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

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Characterisation of rock mass rating (RMR) parameters by geostatistical analysis Orapa Mine Botswana
(2019) Kgomanyane, Joel Thabo
The focus of this research project is on the application of geostatistics to evaluate the rock mass rating (RMR) parameters for Orapa AK1 kimberlite pipe. The RMR parameters evaluated are Uniaxial Compressive Strength (UCS), Fracture Frequency per Metre (FFPM), Rock Quality Designation (RQD) and Dry Density (DD). When applied appropriately, these RMR parameters have the potential to enhance and inform conventional geological models, blast designs and metallurgical plant performances. Compared with other assessment methods such as using the global mean of the RMR parameters, the geostatistical estimates resulted in a more accurate and robust assessment of the geotechnical variables studied herein. Ordinary Kriging has been applied to estimate the RMR values at unsampled locations for the different rocktypes of the Orapa AK1 Kimberlite pipe. Variogram models were generated for the above RMR parameters within the different rocktypes both in the horizontal and vertical directions including an estimate for the nugget effect. The resulting block estimates were compared with sample data for all RMR parameters and bench plots for each rocktype were generated and analysed. Furthermore, geostatistics revealed that, RMR parameters have spatial correlation and these are strongly influenced by the geological environment of the AK1 Kimberlite at Orapa mine in Botswana. It is concluded that the evaluation of the rock mass rating (RMR) parameters using geostatistics is an important future requirement for the success of any mining project. It is recommended that geological and geotechnical data processing and interpretation should be coupled with geostatistical modelling at project pre-feasibility studies to enhance the conventional methods used in geoscience mining projects. The geostatistical estimation approach provides a more reliable and accurate method (low kriging variance) by taking into account the spatial continuity of variables under study as compared to simple averaging of geotechnical parameters for a given volume of rock mass.
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.
Yielding pillar design in South African collieries
(1997) Oldroyd, Oldroyd
Three cases of pillar failure on Southern African Collieries have been studied to analyse the behaviour of both the pillars and the overlying strata. Each of the cases shows a different type ofpillar and strata behaviour during failure and thus provides an opportunity for back analysis. In the first case, pillars failed in a controlled fashion while the overlying strata behaved in an elastic fashion. In the second pillars failed in a controlled fashion while the surrounding strata behaved inelastically. In the third failure was initially controlled but became uncontrolled. Computer models have been run to determine the theoretical critical post peak pillar slopes and the results of these models have been compared with the actual pillar behaviour as derived from in-situ measurements during failure and that which might be predicted from the theory of controlled and uncontrolled pillar failure. Comparisons are also made with the expected behaviour implied from the results of in-situ strength tests carried out on small coal pillars to ascertain their load deformation characteristics. The results indicate that the behaviour of the pillars more closely resemble that predicted from in-situ tests carried out by Van Heerden4• The results also indicate inadequacies in using elastic methodologies to determine whether pillar failure will be controlled or uncontrolled.
Unlocking value through improved production decision making : a trackless mining systems analysis
(2018) Mukonoweshuro, Christopher
This study was based on the hypothesis that there are opportunities to maximize production outputs in many existing underground hard rock trackless mining systems using the same or less resources by improvement in decision making paradigms. This is very important in the current operating environments of uncertainties and continued drop in metal prices. The project main goal was thus to carry out a detailed investigation of trackless mining production systems and test how to maximize output by focusing on three objectives, namely: analyzing key technical factors that impact the production rates in terms of tons per hour, identifying major operational activities which impact effective equipment operating hours, and identifying decision support systems (DSS) to improve operational decision making. Regarding the first objective (production rates), through the analysis of trackless mining as a serial production system, it was shown that production rates could be increased by focusing at system level, process level and work station/equipment level decisions. System level decisions must minimize the total residence time of the material (ore) in transit or work in process(WIP). This will open capacity for generating more ore. Process level decisions must reduce the gross cycle times at the work stations to equal or be below the Takt times inorder to smoothen production flow. Takt time is an important factor in a production system which shows the maximum cycle time allowed to meet the daily demand. The third level focuses on the capability of the mining equipment itself through decisions that improves the reliability, maintainability and capacity. Decision tables based on reducing the equipment failure rates (λ), improving the repair rates (µ) and the cycle times were developed to aid in making the reliability, maintainability or capacity decisions. For the second objective (operational activities), the focus is to maximize effective operating times of the equipment through reduction of delays. The study shows this can be achieved through use of real-time decision support systems (DSS) for better control of the operations. The third objective was able to identify functional modern DSS that can be implemented in trackless mining. Effectively, the study was able to highlight opportunities of generating extra capacity for trackless mines at same or less resources by focusing on the above three objectives.
Reducing risks due to rockbursts: strategic financial considerations
(2018) Moganedi, Kadibetjo Adelaide
Gold mines in the Far West Witwatersrand area experience frequent mining induced seismic events due to dynamic stress changes associated with the depth and extent of mining. Some of these seismic events result in rockbursts, of varying magnitudes, in access tunnels. Geological structures, mine design layout and support system design influence the magnitude of a rockburst damage in an underground excavation. Support systems are the last line of defence and are effective in environments where the mining layout is optimised for dynamic stress changes. The objective of this research is to determine the financial value energy absorbing support systems add to a rockburst prone well designed mine. The research focuses on quantifying indirect consequences of rockburst risk using an Excel model developed as part of this research. The model has three versions, each targeting a specific user. The model is used to evaluate the financial benefits of different support systems in access tunnels prone to seismicity and possible rockbursts. Executive management can use the Executive spreadsheet of the model to facilitate proactive rockburst risk management. Four case studies were evaluated in detail, and the results indicate production loss is the major source of quantifiable financial loss after a rockburst. The tunnels were supported with variations of rigid support systems, even though energy-absorbing support systems were the most suitable for dynamic loading conditions, and were likely to have contained the rockburst events. This is because energy-absorbing support systems are viewed as an unnecessary expense. However, the “extra” cost of energy absorbing support system, as a strategy to minimise effects of rockbursts, will almost always create better value than the less expensive rigid support. This extra cost can be significantly reduced by increasing the spacing between yielding tendons in an energy absorbing support system. In conclusion, it is strategic for rockburst prone mines to install high quality yielding support systems as they have the potential to create substantial long term value for the mine.
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.
A lithological, petrographic and geochemical investigation of the M4 borehole core, Morokweng Impact Structure, South Africa
(2017) Wela, Slindile Sthembile
This study investigates the mineralogical, petrographic and geochemical characteristics of target rocks and impact-formed breccias (impactites) intersected by the 368 m long M4 drillcore located 18 km NNW from the estimated centre of the 145 ± 2 Ma, Morokweng impact structure (MIS), South Africa. M4 is the only core from the central parts of the Morokweng impact structure not to intersect fractionated granophyric impact melt directly beneath 35-100 m of Cenozoic Kalahari Group sediments. Instead it intersects highly fractured, cataclased and shocked, crystalline target rocks that are cut by mm- to m-scale melt-matrix breccia and suevite dykes. The target rocks comprise granitic, granodioritic, trondhjemitic and dioritic Archaean gneisses, metadolerite and dolerite. The gneisses and metadolerite show signs of quartz veining and metasomatism linked to localised mylonitic to brittle fault deformation that predated the impact. The suevite and meltmatrix breccia dykes make up ~10% of the core. All rocks show signs of low-T hydrothermal effects that occurred after the impact. The target rocks contain a complex network of shear fractures that contain cataclasite and which grade into monomict lithic breccia. The cataclasite contains shocked mineral fragments, which indicates that the shear fracturing postdated the initial shock stage of the impact. The melt-matrix breccia and suevite dykes show signs that they intruded along the fractures, although there is also evidence that shear fracturing continued after quenching of the melt. This suggests that the intrusion of the dykes overlapped the brittle deformation of the target rocks. Shock features in the M4 core lithologies include planar fractures, feather features, decorated planar deformation features (PDF), mosaic extinction and toasting in quartz; oblique lamellae, reduced birefringence and patchy (mosaic) extinction in plagioclase, and chevron-style spindleshaped lamellae in microcline, as well as kink bands in biotite and planar fractures in titanite and zircon. Universal Stage measurements of PDF sets in quartz from 8 target rocks and 6 impactite dykes revealed four dominant sets: 0°(0001), 22.95°{ 3 1 10 }, 17.62°{ 4 1 10 }, 32.42°{ 2 1 10 }; with no significant change in shock intensity with depth nor significant differences in PDF orientations or intensity between melt-matrix breccias, suevites and target rocks. Based on these observations the average peak shock pressures are estimated at 10 - 25 GPa. Apart from one suevite dyke that contains exotic clasts and an unusual bulk composition, all suevite and melt-matrix breccia dykes show major, trace and REE compositions and lithic and mineral clasts that indicate that they were formed from the target rocks found in the M4 core. The individual impactite dykes show good compositional correlation with their wallrocks, which supports limited transport of the melt and suevite. This is also supported by evidence of small-scale variation of the melt composition in the melt-matrix breccias, which indicates that not enough time was available for complete mixing to happen. The similarity in matrix composition and in lithic and mineral clast types in the melt-matrix breccias to their wallrocks, is consistent with a friction melt origin. These dykes are thus interpreted as pseudotachylite. Macroscopic and microscopic evidence suggests that the melts intruded cataclasite-filled fractures and that interfingering and infolding between the melts and incohesive cataclasite allowed the melt to assimilate cataclasite. The melt clasts in the suevite show the same composition and clast features as the melt-matrix breccias. Based on this evidence it is proposed that the melt clasts in the suevite in the M4 core are fragments of quenched pseudotachylite that became separated and mechanically mixed into the cataclasite matrix when movement continued along the cataclasite-bearing fractures after the melt quenched. This was possible because the cataclasite was still incohesive and because strong vertical and horizontal displacements of the entire M4 sequence happened during the crater modification stage of the impact, possibly for 1-2 minutes after the impact. The melt-matrix breccias are compositionally distinct from the Morokweng granophyric impact-melt rock intersected in the other central borehole cores. Melt particles are pervasively hydrothermally altered to a secondary mineral assemblage of zeolites and smectites, attributed to impact-induced hydrothermal fluid circulation in the MIS. The upper parts of the core are marked by abundant haematite but in the deeper levels of the core, chlorite-epidote-andradite garnet is found, which may indicate a vertically-zoned hydrothermal system after the impact. The hydrothermal effects also explain the abundance of decorated PDF in shocked quartz grains and the lack of glass in the PDF in quartz. The 10-25 GPa shock levels in the target rocks support them lying close to the transient crater floor and initially close (<10 km) to the point of impact. The high structural position of the rocks relative to the impact-melt sheet suggests that the M4 sequence represents part of the peak ring of the Morokweng impact structure. The rocks of the peak ring would have experienced strong vertical and centrifugal displacement during the crater excavation and modification stages, which can explain the intense shear fracturing and cataclasis, brecciation and friction melting as well as the strong block movements that could disrupt and disperse the pseudotachylite melt dykes to produce suevite. A peak ring radius of 18 km would suggest that the original Morokweng crater rim diameter would have been >70 km, but between 1 and 2 km of post-impact erosion before the deposition of the Kalahari Group means that this could be a minimum estimate.
Prediction of the initiation and orientation of the extension fractures ahead of and around faces and walls of mechanically driven excavations and their effect of stability
(2017) Mokgohloa, Matthews Chuene
Boring of shafts and tunnels in hard rock mines is more prevalent in recent years in South Africa. This normally takes place under substantial stress conditions, where fracturing of rock occurs around the boundaries and ahead of advancing faces of excavations. Fracturing can have a significant impact on boring activities, in some instances causing sidewall spalling which can be extensive, with machine grippers unable to reach the sidewalls. In brittle rock, these fractures are commonly extensional in nature. This research has been undertaken to predict the initiation of extension fractures and their orientations ahead of machine driven tunnels. Furthermore, it will help to assess the stability of the excavations, by evaluating the potential for slab/plate failures. This was based on the typical in situ stress fields for underground deep level mines. The numerical analyses involved the generation of different plots:  Principal stress contour plots, depicting stress distributions around and ahead of tunnel excavation, using cutting planes;  Isosurfaces, showing zones of extension or potential extents of fracturing, applying the extension strain criterion; and  Trajectory ribbons, to demonstrate the orientations of fractures. Based on the results of the stress analyses, potential slab or plate formation was determined. It was noted that the fracture zone is a function of a tunnel size. For instance, a four-metre diameter tunnel is less likely to give boring problems than an eight-metre diameter tunnel. The failure of the tunnels was predicted by employing slab analysis methods. An eight-metre diameter tunnel had slenderness ratio as low as 22.3 as compared with a four-metre diameter tunnel with a slenderness ratio of 27. Looking at buckling stress versus slenderness ratio, this translates to buckling stress values of above 100 MPa for an eight-metre tunnel and to values just below 50 MPa for a four-metre tunnel. The outcome of the research gives a clear indication that boring activities could be undertaken under severe conditions. This could be detrimental to the cutter head, since large slabs and blocks could be encountered during boring. The results of this research can be beneficial in the evaluation of boring conditions prior to and during boring activities.
Combating the effects of rockbursts caused by seismically-induced shock waves
(2017) Mudau, Avhasei
Rockburst occurrences and their consequent damage remain a problem in modern mining, particularly at great depth. The problem of rockbursts has also escalated in deepcivilengineeringtunnelsduetohighlevelsofin-situstressatsuchdepths. Key advancementshavebeenmadetodatetohelpmitigatethedrasticimpactscausedby rockburstdamage,withrocksupportremainingalineofdefensetoprovidestability in rockbursting situations. There is, however, an ongoing inability of support to contain severe rockburst damage, especially conventional support systems. More than two decades ago, a support concept termed “sacriﬁcial support” was proposed as a potential additional method to help inhibit rockburst damage. The philosophy behind a sacriﬁcial support system is that, under dynamic loading conditions, support, in the form of a liner must fail (i.e. be ejected from rock surface), leaving behind, undamaged, what was once supported rock mass. It is because of this reason that this support is referred to as a sacriﬁcial support due to its ability to protect the rock from damage whilst the support itself fails. Since the inception of this support idea, it was only recently that the behaviour of support in real rockburst events manifested the sacriﬁcial behaviour in rockbursting, which warranted the need for further research. The sacriﬁcial support concept stated here is applicable in situations where the source (i.e. seismic event) of the rockburst is located remote from where rockburst damage is likely to occur. To investigate the behaviour of sacriﬁcial support, controlled laboratory experimentsbasedonthesplitHopkinsonpressurebar(SHPB)techniquewereconducted to study some aspects of dynamic rock fracturing in tension at high strain rates, and also the role a sacriﬁcial layer plays in combating dynamic rock failure (i.e. rockburst damage). To achieve this, a single Hopkinson pressure bar conﬁgured for spalling tests, comprised of a relatively long cylindrical intact rock specimen attached at the bar free end, was impacted by a striker on the opposite free end of the bar in order to generate a dynamic stress pulse responsible for spall failure upon reﬂection from the specimen free end. Different liners and/or liner combinations were then introduced at the specimen free end as sacriﬁcial support. This experimental arrangement allowed the role of, and failure mechanisms associated with, sacriﬁcial support under dynamic loading to be demonstrated, and comparisons were made with “sacriﬁcial support” behaviour observed in real rockburst events in a mine. Analysis of experimental results revealed that varying liner thickness and mechanical impedance between rock and support liner plays a signiﬁcant role in helping to limit rockburst damage. Apart from experimental investigations, numerical simulations were undertaken to further probe the behaviour of sacriﬁcial support under dynamic loading. Elastic models subjected to p-wave propagation indicated failure of the sacriﬁcial layer, manifested by ejection of the liner due to reﬂection of compressive wave at the free surface. This failure mechanism was noticed for all the liners, independent of variation in liner thickness, and wavelength characteristic of the applied wave to the model. The sacriﬁcial support method presented in this thesis presents an opportunity to further enhance safety in seismically active mines.
Design of regional pillars for the Khuseleka Ore Replacement Project (KORP) - UG2
(2017) Mutsvanga, Clarence
Depletion of mineral resources is a reality of mining. It is critical that as resources get depleted, new reserves are subsequently opened up continuously if a mine is to continue operating. Failure to open up new reserves will result in a mining operation running out of reserves and ultimately ceasing operations. Besides the economic considerations of an ore reserve such as the grade and tonnage, stability of the mining operation is of equal importance. A mine should remain stable for the entire period that it remains operational. Pillars play a critical role in ensuring the stability of an excavation; actually, regional pillars ensure the overall stability of a mine. It therefore goes without saying, pillar design is an integral component of any successful mine design. This project was undertaken with the objective of ensuring that the new reserves being opened up in the Khuseleka Ore Replacement Project (KORP) section are not only profitable, but also stable. This was done through a) maximisation of extraction ratio, thereby maximising the mines’ profitability. b) designing the regional pillar layout for the KORP section using current empirical and numerical pillar design methods and comparing the results to come up with the most optimal design. c) ensuring the stability of the on and off reef mine infrastructure by determining the Rockwall Condition Factor (RCF) values on the footwall infrastructure due to pillars left above and thus prevent damage to these excavations through stress induced failures. Consideration was given to the standard Khuseleka footwall infrastructure layouts for the design based on the planning department’s layout of haulages and crosscuts for the KORP section. The layout of the footwall excavations indicated that the pillars would be differently sized thereby having an influence on the APS, pillar strength and factors of safety of the regional pillars. d) numerical modelling analysis of the effects of leaving stabilizing pillars on the 27 raise line where the haulages intersect the reef horizon. The methodology employed for this undertaking involved a critical literature review of existing pillar design methods, applying and comparing them, and coming up with an economic and safe design. To be able to design a pillar layout that met the objectives listed above, engineering design principles had to be applied. It involved gathering the relevant geological and geotechnical information required as input parameters for the different empirical and numerical analyses methods. What came out from this project was that each method employed yielded its own set of results. This highlighted the need to understand the context under which a design is carried out and the shortcomings of each method employed. It showed how important it is to have all the relevant information of not only the characteristics of the rock mass in which an excavation will be made, but also on the strengths and limitations of the tools available to design a structure. It highlighted the fact that to minimize uncertainty and have a more robust design, it was necessary to spend time and effort in gathering as much relevant data as possible. In the end engineering judgment was used to decide on the best method or system to employ in the design of the pillars.

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

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