3. Electronic Theses and Dissertations (ETDs) - All submissions

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    Excavating through the kalahari group rock masses: practical experience from a small-scale shaft sinking project
    (2019) Mateveke, Raymond; Mateveke, Raymond
    Tunnelling projects in the weak Kalahari rock masses of the Northern Cape, South Africa present significant design challenges for both large and small-scale excavations. Most of the design of tunnelling and support carried out at present in the Kalahari is based on experience, analytical and empirical methods. The approach typically makes use of limited geotechnical information from the project site.The Kalahari basin is a complex geotechnical environment. Tunnelling projects are sensitive to the variable ground and groundwater conditions. A detailed site investigation to establish the geological and geotechnical model is critical in the selection of the appropriate excavation method and tunnel design. A review of early tunnelling projects revealed that in situ stresses and water infiltration is a long-term stability concern for inadequately lined tunnels through the red clay and weathered rock masses.This project explores the use of numerical modelling to predict the expected failure modes of the weak rock masses, with emphasis placed on concrete liner support for maintaining stability. The support models are analysed using 2D numerical models to determine the Factor of Safety of the liner. A support design criterion for reinforced and unreinforced concrete is introduced and appliedto the models to evaluate the lining thickness. The effect of using 2D plane strain models instead of3D analysis was also investigated. The total displacement of numerical models built in RS2 wascompared to RS3 models.
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    Evaluation of the deformation in the tailrace tunnel of the Ingula pumped storage scheme
    (2018) Moyo, Amanda
    In the design phase of a project it is imperative for engineers to have a solid understanding of the rock mass behaviour. Geotechnical investigations consisting of field and laboratory tests need to be conducted to determine the rock mass parameters in order to predict deformations, during the construction phase. Rock mass classification systems should also be employed during the geotechnical investigation to quantify the quality of the rock mass. Although the modulus of deformation of a rock mass is of critical importance for a geotechnical project, the use of in situ tests to determine the parameter is an extremely difficult, time consuming and expensive process. As a result empirical equations have been used by professionals to determine the modulus of deformation. The implementation of a monitoring programme consisting of geotechnical instrumentation and mappings during the construction phase is also critical in the construction phase to validate design assumptions and long term deformation. Continuous data collection entailing geological mapping and the application of rock mass classification systems assists in reducing geological uncertainties, determining rock mass conditions of the underground excavation and determining parameters that influence the rock mass behaviour. Data from the monitoring program is used as input into numerical models to back analyse and determine parameters. Numerous models are run to reduce the uncertainty in the mechanical behaviour of the rock mass. The main objective of this project is to assess how good numerical models are at determining the deformation in a rock mass as compared to in situ measured deformations, using the tailrace tunnel (TT) at the Ingula Pumped Storage Scheme as a case study. The comparative study was conducted by using the Phase2 elastic numerical analysis to determine stress, strains and deformation in five tunnel sections. Partial deformation readings of instrumentation necessitated the correction of in situ measured deformations using axisymmetric analysis. These results were then compared to the numerically modelled results. The effectiveness of empirical methods based on rock mass classification systems and back analysis methods at determining the modulus of deformation was also examined in the research project. Focusing on the empirical relationships of Serafim and Pereira (1983), Sonmez et al (2006), Hoek and Diederichs (2006), Hoek and Brown (1997) and the back analysis method developed by Kirsten (1976). Hazard Warning Levels were used to determine the stability of the TT. Although the quality of the numerical analysis is based on the input rock mass parameters and in situ stress, results show the elastic numerical analysis was effective in the determination of deformation in the TT. Empirical and back analysis methods were also successful at determining the modulus of deformation, when compared to in situ tests.
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    Current practices for estimation of strength and deformation properties of weak rock masses for geotechnical applications
    (2018) Odendaal, Elaine
    A weak rock mass comprises a collection of material with diverse characteristics and there is thus no single description for weak rock masses. This report summarises developments made in the understanding of weak rock mass, based on measurable parameters. Available tests predominantly measure the compressional strength of intact rock material. The shear strength is then estimated through existing failure criteria, since it is very difficult to obtain the shear strength of rock directly. Wiid (1981) offered an alternative testing technique, ideal for the measurement of shear strength of very soft to soft rock, in the form of a modified vane shear test and this technique is explored further in this report. Additionally, current modelling practices for rock masses generally consider shear strength criteria. However, unexpected failures in major excavations indicate the importance of damage mechanics and the presence of tensile strains in the rock (mass). Through correlations between measurable parameters, a conceptual model for rock strength, is suggested.
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    Application of rock mass classification and blastability index for the improvement of wall control at Phoenix Mine
    (2017) Segaetsho, Gomotsegang Seth Kealeboga
    The study sought to establish the applicability of rock mass classification as a primary input to wall control blasting. Conventional rules of thumb are used to develop blast designs based on parametric ratios with insufficient consideration of the rock mass factors that influence the achievability of final wall designs. Control of the western highwall of the Phoenix pit had proven to be challenging in that the designed catchment berms and wall competence were perpetually unachievable from the pit crest to the current mining levels. This exposed the mining operation to safety hazards such as local wall rock failure from damaged crests, frozen toes and rolling rock falls from higher mining levels. There was also an effect of increased standoff distances from the concerned highwall which reduce the available manoeuvring area on the pit floor and subsequently the factor of extraction that is safely achievable. The study investigated the application of rock mass classification and the Blastability Index (BI) as a means to improve wall control. This was achieved by establishing zones according to rock type forming the western highwall rock mass wherein distinguishing rock mass classification factors were used to establish the suitable wall control designs through a Design Input Tool (DIT). The DIT consolidated rock mass classification methodologies such as the Geological Strength Index (GSI) and the Rock Mass Rating (RMR) and related them to the BI and discontinuities of the rock mass to produce a tool that can be used to develop objective wall control designs. The designs driven by the tool inherently take into account the rock mass characteristic factors at the centre of rock mass classification methods and significantly reduce the dependence on rule of thumb. It was found that this approach yields designs with powder factors that are consistent with the rock breaking effort and the behaviour of discontinuities while remaining biased towards preservation of perimeter wall rock.
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