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

published_or_final_version / Civil Engineering / Master / Master of Philosophy

In fractured rocks of low permeability, the hydraulic properties of the rock mass are strongly influenced by the connectivity and fracture geometry of the fracture system, the stiffness and deformational properties of fracture surfaces and the geostatic stresses. This dissertation demonstrates through theoretical analysis the sensitivity of fracture connectivity and flow rate to fracture radius, fracture density and measurement scale. Percolation factor and percolation frequency are suggested as indices of connectivity and flow rate. Models of hydromechanical coupling, normal closure and simple geometrical joint systems are proposed to study fracture porosity and fracture permeability and are compared with measured values from published papers. Fracture surface characteristics are expressed as indices of JRC and JCS. The relation between fracture aperture and geostatic stress is also examined. Based on the percolation process, a three dimensional discrete fracture model with statistical parameters of fracture geometry is developed to investigate the relations between the connectivity and flow rate and the percolation factor and percolation frequency. This model has the capability to generate a fracture network and to solve for steady state flow. The flow through each fracture is discretized by the boundary element method. By performing numerical simulation, the percolation threshold was found to be in the range of 0.9 to 2.4 for orthogonal joint sets. There is a rapid increase in flow rate with increasing fracture density or fracture length as the percolation factor reaches the percolation threshold. The percolation factor and percolation frequency are scale-dependent. A fracture network with large fractures and a high fracture density has a high percolation frequency and a high percolation factor. A network with high percolation frequency and percolation factor has a high flow rate. A constitutive model linking the initial conducting aperture, mechanical conducting aperture, JRC, JCS, geostatic stress and fracture geometries can be used to predict fracture porosity and fracture permeability in terms of depth. Preliminary comparison with field data shows that models comprising three orthogonal sets and disc-type fracture models can be used to simulate observed behavior. Fracture porosity and fracture permeability based on a model of three orthogonal sets can be used to predict flow through volcanic rocks. For sedimentary rocks a model incorporating a model of three orthogonal sets and one parallel set can be used with varying levels of confidence.

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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, 2017 / 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. / MT 2017

A thesis submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy, Johannesburg, September 2017 / 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. / XL2018

Thesis (Ph. D.)--University of Sydney, 1990. / Includes tables. Title from title screen (viewed November 06, 2009). Bibliography: leaves 360-369. Also available in print form.

Thesis (Ph. D.)--University of Texas at Austin, 2001. / Vita. Includes bibliographical references. Available also from UMI/Dissertation Abstracts International.

Thesis (Ph. D.)--University of Texas at Austin, 2001. / Vita. Includes bibliographical references (leaves 156-166). Available also in a digital version from Dissertation Abstracts.

published_or_final_version / Applied Geosciences / Master / Master of Science