Determination of the hydraulic conductivity-void ratio function (k-e) of sedimenting soft soils can be performed using in situ density and pore-pressure measurements in column tests, centrifuge, and in pilot or field deposits without resorting to back-calculation using numerical methods. The disadvantage of back-calculation is that it requires knowledge of the compressibility curve, which is known to change over time in soft soils. Direct determination of k-e has been employed in previous studies, essentially applying Darcy's law to find k from fluxes and gradients determined by various interpolation strategies of the known density and pore-pressure profiles. This technique is sometimes called the instantaneous profile method (IPM). This paper examines this technique rigorously through both (1) generating artificial data from a large-strain consolidation model and applying the IPM to this artificial data to attempt to recover the consolidation properties inputted into the model, and (2) by comparison with known case studies in the literature. This exercise provides clear requirements and limits on data needs as well as data interpolation methods. It was found that a substantial reduction in the scatter of the estimated k-e values was achieved when a local dependency of k on void ratio was assumed only for the variation of k across an IPM element: k similar to eb. This assumption of local variation did not inhibit IPM from regenerating the global hydraulic conductivity function, even if the power of this global function was very different from the assumed form of the local variance. Screening out fluxes generated by measurement noise also improved the results.

Ultrasonic methods are used for non-destructive detection of changes of low-strain properties in concrete, soils, and other materials. This paper presents the use of ultrasonics to detect density and structure changes in a soft clayey tailings. Such tailings undergo sedimentation and consolidation following their deposition, which may occur simultaneously with flocculation and other fabric-altering processes, resulting in time-dependent consolidation characteristics. Here we attempt to measure structural changes in the clayey tailings using ultrasonics, which presents several challenges due to the material’s relative softness. Ultrasonics testing was attempted using a special 3D printed column with height and diameter of 540 and 110 mm, respectively. Specially fabricated sample holders were designed to repeatedly ensure proper connections of compression (P-wave) and shear (S-wave) wave transducers. The arrival time corresponding to the largest P-wave amplitude showed a strong correlation with density of the tailings, while a specific frequency bandwidth of the P-wave spectra indicates an increase in wave amplitude (signal energy) occurring subsequent to most of the volume change. Ultrasonic measurements appear to be a promising technique to monitor density and stiffness in soft clays, tailings, and similar soft sediments.

The large-deformation consolidation theory is fundamental for many applications involving the poromechanical couplings but faces great challenges in making accurate predictions in practices due to significant uncertainties of material permeability, k. The permeability could take random values at constant void ratio, e, due to random microscopic flow channels, moreover, its statistics vary with evolving microscopic structure subjected to volumetric compression during large-deformation consolidation, which is referred to as permeability's volumetric variability in this work. Current consolidation-related probabilistic models are focused exclusively on the properties' spatial variability and limited to infinitesimal deformations. Here, we formulate a random large-deformation consolidation model, RANDCON, by combining the random field theory with an existing piece-wise linear formulation in the Monte Carlo framework, which will simulate the large-deformation consolidation as a random process. In this model, we treat e as a spatial-like dimension to establish a generalized non-stationary random field where the statistical moments of k changes with e (i.e., random k−e relations) to embrace volumetric variability. We choose the piece-wise linear formulation that directly updates k from the generalized random field, which, being different from the conventional random field approach, is performed at every time step. The statistical results from an example shows that (1) variabilities of k yields an overall delayed consolidation due to local clogging, which could be aggravated by the associated fluctuations of e, (2) “too significant” clogging can, conversely, suppress the clogging by creating a permeable near-surface zone with increased e (k) from fluctuations; (3) the converse clogging effect also explains distinct effects of scale of fluctuation on changes of average consolidation rate in different cases. Practical significances mainly include that the deterministic approach might underestimate the time to reach full consolidation.

Soft soils and clayey tailings deposits can exhibit substantial creep during consolidation. Creep can strongly influence the consolidation and strength development of tailings deposits but is rarely considered in existing large-strain consolidation analyses for design purposes. This paper presents an analysis of a 13 m high deposit of soft clayey tailings using large-strain consolidation coupled with three alternate creep formulations: the Yin-Graham, Vermeer, and overstress formulations. Predictions of settlement and excess pore water pressure using all three consolidation-creep models have better agreement with the measured data compared to predictions from a large-strain consolidation model without creep. The overstress model is more difficult to calibrate for young soft soils in an initially normally consolidated state, owing to the strong dependency on the initial ratio between an unknown small preconsolidation stress and the initial small effective stress. The models calibrated to the pilot study were then used to extrapolate the full-deposit behavior, and the implications of using such consolidation-creep models and their limitations are discussed.

Evidence of ageing has been reported in natural deposits of sedimentary clays, dredged sediments, and in clay-rich oil sands tailings, and such time-dependent behaviour has been observed to influence their long-term set-tlement and strength development characteristics. The ageing phenomenon can, therefore, have important im-plications for the reclamation of such deposits. This research investigated ageing in polymer-amended tailings and Leda clay monitoring relatively small (10 cm high) laboratory samples for over 100 days. Both tailings and natural clay were found to develop increased strength, sensitivity, and higher pre-consolidation pressure and reduced compressibility. The correlation between fall cone shear strengths and pre-consolidation pressures was strong for both the natural clay and the clayey tailings, though this ratio was different in the tailings and the clay. The Leda clay appeared to reabsorb some of its bleed water, which was found to be due to the transition of some free water to absorbed water after 80 days. Using a concept developed from ageing experiments in artificial clays, it was found that shear strengths measured after an "early equilibrium state" correlated strongly with shear strengths measured near the end of ageing. This is promising for quantifying the potential for ageing on compressibility without having to resort to year-long experiments.

The strength and deformation properties of tailing materials are strongly affected by their hydromechanical history including the cycles of desiccation and re‐saturation they undergo due to the often layered deposition process. The current study reviews the fundamental characteristics of some of the commonly used constitutive models for partially saturated soils, namely Barcelona Basic Model (BBM), Glasgow Coupled Model (GCM), and a dilatancy‐based model by Buscarnera and Nova Model (BNM) in predicting the hydromechanical response of tailing materials. In particular, we investigate the strength and shortcomings of each model and compare their capability of capturing the salient trends during drying/wetting cycles, as well as during shearing. Based on comparison with experimental data, modifications are suggested for the models, specifically related to a realistic formulation of the water retention curve and its hysteresis. The results point to a potential trade‐off in the accuracy of the constitutive models in capturing the volumetric responses during the drying/wetting cycles on one hand, and the stiffness and strength characteristics during the shearing phase on the other.

This paper develops the one-dimensional large-deformation consolidation theory for the porous medium under alternating saturated-unsaturated conditions, through a former generalization of the well-established Gibson's nonlinear theory from saturated to unsaturated regime. With the differential relations between the coordinate systems, the equations governing the momentum\mass balances in the unsaturated porous stratum are transported from the moving current to the initial Lagrangian coordinate system. Without loss of generality, the collective forms of various incremental constitutive laws are then incorporated for unsaturated materials, indicating that any particular constitutive law has been embraced. Two forms of final governing equations are derived depending on the primary independents: sigma v - uw (total vertical stress - pore fluid pressure) and w (gravimetric water content); which are solved by two distinct finite-element codes on the initial fixed configuration. Numerical examples demonstrate that the two resulting Total Lagrangian formulations are equivalent, yield virtually identical predictions and agrees well with an existing finite-differential code that solves the equations in the convective configuration. The dynamical consolidation process evolving from a weight-driven saturated stage to an evaporation-driven unsaturated stage is quantitatively captured very well by the predictions. The impacts of evaporation, material's compressibility, and permeability in these two distinct stages are independently explored through several parametric analyses on the consolidation/dewatering rates and the penetration of drying front. Through one factor must affect, in concert with the others, such a hydro-mechanical coupling process, these results can provide a profound understanding of the entire consolidation process of a porous stratum under large deformations.

Application of rheology to the post-deposition behaviour of tailings is important for the design of the slope of the impoundment, control of layering for purposes of strength enhancement through desiccation, properly describing the mixing of different tailings streams and for dam breach consequence analysis.This paper aims to advance the understanding of tailings rheology in the post-deposition context to aid all of these applications.Firstly, thixotropy and its quantitative implications for beach slope and layer geometry are explored using a simple treatment.Secondly, method dependency in yield stress measurement is discussed, along with how such uncertainty can be practically handled by considering the appropriate stress path and timescale for a particular application.Finally, numerical simulation using a thixotropic rheology of channel flow down a beach and runout from a dam breach experiment, conducted in a centrifuge, is used to highlight the utility of advanced rheology to such problems.The paper uses data collected from published work on both hard rock and oil sands tailings over the last 15 years.

Proper application of unsaturated soils to thickened or filtered tailings has substantial potential to optimise such tailings deposition.Unsaturated behaviour bears on both the geotechnical and geo-environmental performance of these deposits.Geotechnically, there is an increase in strength through both additional densification, and the presence of suction, as well as a stress history effect.The latter is especially important to understand in situ measurements in previously desiccated deposits; geo-environmentally, unsaturated behaviour strongly bears on seepage generation, as well as management or mitigation of acid generation.This paper summarises a body of work on experimentation, numerical simulation, and field data relating to different aspects of unsaturated behaviour and their relevance to practical outcomes for design of thickened or filtered tailing deposits.An important new outcome from this summary is the suggestion to characterise the extent of the stiffness imparted by drying, especially due to the possible risk of strength loss in thick deposits when the tailings eventually yield under self-weight.

The one-dimensional (1D) large strain consolidation (LSC) of saturated soft materials that are deposited at very low-density usually exhibit time-variant compressibility (void ratio vs vertical effective stress (e-σv′)) relation. The 1D column-like model test serves as an effective approach for characterizing this consolidation characteristic if all the physical parameters (including the settlement rate, pore pressure and density) are measured. Unfortunately, the density measurement is not always realistic due to its high cost (e.g., with X-rays) and the time-effect must be roughly neglected by using an average compressibility relation. This can further lead to erroneous estimations of the materials’ permeability (k) relation (permeability vs void ratio (k-e)) in the LSC analysis. This paper presents two modifications on two conventional equations for compressibility and permeability, respectively. The first one describes the compressibility curve’s movement in the lne-lnσv′ plane, and the other quantifies the ratio between the permeability calibrated by neglecting time-effect and its true value. These modifications originate from deep comparative analyses of several physical parameters between the column test and numerical prediction. Meanwhile, a simple hand-calculation procedure is proposed to estimate the new constants.

Oil sands fluid fine tailings deposits are challenging to reclaim due to their inherently high natural water content, low permeability, and low strength. Combinations of polymers and (or) coagulants are used by operators to improve the dewatering and strength properties of the tailings. However, considerably more work has been done to evaluate polymer performance with short-term dewatering metrics rather than with long-term metrics such as consolidation properties. This paper evaluates the potential of four novel polymers for use in fluid fine tailings treatment compared to a commercially available polymer and untreated tailings specimen. The performance of the polymers was assessed through initial screening with respect to short-term dewatering, evaluation of the consolidation and strength properties using large-strain consolidation tests, shear sensitivity in pipeline transport, and finally, large-strain consolidation modelling to appraise the relative potential performance under different strategies, such as terrestrial or aquatic reclamation options. One polymer exhibits remarkably fast dewatering at high void ratios, while another demonstrates dense and shear-resistant flocs. The paper discusses each polymer’s distinctive tailings fabrics and how their unique merits and limitations would benefit different reclamation eventualities. Finally, potential improvements of the polymers are suggested for future assessment.

A modified Couette rheometer is used to simulate mixing of polymer into clayey tailings, and to study the effects of shearing due to pipeline transport. It is found that a normalized torque correlates very strongly with performance indicators measured at the end of mixing and /or shearing, such as CST, oscillatory rheometry, and floc size distribution as inferred from optical microscopy, whether to evaluate a particular mixing strategy, or after simulated pipeline shear. With respect to polymer injection, the torque increases until reaching a peak value after which the torque sharply reduces. This point was found to correspond to the optimal dose, as characterized by the afore-mentioned performance indicators. For clayey tailings, it appears that this technique, as opposed to alternative torque-based techniques used in other industries such as wastewater floc conditioning, produces samples with superior performance indicators at the method-specific optimal dose.

Desiccation (water loss by drying or freeze-thaw sufficient to generate matric suction), can influence the performance of a tailings deposit both positively and negatively. The significance of desiccation is largest in tailings that have been dewatered prior to deposition, by thickening or filtration. Such tailings can be "stacked" or deposited with a significant slope, which usually implies that a substantial volume of tailings remain above water. Under such conditions the tailings, by accident or by design, may undergo desiccation before burial by fresh tailings. Desiccation can contribute substantially to strength, above and beyond the contribution arising from increase in density, through stress history effects. For some deposits, it is required practice that at least some tailings undergo desiccation to improve, particularly when those tailings for a structural part of a deposit. If, however, tailings remain exposed to the atmosphere in an unsaturated state for some period of time, this may have potential negative consequences through oxidation of sulphide minerals and the formation of acid drainage. This paper describes previous research on the strength gained through desiccation in tailings, and on modelling work that incorporates unsaturated soil phenomena into consolidation analysis. Both types of research are applied to a real field site, providing an example of how novel improvements to tailings management can arise out of application of principles of unsaturated soil mechanics.

The slow settlement of fine clay particles in oil sands tailings, the by-product from the process of bitumen extraction from the oil sands ores takes many years, leading to the build-up of an enormous volume of fluid fine tailings in tailings ponds over time, which also gives rise to the biggest challenge in tailings management - poor dewatering rate and long consolidation time. Adding polymers can promote the flocculation of clay particles, and thus, can improve the dewaterability of tailings. However, choosing an ideal polymer dosage for a particular tailings composition or fine-tuning the polymer dosage based on the changes in tailings composition has always been a challenge for the oil sands industry. This study examined the potential of two rheological methods, which are based on torque rheology and measure the rheological parameters (i.e., peak torque and totalized torque) to determine the optimum polymer dosage. When the tailings sample was subjected to a shearing force in a torque rheometer and a polymer solution was injected into that sample simultaneously, the rheometer measured the torque on the impeller. The peak torque and the totalized torque were determined from the torque-time rheogram and their interpretations provided useful information on the strength of the elastic floc network, which was formed after adding the polymer, and also on the degree of flocculation. When the polymer dosage was increased in a stepwise manner and the resulting peak torques and totalized torques were plotted against the corresponding polymer dosages (six polymer dosages in total: 0.2, 0.4, 0.6, 0.8, 1.0, and 1.2 g/kg of dry solids), a local decrease was observed in both plots at the dosage of 0.8 g/kg of dry solids, which also corresponded to the shortest suction time determined from the capillary suction time tests and the highest solids settlement obtained from the settling tests at the same dosage. Hence, the results suggested that the measurement of peak torque and totalized torque could be used as a reliable control parameter for the optimization of polymer dosage for the dewatering of tailings.

New approaches are proposed for robustly determining the compressibility and permeability functions for large-strain consolidation (LSC), using limited measurements from one-dimensional tests, such as column or centrifuge experiments. These new methods are developed from several new findings reported in this paper, including (i) new analytical solutions that relate parameters of compressibility functions to the final density profile, (ii) analytical proof of the independence of the shape of the settlement curve from the parameter M, where the permeability or hydraulic conductivity k is given by MeP, e is void ratio, and M and P are material-related constants; and (iii) that when the same k function is employed, for the case of one-dimensional settlement, the optimal values of M and P are located on a unique straight line in the P–lnM space. The third finding was determined from optimization analysis of thousands of LSC simulations. Using different combinations of these new findings, three new methods are developed to estimate the permeability function. The efficiency, robustness, and accuracy of the three methods are investigated by their application to four column tests from the literature. The simplest of the three methods requires only two LSC analyses plus several hand-calculations, demonstrating strong potential for practical use.

Slurried soils or tailings are often deposited in layers that undergo complex stress paths in terms of desiccation and loading, as a given layer may undergo variable degrees of desiccation before burial by subsequent layers, which has implications for geotechnical stability and geo-environmental performance. Proper analysis therefore requires not only an effective coupling of unsaturated flow with large-strain consolidation, but also inclusion of hysteresis effects. This paper presents the development and testing of a coupled unsaturated flow-large-strain consolidation model, UNSATCON, initially formulated for only monotonic dewatering, to include such effects. UNSATCON operates by a novel numerical algorithm that ensures mass conservation and handles the saturated/unsaturated transition smoothly. Differently from conventional integration schemes commonly embedded in finite-element formulations, this algorithm, tracking the void ratio change explicitly for large deformation, is shown to be applicable for most constitutive models of unsaturated soils under one-dimensional or isotropic stress conditions. Three constitutive models are implemented: a modified state surface model and the original and a minor variant of the Glasgow coupled model (GCM). The numerical model is evaluated using a recently published experimental study on multilayer deposition of thickened gold tailings. The model adequately reproduces the essential features of the dewatering processes, including the irrecoverable deformation induced by desiccation, the interlayer water exchange governed by deformation and deformation-dependent hysteretic retention behaviour. All three implemented constitutive models can reasonably simulate these experimental observations, despite some trivial discrepancies in magnitude. Point-level inspection, however, reveals some fundamental differences that may have important practical implications. For example, the additional plastic strain that occurs in GCM type models over multiple wetting-drying cycles may explain observed trends in increased strength in the simulated experiment.

A two-dimensional large strain consolidation model is presented. The model uses a piecewise linear formulation for large strain, which implies nodes that are associated with a constant mass of solids, whose positions are updated over time. Fluxes are calculated based on 2D gradients between adjacent nodes. Regions associated with each of these nodes to calculate fluxes only deform vertically, and can slip past each other. This appears to accommodate large strains without the need for remeshing due to mesh distortion, and appears to retain sufficient accuracy. The model is validated against other analytical and numerical solutions for axisymmetric and 2D consolidation. An example analysis of 2D consolidation in a hypothetical tailings impoundment is shown. The analysis shows the formation of a beach, and how the variable water height may affect overall consolidation. RÉSUMÉ Un modèle bidimensionnel de consolidation à grande contrainte est présenté. Le modèle utilise une formulation linéaire par morceaux pour une grande déformation, ce qui implique des nœuds associés à une masse constante de solides, dont les positions sont mises à jour au fil du temps. Les flux sont calculés en fonction des gradients 2D entre les nœuds adjacents. Les régions associées à chacun de ces nœuds pour calculer les flux ne se déforment que verticalement et peuvent se glisser l'une sur l'autre. Cela semble s'adapter à de grandes contraintes sans avoir besoin de remaillage en raison de la distorsion du maillage, et semble conserver une précision suffisante. Le modèle est validé par rapport à d'autres solutions analytiques et numériques pour la consolidation axisymétrique et 2D. Un exemple d'analyse de la consolidation 2D dans un bassin de retenue hypothétique est présenté. L'analyse montre la formation d'une plage et comment la hauteur d'eau variable peut affecter la consolidation globale.

The cost of sourcing material for cover material is often a barrier to implementation of multilayer covers to reclaim mined waste or other disturbed land. This study investigated the use of municipal biosolids blended with stabilizing materials as a low-permeability material in covers with capillary barrier effects (CCBEs) for reactive mine tailings. A blended biosolids referred to as Custom Reclamation Mix or CRM (1:1 volumetric mix of anaerobically digested biosolids and leaf and yard waste) was evaluated as a candidate low-permeability layer in a CCBE using material characterization, physical simulation of the CCBE in columns, and unsaturated flow modelling. The CRM exhibited low saturated hydraulic conductivities (k = 4 x 10(-7) cm/s @ e = 4.0) and an airentry value of approximately 300 kPa. During laboratory column testing comparing a monolayer biosolids cover to a biosolids CCBEs, biosolids layers within the CCBEs remained highly saturated, acting as a barrier to oxygen diffusion and water flux. Pore-water samples in the tailings of the monolayer cover show some evidence of increased concentrations of nitrate relative to CCBEs columns. No significant change in pH was observed in any of the columns. Numerical models showed reduction in water flux at the tailings surface by 98% and a reduction in oxygen diffusion by up to three orders of magnitude when using biosolids CCBEs relative to uncovered tailings. Cover function was not significantly impacted by the characteristics of the underlying tailings. Whereas substantial cracking occurred in the biosolids monolayer cover, the low-permeability layers in the CCBEs remained sufficiently intact to remain both saturated and apparently retained low hydraulic conductivity. The use of amended biosolids as a low-permeability layer in a CCBE therefore appears promising and should be evaluated at larger scales.

The measurement or estimation of the permeability of soft sediments remains a difficult albeit necessary task for several geotechnical applications, such as land reclamation and tailings management. This note presents a new method for determining the two parameters (the multiplier and power) of the power permeability function for high water content sediments, using settlement data from a column experiment, where sedimentation and consolidation occur synchronously. The proposed method stems from the distinct effects of the multiplier and power on the geometric features of numerically predicted settlement curves in the height versus log-time plane. The predicted settlement curves shape is multiplier-independent but controlled by the power; whilst, for a prescribed power, the position of settlement curve depends on the multiplier and can be shifted according to a change in the multiplier without using additional simulation. These effects were recently observed to apply to consolidation analysis only (i.e. without sedimentation), but here are shown to be more general. Further, it is newly shown that the mathematical variance of logarithms of times at a set of random points on the predicted settlement curve correlates very well with the power value. Together, these correlations lead to the development of a new method for calibrating the power and then multiplier that is both accurate and inexpensive (computationally and experimentally), as demonstrated using a well-known case study.

Consolidation parameters are required to support the disposal management of soft soils or mine tailings. The estimation of these parameters from simple correlations using easily measured properties can be advantageous, and considerable work has been done on this topic. This paper proposes two innovations that advance this work: (1) hydraulic conductivity-void ratio (k-e) estimation can be substantially improved by using a single measured value at a high-void ratio, and (2) the compressibility curve itself can be a useful predictor of k-e. Using these findings, general equations are derived that describe k-e using a power law, where the power is either 4 or 5. Examining 79 k-e data sets from clays, clayey tailings, and dredged materials, 94% of all predicted k values are within an order of magnitude of measured k-e values. This level of accuracy, coupled with the advantage of anchoring the k-e function by a measured value at a high-void ratio, is shown to result in robust predictions of settlement in large strain consolidation analyses. DOI: 10.1061/(ASCE)GT.1943-5606.0002344. This work is made available under the terms of the Creative Commons Attribution 4.0 International license, https://creativecommons.org/licenses/by/4.0/.