Energy consumption during electroosmotic dewatering () represents the cornerstone of assessing the financial viability of this method for widespread adoption, including implementation in large-scale field applications. Given the significance of this parameter, the available methods for energy consumption, in retrospect, are somewhat incomplete. This paper first discusses the existing methods of interpreting energy consumption and their limitations. This is followed by presenting an alternative approach for interpreting unambiguous values of energy consumption rates during EOD, evaluating the approach by applying it to experiments of various tailings materials and finally comparing the results with prominent case histories. One of the key limitations associated with the existing methods of reporting energy consumption is that they do not incorporate the response of the material being dewatered to the treatment process. To address this issue, the proposed method takes the liquidity index variation into account, thereby enabling the interpretation of energy consumption to provide a unit change in liquidity index for a given unit of dry tailings. Compared to currently available metrics, the proposed approach provides a better indication of the extent of dewatering achieved with respect to kWh utilised.

Electroosmotic (EO) dewatering is a technology that may be employed for the treatment of fine-grained soils with undesirable geotechnical characteristics, such as high compressibility and low undrained shear strengths. Historically, the soil types that have been subjected to EO treatment encompass a broad variety of soils ranging from soft clays with initial water contents close to the liquid limit to low solids content slurries (e.g., dredged sludge and slurry deposited tailings), which typically have a water content far higher than the liquid limit. Despite being a fundamental factor that governs the consolidation behaviour of each soil type, the effect of variation of the pre-treatment water content, to the overall performance of the EO treatment process, has not been quantified. In this study, EO dewatering laboratory-scale experiments were performed considering three consistency levels, corresponding to the water content values of 1, 1.5 and 2 times the liquid limit. The results indicated that the effectiveness of the EO treatment increased with the increase in pre-treatment water content, in terms of proportion of water removed and relative strength gain changes. A major finding of the present study is that the EO dewatering effectiveness significantly increases after the water content of the material increases beyond 1.5 times the liquid limit. Additionally, a new approach to evaluate the energy consumption is presented. The influence of adopted voltage gradient and initial water content on the energy consumption is then evaluated using the results of this study.

ABSTRACT A laboratory study was carried out on sandy silt gold tailings to investigate the effect of sample homogenization methods on the measured critical state line (CSL) in triaxial compression (TC) tests. Initially, 22 drained and undrained TC tests were performed on loose moist-tamped specimens prepared from the tailings that had been prepared using four different levels of homogenization: (a) without thorough mixing of the initial sample; (b) mixing during tamping of the specimens; (c) homogenization via sieving the material through 2.36 or 1.18 mm sieves; and (d) mixing the material as a slurry, air-drying it, and then sieving the material through 2.36 and 1.18 mm sieves. The results indicated fairly consistent CSL results within a void ratio of 0.03 for the first three sample homogenization methods employed, but showed a significant lowering of the CSL for the specimens homogenized by mixing as a slurry. An additional eight drained TC tests were carried out on dense moist-tamped and air-dried (AD) specimens to assess the effect of the lowering of the CSL on stress- and state-dilatancy behavior, indicating a lower state-dilatancy parameter, χtc, estimated from the AD tests than from the DMT tests.

Stability of base-exposed backfill roof in underhand drift-and-fill mining is crucial for the safety of those working beneath. Given the commonly used primary-and-secondary mining sequence, interfaces are formed between adjacent filled drifts, which can weaken the integrity of the backfill roof. These interfaces also lead to two common drift layouts: aligned drifts and staggered drifts. However, less attention has been paid to the interfaces and the two drift layouts were not adequately distinguished in previous studies. In this paper, the interfaces between filled drifts were firstly considered to investigate the stability of backfill roof. Failure modes and strength requirements of backfill roof in aligned and staggered drifts are comprehensively investigated by FLAC3D, with a focus on considerations of varied shear parameters of the interfaces. Results show that failure modes in aligned drifts transition from block sliding to top caving, bottom caving or sloughing as the interface cohesion increases from zero to at least half of the backfill cohesion. Further increases in interface cohesion allow aligned drifts to behave as if there are no interfaces between them. The critical stability conditions of backfill roof in aligned drifts were mostly determined by the interface strength instead of the backfill strength. However, the stability of backfill roof in staggered drifts is barely affected by the interface strength. The outcomes are expected to provide references for mining engineers to optimize drift layouts and perform cost-effective backfill roof strength design at mines using underhand drift-and-fill mining method.

Tailings storage facilities (TSFs) worldwide have recently experienced a number of failures, with insufficient undrained shear strength identified as a contributing factor in certain instances. The undrained shear strength of tailings is commonly assessed through element testing, such as the triaxial test, typically performed on loose samples under complete saturation. However, a substantial portion of tailings facility deposits often exist in partially saturated conditions. Despite the recognized increase in tailings strength with decreasing saturation levels, the threshold for the potential occurrence of excess pore pressure generation and strain-softening failure remains unclear. This study introduces a triaxial testing program designed to assess variations in undrained (closed system) shear strength for loose samples of low-plasticity gold tailings, prepared at different degrees of saturation using distinct saturation methods to address their impact. The findings reveal that as saturation decreases from complete saturation to partial saturation, both peak and remoulded strength increase, accompanied by a reduction in brittleness. Furthermore, the saturation history of the sample is observed to influence the brittleness of tailings under unsaturated behavior. A methodology to determine unsaturated brittle areas in a TSF is discussed.

The water retention properties of tailings, typically represented by a Water Retention Curve (WRC), influence several aspects of tailings behaviour, including drainage, infiltration, drying, stiffness, and strength. WRCs are influenced by factors such as drying/wetting hysteresis, void ratio, and fabric. Therefore, laboratory-determined WRCs should aim to replicate those observed in the field. This study compares the WRCs of samples taken from high quality blocks and those remoulded using various techniques, obtained from tailings storage facilities (TSFs) with varying ore types, fines content, and plasticity. Results indicate that material void ratio primarily affects the position and shape of the WRC. The preparation method can also influence the transition zone between saturation and residual areas, even when void ratios are identical. For coarse tailings, saturation processes can alter density and thus impact the resulting WRC; evaluating the WRC starting from block sample water content is recommended before conducting tests from a saturated state, especially if the material has not previously experienced wetting collapse. For fine tailings, air drying and slurry preparation methods generally provide similar WRC results to intact samples, provided that the void ratio is appropriately matched.

Bauxite residue (red mud) is a highly alkaline byproduct of the alumina refining process generated in large volumes worldwide. Most alumina operations currently rely on long-term storage facilities due to the high costs and technical constraints of residue reuse technologies. Therefore, understanding the mechanical behaviour of bauxite residue is critical to minimising environmental contamination risks, preventing structural failures, and ensuring the long-term stability of storage facilities. However, obtaining reliable geotechnical parameters is challenging due to the residue’s complex composition and chemical characteristics, particularly given the current fragmented state of the literature. This review compiles an extensive database of bauxite residue parameters across alumina operations worldwide, synthesising existing knowledge and research needs. The database encompasses 940 data points extracted from 63 studies across 25 countries. The findings reveal considerable variability in parameter values across countries, potentially exacerbated by inconsistencies in experimental methodologies. Few studies have integrated geochemical conditions, such as pH and salinity, with geomechanical behaviour, thereby limiting understanding of the mechanisms that govern key parameters, including strength and deformability. Enhancing the predictability of bauxite residue behaviour requires the development of standardised experimental protocols that explicitly account for the coupled effects of chemistry and mechanics to support reliable risk assessments of storage facilities.

The behaviour of sandy silt tailings under general loading conditions is important for assessing slope stability, below slope conditions and other geotechnical applications. Constitutive models used in numerical applications are usually calibrated and validated under axisymmetric loading. It is therefore essential to examine whether they can also predict soil behaviour in other relevant and more general loading scenarios. This study tests the hypoplastic constitutive model of von Wolffersdorff (1996) for sandy silt gold tailings under a wider range of stress and strain paths. The model was calibrated using drained and undrained triaxial compression tests. Validation used independent drained and undrained triaxial extension, torsional shear at constant Lode angle with fixed principal stress direction, and simple shear tests. The loading paths cover consolidation, peak states, and critical states. Our results show that hypoplasticity reproduces key trends in tailings behaviour across various paths. The simulations capture the evolution of the Lode angle and the rotation of principal stresses under non-axisymmetric loading, including plane-strain conditions often used to idealize slopes, which supports applying hypoplasticity to problems involving principal stress rotation in plane-strain, such as tailings dams.

The effect of clay fraction on drained and undrained strain-weakening and subsequent recovery in strength through reconsolidation was investigated using a series of constant volume direct simple shear (DSS) tests and drained ring shear (DRS) tests. This study examined three different materials with a range of plasticity index and fines content values: a mixture of 80% silica fine sand and 20% kaolin by dry weight (20K80SFS), iron ore tailings (IOT), and kaolin. The specimens were monotonically sheared in multiple stages or alternatively cyclically sheared under stress-controlled conditions to various values of shear strain in DSS tests. After the completion of initial shearing, the specimens were reconsolidated to three different vertical effective stresses, and monotonic shearing was recommenced. The lowest fines content 20K80SFS mixture exhibited strength recovery post undrained shearing. The initial degree of shear strain and reconsolidated vertical effective stress was found to influence the post-reconsolidation undrained strength of soils with a higher plasticity index (IOT and kaolin). The shear strength of kaolin specimens after significant undrained shearing showed a reduction that was consistent with that seen in the DRS tests at high shear strain, suggesting a similitude between the process of undrained and drained strain-weakening in predominantly clay soils. This study highlights (i) the potential for undrained strength recovery for pre-sheared specimens and (ii) the undrained frictional-weakening process depending on the clay fraction of the material, consistent with previous studies of drained frictional weakening.

Placement of soil covers over slurry deposited tailings, which are typically characterised by poor undrained shear strength involves significant risks and challenges, from the geotechnical viewpoint. Dewatering by means of applying an electric current, also referred to as electroosmotic (EO) dewatering technology is known to improve weak soil deposits. A laboratory study was undertaken to study the improved geotechnical characteristics of electroosmotically treated (EO treated) material through testing of sand washery tailings. To this end, EO dewatering experiments were carried out at varying voltage gradients using a purpose built EO cell. The properties of the EO treated tailings, including strength and compressibility characteristics were then characterised using laboratory element testing such as direct simple shear and oedometer tests to compare with the typical characteristics of untreated material. The results indicated that EO treatment increased the preconsolidation pressure of the tailings by three to nine times in the cathode region and six to fifteen times in the anode region, depending on the adopted voltage gradient. Owing to the overconsolidation effects induced by the EO treatment, strength gains of up to 664% were observed in the anode region of the experiment conducted with the maximum voltage gradient of 150 V/m.

Tailings within a tailings storage facility (TSF) are subjected to monotonic loading and consolidation following tailings depositions into the impoundment and perimeter embankment raises. The aim of the current study was to identify how monotonic loading followed by consolidation influences the behaviour of a loose silt tailings deposit. The study quantified the effect of magnitude and duration of an applied monotonic load on the evolving undrained shear strength using triaxial testing. It also showed that peak undrained shear strengths are reached under lower shear-induced excess pore pressures and axial strains than inferred from standard triaxial testing. While the study showed how tailings benefitted from gains in peak undrained shear strength, it also highlighted some potential negative implications for timely detection of precursors to failure of TSFs by means of conventional monitoring.

A series of full-flow penetrometer (T-bar and ball penetrometers) and piezocone tests were performed at varied penetration rates in a geotechnical centrifuge to investigate rate effects on penetration resistance, with a focus on the remoulded resistance of a clayey iron ore tailings. The effect of penetration rate on sleeve friction, pore-water pressure generation, and tip resistance is studied in terms of normalised penetration velocity from the cone penetration tests with pore pressure measurement. The effect of penetration rates on the remoulded resistance was studied by conducting cyclic tests across two different depth ranges with the full-flow penetrometers. The strong dependence of remoulded strength on the penetration rate was identified in terms of normalised penetration velocity. In addition, the shear strength inferred from in situ vane shear tests after remoulding was compared with the penetrometer results. The shear rate effect was consistent with the previous rate effects conducted on the same material using different testing techniques. The potential for partial drainage and viscous effects highlights the importance of considering rate effects for improved in situ characterisation of mine tailings.

Electroosmotic (EO) dewatering has gained substantial attention as an alternative method for improving weak soil deposits. One aspect that has not received sufficient attention in this approach is the influence of initial water content and voltage gradient on the EO treatment. This study aimed to provide a comprehensive understanding of the key operational parameters and post-dewatering performance metrics of EO treatment with varying initial water contents and voltage gradients. A number of electrical, hydraulic and soil water retention parameters were monitored during the treatment process, and the material response to the EO treatment was evaluated by assessing various geotechnical and physiochemical parameters. The results indicate that, regardless of the voltage gradients, overall, the EO dewatering led to a reduction of at least one-third of the original water content. The EO treatment resulted in a sevenfold, ninefold and twentyfold relative increase in undrained shear strength for the experiments with the lowest, intermediate and highest initial water contents, respectively, indicating an exponential increase as the initial water content of the slurry increases. The implications of the findings to improve the viability of EO treatment for large-scale applications using electrokinetic geosynthetics are also discussed.

Differences in the fabric of reconstituted triaxial samples may increase the difficulty of achieving a unique critical state for some soils. Moreover, changes in the salt concentration of bauxite residue can result in non-unique critical state lines (CSLs). To evaluate the effects of fabric and pore-water chemistry on the critical state of bauxite residue, this paper compares the triaxial compression behavior of intact, slurry consolidated, and various forms of moist tamped samples, at a range of salt concentrations. The variations in fabric were also investigated using scanning electron microscopy and nuclear magnetic resonance (NMR) tests. The changes in pore-water chemistry were analyzed using X-ray methods and changes in salt concentration. The results showed that the particle agglomeration induced during reconstitution resulted in a more significant shift of the CSL than the decrease in salt concentration. Microimaging and shear behavior of samples showed that the slurry consolidation method may be the most suitable method for representing in situ behavior of clayey bauxite residue. NMR findings suggest that variations in water retained in the micropores of reconstituted samples stem from differences in microstructure. The implications of changes in salt concentration and sample microstructure on the design of clayey bauxite tailings storage facilities are discussed.

The direct simple shear (DSS) test carried out under constant volume (CV) conditions forms one of the primary laboratory techniques to characterise soils and tailings. The use of CV conditions to simulate undrained shearing is supported by historical evidence on the testing of a saturated clay and sands, with this evidence being incorporated into current guidelines and state of practice procedures. However, some recent comparisons of the results of undrained hollow cylinder simple shear (HCSS) and CV DSS tests on predominately silt gold tailings adopting state of practice test procedures (i.e. inundation of the sample after loose moist tamping) showed much less post-peak strength loss in the gold tailings than the undrained HCSS tests. The current study investigated this discrepancy further by carrying out DSS tests under high back pressures, undrained simple shear tests with flexible membrane and constant cell pressure and DSS tests after flushing with carbon dioxide and with use of a small back pressure. In all cases, the undrained tests or DSS tests with greater effort put towards saturation exhibited greater post-peak strength loss more consistent with the HCSS and the critical state line. The importance of these results on the estimation of tailings brittleness in engineering practice was outlined.

Experimental results are presented in this technical paper to investigate the mechanisms of plane strain consolidation and drained shearing typical of below slope conditions. Five torsional shear hollow cylinder tests were conducted on a sandy silt gold tailings, where consolidation was performed under at-rest (K0) conditions, and by applying a horizontal shear stress, while maintaining plane strain conditions. The tests were carried out under drained simple shear conditions (strain controlled) on tailings specimens prepared in loose and dense states and along a constant shear stress drained stress path (stress controlled) on a loose specimen, using an automated computer-controlled testing procedure. The evolution of static stresses in the loose and dense specimens during principal stress rotation, while maintaining plane strain conditions, were examined. These tests provide valuable insights into the behavior of tailings under plane strain conditions, contributing to the calibration of numerical models for slope analysis and more broadly for plane strain problems.

The construction of filtered tailings storage facilities (TSFs) provides an alternative deposition strategy to mitigate the risk of liquefaction of loose and saturated hydraulically placed tailings through the processes of filtration and compaction. However, challenges in depositing and compacting filtered tailings can impede efforts to achieve unsaturated and/or dilative conditions. For example, it is shown that for the same compaction degree and initial void ratio, clays compacted wet of optimum moisture content (OMC) are more compressible than those compacted dry of OMC. Further, increasing stress in initially dilative soils can result in intersection of the normal compression line with the critical state line (CSL), thereby transitioning from a dilative to a contractive state. To assess the effect of compressibility on this transition stress of filtered tailings, one-dimensional compression behavior of wet and dry compacted bauxite residue (red mud) was compared with the CSL for the same material. Soil-water retention tests were also carried out on one-dimensionally compressed samples to estimate the matric suction and effective stress while considering the effects of salinity in red muds. The results showed that for the same compaction conditions, dry filtered tailings transitioned from dilative to contractive conditions at lower transition stress than wet filtered tailings. The implications of these findings on minimizing the risks of flow liquefaction in filtered TSFs were outlined.

Recent failures of iron ore tailings storage facilities highlight gaps in understanding the behaviour of these materials. Stiffness is an important tool in the assessment of static liquefaction triggering and the monitoring of potential dam degradation that may result. This study evaluated the variation of the elastic stiffness across a wide range of void ratios and fines content and proposes a new model for the variation of shear modulus with mean effective stress. As commonly found, the shear wave velocity is more stress dependent for tailings in a loose state than in a dense state, but compared to "natural" soils, the shear modulus is significantly affected by the heavy (high specific gravity) fines content. These outcomes suggest that the addition of heavy fines (specific gravity > 3.5 vs. similar to 2.7 of most natural soils) can increase the shear modulus without increasing the shear wave velocity. From these results, a boundary anchored to the state parameter that separates contractive and dilatative behaviour can be found using V-s measurements. This provides an important monitoring tool for iron-ore tailings dams in light of two recent failures (Fund & atilde;o and Feij & atilde;o) that displayed minimal prior warning.