Adopting yield pillars has been considered an effective way of alleviating ground control problems and increasing production. The purpose of this research was to study the behavior of yield pillars and to develop the design criteria. After a literature review, two 2-D finite element models were developed, each following a different non-linear approach. The first model adopted the successive iteration technique incorporated with the Mohr-Coulomb yield criterion. The second followed the elastic—plastic approach, implementing a generalized Von Mises yield criterion. Extensive underground monitoring was conducted and the finite element models were compared with the field data, both yielding promising results. Three different longwall entry layouts were investigated. The yield-stable-yield pillar system was considered to be the best design. A parametric analysis was also performed. The triaxial factor and Poisson's ratio were found to be the most important material properties affecting pillar yielding. The progressive failure hypothesis for pillar design was critically examined. The analysis suggested that the formulation defining the stress distribution in the yield zone under this hypothesis may be satisfied only in extreme cases and, therefore, the actual distribution can be different. An improved equation, describing the stress distribution in the yield zone, was derived by statistically analyzing the results of finite element simulations. The latter equation fitted the observed field data better than did the original equation, and it was further developed for estimation of yield zone width. Consideration was also given to yield pillar design. Three possible yield pillar sizes were proposed in this paper. The maximum yield pillar size was considered to be twice the width of the yield zone. Based on the pressure arch concept, the minimum yield pillar size was determined by accepting that yield pillars were only supporting the rock strata under this pressure arch. A suggested yield pillar size was obtained by selecting a size which would force the peak stress at the center of the yield pillar to equal the average tributary stress. The case studies conducted in this research indicated that the predicted yield pillar sizes were reasonably accurate. / Ph. D.

In general, as the uniaxial compressive strength of rock samples is tested, the uniaxial strength of the rock decreases with increasing sample size until a strength is reached beyond which no further decrease in strength is observed for further increases in size. The size at which this occurs was termed the critical size by Bieniawski (1968) and the corresponding strength the critical strength. Once these values are obtained no significant changes in strength may be expected as a result of further volume changes. For the purposes of pillar design, this strength should be adjusted to account for other factors that affect pillar strength, the main factors being the width to height ratio (w/h) effect, jointing and contact conditions. Further test work on Merensky Reef was required to clarify the: 1. Values of it’s critical size and strength 2. Effect of the w/h on it’s strength 3. Effect of the frictional contacts between the reef and the surrounding rock on the reefs uniaxial strength. These results could then be integrated into a holistic pillar design methodology to improve current pillar designing practices. These effects were examined through the laboratory testing of samples originating from Amandelbult Platinum mine. A critical strength of approximately 110 MPa was obtained for samples with diameters, 130 - 250 mm (w/h =1). Increasing the frictional contacts between sample and loading platens was found to increase the sample's strength. A marked difference was found between the insitu and laboratory contact friction angles for Merensky Reef. The insitu contact friction angle was found to be approximately 2.5 times larger then the laboratory contact friction angle. The uniaxial strength increased linearly with increasing w/h ratios up to a w/h ratio of 6. For w/h ratios greater then 6 the strength continued to increased with increasing w/h ratios, but no curve could be acceptably fitted to the data to describe this trend. The results of this study can be applied to mine pillar design in the Bushveld Igneous complex. / Dissertation (MEng (Mining Engineering))--University of Pretoria, 2007. / Mining Engineering / unrestricted

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 2016 / The aim of this research was to investigate the bord and pillar design procedure in use at the time on chrome and platinum mines and subject it to a critical appraisal and, if necessary, propose an improved methodology. An analysis of the current method and some of the alternatives proposed in the literature has shown that the methodologies suffer from drawbacks that can be detrimental to the mining industry due to overdesign or rendering an excavation unsafe. The conclusion was that improvement is essential. The influence of the variability of the rock mass properties input parameters on the factor of safety in the current equation was calculated and the findings were that the value of the factor of safety can vary by up to 30 percent due to these variation. The proposed process adopted FLAC2D Hoek-Brown simulations to develop full stress deformation curves for typical pillars. The mine stiffness concept was introduced to determine the pillar load which automatically included the influence of the pillar and strata stiffness, excavation spans, pillar yield and failure. The factor of safety was obtained by dividing the pillar strength by the stress value of the intersection point of the two linear equations for the stiffness of the system and the pillar respectively. The proposed methodology was calibrated by applying it to two mines in the Bushveld. The conclusion was that the methodology is a significant improvement over the one in use. It was shown that a combination of the FLAC2D Hoek Brown and the System Pillar Equilibrium Concept can predict the extent of the fracture zones and, to certain extent, the pillar stresses. The stage has been reached where the methodology can be used to predict the most likely commencement of failure of pillars at greater depth and alternative pillar mining methods can be modelled. / MT2017

A research report submitted to the Faculty of Engineering and Built Environment, University of the Witwatersrand, Johannesburg, in partial fulfilment of the requirements for the degree of Master of Science in Engineering, Johannesburg 2017 / 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. / XL2018

M.Ing. / Safety within the mining industry is a primary concern for everyone involved. More specifically, active below-ground stope support for South African Mines is becoming increasingly important due to a renewed emphasis on the safety and well-being of the people actually working underground. It is imperative that all stope support systems are rigorously tested, continuously, both under laboratory conditions and in-situ to prove their performance and manufacturing standards. The Rocprop was initially manufactured in 1995 with the first two hundred props being installed at East Driefontein Consolidated Gold Mine in the Carletonville area. In the three years since the first introduction over three hundred thousand Rocprops have been manufactured and sold to South African Mines with the number steadily increasing. The Rocprop is a tubular support consisting of two tubes — a Ø139mm 'inner' tube and a Ø152mm 'outer' tube. One end of each tube is sealed by dome ends which are welded onto the tube mouths. The two tubes, cut to identical lengths, fit inside one another and extend telescopically during installation. Once the desired height has been reached, leaving enough tube overlapping to ensure the support does not buckle, the wedge is hammered in locking the prop at that height. The water is then removed after which the prop will provide active support of the rock mass above it. One of the components responsible for the Rocprops success is a dome end. This is either a forging or a pressing welded onto each end of the support and allows continual concentric loading throughout the life of the Rocprop. At present the dome ends are pressings, manufactured into hemispheres from 10mm mild steel plate in one action. The reason for the Rocprop's success is its performance characteristics. It's all metal construction, ease of installation, reliability and predictability in both seismic and static conditions, fire resistance, blast resistant, economically viability and versatility have made the prop successful. Reasons for the research were to investigate the dome end forming process in general and to investigate current numerical analysis techniques ability to predict loads during manufacture, the final shape, spring-back and other local deformation areas. Also to investigate alternate manufacturing methods such as cold forming, which provides advantages such as better mechanical properties and higher structural capabilities. The use of alternate materials in the Rocprop manufacture has been an ongoing process for MSP, manufacturer and current licensee holder of the Rocprop. A substitute for the current dome end manufactured from mild steel was investigated. For the substitute to be viable the material should be stronger, weigh less and be cost effective. In depth knowledge about the forming of the dome end at various velocities was gathered, providing information for further optimisation of the component.