” soil strength is used in the stability assessments and such scenarios typically governs the tailings dam design. The most challenging part of the liquefaction potential assessment is to investigate if the tailings are loose, which in this context refers to “looser” than its critical state at the current stress level indicated by contractive behaviour during shearing.

Assessment of tailings state (i.e. loose or dense) is thereby crucial and engineering practice relies on in-situ testing, since undisturbed sampling in sandy soils is challenging. The Cone Penetration Test (CPT) is today the most used in-situ test for liquefaction assessments in sandy soils, including tailings deposits. However, existing CPT related interpretation methods were mainly developed based on natural clean sands and CPT conducted in calibration chambers. Using these interpretation methods on the CPT response in a loose silty-sandy tailings is thereby outside the original context in which the methods were derived. Concludingly, there are uncertainties in CPT interpretation of tailings state which become uncertainties inherent in the tailings dam design.

In this thesis the usage of CPT for assessing the tailings state is further discussed, from physical measured CPT data, like cone tip resistance or pore pressure development during penetration, to interpreted parameters used in static liquefaction assessments. CPT as well as static liquefaction is highlighted from a perspective in close connection to the framework of Critical State Soil Mechanics. Uncertainties in the CPT interpretations in tailings are highlighted, with focus on deviating characteristics between tailings and natural sands that motivates the need for CPT calibration chamber testing on tailings. As part of the ongoing research a new calibration chamber has been designed and developed at LTU. The chamber is in detail presented in this thesis. In the chamber a sample with 0,6 m diameter and 1 m height can be prepared. Sample saturation and consolidation to desired conditions are conducted in the chamber prior to pushing a CPT. Details on how the CPT calibration chamber testing will be utilized in forthcoming research are presented in the thesis. Loose (contractive) silty sandy tailings will initially be tested to investigate the relation between tailings state and CPT data. In addition, other possibly relations between conditions for static liquefaction potential in tailings and CPT data are also of interest in the upcoming research.

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.