published_or_final_version / abstract / Civil Engineering / Doctoral / Doctor of Philosophy

published_or_final_version / Applied Geosciences / Master / Master of Science

published_or_final_version / Applied Geosciences / Master / Master of Science

A fracture and permeability analysis was performed on the Santana Tuff because of its similarity to the Topopah Springs unit at the Yucca Mountain site. The Topopah Springs unit is the proposed horizon for the spent nuclear fuel repository. Because of the impossibility of completely characterizing the flow properties of the unit without destroying the characteristics that make it desirable as a repository, other ash flow tuffs must be studied. The Santana Tuff and the Topopah Springs tuff both are rhyolitic in composition, nonwelded to densely welded and fractured. Fractures were examined at six outcrop locations spanning a five mile area. Stereonets and rose diagrams were constructed from over 312 fracture orientations. Although the composite data showed two major orientations of nearly vertical fractures, fracture trends at individual outcrops showed a variety of preferred orientations. Over 900 surface permeability measurements were taken using a mini-permeameter. The samples were categorized by three observed types of surface weathering: fresh, weathered, or varnished. Fracture surfaces were generally classified as weathered. The average permeabilities for the samples are 55.33 millidarcies, 5.03 millidarcies, and 3.31 millidarcies, respectively. The one-way statistical analysis performed on the data indicated that the permeability of fresh tuff surfaces is significantly different than both the permeabilities of the weathered and varnished tuffs, using both a least significant difference and greatest significant difference test. However, no difference was shown to exist between the weathered and varnished tuff permeabilities. Samples of fresh, weathered, and varnished tuffs were examined by X-Ray Defraction, the Scanning Electron Microscope, and in thin section. The SEM analysis showed surface differences between the three weathering classifications. The weathered and varnished samples were similar, exhibiting a platy, lamellate texture. The fresh surfaces were irregular and jagged. In thin section, a thin rind of dark minerals (FE-oxides) is observed on the edges of the varnished samples and in microcracks. This fills surface pores and causes the reduction in permeability. Two other zones of weathering have been identified in some of the samples, which may also cause changes in permeability. Tuff permeabilities were also analyzed for directional dependence. After an ash flow tuff is deposited and cooled, it may undergo flattening of pumice fragments and glass shards. These flattened fragments can be identified in handsamples, and are indicative of the direction of flow emplacement. The analysis showed that permeability is enhanced parallel to the emplacement direction, which is generally horizontal. Cut surfaces showed a 30% decrease in permeability perpendicular to flow direction. On varnished surfaces, this trend is still evident, although decreased in magnitude. This is expected because of the clay particles which make up the desert varnish. This study indicates that the formation of low permeability weathering rinds in association with vertical fractures may inhibit infiltration at the surface. It may accelerate infiltration at depth and allow more fluid to penetrate vertically into the tuff. In the event that fluid is absorbed into the matrix, it will travel horizontally, along the enhanced permeability parallel to the emplacement direction. / text

Equilibrium properties of the MgO-Al₂O₃-SiO₂ (MAS) system are modeled using techniques from statistical and quantum mechanics. The fundamental structural units in this model are the closed shell ions: Mg²⁺, Al³⁺, Si⁴⁺, and O²⁻. The equilibrium properties of the MAS system are determined by the interactions among these ions and by the environment (i.e. temperature and pressure). The interactions are modeled using coulombic, dispersion, and repulsive forces. Two parameters appearing in the repulsive terms for each cation-oxygen interaction are fitted from properties of quartz, corundum, and periclase crystals. The effects of the environment on the liquid and solid compositions found in this system are calculated using a Monte Carlo technique involving the generation of a Markov chain of configurations; each configuration being a "snapshot" of the particles in the liquid or solid material being studied. The properties of the material are derived from averaging appropriate quantities over all the configurations. Enthalpies of formation, heat capacities, and volumes of seven compositions in the MAS system have been calculated using this method. All are within three percent of the corresponding experimental values. Radial distribution functions for these runs show the competition among the cations for the common anion, oxygen, under charge and mass balance constraints. The electronic structure of several molecular clusters in the MAS system are examined using ab initio linear combinations of atomic orbitals (LCAO) techniques. The assumptions used in LCAO calculations are examined and a small, balanced basis set for the MAS system is presented. The Mg-, Al-, and Si-O interactions are all found to be highly ionic using this basic set. Using a first principles technique, the two body effective pair potentials assumed for the Monte Carlo calculations were shown to be physically reasonable.

The company Rio Deserto is currently mining the famous Barro Branco coal seam in the state of Santa Catarina located in the south of Brazil. One of their coal mines, #101, is experiencing problems related to the pillars in one panel. The coal seam is slightly inclined and several pillars have developed damages on the down-slope side with focus in the top corner. Damage inspections revealed a thin clay layer located between the coal pillar and the overlying siltstone. The clay layer is believed to affect the pillar strength and possibly be a source for the observed damages. Aim of this report has been to evaluate different theories behind the damages, focusing on the clay interface using numerical modelling with FLAC. Using convergence data, a calibration of the model is initially done before evaluating the combination of different interface and coal strength while observing the pillar. In addition is an evaluation of influence from structures such as cleats/joints. The results clearly show that with a small shear displacement (1-4 mm) the pillar damages are almost symmetrical on the up-slope and down-slope side of the pillar. Structures can influence and contribute to non-symmetrical pillar damages although not perfectly matching the field observations. Furthermore, the results show that a forced shear movement (8-25 mm) best reproduce the observed damages. A shear movement along the interface is therefore believed to be source mechanism behind the pillar damages. The forced shearing can potentially be explained by factors not considered in the model such as horizontal stresses, disturbances by mining and presence of water within the clay.

A project report submitted to the Faculty of Engineering, University, of the Witwatersrand, Johannesburg, in partial fulfilment of the requirements for the degree of Master of Science in Engineering Johannesburg, 1991 / As mining proceeds deeper on Elandsrand Gold Mine scattered mining will no longer be viable due to the excessive stress levels which would occur during mining of the final remnants between raises. Longwall mining with strike stabilizing pillars would eliminate this need for remnant mining. However, since the Ventersdorp contact Reef on Elandsrand has a relatively large number of faults and dykes and highly variable grade, longwall mining would result in a excessive amount of off-reef mining and mining of unpayable reef. / GR 2017

Thesis is submitted in partial fulfilment of the requirements for the degree of Master of Science in Engineering to the Faculty of Engineering and the Built Environment, School of Mining Engineering, University of the Witwatersrand, Johannesburg, 2017 / 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. / MT2018

A Dissertation submitted to the Faculty of Engineering, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the Degree of the Master of Science / Drilled specimens from four types of rocks, namely Quartzite, Norite, Sandstone and Coal encountered in the South African mining environment were tested to determine their fracture toughness coefficients, using International Society for Rock Mechanics (I...) standards. The tests were conducted using a MTS servo-controlled testing machine in displacement control mode. (Abbreviation abstract) / Andrew Chakane 2018

A research report submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, in partial fulfilment of the requirements for the degree of Master of Science in Engineering. Johannesburg, 2016 / Since mining platinum started at Unki Mine in 2005, large and small geologically controlled falls of ground (FOG) have been problematic especially in 2011 where a FOG caused a fatality. This study is about determining the rock properties at Unki Mine and then using the results to design and recommend robust support to reduce the FOG problems that are continuously happening. The study analysed all the FOG data from the mine database from 2010 to 2015 using statistical methods. Rock properties of the hangingwall, ore zone and footwall were determined from the laboratory tests. The geological structures were also mapped carefully. The results were then used as input data to the numerical modelling softwares Phase2 and J-Block. The J-Block program was used to determine the number of keyblocks that were stable, unstable and failed with support in designated and specified bords. A probabilistic approach was used to evaluate the stable span with special reference from small to large hangingwall instabilities for different mining scenarios. It was found out using Phase2 that large spans at Unki Mine are possible provided appropriate and robust support system is adopted. To fully address the issue of FOG problem at Unki Mine, a probabilistic approach is recommended as this is considered to be more appropriate than a deterministic approach that has been the traditional design approach so far. / MT2017