Andy Fourie

Dewatering of mine tailings using electrokinetic geosynthetics

Many mining operations produce tailings that dewater very slowly under self-weight consolidation. One way of reducing the water content of such tailings is by electroosmotic dewatering. Although the technique has been used with some success in civil engineering applications, it is still largely seen as a solution of last resort. This is probably due to the high energy costs reported in the literature, as well as problems of very rapid corrosion of metal electrodes. This paper describes a study using newly developed electrokinetic geosynthetics (EKGs) as electrodes for the in situ dewatering of mine tailings. Laboratory tests were undertaken on mineral sands tailings in both a purpose-built testing cell and a laboratory testing tank using EKGs, followed by an outdoor experiment in a tank containing approximately 9m 3 of the tailings. This test was run for over 2 months. Energy consumption in the outdoor test was less than 1 kWh per dry tonne of material dewatered and there was no sign of electrode deterioration even after 2 months of usage. The results point to a potentially powerful technique for reducing the water content of tailings ponds in situ, thus increasing storage space, improving stability, and facilitating closure of these facilities.

Some aspects of the mechanics of arching in backfilled stopes

In current underground mining using “open stoping” methods, it is common to backfill mined-out voids (“stopes”) using hydraulically placed backfill, which is commonly composed of tailings, to which cement is often added. Knowledge of the stress state within a backfilled stope is required for safe design of drawpoint barricades and for other operational reasons. This stress state depends, inter alia, on the degree of “arching” that occurs, resulting from the development of shear stress between the fill and the stope walls. This paper presents a numerical modelling study of aspects of the arching phenomenon, carried using the computer code Plaxis. The backfill is characterized using the Mohr–Coulomb soil model, and both dry backfill and saturated backfill are considered to completely cover the full spectrum of backfill types that are used in practice. It is shown that even with dry backfill, the behaviour is governed by a complex interaction between the soil parameters. The behaviour is more complex with sa...

Coupled two-dimensional finite element modelling of mine backfilling with cemented tailings

Mine backfilling is a process whereby mine tailings mixed with small amounts of cement are placed hydraulically into mined-out voids (“stopes”) to stabilize the rockmass and allow full extraction of adjacent ore. A containment barricade is constructed to block the access point at the base of the stope, the design of which requires calculation of the total stress on the barricade during and following filling. For fine-grained backfill containing cement, the rate of development of stresses is governed by the rates of filling, consolidation, and cement hydration, each with its own timescale. As “consolidation” in backfill undergoing hydration can be dominated by “self-desiccation”, this mechanism must also be incorporated. Interaction between the backfill and the stope walls (“arching”) also has an influence. The paper describes a finite element (FE) model (“Minefill-2D”) that can model these interactions, although only in a two-dimensional (plane–strain or axisymmetric) fashion. It is shown that arching sig...

Assessment of the modified slump test as a measure of the yield stress of high-density thickened tailings

Yield stress values of four thickened (high-density) mineral tailings at varying solids concentrations were determined using three different techniques. The first set of values was measured using the modified slump test approach with an open-ended cylinder having an aspect ratio of 1.2. A second set was derived from measurements obtained from a coaxial cylinder fitted to a Rheolab® MC1 rheometer. The results were graphically compared with those obtained using the miniature vane technique, a popular and well-accepted method of measuring yield stress. Empirical relations developed from the modified slump test appear to predict reasonably accurate yield stress values up to about 200 Pa when compared with the vane and rheometer results. It is concluded that, although the time-dependent nature of the tailings tested may induce errors as much as ±30% for some samples, the modified slump test provides a reliable and simple test for evaluating the yield stress of thickened tailings. The method can therefore be em...

Electrokinetic in situ oxidation remediation: Assessment of parameter sensitivities and the influence of aquifer heterogeneity on remediation efficiency

A newly developed groundwater and electrokinetic (EK) flow and reactive transport numerical model was applied to simulate electrokinetic in situ chemical oxidation (EK-ISCO) remediation. Scenario simulations that considered the oxidation of a typical organic contaminant (tetrachloroethene) by permanganate were used to gain a better understanding of the key processes and parameters that control remediation efficiency. In a first step a sensitivity analysis was carried out to investigate a range of EK, hydraulic and engineering parameters on the performance of EK-ISCO. While all investigated parameters affected the remediation process to some extent, the duration and energy required for remediation were shown to be most dependent upon the applied voltage gradient, the natural oxidant demand and the concentration of the injected oxidant. Secondly, the efficacy of EK-induced oxidant transport was further examined for a heterogeneous aquifer system with random permeability fields. Oxidant migration under EK was slower in low-permeability media due to the increased oxidant consumption of competing reductants. Instead of injecting oxidant only at the cathode, locating injection wells between the electrodes greatly increased the contaminant degradation by decreasing the distance the amendment had to migrate before reaching the contaminant.

Comparative Study of Hybrid Artificial Intelligence Approaches for Predicting Hangingwall Stability

AbstractFive hybrid artificial intelligence (AI) approaches based on machine learning (ML) and metaheuristic algorithms were proposed to predict open stope hangingwall (HW) stability. The ML algori...

Rectangular foundations on a sand embankment over mine tailings

AbstractThis paper reports results from centrifuge model tests that provide insight into the behavior of rectangular foundations (representing earthmoving equipment) during penetration, with freedom in rotation, adjacent to a sand embankment (cover layer), into a weaker clay layer (representing mine tailings). A parametric study was used to explore the relevant range of the setback ratio, slope height ratio, normalized clay strength and its nonhomogeneity, and foundation base geometry. The effect of these nondimensional parameters on the penetration resistance profile is discussed in the context of the likelihood and severity of failure. The failure of a flat-based rectangular foundation was less severe than that associated with a strip foundation, and the corresponding measures reduced with the increasing setback ratio (λ) and slope height ratio (η). No rotational failure occurred for λ/(su,av/γcB)cr>8 (where su,av/γcB is the average normalized strength of the lower clay layer) or η≥0.604, regardless of ...

Experimental study of the evolution of the soil water retention curve for granular material undergoing cement hydration

AbstractThe evolution of the soil-water retention curve (SWRC) for cemented paste backfill (CPB) undergoing cement hydration is evaluated in this paper. A pressure plate apparatus and insertion tensiometers in a continuous drying configuration were used to apply and measure suction, respectively, at different CPB curing ages. In addition to this, a novel experimental setup that combines the previously mentioned techniques was developed and validated in an effort to reduce the time associated with SWRC determination. Unconfined compressive strength (UCS) tests and shear wave velocity measurements on CPB specimens provided complementary information regarding the rate of change of material properties attributable to cementation. An exponential maturity relationship commonly used for cement hydration was used to fit this data. The results showed that the SWRC evolved in a continuous manner from low to high suction ranges as cement hydration progressed. The evolution of the air entry value with hydration time ...

Overcoming Permanganate Stalling during Electromigration

AbstractElectrokinetic experiments were undertaken to transport permanganate (MnO4−) through a low permeability porous media. The experiments employed a one-dimensional apparatus in which MnO4− was electromigrated through a central porous media core. Two outer porous media cores separated the electrode reservoirs from the inner permanganate source and permanganate target reservoirs. By utilizing a pH-isolation technique, whereby electrolysis reactions occurring at electrodes were isolated, uniform and repeatable MnO4− electromigration was achieved. This result was compared with non-pH-isolated experiments (normal mode), which resulted in a stalled MnO4− electromigration front. The research also investigated potential stalling mechanisms, including voltage gradient nonlinearity through the central porous media core and the reduction of MnO4− to Mn2+. It was observed that the voltage gradient decreased as a result of MnO4− stalling; however, it was not considered a stalling mechanism. Results from Mn2+ anal...

Geotechnical systems that evolve with ecological processes

Geotechnical systems, such as landfills, mine tailings storage facilities (TSFs), slopes, and levees, are required to perform safely throughout their service life, which can span from decades for levees to “in perpetuity” for TSFs. The conventional design practice by geotechnical engineers for these systems utilizes the as-built material properties to predict its performance throughout the required service life. The implicit assumption in this design methodology is that the soil properties are stable through time. This is counter to long-term field observations of these systems, particularly where ecological processes such as plant, animal, biological, and geochemical activity are present. Plant roots can densify soil and/or increase hydraulic conductivity, burrowing animals can increase seepage, biological activity can strengthen soil, geochemical processes can increase stiffness, etc. The engineering soil properties naturally change as a stable ecological system is gradually established following initial construction, and these changes alter system performance. This paper presents an integrated perspective and new approach to this issue, considering ecological, geotechnical, and mining demands and constraints. A series of data sets and case histories are utilized to examine these issues and to propose a more integrated design approach, and consideration is given to future opportunities to manage engineered landscapes as ecological systems. We conclude that soil scientists and restoration ecologists must be engaged in initial project design and geotechnical engineers must be active in long-term management during the facility’s service life. For near-surface geotechnical structures in particular, this requires an interdisciplinary perspective and the embracing of soil as a living ecological system rather than an inert construction material.