Experimental and Numerical evaluation of mine pillar design

Mining is an art of extracting valuable minerals or other geological materials from the earth, usually from an ore body, vein or (coal) seam with minimum unit cost within acceptable social, legal, and regulatory constraints. There are two major methods of underground mining of coal: Bord& Pillar Method and Longwall Method. Pillars are mostly encountered in the former method. Pillar is the structural element and form an integral part of a mine on which the stability of the mine depends. A pillar support is intended to control rock mass displacement right through the zone of influence of mining, while mining activities proceeds. If pillars are made too small increasing the extraction percentage, it would affect the stability of the mine and vice-versa. An economic design of a support system implies that ore committed to pillar support be minimum, while fulfilling the vital requirements of assuring the global stability of the mine structure. This project critically studies the different optimum combination of pillar dimensions that could be effectively incorporated in the mines. Geotechnical factors of a nearby underground coal mine has been determined in the laboratory. Different approaches of pillar design have been compared. Variation of safety factors with width to height ratio of pillar, extraction percentage and depth of cover has been determined and conclusion has been made. The safety and feasibility of mining method is obtained through an optimum correlation between safety factor and extraction percentage. Numerical modeling has been done to evaluate the maximum stress induced over the pillar and gallery and also to calculate the deformation in the pillar and the sagging in gallery due to induced stress. ANSYS.13 3-D software was used in numerical modeling. Different mining parameters were changed to measure the effects on stress behavior, deformation and sagging in gallery

INVESTIGATION INTO MINE PILLAR DESIGN AND GLOBAL STABILITY USING THE GROUND REACTION CURVE CONCEPT

Pillars form an important support structure in any underground mine. A bulk of the overburden load is borne by the mine pillars. Thus, the strength of pillars has been a subject of detailed research over more than 6 decades. This work has led to the development of largely empirical pillar design formulations that have reduced the risk of pillar failures and mine collapse. Current research, however, has drawn attention to the fact that some of the assumptions used in the development of conventional pillar design methodologies are not always valid. Conventional pillar design methodology assumes that the pillars carry the dead weight of the overburden. This conventional method treats the pillars as passive structures. The limitation of this approach is that the self-supporting capacity of the overburden is not incorporated in pillar design. This suspension theory of pillar design treats the strata-pillar interaction problem as a classic case of static equilibrium, without detailing the interactions of the two structures. Globally, multiple pillar design methods have been developed, based on this suspension theory. Each of these methods approaches the calculation of pillar stability a little differently with respect to material properties, excavation geometries and stress conditions. Most of these design methods are derived empirically and lack a mechanics-based approach. Moreover, there is a lack of a unified pillar design methodology that can be used to design all types of mine pillars using a mechanics-based approach. The Ground Reaction Curve has been used as a means of correlating strata displacements to stress conditions. In addition, the Support Reaction Curve has been used in modeling the response of a support system under load, as a function of support properties and installation time with respect to opening development. In comparing the Ground Reaction Curves and Support Reaction Curves for different support systems, one can evaluate the effectiveness of installed support systems in maintaining the integrity of the excavated area(s). This approach has been widely used in designing secondary (artificial) support systems in both civil tunneling and the mining industry. Encouraged by the successful use of this single method in designing secondary support systems, this research revisits this concept for mine pillar design. This research investigates the utilization of the Ground Reaction Curve and Support Reaction Curve for the design of mine pillar support systems with respect to anticipated pillar loading and opening convergence. In addition, a conceptual three-tier solution to the pillar design problem, using a proper combination of numerical, analytical and data-driven methods is suggested, and a flowchart for the pillar design methodology is proposed. At the focus of this proposed method lies the Ground Reaction Curve (GRC) Concept. This research effort tries to verify the proposed pillar design flowchart using in-mine instrumentation and numerical modeling. For the purpose of this research, a deep longwall coalmine is instrumented to measure changes in pillar stress and associated roof convergence, due to mining activity. Subsequently, numerical models were developed in FLAC3D to model the geomechanical effects of underground longwall mining. The numerical modeling results are validated and calibrated using instrumentation data and a surface subsidence profile. The calibrated numerical models are further used to generate the Ground Reaction Curve for the overburden and Support Reaction Curve for the coal pillar. The comparison of both curves gives a detailed view of the overburden stability with respect to the mine pillar loading, in a more mechanics-based sense. The developed numerical approach can be used in future research and further development of this methodology for various mine types and different pillar support systems

Geotechnical properties of coal and mine pillar design in the Greymouth and Reefton coalfields, West Coast, South Island

This project has estimated coal strength in selected parts of the Greymouth and Reefton Coalfields in order to better estimate the size of pillars needed to maintain stability of the underground workings. Coal strength in known to decrease with increasing rank, and the Greymouth Coalfield displays to a very high rank gradient increasing from west to east. The mines assessed by this study were the Bishop Block, Strongman No. 2, Spring Creek, Roa and Terrace (Reefton Coalfield) Mines. Core samples could not be obtained from all locations so 63.5mm cubes and point load tests were used, and compared to a control group of with a known UCS/cube relationship, in order to develop an equation from which a UCS equivalent value could be determined. Coal strength drops from 24 MPa in the west (Strongman No. 2 E seam) to 1.3 MPa in the east of the coalfield (Roa mine Kimbell seam). Other coal properties also shows changes corresponding to changes in coal strength including carbon, volatile matter, ash, and the degree of cleating. Ash is the only one of these which is not related to increasing rank. Cleat frequency, which increases with coal rank has the most significant effect on coal strength. The equations of Bieniawski and Salamon-Munro have been used for pillar strength calculations with panel pillars designed to a factor of safety of 1.6. Optimum pillar sizes for each of the locations in this study have been calculated, but small changes to these sizes may be necessary depending on local conditions such as faults and sheared zones. Pillar design must take into account the chance of pillar shearing once seam dip increases above 20° as the shear strength becomes greatly reduced with increasing seam dip. Coal from the Spring Creek Mine shows a high degree of anisotropy and so pillars have been designed for specific seam dips at this location. Bearing capacity of the fireclay in the Terrace Mine is greatly reduced with increasing seam dip and overburden thickness, thus increasing the chances of floor heave. Pillars need to be of adequate size so as not to transfer excess overburden load to the mine floor, which would result in floor heave

Geotechnical properties of coal and mine pillar design in the Greymouth and Reefton coalfields, West Coast, South Island

This project has estimated coal strength in selected parts of the Greymouth and Reefton Coalfields in order to better estimate the size of pillars needed to maintain stability of the underground workings. Coal strength in known to decrease with increasing rank, and the Greymouth Coalfield displays to a very high rank gradient increasing from west to east. The mines assessed by this study were the Bishop Block, Strongman No. 2, Spring Creek, Roa and Terrace (Reefton Coalfield) Mines. Core samples could not be obtained from all locations so 63.5mm cubes and point load tests were used, and compared to a control group of with a known UCS/cube relationship, in order to develop an equation from which a UCS equivalent value could be determined. Coal strength drops from 24 MPa in the west (Strongman No. 2 E seam) to 1.3 MPa in the east of the coalfield (Roa mine Kimbell seam). Other coal properties also shows changes corresponding to changes in coal strength including carbon, volatile matter, ash, and the degree of cleating. Ash is the only one of these which is not related to increasing rank. Cleat frequency, which increases with coal rank has the most significant effect on coal strength. The equations of Bieniawski and Salamon-Munro have been used for pillar strength calculations with panel pillars designed to a factor of safety of 1.6. Optimum pillar sizes for each of the locations in this study have been calculated, but small changes to these sizes may be necessary depending on local conditions such as faults and sheared zones. Pillar design must take into account the chance of pillar shearing once seam dip increases above 20° as the shear strength becomes greatly reduced with increasing seam dip. Coal from the Spring Creek Mine shows a high degree of anisotropy and so pillars have been designed for specific seam dips at this location. Bearing capacity of the fireclay in the Terrace Mine is greatly reduced with increasing seam dip and overburden thickness, thus increasing the chances of floor heave. Pillars need to be of adequate size so as not to transfer excess overburden load to the mine floor, which would result in floor heave

LONG-TERM STABILITY MONITORING OF THE MINING BLOCKS IN ESTONIAN OIL SHALE MINES

This paper deals with long-term stability prediction and monitoring methods by room-and-pillar mining system. Roof-to-floor convergence and conditional thickness methods suit for calculations. They allow determination of the location, area and time of the collapse in a mining block. The uncertainty in time is less than 10 % at the 95 % confidence level. Roof-to-floor convergence method is preferred; it takes into consideration all the geological and mining feature in the critical area. Conditional thickness method demands supplementary investigations, determination of the influence factors on the process. The applicability of these methods is clearly demonstrated

MINING BLOCK STABILITY PREDICTION BY THE MONTE CARLO METHOD

This paper analyses the stability of the mining blocks by the Monte Carlo method in Estonian oil shale mines, where the room-and-pillar mining system is used. The pillars are arranged in a singular grid. The oil shale bed is embedded at the depth of 40-75 m. The processes in overburden rocks and pillars have caused the subsidence of the ground surface. Visual Basic for Application was used for the modeling. Through Monte Carlo simulation, room-and-pillar stable parameters can be calculated. Model allows determination of the probability of spontaneous collapse of the pillars and surface subsidence by the parameters of registered collapsed mining blocks. Proposed method suits as an express-method for stability analysis and failure prognosis. It is applicable in different geological conditions, where the room-and-pillar mining system is used

MINING BLOCK STABILITY ANALYSIS IN ESTONIAN OIL SHALE MINES BY STATISTICAL METHODS

This paper analysis the stability o f the mining blocks in Estonian oil shale mines, where the room-and-pillar mining system is used. The pillars are arranged in a singular grid. The processes in overburden rocks and pillars have caused the subsidence of the ground surface. Statistical analysis o f the pillars cross-sectional area evaluated the calculations. Normal distribution control allows determing the stability of a mining block. By normal distribution of the pillars cross-section area a potential collapse of a mining block can be expected. Theoretical and in situ investigations in Estonian oil shale mines showed that their results are close to the modeling ones. The surface subsidence parameters will be determined by conventional calculation schemes. Presented method suits well for mining block stability analysis and spontaneous failure prognosis

Min Metall Explor

Pillar stability continues to be a significant concern in multiple-level mining conditions, particularly for deep mines when pillars are not stacked or the thickness of interburden between mining levels is thin. The National Institute for Occupational Safety and Health (NIOSH) is currently conducting research to investigate the stability of pillars in multiple-level limestone mines. In this study, FLAC3D models were created to investigate the effect of interburden thickness, the degree of pillar offset between mining levels, and in situ stress conditions on pillar stability at various depths of cover. The FLAC3D models were validated through in situ monitoring that was conducted at a multiple-level stone mine. The critical interburden thickness required to minimize the interaction between the mining levels on top-level pillar stability was explored, where the top level mine was developed first followed by the bottom level mine. The model results showed that there is an interaction between numerous factors that control the stability of pillars in multiple-level conditions. A combination of these factors may lead to various degrees of pillar instabilities. The highest degree of local pillar instability occurred when pillar overlap ranges between 10 and 70%. On the contrary, the highest degree of stability occurs when the pillars are stacked, the underlying assumption is that the interburden between mining levels is elastic (never fails). Generally, for depths of cover investigated in this study, the stability of top-level pillars shallower than 100 m (328 ft) or with interburden thicknesses greater than 1.33 times the roof span-16 m (52.4 ft) in this study-does not appear significantly impacted by pillar offset. The results of this study improve understanding of multiple-level interactions and advances the ultimate goal of reducing the risk of pillar instability in underground stone mines.CC999999/ImCDC/Intramural CDC HHSUnited States

Laboratory and numerical investigation on strength performance of inclined pillars

Pillars play a critical role in an underground mine, as an inadequate pillar design could lead to pillar failure, which may result in catastrophic damage, while an over-designed pillar would lead to ore loss, causing economic loss. Pillar design is dictated by the inclination of the ore body. Depending on the orientation of the pillars, loading can be axial (compression) in horizontal pillars and oblique (compression as well as shear loading) in inclined pillars. Empirical and numerical approaches are the two most commonly used methods for pillar design. Current empirical approaches are mostly based on horizontal pillars, and the inclination of the pillars in the dataset is not taken into consideration. Laboratory and numerical studies were conducted with different width-to-height ratios and at different inclinations to understand the reduction in strength due to inclined loading and to observe the failure mechanisms. The specimens’ strength reduced consistently over all the width-to-height ratios at a given inclination. The strength reduction factors for gypsum were found to be 0.78 and 0.56, and for sandstone were 0.71 and 0.43 at 10? and 20? inclinations, respectively. The strength reduction factors from numerical models were found to be 0.94 for 10? inclination, 0.87 for 20? inclination, 0.78 for 30? inclination, and 0.67 for 40? inclination, and a fitting equation was proposed for the strength reduction factor with respect to inclination. The achieved results could be used at preliminary design stages and can be verified during real mining practice

Crown Pillar Optimization for Surface to Underground Mine Transition in Erzincan Bizmisen Iron Mine

Legal license boundary restrictions and economic considerations for extraction of vein type iron orebody in Erzincan/Bizmişen area of Turkey limit the overall production by open pit mining. Based on overall Stripping Ratio (OSR) assessments, ore production is decided to proceed by open pit mining to a certain depth and continue with a cut and fill type underground mining operation. A crown pillar is to be designed in the transition region from surface to underground operations. Crown pillar dimensioning effects the overall ore recovery in such small-scale ore bodies. Proper dimensioning has to ensure the stability of open pit mine slopes and underground mine structural units as underground mining progresses to upper levels. Dimensional optimization of the crown pillar is performed by empirical, deterministic and numerical modeling approaches. First, scaled span method was employed to estimate the safe crown pillar thickness providing the required service life. Later, rigid analysis method was used to check shear type failure from the abutments. Finally, 2D finite element modeling analysis was used to ascertain the stable crown pillar thickness. Input parameters of numerical models were produced from geotechnical site investigation work and laboratory experiments. Disturbance effect of surface mine production blasting on the slopes of rock mass above the underground workings was taken into account in deciding on the rock mass properties