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Recent Submissions

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    Statistical modelling of the leaching of Polycrystalline Diamond Compacts (PDCs) for integration into the product design process
    (University of the Witwatersrand, Johannesburg, 2025) Ndlovu, Andrew; Cornish, Lesley A.
    This thesis investigated the optimisation of the high-pressure high-temperature (HPHT) sintering process for manufacturing polycrystalline diamond compacts (PDCs) with enhanced leachability and performance. A primary focus was the precise control of the key sintering parameters: pressure, temperature and holding time. The research addressed the challenges of accurate parameter measurement under extreme HPHT conditions by developing and validating a novel in situ pressure calibration method using a cubic press. The method, based on real-time volume change measurements of the PDC sample and the Murnaghan equation of state, gave improved accuracy and material specificity than conventional fixed-point techniques. Furthermore, the study examined the influence of thermocouple placement on temperature measurement accuracy, demonstrating the critical importance of optimal positioning for reliable temperature monitoring. The individual and combined effects of pressure, temperature and holding time on PDC leach rate were extensively analysed. Experiments showed that increased pressure significantly reduced leach rate, attributed to enhanced densification, reduced porosity and smaller cobalt pools. Similarly, higher sintering temperatures decreased leachability due to greater densification and altered pore morphology. Holding time also played a role, with longer durations promoting densification and thus reducing leach rate, particularly at lower temperatures. Statistical models, including linear and polynomial regression, were developed and validated to predict leach rate based on individual parameter variations. To further understand the complex interaction between the sintering parameters, a multiple regression model was developed, revealing a significant interaction between holding time and temperature. Dominance analysis was employed to establish a hierarchy of parameter importance, identifying pressure as the most influential factor in controlling leach rate, followed by temperature, with holding time having a lesser, yet significant, indirect effect. The developed models coupled with the insights into parameter interactions and relative importance provided a comprehensive framework for optimizing the HPHT sintering process of PDCs.
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    A dynamic long-term and medium-term integrated open-pit mine production scheduling system based on the genetic algorithm
    (University of the Witwatersrand, Johannesburg, 2025) Muke, Pathy Musema; Tholana, Tinashe; Musingwini, Cuthbert; Ali, Montaz
    Open-pit mine production scheduling (OPMPS) is systematically divided into long-term (LT), medium-term (MT) and short-term (ST) scheduling. There is a clear interdependence between these scheduling horizons. An integrated optimisation approach to these scheduling horizons is essential since the horizons can be linked together to ensure spatial and temporal scheduling alignment. The linkages include feedback loops to enable dynamic updates of the scheduling system to incorporate changes whenever they occur. Traditionally, optimisation techniques have often focussed on solving LT, MT and ST scheduling horizons independently from each other, despite there being some shortcomings associated with the independent optimisation of LT, MT and ST production schedules. Current studies have not adequately presented an integrated scheduling system in which all horizons are incorporated and correlated to mitigate scheduling misalignment. Generating independent mine production schedules is known to be detrimental to a project’s net present value (NPV) due to scheduling misalignment and inconsistency. This raises the question of whether a solution approach can be developed to dynamically integrate scheduling horizons to overcome challenges associated with spatial and temporal misalignment and inconsistency in scheduling. This thesis developed a new approach to dynamically integrate LT and MT production scheduling to improve temporal and spatial alignment while attempting to maximise NPV as opposed to when these scheduling horizons are optimised independently. The reason for integrating LT and MT production schedules is to provide a framework to support ST scheduling which introduces more complexity and variability to account for operational planning. Firstly, the thesis expressed the scheduling problem as a mixed integer programming (MIP) problem to integrate LT and MT production scheduling horizons. The MIP model was employed in this thesis because it can formulate practical extraction sequences and incorporates continuous variables, compared to linear programming (LP) and integer programming (IP) models. Then, the thesis developed an integrated solution approach based on the genetic algorithm (GA), which is a metaheuristic algorithm, to solve the integrated LT and MT MIP scheduling problem. GA was selected because it can provide solutions to problems that are unattainable with exact or deterministic techniques. GA can also incorporate geological, economic and operational uncertainties, although in this thesis the focus was on operational uncertainty. Since GA is a stochastic technique, the combined MIP model and GA approach generated stochastic solutions that accounted for constraints and uncertainties associated with mining rate, processing rate and ore grade target. The combined MIP model and GA approach was coded in Python programming language and validated by applying it to a block model downloaded from Geovia Surpac® software. Results show that when the integrated LT and MT production scheduling system was solved using the combined MIP model and GA approach, NPV improved by 2.60% compared to when the scheduling horizons were independently optimised. In addition, spatial and temporal alignment between the LT and MT production schedules was achieved. Therefore, the thesis demonstrated that a combined MIP model and GA approach can dynamically integrate LT and MT production scheduling horizons to improve NPV as opposed to when these scheduling horizons are independently optimised. The combined MIP model and GA approach was also applied to the Newman and Zuck Small block models downloaded from MineLib, to validate the solution approach. The results were compared to some of the MineLib existing solutions obtained by applying the TopoSort heuristic algorithm to solve a LP relaxation model of the scheduling optimisation problem. The integrated LT and MT scheduling system solved using the combined MIP model and GA approach had a 10.86% higher and a 4.86% lower NPV compared to the TopoSort algorithm when applied to the Zuck Small and Newman block models, respectively. Although the aim was to improve NPV, a lower NPV was generated during the validation using the Newman block model. This is because GA can generate a different outcome each time it is run on the block model, due to its stochastic solution process. Another run of the combined MIP model and GA approach on the same block model with the same inputs may generate a higher NPV. However, in both instances, the combined MIP model and GA approach delivered an integrated production schedule that ensured that spatial and temporal alignment was achieved between LT and MT production schedules. Future research may focus on developing solution approaches that can fully integrate LT, MT and ST production scheduling horizons, and incorporate factors like stockpiling and blending to further improve scheduling effectiveness. Further research could also investigate the fine-tuning of GA parameters to ensure that the application of the combined MIP model and GA approach can consistently generate higher NPVs.
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    Estimating the Potential Residual Expansion Due to Alkali Silica Reaction (ASR) in In-Service Reinforced Concrete Structures
    (University of the Witwatersrand, Johannesburg, 2025) Kanjee, Janna Prakash; Otieno, Mike; Ballim, Yunus
    This thesis investigated the extent of deterioration caused by alkali-silica reaction (ASR) in a 78-year-old reinforced concrete (RC) railway bridge in Johannesburg. Five assessments were conducted to characterise and quantify the extent of ASR damage. These included 2D potential residual expansion measurements at a micro-scale level using digital image correlation (DIC) and manual strain readings, focusing on localised expansion at the aggregate and cement paste interface. The study also explored correlations between micro-scale ASR strain and macro scale deformations in the concrete. The results showed that an on-site visual assessment identified varying degrees of ASR damage, leading to the extraction of concrete cores from different locations on the bridge. A modified damage rating approach, using a grid of 2 cm squares and the naked eye, was used to assess the extent of ASR damage based on 2D cylindrical images of each core's surface. Petrography and micro-XRF spectrometry confirmed that ASR was the primary cause of deterioration, characterised by cracks radiating from reactive aggregates and ASR gel presence. With respect to alkali reserves, concrete from the slightly damaged interior region had the highest sodium oxide equivalent content, 3.2 kg/m³. This value falls above the 2.0 kg/m3 limit for Witwatersrand aggregates, thereby validating the susceptibility of the concrete to ASR deterioration. The elastic modulus decreased with increased ASR damage, aligning with existing literature. Compared to slightly damaged concrete 18.0 GPa, moderately and severely damaged concrete had a 22.2% and 31.1% reduction in elastic modulus, respectively. Due to technical challenges, efforts to employ DIC for micro-scale residual expansion strains were unsuccessful. However, manual strain readings indicated that local equivalent normal strains were generally higher, and shear strains were generally lower than their respective bulk strains. The study found that the potential residual expansion of concrete displayed similar magnitudes of bulk expansion despite varying surface damage and alkali availability. Surprisingly, the potential residual micro-expansion displayed a similar trend. The five assessments conducted in this study provided a general basis for quantifying the extent of ASR deterioration in a 78-year-old RC railway bridge. This approach can be used as a basis for the condition assessment of existing structures subject to ASR deterioration. Engineers can use the results to model and understand how ASR-affected concrete impacts the structure's behaviour and its remaining service life. The findings, in particular of the residual potential expansion, emphasised the crucial role of moisture in facilitating future expansion and damage in the concrete due to ASR. Furthermore, owners of RC structures can obtain information about how long their RC structures, subject to ASR deterioration, may last before any repair is envisaged.
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    Intersecting rationalities: Negotiating a process of inclusion between the minibus taxi industry and the BRT system
    (University of the Witwatersrand, Johannesburg, 2024) Gavrilov, ZethuZonke Bella; Todes, Alison; Harrison, Philip
    The thesis investigates the politics and everyday processes of public transport reform in South Africa through a case study of the City of Ekurhuleni’s Integrated Rapid Public Transport Plan (IRPTP) process and the interim negotiations between the City and the Ekurhuleni Taxi Industry between 2016 and 2022. The process and negotiations that unfolded aimed at the implementation of a BRT system in South African cities including Ekurhuleni. The study explores how differences and conflicts were addressed between the two most powerful stakeholders involved in this process, the City of Ekurhuleni and the local taxi industry. The analysis takes on a “sympathetic critique” of Watson’s (2008) notion of conflicting rationalities and a later review by De Satgé and Watson (2018). The study is positioned closer to the idea of entangled rationalities in De Satgé and Watson (2018) It shows that there is more intersection and mutuality between the state and informal actors than is presented by Watson (2008). The study adopts a qualitative approach and various methodologies such as participant observation, interviews, and archival analysis. To unveil the crevices between these two powerful stakeholders, the researcher employed participant observation as one of the main methods. The researcher spent a year each in both institutions; first as a representative of the city government and subsequently as a representative of the taxi industry. The study finds out that disagreements between the city government and the taxi industry had less to do with conflicting views and values about planning and development, as proposed in the theories of conflicting rationality, and more to do with competition and power struggles. There was a limitation for the potential for mutuality and collaboration to take place as the scope of the transit reform had already been decided by the City of Ekurhuleni prior to consultation, and that the negotiations were around the implementation of a pre-defined idea without the possibility of open debate. Nevertheless, while the negotiation stalled it did not spiral into violence. This case study is significant in that serves as a case study for conflict mitigation and exploring potential for mutuality and more collaborative forms of engagement in cases of extreme conflict in urban development projects.
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    Facilitators and barriers to antiretroviral therapy adherence among adolescents and young adults in rural South Africa
    (University of the Witwatersrand, Johannesburg, 2025) Sithole, Samkelo Bhekinkosi; Sodo, umla Pamella
    Background: South Africa’s antiretroviral therapy (ART) programme is regarded as one of biggest in the world, to combat the effects of Human Immunodeficiency Virus (HIV) and Acquired Immuno-deficiency Syndrome (Aids) among infected and affected rural communities. The notable challenge on the programme was related to the sub-optimal adherence to ART among vulnerable groups including adolescents and young adults (AYA), who remained viral unsuppressed despite being on ART. The available literature has made recommendations for future research studies to explore factors hindering adherence to ART to adolescents and young adults living in rural areas. Aims: To explore multi-level facilitators and barriers to ART adherence among AYA living with HIV/Aids in rural South Africa. Methodology: A cross-sectional study using a qualitative approach was used to explore the phenomenon. A convenient sampling method was employed to recruit a maximum of 20 participants or until data saturation was reached. Data was collected through in-depth interviews using audio recorder tape lasting about 25-45 minutes per session, which were transcribed verbatim. Thematic analysis using deductive and inductive approaches was used to analyse data and interpret the research study findings or results. As an Operational Manager at Othobothini Community Health Centre, I approached this study with insider knowledge of the healthcare system, while using reflexive and participant-centred strategies to minimise bias and ensure that the voices of adolescents and young adults living with HIV/AIDS remained central to the research. Findings: The most evident experiences explored in the study relating to ART adherence barriers from intrapersonal to structural level were medication related side effects and lack of autonomy, non-disclosure of HIV status, stigma and discrimination, overarching priorities, and lastly, adolescence stage largely influenced the ability to efficient adhere to ART among targeted groups. The common facilitators explored among AYA from intrapersonal to structural level were fear of getting sick or death, family support system, normalisation and shared experiences, positive attitude of healthcare workers, and lastly, technology innovation such as manual and digital reminders played a significant role in promoting optimal ART adherence. iii Conclusion: Facilitators and barriers to ART adherence have several sources of origin, some are health system and process induced. Therefore, creating an environment enabling full disclosure, maintaining healthcare worker professionalism, and demand creation for HIV information can help eradicate stigma related factors. Furthermore, multisectoral stakeholders need collaboration to strengthen adherence counselling, family support, education, and continuous supervision at homes.