Recent tailings dam failures around the world have highlighted the real risk posed by undrained slope failures. Undrained failures are fundamentally different to drained failures in the sense that different mechanisms are involved (i.e. a slope may be stable against drained failure but unstable against undrained failure). Popular methods to assess the stability of slopes against undrained failure involve the use of limit equilibrium analyses with both drained and undrained strengths assigned in the same analysis. A potential shortcoming of these methods is that no consideration is given to strain compatibility. In this study, a limit equilibrium based method where strain compatibility is maintained on the failure surface was developed. The method, referred to as the strain mobilisation method, considers a Mohr circle of stress at failure to determine the shear strength mobilised on the failure plane for use in the stability analysis as a function of the deviator stress imposed on the triaxial test result. The mobilisation of stress on a failure plane with strain was determined based on the stress-strain relationships observed during triaxial tests. A Factor of Safety (FoS) was used to express the stability of the slope as a function of the mobilised strain and the calculated FoS results obtained using the proposed method were compared to calculated FoS results using traditional methods. This was done for three tailings materials (gold, iron and platinum) for three specific hypothetical slopes. As an additional check, the proposed method was tested on Nerlerk sand, a well-known sand showing strain-softening behaviour during undrained shearing. It was found that, in general, as mobilised strain is increased, the FoS calculated using the proposed method converges to that of traditional methods so that there was no significant difference in calculated FoS between the current methods and the proposed method that does consider strain compatibility. This indicates that the proposed method provides FoS values comparable to those calculated using currently accepted methods where the failure surface passes predominantly through a single material type. For such a case, there does not appear to be a need to consider limit equilibrium methods where strain compatibility is maintained. The proposed method provides an indication of the amount of strain that may be expected to mobilise to provide the FoS. Given that this amount of strain is not excessive, the current methods which do not consider strain compatibility perform satisfactorily and can continue to be used / Dissertation (MEng (Civil Engineering))--University of Pretoria, 2021. / Civil Engineering / MEng (Civil Engineering) / Unrestricted

Orientador: Cesar Augusto Moreira / Resumo: Uma barragem de rejeitos é constituída por uma barreira física, cuja função é a acumulação de produtos residuais do processo industrial de beneficiamento mineiro. Os métodos de projeto para barragens de rejeitos diferem das barragens de retenção de água pela construção do barramento em diferentes etapas. Deste modo, os problemas de estabilidade em uma barragem são de grande importância, principalmente quando se trata de barragens construídas para armazenamento destes rejeitos de mineração, uma vez que a possibilidade de falhas não apenas ameaça a segurança das pessoas e da propriedade industrial, mas também pode causar impactos ambientais substanciais. No Estado de Minas Gerais, aproximadamente 30 km a sudoeste do município de Caldas, está localizada a Mina Osamu Utsumi de responsabilidade das Indústrias Nucleares do Brasil (INB), a qual tem um sistema para contenção de rejeitos provenientes da exploração e processamento de urânio, atualmente desativado. O Sistema consiste em uma barragem de rocha e terra a qual apresenta a existência de surgências no maciço rochoso à jusante do barramento, cujo fluxo ocorre em sistema fraturado. O objetivo do trabalho é reconhecer possíveis zonas de fluxo no interior e na base da barragem por meio de um conjunto de ensaios geofísicos de eletrorresistividade adquiridos com a técnica de tomografia elétrica através de modelos de inversão 2D e modelamento 3D. Foram adquiridas 5 linhas de tomografia elétrica com espaçamento entre eletrodos de 6 m... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: A tailings dam is constituted by a barrier, whose function is the accumulation of residual products from the mining beneficiation industrial process. Project methods for tailings dams differ from water retention dams by the construction of the dam in different stages. Thus, the stability problems in a dam are of great importance, especially when it comes to dams built for the storage of these mining waste, since the possibility of failures not only threatens the safety of people and industrial property, but also cause negative and substantial effects to the local environment. The Osamu Utsumi uranium mine, administrated by the state company Industrias Nucleares do Brasil (INB), is located 30 km southwest of Caldas, in the State of Minas Gerais, and its contention system of tailings, generated by the exploration and processing of uranium ore, is currently out of operation. The system is composed of a rock-soil dam, which presents the existence of water upwellings downstream in the bedrock, with water flux confined in fractures. This research aims the recognition of possible flux zones into the base of the dam through geophysical surveys applying DC Resistivity, in addition to Electrical Resistivity Tomography technique in the generation of 2D inversion models and 3D modeling. The acquisition was composed of five lines of electrical resistivity tomography with 6 m of spacing between electrodes, using Schlumberger array. The results are presented by 2D and 3D geophysical models ... (Complete abstract click electronic access below) / Mestre

Large quantities of tailings are produced during gold and coal mining activities. These tailings consist of ash dumps, waste rock dumps, in-pit deposits and any other heap, pile or accumulation of residue in the tailings or slimes dams. It has been reported that these tailings can have a significant impact on water quality in the vicinity of gold and coal residues in South Africa. Water quality deterioration in the vicinity of gold and coal mines in the Johannesburg and other areas has been reported. However, little information is available on the potential impact of residues on water quality near Roodepoort and Newcastle where gold and coal, respectively, are mined. The objective of this investigation was therefore to determine the potential impact of gold and coal mine residues on the environment in the vicinity of Roodepoort and Newcastle. Secondary objectives were to identify the metal constituents of gold and coal mine residues, to evaluate and define the current knowledge with regard to the short-term water quality impact of gold and coal residues in terms of concentration of metals leaching from the residues, to assess the potential impact of gold and coal tailings on the water environment within the study areas and to suggest methods to prevent pollution from taking place. Acid Base Accounting (ABA), Toxicity Characteristics Leaching (TCLP), Acid Rain Leaching Procedure (ARLP) and Inductively Plasma Coupled – Mass Spectrometry (IPC-MS) were used as tools to determine the potential impact of gold and coal tailings on the environment. Acid Base Accounting comprises two components that show the potential of the mine residue to produce acid mine drainage, that is, the total sulphur and the net neutralisation potential (NNP). It has been reported that any pyrite mine residue containing more than 0.5% total sulphur may generate acid mine drainage. Mine residues with a net neutralisation potential of less than zero ppt CaCO3 produce acid drainage. The acid base accounting results show that the gold and coal mine residues contain sulphur which has the potential to produce acid mine drainage. Lithium (Li), sodium (Na) magnesium (Mg), aluminium (Al), potassium (K), calcium (Ca), iron (Fe), manganese (Mn) and nickel (Ni) were identified to be present in the gold mine residue. The concentrations of some of the metals that leached from the gold residue according to the TCLP tests were as follows: Al (22 mg/L); Ca (242 mg/L); Fe (29 mg/L); Mn (88 mg/L) and Ni (87 mg/L). The metals that leached from the gold residue according to the ARLP results were as follows: Na (43 mg/L); Al (169 mg/L); Ca (246 mg/L); Fe (771 mg/L); Mn (16 mg/L) and Ni (11 mg/L). Higher concentrations of metals generally leached from the gold residue with the ARLP test than with the TCLP test. The sulphate concentration up-stream of the gold residue was determined at 225 mg/L. This concentration increased to 3 490 mg/L at the decanting point and to 11 577 mg/L downstream of the decanting point. The surface and possibly groundwater are therefore polluted with sulphates. Lithium (Li), sodium (Na), magnesium (Mg), aluminium (Al), potassium (K), calcium (Ca), iron (Fe), manganese (Mn) and nickel (Ni) were identified to be present in the coal mine residue. The concentrations of some of the metals that leached from the coal residue according to the TCLP tests were as follows: Al (3 mg/L); Ca (56 mg/L); Fe (0.21 mg/L); Mn (1 mg/L) and Ni (0.082 mg/L). The metals that leached from the coal residue according to the ARLP test results were as follows: Na (3 mg/L); Al (15 mg/L); Ca (136 mg/L); Fe (0.91 mg/L); Mn (1 mg/L) and Ni (0.07 mg/L). Higher concentrations of metals generally leached from the coal residue with ARLP test than with the TCLP test. The sulphate concentration up-stream of the coal residue was determined at 26 mg/L. This concentration increased to 3 615 mg/L at the decanting point and to 6 509 mg/L downstream of the decanting point. The surface and possibly groundwater are therefore polluted with sulphate. The upstream Na (26 mg/L), Ca (41 mg/L), Fe (0,02 mg/L), Mn (3 mg/L) and Ni (0.065 mg/L) concentrations were low in the case of the gold residues. These concentrations at the decanting point were: Na (289 mg/L); Ca (266 mg/L); Fe (0.2 mg/L); Mn (0.01 mg/L) and Ni (2 mg/L). Fifty metres downstream these concentrations were: Na (140 mg/L); Ca (389 mg/L); Fe (722 mg/L); Mn (395 mg/L) and Ni (15 mg/L). There was a significant increase in the metal concentration from up-stream of the gold residue, to the decanting point and further downstream of the gold residue. The surface and possibly ground water are therefore polluted by the metals leaching from the gold residue. The upstream Na (5 mg/L), Ca (8 mg/L), Fe (0,12 mg/L), Mn (0.015 mg/L) and Ni (0.05 mg/L) concentrations were low in the case of the coal residues. These concentrations at the decanting point were: Na (189 mg/L); Ca (337 mg/L); Fe (68 mg/L); Mn (13 mg/L) and Ni (0.06 mg/L). Fifty metres downstream these concentrations were: Na (65 mg/L); Ca (129 mg/L); Fe (0.48 mg/L); Mn (5 mg/L) and Ni (0.06 mg/L). There was a significant increase in the metal concentration from up-stream of the coal residue, to the decanting point and further downstream of the coal residue. The surface and possibly ground water are therefore polluted by the metals leaching from the coal residue. The gold and coal mine residues are polluting the surface and possibly ground water. Therefore, in order to ameliorate the current status within the Roodepoort and Newcastle catchments, mitigation and management measures such as that the residues should be covered and capped with soil material that would prevent infiltration of the oxygen and rain water into the soil, are recommended. A more comprehensive water quality analysis of the surroundings of the residues is also suggested to be able to better quantify the extent of the problem. Copyright / Dissertation (MSc)--University of Pretoria, 2011. / Chemical Engineering / Unrestricted

The enrichment of ore produces large amounts of sulfur and metal-rich residual waste called tailings, which need to be deposited and stored for a long time. When the tailing is oxidized, large amounts of protons and metals are dissolved and diffuse to the groundwater. This poses a major environmental threat to biological life forms in the downstream ecosystem (Karltorp, 2008). In this study, leachate plumes and geological structures surrounding the tailings impoundment at the Kringelgruvan mine in northern Sweden have been successfully mapped using geophysical methods. Three methods have been used in parallel, slingram, ground penetrating radar (GPR) and electrical resistivity measurements, known as continuous vertical electrical sounding (CVES). The resulting data from GPR and CVES have been co-analyzed using Matlab. Algorithms have been produced that plots underground structures from CVES and compares them with interpreted structures from GPR. Studies have shown that the GPR is more sensitive than CVES to local variations of substructures when used in shallow soil cover, while CVES gives considerably more information regarding localization of the leachates and other electrically conductive materials, such as ore. Slingram EM31 has been shown to be the most time-efficient method to localize groundwater flow.

Cemented paste backfill (CPB) is a novel technology developed in the past few decades to better manage mining wastes (such as tailings) in environmentally friendly way. It has received prominent interest in the mining industry around the world. In this technology, up to 60% of the total amount of tailings is reused and converted into cemented construction material that can be used for secondary support in underground mine openings (stopes) and to maximize the recovery of ore from pillars. CPB is an engineered mixture of tailings, water, and hydraulic binder (such as cement), that is mixed in the paste plant and delivered into the mine stopes either by gravity or pumping. During and after placing it into the mine stopes, the performance of CPB mainly depends on the role of the hydraulic binder, which increases the mechanical strength of the mixture through the process of cement hydration. Similar to other fine-grained soils undergoing cementations, CPB’s behavior is affected by several conditions or factors, such as cement hydration progress (curing time), chemistry of pore water, mixing and curing temperature, and filling strategy. Also, it has been found that fresh CPB placed in the mine stopes can be susceptible to many geotechnical issues, such as liquefaction under ground shaking conditions. Liquefaction-induced failure of CPB structure may cause injuries and fatalities, as well as significant environmental and economic damages. Many researches studied the effect of the aforementioned conditions on the static mechanical behavior of CPB. Other researches have evaluated the liquefaction behavior of natural soils and tailings (without cement) during cyclic loadings using shaking table test technique. Only few studies investigated the CPB liquefaction during dynamic loading events using the triaxial tests. Yet, there are currently no studies that addressed the liquefaction behavior of CPB under the previous conditions by using the shaking table technique. In this Ph.D. study, a series of shaking table tests were conducted on fresh CPB samples (75 cm × 75 cm ×70 cm), which were mixed and poured into a flexible laminar shear box (that was designed and build for the purpose of this research). Some of these shaking table tests were performed at different maturity ages of 2.5 hrs, 4.0 hrs, and 10.0 hrs, to investigate the effect of cement hydration progress on the liquefaction potential of CPB. Another set of tests were conducted to assess the effect of the chemistry (sulphate content) of the pore-water on the cyclic response of fresh CPB by exposing cyclic loads on couple of CPB models that contain different concertation of sulphate ions of 0.0 ppm and 5000 ppm. Moreover, as part of this study, series of shaking table test was conducted on CPB samples that were prepared and cured at different temperatures of 20oC and 35oC, to evaluate the effect of temperature of the cyclic behavior of CPB. Furthermore, the effect of filling strategy on the cyclic behavior of fresh CPB was assessed by conducting set of shaking tables tests on CPB models that were prepared at different filling strategies of continuous filling, and sequential or discontinuous (layered) filling. The results obtained show that CPB has different cyclic behavior and performance under these different conditions. It is observed that the progress of cement hydration (longer curing time) enhances the liquefaction resistance of CPB, while the presence of sulphate ions diminishes it. It is also found that CPB mixed and cured in low temperature is more prone to liquefaction than those prepared at higher temperatures. Moreover, the obtained results show that adopting the discontinuous (layered) filling strategy will improve the liquefaction resistance of CPB. The finding presented in this thesis will contribute to efficient, cost effective and safer design of CPB structures in the mine areas, and will help in minimizing the risks of liquefaction-induced failure of CPB structures.

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The mining industry is a major economic driver and job creator for many countries. However, mining is associated with geo-hazards and environmental issues, such as the disposal of large volumes of waste, acid mine drainage, and ground subsidence. As a result, efficient mining waste management is crucial for sustainable development. The geotechnical, economic, and environmental benefits of cemented paste backfill (CPB) have piqued the interest of researchers and academicians worldwide, making it an essential aspect of underground mining management. CPB is a thickened cementitious combination of dewatered tailings (70 - 85 wt.%), binders (usually 3 to 8% wt.%), and water used to backfill mine waste into underground mining stopes. Despite being used in small amounts, the cost of cement makes up to 80% of the cost of backfilling operations. In addition, clinker production accounts for 5-8% of global human created carbon dioxide (CO₂) emissions. This predicament necessitates the development of a viable alternative to cement. Partially substituting cement with supplementary cementitious materials like fly ash, blast furnace slag, natural pozzolans, and other materials has been increasingly prevalent in CPB. It is evident that the addition of slag to cement can increase the mechanical strength of CPB at the advanced ages but decreases the strength and suction development due to the slow reaction kinetics in the CPB at the early ages, which may negatively affect the mechanical stability of the CPB, mining cycle, and safety of mineworkers. Moreover, the supply of these materials is limited and may not be enough for the future needs of the industry. Furthermore, there has been a surge in interest in using limestone powder (LS) owing to its abundance, low cost, and lack of environmental costs which are associated with Portland cement - Type 1 (PCI). The addition of LS accelerates hydration at the early ages, thus resulting in high early strength, but the dilution effect can reduce the late strength. The combination of LS and slag in a ternary blended cement can be potentially used as a binder for CPB with acceptable strength development at the early and advanced ages while lowering the cost of the CPB and the carbon footprint of the mining industry. Nevertheless, the rheology, mechanical strength, and stability are important key performance quality criteria for CPB; however, the effect of ternary cement blends on these parameters is not well known. In this research program, the impact of the binary and ternary cement blends on (i) the fresh properties of CPB, such as the rheological properties (yield stress, viscosity) and setting time, and (ii) the strength and suction development of CPB are investigated. To understand the effect of substituting slag with LS in the binary binder in the first phase of the study, binary binders with two differ-ent PCI: Slag proportions of 50/50 and 80/20 are examined with no limestone, followed by replacing slag with an increasing amount of LS from 0 to 20 wt. % of the total binder, with a constant cement content, over a period of 4 hrs (0, 0.25, 1, 2, and 4 hrs) of curing at room temperature. In the second phase, the effect of a ternary binder (PCI-Slag- LS) with varying proportions on the suction development and the mechanical behavior of hardened CPB is investigated over a curing period of up to 90 days. The changes in strength of these binary and ternary binders on the CPB sample are tested for 1, 3, 7, 28, 60, and 90 days. An unconfined compression test (UCS) is conducted to evaluate the strength development. The microstructure of the mixes is examined through mercury intrusion porosimetry (MIP) for changes are validated through monitoring for the development of hydration and suction, electrical conductivity (EC), and temperature, which is carried out for up to 30 days. This is followed by a microstructure analysis with a thermogravimetric/differential thermogravimetric test on fresh and hardened samples. The results of the first phase show that an increase in the percentage of substituted cement in the binary binder (from PCI/Slag 80/20 to 50/50) increases the yield stress of the CPB but decreases the viscosity of the mix. However, the addition of LS as a substitution for slag shows a considerable decrease in the yield stress of the control mix with an increase in viscosity with increasing dosages of LS, thus indicating an improvement in the flowability of CPB. The second phase results indicate that the slow hydration kinetics of slag influences early age suction and strength changes in the binary sample with a high slag content (50/50); however, its latent hydraulic and pozzolanic properties enhance strength gain after 28 days. The addition of 5% limestone to the ternary blend increases the strength gain by up to 7 days compared to the binary control samples. Indeed, the presence of LS influences the rate of hydration of cement and slag through both physical (filler, nucleation, dilution) and chemical (hydrate) effects. However, substituting more than 10% LS for slag affects the mechanical performance at all ages. Overall, up to 50 wt.% slag and 10 wt.% limestone with cement as a ternary binder can be used without significant compressive strength loss. This study demonstrates that the partial substitution of ordinary Portland cement (OPC) with varying percentages of slag and LS is complementary, and overall, the interaction of slag and LS is observed. The optimal use of LS and slag in a ternary system may serve as a sustainable alternative to the commonly used OPC and PCI/Slag binders, thereby reducing the energy consumption and carbon footprint associated with cement. The findings of this study will ultimately help to develop a better understanding of the impact of ternary blends with increasing percentages of LS on the rheology and setting time of CPB mixes and mechanical strength changes in designing an efficient mixing plant, particularly its transport system.

The mining industry contributes to enormous amounts of waste all over the world, which places high demands on tailings dams. In Sweden, there are strict regulations regarding the management and treatment of tailings dams, but some dams that were built before these regulations existed still pose a threat to the environment. Lilla Bredsjön in Dalarna, Sweden, is an example of an old tailings dam that has not been treated properly. Boliden, which is the owner of the dam, and the county board of Dalarna are currently in meetings regarding the responsibilities of old polluted sites in Garpenberg, where Lilla Bredsjön is included. This study aims to investigate possible remediation measures of the dams at Lilla Bredsjön in order to achieve long- term stability upon closure of the facility. Lilla Bredsjön is 350 000 m2 large and contains three impoundments; dam 1 to the east, dam 2 to the south and dam 3 to the north. Dam 1 is seven meters at its highest point whereas dam 2 and 3 both are three meters high. All of the dams are classified to a consequence level of 3, meaning that the consequences in case of a failure would be negligible. They are built using the centerline method with impermeable cores of moraine. The tailings are partly covered with sludge mixed with biomass ash, but no proper closure method have been implemented. A geotechnical investigation was conducted to obtain more information about the geometry, construction and foundation of each dam. It could be observed that dam 1 consisted of a moraine core and support fill, both constructed using the Christmas tree principle. Dam 2 consisted of a coarse and fine filter in the upstream part of the dam, whereas the rest of the dam consisted of a moraine core. Dam 3 consisted of tailings and a moraine core, and was like dam 1 also constructed using the Christmas tree principle. The downstream slopes of all dams are moreover very steep with slope angles of approximately 34o. Seepage and stability calculations were therefore performed in the software GeoStudio. In GeoStudio, two different analysis tools were used; SEEP/W to calculate seepage and SLOPE/W to calculate slope stability. Four hydraulic load cases were analyzed in the calculations: present conditions, flooded conditions, potential maximum seepage and present conditions with lower permeability of the tailings. The result shows that all of the dams are unstable in their present con- ditions, both with original and lower permeability of the tailings, as well as at flooded conditions. The required safety factor is however fulfilled at potential maximum seepage. The stability highly depends on the pore pressure inside the dams, which in turn depends on the choice of closure method. Based on the result from the stability calculations it can be concluded that if a dry cover is chosen as the closure method, it will probably be enough to reinforce the existing dams to achieve long-term stability. If, on the other hand, a raised groundwater level is chosen, more extensive solutions in terms of for example completely new impoundments directly downstream of the existing ones are most likely needed. Before a decision regarding the closure method is taken, it is hence very difficult to come to a conclusion regarding suitable remediation measures of the dams. / Gruvindustrin bidrar med enorma mängder avfall över hela världen, vilket ställer höga krav på gruvdammar. I Sverige finns det strikta förordningar gällande hanteringen och efterbehandlingen av gruvdammar, men en del dammar som byggdes innan dessa förordningar existerade utgör fortfarande ett hot mot miljön. Lilla Bredsjön i Dalarna är ett exempel på en gammal gruvdamm som inte blivit efterbehandlad tillräckligt. Boliden, som äger dammen, har möten med Länsstyelssen Dalarna angående ansvarsförhållanden gällande gamla förorenade områden i Garpenberg där även Lilla Bredsjön ingår. Denna studie syftar till att undersöka möjliga efterbehandlingsmetoder av dammarna vid Lilla Bredsjön för att uppnå långtidsstabilitet i samband med nedläggning av anläggningen. Lilla Bredsjön är 350 000 m2 stor och består av tre uppdämningar; damm 1 i öst, damm 2 i söder och damm 3 i norr. Damm 1 är sju meter som högst medan damm 2 och 3 båda är tre meter höga. Samtliga dammar är klassificerade till konsekvensklass 3, vilket betyder att konsekvenserna i händelse av ett dammbrott är försumbara. Dammarna är byggda med uppåtmetoden och har tätkärnor av morän. Magasinet är delvis täckt med slam utblandat med biobränsleaska, men ingen riktig efterbehandling har utförts. En geoteknisk undersökning genomfördes för att erhålla mer information om geometrin, konstruktionen och grundläggningen av varje damm. Det observerades att damm 1 består av en tätkärna av morän samt stödfyllning som båda konstruerades med julgransprincipen. Damm 2 består av ett grov- och finfilter i uppströmsdelen av dammen medan resten av dammen består av en tätkärna av morän. Damm 3 består av anrikningssand och en tätkärna av morän, och var liksom damm 1 konstruerad med julgransprincipen. Nedströmsslänterna på alla dammar är väldigt branta med en vinkel på ungefär 34o. Genomströmnings- och stabilitetsberäkningar genomfördes därför i programmet GeoStudio. I GeoStudio användes två olika verktyg; SEEP/W för att beräkna genomströmning och SLOPE/W för att beräkna släntstabilitet. Fyra olika hydrauliska belastningsfall analyserades i beräkningarna: normalfallet, överdämning, dimensionerat läckage och normalfallet med lägre permeabilitet på anrikningssanden. Resultatet visar på att samtliga dammar är instabila i deras nuvarande skick, beräknat på normalfallet med både ursprunglig och lägre permeabilitet på anrikningssanden, samt vid överdämning. Den erforderliga säkerhetsfaktorn är däremot uppfylld vid dimensionerat läckage. Stabiliteten är starkt beroende av portrycket inuti dammarna, vilket i sin tur är beroende av vilken efterbehandlingsmetod som väljs för magasinet. Baserat på resultatet från stabilitetsberäkningarna kan det konstateras att om torrtäckning väljs som efterbehandlingsmetod så räcker det förmodligen med att förstärka nuvarande dammar för att uppnå långtidsstabilitet. Om, å andra sidan, en förhöjd grundvattennivå väljs behövs med största sannolikhet en mer omfattande lösning, till exempel helt nya dammar direkt nedströms de nuvarande. Innan ett beslut gällande efterbehandlingsmetod av magasinet har tagits är det alltså väldigt svårt att dra en slutsats gällande den mest lämpliga efterbehandlingsmetoden av dammarna.

Assessment of post-liquefaction residual strength is needed for the development of empirically-based, predictive correlations for earthquake engineering design. Previous practice commonly assigned negligible strengths to liquefied materials for engineering analysis, producing overly-conservative designs. Increasingly available case history data, and improved analytical tools have allowed for more accurate and less overly-conservative estimation of soil residual strength, improving empirical predictive models. This study provides a new case history to the limited suite of (approximately 30) liquefaction failure case histories available for post-liquefaction in-situ strength predictive correlations. This case history documents the Las Palmas gold mine tailings dam failure, resulting from seismic-induced liquefaction during the moment magnitude 8.8 February 27, 2010 Maule, Chile earthquake; the sixth largest since 1900. Forensic analysis provides reasonably well-constrained values of 1) back-calculated representative post-liquefaction residual strength, 2) representative penetration resistance, and 3) representative vertical effective stress along the suspected liquefied failure surface. This study employs the incremental momentum method to incorporate momentum effects of a moving soil mass. The incremental momentum method requires a series of cross sections animating the geometry of failure progression from initiation to termination, converging on the observed final geometry. Using interpreted soil strength characteristics, an iterative procedure approximates the back-calculated value of post-liquefaction residual strength. Findings of this case history plot well with existing empirical deterministic regression charts and are in general agreement with previous, related efforts. Results yield representative, well-constrained values of: 1) post-liquefaction residual strength ≈ 173 psf, 2) penetration resistance of N1,60,CS ≈ 5 and N1,60 ≈ 2.5, and 3) vertical effective stress ≈ 4,300 lb/ft2, or ≈ 2.0 atm.

Cemented paste backfill (CPB) technology is widely used in the mining industry as an effective means of tailings disposal. CPB is a mixture of tailings, binder, water, and additional admixtures when required. It is prepared in a mixing plant on the ground surface and then transported into the mine cavities through pipelines either by gravity and/or using pumps. To ensure efficiency during transportation and avoid pipe clogging (which can cause unnecessary delays and loss of productivity), fresh CPB must have sufficient flowability. To achieve that, high-range water reducing admixtures, also known as superplasticizers, are usually added to the CPB during mixing. These admixtures are widely used in the construction industry due to their ability to improve flowability without undermining other important engineering properties. However, their influence on the rheology, mechanical strength and environmental performance (reactivity and permeability) of CPB is not fully understood. Thus, experimental studies were conducted to investigate the effects of superplasticizers on the performance properties of cemented paste backfill at different curing temperatures. Yield stress and viscosity of fresh CPB cured for 0, 1, 2, and 4 hours were measured using a vane shear device and a Brookfield Viscometer respectively. Unconfined compressive strength (UCS) of samples cured for 1, 3, 7, and 28 days was determined in accordance with ASTM - C39. Superplasticizer contents were varied as 0%, 0.125%, and 0.25% of the total weight of the CPB. Preparations and curing of the specimens were performed at controlled conditions of 2, 20, and 35 °C to investigate the effect of ambient or curing temperatures. To have a better understanding of the environmental performance of CPB containing superplasticizer, reactivity, and hydraulic conductivity up to 90 days of curing were also investigated. The reactivity was measured using oxygen consumption test while hydraulic conductivity was measured using flexible wall permeability test. Microstructural analyses (thermogravimetric analyses, X-Ray diffraction, and mercury intrusion porosimetry) and monitoring tests (pH, zeta potential, electrical conductivity, and matric suction) were carried out to understand the principles behind the changes of the observed properties. The obtained results show that superplasticizer dosage and temperature variation have significant effects on the rheology, strength development, hydraulic conductivity and reactivity of the CPB. The polycarboxylic ether-based superplasticizer significantly reduces the yield stress and viscosity by creating strong electrostatic repulsion between the solid particles in the CPB and by steric hinderance. The CPB containing the superplasticizer remains fluid for longer period (as compared with the CPB without superplasticizer) due to the retardation of binder hydration. However, high curing temperature induces faster cement hydration, which thickens the fresh CPB. The unconfined compressive strength (UCS) of the CPB containing superplasticizer was observed to be lower in the early age (up to 7 days), which is also attributed to retardation of the binder hydration. At later ages, the superplasticizer improves the mechanical strength as the binder hydration accelerates and the solid particles self-consolidate. Coupled THMC processes in the CPB showed the role played by the changes in electrical conductivity, volumetric water content, matric suction, and temperature on the development of mechanical strength of the CPB containing superplasticizer. Similarly, addition of the superplasticizer in the CPB decreases both the hydraulic conductivity and reactivity of CPB, thus improving its environmental performance. The improvement is largely attributed to enhanced binder hydration and self-consolidation which decrease the porosity of the CPB. Increasing the curing temperature was found to magnify the improvement of the CPB properties by inducing faster binder hydration. The findings from this study will undoubtedly inform the design of CPB structure with better mechanical stability and environmental performance.