The principal functions of underground mine pillars are to stabilize openings and to carry the load of overlying rock strata. They are often (partially or completely) recovered at a later stage when their stabilizing effect is no longer required. For economic reasons, an optimum-sized pillar is the smallest one satisfying safety requirements. Thus the pillar design problem consists of determining the pillar's minimum dimensions as the load approaches the ultimate pillar strength. Because the pillar's strength and the load acting upon it are both functions of many interrelated factors, which may vary as mining progresses, the determination of pillar dimensions is a complex task. Furthermore, the multiplicity of pillar shapes, sizes, rock material and functions add to the designers' problem. Consequently, pillar design programs are still generally performed as a trial-and-error process. In order, to improve the present pillar design practices (1) -AApillar classification is proposed to standardize the design procedure (2) The principal design methods, divided into four groups, are summarized and their applicability is is defined (3) A five-phase design procedure with design charts is developed (4) The design procedure is applied in analysing two case histories / Applied Science, Faculty of / Mining Engineering, Keevil Institute of / Graduate

The design of open stope rib pillars has been done using many empirical methods, but none of the methods has been verified with a design survey. This thesis uses data collected in the "Integrated Mine Design Study" to develop an empirical rib pillar design method for open, stope mining. The method is called the "pillar stability graph". The design variables in the method are: the compressive strength of the intact pillar material, the average pillar load determined by numerical modelling, the pillar width and the pillar height. The graph has been refined with the use of more than 80 literature case histories of hard rock pillars in room and pillar mining. The pillar stability graph and the pillar data base are used to examine the applicability of empirical methods commonly used in open stope rib pillar design. The investigation found the pillar strength curves developed by Hoek and Brown (1980) may be useful under some conditions for the design of open stope rib pillars but formulas by Hedley (1972), Obert and Duvall (1967) and Bieniawski (1983) are not applicable. Guidelines, using the pillar stability graph method, are proposed for the design of permanent open stope rib pillars, stable temporary open stope rib pillars, and failing temporary open stope rib pillars. / Applied Science, Faculty of / Mining Engineering, Keevil Institute of / Graduate

A research report submitted to the Faculty of Engineering and Built Environment, University of the Witwatersrand, Johannesburg, in the partial fulfilment of the requirements for the degree of Master of Science in Engineering, January 2018 / The higher quality virgin coal resources in the Witbank Coalfield are near completion, hence new methods of mining the underground coal pillars that have been left insitu as primary support becomes attractive to mine using opencast methods. Up until 2006 more than three million pillars have been created (van der Merwe, 2006) and have been growing since. There are various challenges associated with pillar extraction via opencast mining method. These challenges relate to spontaneous combustion, underground water and the exact spatial location of underground pillars that have been mined in the early to mid-1900. The reliability of old underground mine plans pre-1960, before the Coalbrook disaster, saw underground pillars not being offset which resulted in unreliable survey plans (van der Merwe, 2006). This report focuses on possible techniques that can be used to identify underground pillars where no water and no spontaneous combustion are evident. The following two methods have been tested: Ground Penetrating Radar (GPR) technique which is based on geophysics and a down the hole 3D laser method using the Cavity, Auto-scanning, Laser System (CALS Tool) which uses reflectorless principles to measure the geometries of the underground pillars. The GPR did not provide any conclusive data, whilst the CALS Tool provides detailed information of the workings. The CALS Tool is not practical to identify every single pillar in a reserve but can be used on a larger extent as a short term mine planning mechanism. The CALS Tool proved to have the ability to identify the spatial location of the underground workings as confirmed by the test done at TOC. / XL2018

No description available.

Thesis (Ph. D.)--West Virginia University, 2003. / Title from document title page. Document formatted into pages; contains xi, 187 p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 181-187).

Thesis (M.Eng.(Mining Engineering))--University of Pretoria, 2000. / Includes abstract in English. Includes bibliographical references.

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.

The finite element method of stress analysis was utilized throughout this study to model single- and double-entry longwall systems. The influence of the geology, mining depth and mine-layout on barrier pillar stability was examined and design guidelines for determining sizes for such pillars were established using the "pillar core concept". The yield zone was better estimated by the use of the Coulomb failure/strength approach than the Drucker-Prager failure criterion. The width of the yield zone surrounding a barrier pillar is not constant as originally assumed, but depends on the size of the panel width (width of extraction). However, its influence can be ignored for face lines in excess of 800 feet. Although this analysis was based on information pertaining to a particular mine plan and specific geologic conditions, the recommended values of pillar sizes are in agreement with those suggested in other case studies. The results presented in this thesis, therefore, could serve as a general approximation of barrier pillar sizes if a more precise analysis is not available. / M.S.

The mechanisms controlling pillar load transfer from an overlying seam to subadjacent workings is investigated. Mine geometry and geology are researched as major variables affecting interaction between contiguously placed seams. The thesis includes photoelastic experimentation, finite element modeling, and analysis of multi-seam room and pillar mine case studies from the Appalachian region. A structured approach first establishes theoretically based trends, confirms their existence in the field, and finally results in invaluable multi-seam mine design guidelines. Stratification, lithology, and size of the interval between seams, and the extraction ratios of the seams are parameters found responsible for defining the stability of a lower seam overlain by either past or present mine operations. / Master of Science

As the coal reserves at shallow depths become exhausted companies have to develop deeper deposits and increase percentage extraction to maintain production levels. Total extraction for room and pillar mines can only be achieved by pillar extraction. The unsupported roof increases during pillar extraction and hence the cost of ground control also increases. Nevertheless, pillar extraction where possible has many potential advantages such as decreased operating cost, increased utilization of reserves, and extended life of the mine. There are several variables such as depth, mining height, rock strength, mining geometry, roof and floor conditions, and retreat mining methods, which affect pillar extraction cost. Cost components of pillar extraction are classified as direct, indirect, fixed, and subsidence compensation costs. A discounted cash flow pillar extraction cost simulator has been developed and used to compute total pillar extraction cost for a variety of conditions and to explore the possibilities of optimizing ground control and retreat mining techniques to maximize extraction ratio. The computer program computes the safe and optimum pillar dimensions and determines the suitable pillar extraction method for the computed pillar width. Pillar extraction cost components are generated and totalled using the net present value method by the simulator. The total extraction cost simulator evaluates the potential advantages of pillar extraction and tests individual variables for sensitivity to changes in other variables attributable to ground control and pillar extraction techniques. Cost of pillar extraction per ton of coal versus depth is presented in the form of a simple nomogram by the simulator. The simulator can be used to determine the economic feasibility of pillar extraction at a particular depth, geologic and mining environment when the market price of mined coal is known. / Ph. D. / incomplete_metadata