Design practices for pillars in stone operations are notably different from coal. In general, stone pillars are extremely strong. However, when the width-to-height ratio falls, the potential for rib instabilities and pillar failures increases. Pillars are frequently encountered with width-to-height ratios below 1.5. Discontinuities within the pillars are much more significant with regard to pillar stability, and bench mining produces slender pillars which are more likely to be adversely affected by discontinuities. The work in this topic area is supported by the NIOSH Mining Ground Control program. See the NIOSH Mining Products page for software, guides, training materials or other items related to this topic. |
The science of pillar design in the U.S. goes back nearly a century. Interest in pillar design stems from concerns that inadequately designed pillars will result in pillar failure and associated roof and/or floor failure, sometimes in a catastrophic manner such as a bump or burst. In Longwall mining, the failure of a pillar may not mean that the pillar has no load-bearing capacity, but rather that the gate roads are no longer serviceable. A variety of reliable methods like those employed in the ALPS and ARMPS computer software have proven their value, as have the displacement-discontinuity models, MULSIM/NL and LAMODEL.History
The Evolution of Intelligent Coal Pillar Design: 1981-2006 (PDF, 664 KB, 2006)
This paper discusses progress in the science of coal pillar design over the past 25 years and some of the current research needs.
Software
ARMPS-HWM - Analysis of Retreat Mining Pillar Stability - Highwall Mining
ARMPS-HWM is a modification of Analysis of Retreat Mining Pillar Stability (ARMPS) software for designing web and barrier pillars for Highwall Mining (HWM).
ALPS - Analysis of Longwall Pillar Stability
ALPS is a computer program for use in the design of pillars for longwall mines.
ARMPS - Analysis of Retreat Mining Pillar Stability
ARMPS is a software tool for designing pillars for room-and-pillar retreat mining.
CMRR - Coal Mine Roof Rating
The Coal Mine Roof Rating is a roof classification system that was first introduced to the mining community in 1994. The CMRR is a rock mass classification system which calculates the inherent strength of coal mine roof. It can be used in the design of gateroads, roof bolt selection, extended-cut decisions, and numerical modeling.
Measurement & analysis
Analysis of Pillar Design Practices and Techniques for U.S. Limestone Mines (PDF, 1315 KB, 1999-12)
As underground stone mining expands to deeper cover situations, and more efficient mining methods are utilized, effective stone pillar design methods become more important. Seventy-two stone-mine pillar designs were examined or factors effecting stability and results are presented.
An Analysis of Rock Failure Around a Deep Longwall Using Microseismics (PDF, 442 KB, 2001-08)
A state-of-the-art, three-dimensional, full waveform, microseismic system was used to analyze the rock failure around a deep bump-prone longwall panel. Analysis of the recorded events is discussed.
Behavior of a Coal Pillar Prone to Burst in the Southern Appalachian Basin of the United States (PDF, 516 KB, 1990-08)
This paper describes research designed to quantify the behavior of a particular size abutment pillar prone to burst or bump (local terminology). This information reflects the overriding influence of the local geologic characteristics on bursts, provides the field measurements needed to verify concurrent modelling studies, and suggests some basic mechanisms associated with coal pillar bursts.
Coal Mine Seismicity and Bumps: Historical Case Studies and Current Field Activity (PDF, 301 KB, 2000)
Research on techniques that will reduce the hazards in the mining workplace associated with coal bumps.
The Effects of Roof and Floor Interface Slip on Coal Pillar Behavior (PDF, 531 KB, 1990)
This U.S. Bureau of Mines paper discusses the importance of an interface slip mechanism between the coalbed and the surrounding strata in controlling the extent and pattern of stresses and deformations in a coal pillar.
Effects of Weak Bands on Pillar Stability in Stone Mines: Field Observations and Numerical Model Assessment (PDF, 1398 KB, 2007)
Observations of pillar conditions in limestone mines showed that the presence of weak bands in the limestone can result in pillar damage at stresses that are lower than one would otherwise expect. The objective of this NIOSH study was to investigate the mechanism of failure caused by the presence of weak bands by using a series of numerical models based on realistic physical properties.
Evaluation of the Effects of Length on Strength of Slender Pillars in Limestone Mines Using Numerical Modeling (PDF, 164 KB, 2007)
Numerical modeling is used to evaluate the degree the pillar strength is increased with a rectangular pillar over a square pillar.
Evaluation of the Strength of Slender Pillars (PDF, 1206 KB, 2006-01)
This paper investigates some of the issues affecting pillar strength at low width-to-height ratios in hard, brittle rock. The investigation includes an evaluation of empirical pillar strength data presented in the literature and observations of pillar performance in underground limestone mines in the Eastern United States, supplemented by numerical modeling in which failure processes and sensitivity of slender pillars to variations in rock mass properties are evaluated. The results showed that the strength of slender pillars is more variable than that of wider pillars.
An Examination of the Loyalhanna Limestone's Structural Features and their Impact on Mining and Ground Control Practices (PDF, 2678 KB, 2002)
A close look at the Loyalhanna Limestone of southwestern Pennsylvania reveals a complex structural environment. Geologic and engineering analyses were performed using gas well and core logs, outcrop examinations, underground observations, and mine maps. Many of the structural conditions cause ground control problems at local quarries.
The Forgotten Denominator: Pillar Loading (PDF, 588 KB, 2000)
In the past few decades, a considerable amount of effort was directed at accurately determining the coal/pillar strength to use for safely designing coal mines. This paper addresses the deficit of pillar loading research by exploring the accuracy of the empirical abutment load calculations using insight provided by an elastic overburden model, a laminated overburden model and field obserbvations.
International Experience with Longwall Mining into Pre-Driven Rooms (PDF, 663 KB, 1998)
Unusual circumstances may require that a longwall retreat into or through a previously driven room; there have been a number of spectacular failures which exposed miners to serious roof fall hazards. To help determine what factors contribute to such failures, NIOSH compiled a comprehensive international database of 130 case histories.
The NIOSH Shield Hydraulics Inspection and Evaluation of Leg Data (Shield) Computer Program (PDF, 224 KB, 2002)
Longwall shields provide essential ground control in longwall mining, yet a high percentage of shields are operating at less than peak capacity and many at well below the rated support capacity due to defective hydraulic cylinders or malfunctions in other hydraulic components. NIOSH developed a computer program that is designed to analyze leg pressure data and identify shields that are not performing to rated specifications.
Numerical Modeling for Increased Understanding of the Behavior and Performance of Coal Mine Stoppings (PDF, 204 KB, 2004)
In underground coal mines, concrete block stoppings are widely used to control mine ventilation. A combination of numerical simulations and large scale physical tests were employed to develop a scientific understanding of stopping performance. The product of this study is a numerical model that can be used to evaluate the performance of stopping materials and different wall geometries in a controlled environment.
Optimum Mine Designs to Minimize Coal Bumps: A Review of Past and Present U.S. Practices (PDF, 813 KB, 1992)
This presents a review of past and present Unites States mining practices for optimum design to minimize coal bumps.
Pillar Mechanics of Coal Mine Bursts: A Control Strategy (PDF, 1195 KB, 1995)
In this study, data available for a range of U.S. bursts were considered in an effort to evaluate both common and distinctive features of coal mine bursts.
Pillar Stability Issues Based on a Survey of Pillar Performance in Underground Limestone Mines (PDF, 1137 KB, 2006)
A survey of pillar conditions was carried out at 21 operating limestone mines that use the room-and-pillar method. Nine cases of pillar instability were recorded. It is concluded that pillar instability is most likely to be caused by unfavorable geological structures in pillars with width to height ratios of less than 1.5. Stress related instability such as rib spalling becomes more prevalent when the average pillar stress approaches approximately 20 MPa.
Pillar Strength and Design Methodology for Stone Mines (PDF, 690 KB, 2008)
This paper presents a pillar design methodology that was developed from a study of pillar performance in operating stone mines and concludes with guidelines for applying a developed equation and selecting appropriate input parameters.
Possible Mechanism for Surface Vibrations Near Maxwell Hill, West Virginia (PDF, 264 KB, 1990-08)
This presents details of a research program initiated in Maxwell Hill, WV to determine whether subsurface ground movements associated with the abandoned mines could be responsible for the seismic activity experienced at the surface.
Toward Pillar Design To Prevent Collapse of Room-and-Pillar Mines (PDF, 866 KB, 2001)
Three alternative design approaches to decreasing the risk of large-scale catastrophic collapses are described: the containment approach, the prevention approach, and the full-extraction approach. Until good data on the post-failure behavior of pillars become available, the containment and full-extraction options are the safest. The limitations in our ability to evaluate both the stability of old workings and the long-term performance of room-and-pillar mines are described.
The Unpredictable Life Cycle of a Coal Pillar (PDF, 1166 KB, 2007)
A unique circumstance created by monitoring a pre-driven longwall recovery room permitted measuring the stresses of a coal pillar throughout its entire life cycle in less than a week. The complete transformation, or life cycle, took place in less than 12 hours. Roof to floor closure measurements, combined with the timing of the pillar behavior, provides a detailed look at the uncontrollable convergence of underground mine openings.
Case studies
Development of a Statistical-Analytical Approach for Assessing Coal Bump Potential (PDF, 331 KB, 1997)
Researchers from private industry and NIOSH have studied mechanisms of violent failure and have identified individual factors that contribute to coal bumps. In an attempt to develop tools for assessing coal bump potential, the authors initiated a comprehensive study using information from 25 case studies undertaken in U.S. mines.
A Study of the Ground Control Effects of Mining Longwall Faces Into Open or Backfilled Entries (PDF, 1525 KB, 2001-01)
This paper summarizes case studies where unusual circumstances required that a longwall retreat into or through previously driven rooms. Two types of roof failure are identified and discussed.
Engineering controls
Design Methodology for Standing Secondary Roof Support in Longwall Tailgates (PDF, 666 KB, 1999)
This paper discusses a design methodology for standing secondary tailgate supports. The methodology uses the performance characteristics generated in the NIOSH Mine Roof Simulator (MRS) to match the stiffness and load characteristics of various supports to the measured ground reaction behavior.
Design Methods to Control Violent Pillar Failures in Room-and-Pillar Mines (PDF, 624 KB, 1997)
Methods are described to limit "cascading pillar failure", which occurs when one pillar in a mine layout fails, transferring its load to neighboring pillars, which causes them to fail, and so forth. Design approaches are illustrated with practical examples for coal mining at shallow depth.
Design Parameters of Roof Support Systems for Pre-Driven Longwall Recovery Rooms (PDF, 329 KB, 2004-02)
A calibrated three-dimensional finite element model, developed in conjunction with an underground test area, provided the critical components and design principals for a comprehensive parametric study.
Examination of Design and Operation Practices for Longwall Shields (PDF, 1569 KB, 1992)
This report examines shield design and operation practices and their consequences for the utilization of high-capacity shield support systems. An optimization goal is to minimize support loading by selecting an active shield setting force that is compatible with strata behavior and shield loading characteristics.
Pillar Design and Strategies for Retreat Mining (PDF, 8693 KB, 1994)
The U.S. Bureau of Mines is field testing and refining a "user friendly" computer program called Analysis of Retreat Mining Pillar Stability (ARMPS) to estimate abutment pressures developed during pillaring. The goal is to provide miner safety and an efficient recovery of reserves through design of sufficiently sized production pillars that will prevent pillar squeezes, excessive pillar spalling, severe floor heave, roof falls, and pillar bumps.
Pillar Design Issues for Underground Stone Mines (PDF, 859 KB, 1999-01)
This paper examines current design practices for stone mines and discusses issues for safe mine layouts so that a rational first approach towards balancing the demands for increased production can be weighed against increased risk to worker safety from rib instabilities and pillar failures.
Proceedings of the Second International Workshop on Coal Pillar Mechanics and Design (PDF, 3559 KB, 1998-06)
A collection of 15 papers from leading ground control specialists in the United States, Canada, Australia, the United Kingdom, and the Republic of South Africa. The papers address the entire range of issues associated with coal pillars and have a decidedly practical flavor. Topics include numerical modeling, empirical design formulas based on case histories, field measurements, and postfailure mechanics.
Proceedings: New Technology for Ground Control in Retreat Mining (PDF, 5087 KB, 1997-03)
This proceedings volume contains papers presented at technology transfer seminars sponsored by the National Institute for Occupational Safety and Health (NIOSH) on New Technology for Ground Control in Retreat Mining.
Reducing the Risk of Ground Falls During Pillar Recovery (PDF, 1948 KB, 2003-12)
Pillar recovery has been associated with nearly one third of roof fall fatalities in underground coal mines during the past decade. This paper discusses design methods and technologies that have been transferred to the mining community and implemented.
Sizing of Final Stumps for Safer Pillar Extraction (PDF, 127 KB, 2001-08)
The risk of major pillar falls can often be reduced by leaving final stumps that are large enough to protect the intersection, but small enough that they do not inhibit the caving of the gob. Analyses were conducted to assess the effect of seam height and depth of cover on the potential variation in the size of remnant stumps.
The State-of-the-Art in Coal Pillar Design (PDF, 502 KB, 1999)
Recently, powerful design methods have emerged from analysis of large data bases of real-world pillar successes and failures. In the process, our understanding of pillar mechanics has been greatly enriched.
Technology News 492 - Proceedings of the Second International Workshop on Coal Pillar Mechanics and Design (PDF, 230 KB, 2001-08)
An announcement of the availability of the proceedings of the Second International Workshop on Coal Pillar Mechanics and Design. Included are 15 papers from leading ground control specialists in the United States, Canada, Australia, the United Kingdom, and the Republic of South Africa. The papers address the entire range of issues associated with coal pillars and have a decidedly practical flavor.
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