Examination of the effects of solids content on thickened gold mine tailings sedimentation and self-weight consolidation

Abstract: Thickening is being increasingly adopted by the mining industry because of its economic and environmental attributes, such as decreased amounts of water released following deposition and a smaller footprint of the tailings site. This study presents an assessment of the continuous process of sedimentation and the self-weight consolidation of slurry and thickened mine tailings. The results of a series of settling column tests performed with specimens with solids contents between 50 % and 72 % are presented and discussed. Lower solids contents that are more characteristic of slurry tailings were also included to cover a wide range of settling behavior. High-precision monitoring of pore water pressure was used to identify the transition from sedimentation to self-weight consolidation, which likely occurs between solids contents of 65 % and 68 % for this material, and it highlighted the fact that the combination of these two settlement processes accelerates ue dissipation. The displacement results for the tailings–water interface corroborate values in the technical literature related to the settlement of suspensions. Equilibrium was reached within a narrow time range (i.e., 400 to 500 min) despite the wide range of initial solids content in the slurries tested (i.e., 50 % S to 65 % S)

Investigations on the mechanical behavior of a heat exchanger pile

The geothermal use of concrete geostructures (piles, walls and slabs) is an environmentally friendly way of cooling and heating buildings. With such geothermal structures, it is possible to transfer energy from the ground to fluid-filled pipes cast in concrete and then to building environments. A comprehensive research work was carried out at the EPFL (Switzerland) to improve the knowledge in the field of geothermal structures and to quantify the thermal influence on the bearing capacity of heat exchanger piles. In this paper, some features of the behaviour of a pile subjected to thermo-mechanical loads are presented. Numerical finite element results are supported by in-situ measured values

Intelligent realisation of ground energy

There is a need to reduce the economic cost of the energy geostructures and increase the efficiency of such systems, which will also rationalise the environmental impact

Experimental study and numerical reproduction of self-weight consolidation behavior of thickened tailings

Abstract: Thickened tailings, defined as mineral wastes that behave as a non-Newtonian fluid, show a small yield stress and release a small amount of water following deposition. Thickening has become an increasingly used option in tailings management. This paper presents a detailed examination of gold mine thickened tailings undergoing self-weight consolidation, which is an important mechanism affecting soft soils immediately after deposition. Self-weight consolidation was evaluated using a column equipped with water pressure transmitters whereas a slurry consolidometer was employed to obtain the compressibility relationship under low vertical effective stresses. The piecewise-linear model CS2 was used to model the experimental self-weight consolidation test. This model proved very accurate in reproducing the observed behavior. Both the test results and the model results also confirmed the absence of sedimentation in the thickened tailings, which is in agreement with values reported in the literature related to similar materials

Effective Stress Concept in Unsaturated Soils: Clarification and Validation of an Unified Framework

The effective stress principle, conventionally applied in saturated soils, is reviewed for constitutive modelling purposes. The assumptions for the applicability of Terzaghi’s single effective stress are recalled and its advantages are inventoried. The possible stress frameworks applicable to unsaturated soil modelling are reassessed in a comparative manner, specifically the Bishop’s single effective stress, the independent stress variables approach and the generalized stress framework. The latter considerations lead to the definition of a unified stress context, suitable for modelling soils under different saturation states. In order to qualify the implications brought by the proposed stress framework, several experimental data sets are re-examined in the light of the generalized effective stress. The critical state lines (CSLs) at different saturation states tend to converge remarkably towards a unique saturated line in the deviatoric stress versus mean effective stress plane. The effective stress interpretation is also applied to isotropic paths and compared with conventional net stress conception. The accent is finally laid on a second key feature for constitutive frameworks based on a unified stress, namely the sufficiency of a unique mechanical yield surface besides the unique CSL

Constitutive modelling of unsaturated soils with hydro-geomechanical couplings

Even though the saturation in water of most natural and engineered soils is partial, constitutive models in geotechnical engineering have long made the assumption of complete saturation. The increasing knowledge of the rheological behaviour of unsaturated soils is now favourable to new constitutive formulations to understand the stress-strain behaviour under the effect of suction. Yet, it is not agreed that there is a unique way to account for capillary effects, and several families of constitutive models appear to be either insufficient to describe the complete soil behaviour or else far too complex to be handled in practical engineering. The principal objective of the PhD work is to overcome those limitations by formulating a new unified and advanced constitutive model to understand and predict the behaviour of unsaturated soils. It is specified that the model shall be applicable to the broadest panel of civil engineering cases involving changes in water content of the soil, for instance landslides and earthdams. The methods and contributions of the presented work can be summarized in four main themes that are (i) the effective stress (ii) the constitutive model for stress-strain behaviour, (iii) the model for water retention behaviour (iv) the integration of the coupled model in a finite element code. A review and clarification of the possible effective stresses for unsaturated soils is proposed. The generalized effective stress is identified and justified as the most suitable and convenient unique mechanical effective stress for stress-strain behaviour. It combines the mechanical stress and the capillary variables that are degree of saturation and matric suction. The complete stress-strain framework is built on the basis of the generalized effective stress by relying on work input equation. A new constitutive model named ACMEG-s is formulated from a reference critical state elasto-plastic model which features two mechanisms of plasticity. The proposed model uses bounding plasticity. The effects of capillarity are introduced by the means of a modified yield limit, and the internal couplings due to the effective stress itself. The mechanical behaviour is thus dependent on the matric suction and degree of saturation. The model shows particularly good performances in modelling wetting pore collapse and swelling pressure tests. On the basis of experimental results obtained with an unsaturated oedometer, new contributions to the understanding of the soil water retention curve were produced. A new model linking the degree of saturation and the matric suction has been formulated and validated on a number of experimental datasets. An innovative interpretation of the coupling between the void ratio and air entry value is proposed, in parallel with discussions on the capillary hysteresis. The retention model, based on kinematic hardening, is thus added to the stress-strain model which requires the management of double-way coupling. The coupled model ACMEG-s has been implemented into the finite element code LAGAMINE, in the objective of application to boundary value problems. The implementation required the development of specific integration routines for the constitutive models with a constant updating of the terms of coupling. The numerical code enables a three-level coupling between mechanical, retention, and fluid flow behaviours. Two case studies are presented with innovative insight into the spatial distribution of pore water pressures: the earth dam of Mirgenbach and the shallow landslide experiment of Rüdlingen. Recommendations are also formulated with respect to parameter determination

Un nouveau cadre constitutif couplé pour la modélisation avancée des sols non saturés

Most natural and engineered soils usually exhibit a state of partial saturation in water. Indeed, fast and heavy water flowing into the soil is known to cause collapse or soil plasticization which may be at the origin of harmful settlements. The mechanical loading conditions and the amount of confining have a pre-eminent influence on the intensity of these plastic phenomena. The presented work sets up an innovative constitutive framework for modelling soils partially saturated in water. The framework features (i) a stress-strain model with two mechanisms of plasticity, (ii) an advanced definition of the coupled stress and strain variables, and (iii) an accurate model of the soil water retention curve. The model is validated over different sets of experimental data, in particular with swelling pressure tests. The innovative concepts are intended for instance for the modelling of earth dams and landslides

Evaluation of capillary water retention effects on the development of the suction stress characteristic curve

The suction stress characteristic framework is a practical approach for relating the suction and the water-filled pore volume to the stress state of unsaturated soils. It predicts the effective stress by developing the suction stress characteristic curve from the soil-water retention curve. In this framework, the effective degree of saturation is usually calculated by the empirical water retention model of van Genuchten (published in 1980). In this paper, the use of a generalized soil-water retention model proposed by Lu in 2016, which differentiates the role of capillary and adsorption mechanisms, in the suction stress characteristic framework is studied. A redefinition of the effective degree of saturation is suggested, by choosing the retention state where capillarity approaches zero instead of the residual retention state. The validity of this assumption is examined using experimental data obtained by unsaturated shear strength and retention tests and datasets collected from the literature. The proposed definition is applicable for a variety of soils where capillarity is the dominant mechanism in producing suction stress within the range of suction 0–1500 kPa. In addition, it is observed that the generalized soil-water retention model presents a more realistic prediction of unsaturated shear strength compared with empirical water retention models.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author

Advances in modelling hysteretic water retention curve in deformable soils

Experimental findings on the hysteretic nature of the soil water retention curve, relating the degree of saturation to the matric suction, have generally to be superimposed with the aspects due to the deformability of the soil matrix. Indeed, most state-of-the-art models for retention curves only feature one of these two essential features, that is either capillary hysteresis or void ratio dependency. In an effort to set an advanced comprehensive model for the retention curves, it is proposed to review some recent results of the capillary hysteresis and focus on the elasto-plastic analogy in the degree of saturation versus suction relationship. The paper also contributes to quantifying the effects of mechanical straining on the retention curve on the basis of experimental data from the literature besides those obtained by the authors. The intrinsic shape of the soil water retention curve is first defined, followed by the empiric relationship between air entry value and void ratio. The retention sub-model of a complete constitutive model for unsaturated soils is described, the mathematical formulation being based on kinematic hardening and featuring direct coupling with the mechanical stress-strain module. Model capabilities are assessed on complex retention outlines, displaying the added value of the proposed framework for prediction issues