Pillar Strength Research Articles

Understanding roof deformation mechanics and parametric sensitivities of coal mine entries using the discrete element method

Although conventional coal mine designs are conservative regarding pillar strength, local failures such as roof-falls and pillar bursts still affect mine safety and operations. Previous studies have identified that discontinuous, layered roof materials have some self-supporting capacity. This research is a preliminary step towards understanding these mechanics in coal-measure rocks. Although others have considered broad conceptual models and simplified analogs for mine roof behavior, this study presents a unique numerical model that more completely represents in-situ roof conditions. The discrete element method (DEM) is utilized to conduct a parametric analysis considering a range of in-situ stress ratios, material properties, and joint networks to determine the parameters controlling the stability of single-entries modeled in two-dimensions. Model results are compared to empirical observations of roof-support effectiveness (ARBS) in the context of the coal mine roof rating (CMRR) system. Results such as immediate roof displacement, overall stability, and statistical relationships between model parameters and outcomes are presented herein. Potential practical applications of this line of research include: (1) roof-support optimization for a range of coal-measure rocks, (2) establishment of a relationship between roof stability and pillar stress, and (3) determination of which parameters are most critical to roof stability and therefore require concentrated evaluation.

Electro-chemo-mechanical coupling of nanoporous gold at the microscale

The observation of reversible strengthening and stiffening of nanoporous gold (NPG) under electrochemical potential has opened opportunities to exploit this material for multifunctional applications. Yet the complex structural geometry and length-scales involved make a definitive understanding of structural correlations to the behaviors difficult at best. Achievement of coupled electro-chemo-mechanical testing at the micrometer scale is a key step toward this goal. Here, we introduce an experimental approach to investigate the elastic and plastic behaviors of NPG under electrochemical potential at the microscale using a modified nanoindentation setup and multiple load function. The in situ experiments in electrolyte show a significant increase by 32% in strength of pillars in a positive potential regime where oxygen adsorption occurred. This response was found to be reversible, which agrees with macroscopic results, while the elastic modulus was shown to be insensitive to the applied potential—an observation inconsistent with recent bulk dynamic mechanical analysis results.

Effect of Shear Stresses on Pillar Stability: A Back Analysis of the Troy Mine Experience to Predict Pillar Performance at Montanore Mine

This paper describes the results of a back analysis of pillar failures at Troy Mine, Montana, and the use of this experience to make forward predictions on pillar stability in the nearby Montanore deposit which lies in a similar geomechanical setting. At Troy Mine, a progression of pillar failures in areas within the Middle Quartzite of the Revett formation led to the observed surface subsidence. The Troy Mine experience was used to understand the level of stresses and failure mechanism leading to the collapse of some pillars in the North Orebody to estimate pillar strength in quartzite beds within Troy’s mountainous terrain. The model elucidated that the dipping orebody geometry in relation to topography led to shear stresses in pillars at Troy Mine. Shear stresses resulted in significant loss of confinement in pillar cores (many theoretically in tension), even at width-to-height ratios that would be deemed stable under zero shear stress (flat seam under flat topography). A calibrated model was achieved, which allowed us to evaluate the impact that different pillar geometric characteristics (such as width, length, height, and shape) have on pillar performance under shear conditions for different depths and extraction ratios. Design charts were then generated to provide guidance on pillar geometry based on expected demand. Mine-wide models were developed to predict the level of vertical stress and horizontal shear stress for pillars in the different ore-bearing beds at Montanore. A sensitivity study was performed for various conditions, including extraction ratio, spatial location under the mountainous terrain, and local orebody geometry with the aim of performing a mine-wide evaluation of the factor of safety against shear. The results of the analyses performed in the present work show that the use of design methods that do not take the effect of shear stresses into account may result in under-designed pillars, while a false impression of rock mass strength could be derived from back analysis.

An approach for support design in depillaring panels of coal mines

Among different methods of underground mining technology adopted for coal extraction, bord and pillar mining is the most popular and predominant method of underground mining in India. This method is characterised with small capital investment and faster rate of return on capital. Varying geomining conditions demand scientific approach for design of supports in depillaring panels. In view, this there is a decisive lack of judgement on support design, given the geomining conditions. Support design methods in vogue for depillaring districts do not takes into account, among other parameters, the inclination of the coal seam and direction of in situ stress. Thus, there exists a need for research work aimed at investigating the effective role, inter-alia, of direction of in situ horizontal stress and inclination of coal seams to ensure a better condition of roof and efficient design of effective support system. The authors through this paper intend to address similar problems being faced by the industry. Detailed numerical models for one depillaring panel in Thesgora Mine and in two depillaring panels of Kakatiya Khani (KTK)-KTK-1 and KTK-5 Incline mines respectively of Bhupalpalli area under study have been developed. The models are then calibrated using the empirical relations of pillar strength. Stage-wise extraction in the models is carried out to simulate the change in stress and displacement as the mining activity progresses. Based on the geo-mining data and the numerical models, the rock load heights at four strategic locations namely split gallery, slice, junction and goaf edge have been evaluated in all the depillaring panels under study. Generalised equations for rock load height at different strategic locations in a depillaring panel have been developed. The said empirical relations for rock load height have been derived using parametric study of the numerical models developed for depillaring panels of the selected mines. The equations so developed have a correlation coefficient of maximum 87.8 % and a minimum of 80.5%.

Risk assessment for the cascading failure of underground pillar sections considering interaction between pillars

The cascading failure of underground pillar sections poses a risk to miners and surface structures. Assessing the risk of underground mining panels contributes to the prevention and control of the catastrophic failure. To resolve the challenge in quantifying the effect of failure of a single pillar on the risk of an entire pillar section, a new risk assessment model incorporating the stability of individual pillars and the load transfer between pillars was proposed to investigate the cascading failure of pillar sections. The load transfer process from failed pillars to adjacent ones was effectively quantified by the relationship between transferred incremental load and transferring distance. The influence of uncertainties of pillar strength, caused either by intrinsic strength variability of a single pillar (quantified by coefficient of variation COV) or the non-uniform deterioration process of pillar sections (quantified by correlation coefficient ρ), was investigated on the probability of cascading failure. Risk mapping was also performed on two representative historical collapsed pillar sections using the risk evaluation method proposed to illustrate the risk level of each pillar to trigger cascading failure. The proposed model could represent realistic load transfer process and provide reliable risk assessment results for pillar sections. The results showed that the reliability of pillar sections is significantly influenced by both intrinsic variability in pillar strength and intercorrelation of strength between pillars. The probability of cascading failure increases with increasing COV, which verifies that geological uncertainties increase the risk of collapse in pillar sections. The impact of COV on probability gradually decreases to negligible levels as ρ increases to 1, which means that the synchronous variation of pillar strength among a pillar section can significantly compensate for the impact of COV. The proposed approach provides a distinct perspective on understanding sudden failure of high-risk pillars and contributing to the risk control for abandoned pillar sections.

New coal pillar strength formulae considering the effect of interface friction

Coal pillars perform the vital function of sustaining the weight of the overburden and protecting the entries and crosscuts during mine development and production. The strength of coal pillars is known to be greatly influenced by the friction of coal/roof and coal/floor interfaces. Unfortunately, none of the current empirical formulae for pillar strength has considered the effect of this so-called interface friction. This paper develops new coal pillar strength formulae considering the effect of interface friction. The formulae, which are in linear and power forms, are derived based on a series of UCS tests on coal specimens at three different interfaces (high: c = 124 kPa and μ = 0.40; medium: c = 76 kPa and μ = 0.22; and low: c = 55 kPa and μ = 0.13) and at ten width-to-height (w/h) ratios (w/h = 2–8, 10, 12, and 16). Having compared the formulae against stable and unstable pillar cases from different areas around the world, this paper finds that: (1) the low-friction formula of the linear form Sp=2.7(0.12+0.88wh) provides the best representation of the pillar cases database; (2) to improve pillar strength prediction, a safety factor = 1.5 is suggested to be used for all pillar designs using the linear low-friction formula (except for USA coal pillars with w/h ≤ 6 that need a higher SF up to SF = 2.5); (3) when the friction of the coal/roof and coal/floor interfaces is not known, it is beneficial to assume that the interface has a low friction value and thus to use the linear-low friction formula to predict pillar strength.

A Case Study of the Collapse of Slender Pillars Affected by Through-Going Discontinuities at a Limestone Mine in Pennsylvania

The sudden collapse of approximately 3 Ha of room-and-pillar workings at a limestone mine in southwestern Pennsylvania in 2015 resulted in an air blast that injured three mine workers. Subsequent investigations showed that an area encompassing 35 pillars had collapsed. The pillars were 9-10 m wide and up to 18 m high. A notable geologic feature is the through-going joints that dip at 50-80° and can extend from the roof to the floor of the pillars. These structures are thought to have weakened the pillars well below the strength that is predicted by empirical equations for hard-rock pillar design. This paper presents the relevant geotechnical data related to the collapsed area and numerical model results that were used to estimate the pillar loading underneath the variable topography, and compares the pillar loads to some established hard-rock pillar strength equations. The outcome is also compared to a strength equation that was developed specifically for limestone mines in which the negative impact of large angular discontinuities is explicitly accounted for. The results show that established hard-rock pillar strength equations do not adequately account for the impact of large through-going discontinuities on the strength of slender pillars. The equations would have significantly overestimated the strength of the pillars at the case study mine. The critical state of the workings would have been predicted correctly by the limestone pillar strength equation that accounts for the large discontinuities.

Assessment of the Strength of Inclined Coal Pillars through Numerical Modelling based on the Ubiquitous Joint Model

The inclined coal pillars, formed during the underground extraction of the inclined coal seam, are different from the flat coal pillars due to the high strength anisotropy of the inclined bedded rock, and the asymmetrical stress distribution. To ease the manoeuvring of the men and machinery in the inclined coal mine, the pillars are developed along the apparent dip. Thus, the pillars become rhombus-shaped with acute-angled corners which crush rapidly due to the high stress concentration. No suitable formula is found to estimate the strength of the inclined coal pillar that incorporates all these factors. Thus, the use of the available coal pillar strength formulae may endanger the extraction of the inclined coal seams. This study elucidates the procedures to estimate the strength of the inclined coal pillars by the numerical modelling technique. The ubiquitous joint model is used to simulate the shearing characteristics of the inclined strata. The parametric study shows that the strength of the inclined coal pillars decreases with the increase of the dip of the coal seams. It is also obtained that the strength of the pillars decreases with the decrease of the value of the acute angle of the corners. The peak stress distribution and the strain accumulation over the inclined pillars at the time of failure are plotted in three-dimensional graphs which show the asymmetrical characteristics of the inclined coal pillars. The analysis of variance shows that the dip of the coal seams and the acute angles of the corners have a statistically significant effect on the strength of the inclined coal pillars. Based on the results of the simulation, the best fit relation is established by the multivariate non-linear regression technique to estimate the strength of the inclined coal pillars. The coefficient of determination (R2) and the root mean square error of the model are obtained as 0.92 and 0.065, respectively. The validation of the developed model has been carried out by the stable and failed inclined pillar cases of different underground inclined coal mines in India. Comparisons of the safety factors, obtained from the developed model and the flat pillar strength formula, indicate that the flat pillar strength formula overestimates the strength of the inclined coal pillars. The developed model can be used to design the inclined coal pillars for safe extraction of inclined coal seams.

Generalised Analytical Models for the Strength of the Inclined as well as the Flat Coal Pillars using Rock Mass Failure Criterion

The assessment of the strength of the coal pillars is essential for the safe extraction of the coal seam. All the pillar strength formulae used worldwide are developed for the flat coal pillars. Therefore, their adoption in evaluating the strength of the inclined coal pillars may endanger the workings of the inclined coal seams. The strength of an inclined coal pillar should be estimated by considering the inclination of the coal seam and its associated behaviour, because the shearing effect along the true dip aggravates the instability of the inclined coal pillars. In this paper, generalised analytical solutions have been developed to estimate the strength of the coal pillars which can be applied for both the inclined and flat coal pillars. The mathematical models are derived to obtain the confining stress in the coal pillar and the corresponding peak stress at the time of its failure using a rock mass failure criterion. The mathematical expressions are also developed for the stress distribution over the pillar considering the increase of the shearing effect with the dip of the coal seam. The Mohr–Coulomb criterion is considered for the shear characteristics at the interfaces of pillar–floor and pillar–roof. The asymmetrical stress distribution and failure along the dip-rise and the strike directions of the inclined coal pillars are addressed in this study. The concept of the confined core and three-dimensional stress distribution over the coal pillar are used to derive the strength formulae for the square, rectangular, and very long pillars. The performance of the derived strength formulae is assessed by the stable and failed cases of the flat and the inclined coal pillars. It is observed that all the inclined and flat coal pillars cases are correctly predicted by the derived generalised strength formulae. According to the derived strength formulae, the strength of the inclined coal pillar decreases with the increase of the inclination of the coal pillar. This paper also describes the variation of the strength of the inclined coal pillars with respect to the coal seam inclinations and the frictional properties of the contact planes for the different width-to-height ratios.

Evolution of Local Mine Stiffness with Mining Process: Insight from Physical and Numerical Modeling

It is well known that local mine stiffness (LMS) plays a role in the violent failure of a pillar prone to strainburst. Very limited laboratory experiments have been carried out to simulate bursts caused by a gradual decrease in the local mine stiffness resulting from excavation. In this study, an experimental study is performed aiming to provide insight into the coal burst caused by a decrease in the LMS. Coal bursts were produced using a physical model in which excavation was performed stage by stage to produce a gradual decrease in the LMS. A numerical study was then carried out to study the evolution of the LMS with excavations. The effect of pillar strength, immediate roof stiffness, and excavation sequence was examined using the numerical model. The result suggests that the LMS at a given condition depends on a combination of various factors including pillar strength, pillar stiffness, host rock stiffness, etc. The interaction between these factors may play a key role in the LMS. It is also found that mining and excavation at the far field (e.g., mining at the adjacent panel) may not cause a great decrease in the LMS and thus is less likely to produce coal bursts at distant coal pillar. Great concern should be paid when mining and excavation activities are carried out at the current panel.

Strength of inhomogeneous rib pillars

The author analyzes stress state of a piecewise-homogeneous pillar with a curved interface. Based on the system of singular integral equations, relations are derived for stresses and displacements at the boundary of the pillar and at the interfaces of the pillar components. Examples of numerical implementation are given.

Assessing the effects of rock mass gradual deterioration on the long-term stability of abandoned mine workings and the mechanisms of post-mining subsidence – A case study of Castle Fields mine

Post-mining subsidence is a serious problem in urban areas constructed above abandoned mine workings. Subsidence may severely damage the environment and threaten the stability of surface structures and infrastructure. This study aims at analysing the role of gradual weakening of rocks on chimney collapse and sinkhole subsidence formation, in shallow mine workings. The factors leading to the degradation of the rock masses mechanical parameters are explained. The long-term stability of mine workings at Castle Fields mine, in West Midland, UK, is investigated as a case study. Progressive chimney collapse resulted in a large sinkhole in this area, and damaged the Dudley cricket ground, in 1987. The modified Tributary Area Method, TAM, is used to assess the effects of deterioration on the long-term loading of pillars. The long-term strength of pillars is also assessed based on empirical and theoretical approaches. Then, three-dimensional (3D) numerical modelling is used to assess the role of deterioration on the mechanisms of chimney failure and sinkhole development. A deteriorating model is proposed to incorporate the effects of gradual deterioration of rock mass mechanical parameters. Results of this study indicate that gradual deterioration due to weathering plays a significant role in the propagation of cavity and the formation of sinkholes above abandoned mine workings. Ignoring the gradual softening and the effects of deterioration, in the stability analysis of mine workings, may result in catastrophic consequences.

In Situ Observation Reveals Local Detachment Mechanisms and Suction Effects in Micropatterned Adhesives

AbstractFibrillar adhesion pads of insects and geckoes have inspired the design of high‐performance adhesives enabling a new generation of handling devices. Despite much progress over the last decade, the current understanding of these adhesives is limited to single contact pillars and the behavior of whole arrays is largely unexplored. In the study reported here, a novel approach is taken to gain insight into the detachment mechanisms of whole micropatterned arrays. Individual contacts are imaged by frustrated total internal reflection, allowing in situ observation of contact formation and separation during adhesion tests. The detachment of arrays is found to be governed by the distributed adhesion strength of individual pillars, but no collaborative effect mediated by elastic interactions can be detected. At the maximal force, about 30% of the mushroom structures are already detached. The adhesive forces decrease with reduced air pressure by 20% for the smooth and by 6% for the rough specimen. These contributions are attributed to a suction effect, whose strength depends critically on interfacial defects controlling the sealing quality of the contact. This dominates the detachment process and the resulting adhesion strength.

Strengthening and deformation mechanisms in nanolaminated graphene-Al composite micro-pillars affected by graphene in-plane sizes

Graphene is considered to be an ideal reinforcement in metals, and its strengthening to the metal matrix is found to be the combined effect of load-sharing and the alteration of dislocation activity in the matrix. In this study, uniaxial compression tests were performed on graphene (reduced graphene oxide, RGO)-Al nanolaminated composite micro-pillars with different RGO in-plane sizes and laminate orientations. We elaborately controlled the processing parameters so that the composites with different RGO in-plane sizes shared the same RGO volume concentration and total RGO/Al interface area. It was found that the strength of RGO-Al composite pillars increased with decreasing RGO in-plane size, regardless of the laminate orientations relative to the loading direction (iso-strain or iso-stress). A clear transition in deformation mechanism existed in RGO-Al composite pillars having iso-strain configuration, from shear fracture in pillars with larger RGO in-plane size, to kink banding in pillars with smaller RGO nanosheets. These observations were interpreted by isotropic and kinematic hardening mechanisms, the superior constraining effect of smaller RGO nanosheets over dislocation transmission and motion, and the crack propagation in Al lamellae across the pillar interior. This work indicates that the mechanical behavior of graphene-reinforced metal matrix composites (MMCs) can be tailored by carefully tuning the geometry of the graphene nanosheets, without changing their type and concentration.

Coal pillar strength analysis based on size at the time of failure

Coal pillar strength analysis based on size at the time of failure

Limitations and potential design risks when applying empirically derived coal pillar strength equations to real-life mine stability problems

The method of determining coal pillar strength equations from databases of stable and failed case histories is more than 50 years old and has been applied in different countries by different researchers in a range of mining situations. While common wisdom sensibly limits the use of the resultant pillar strength equations and methods to design scenarios that are consistent with the founding database, there are a number of examples where failures have occurred as a direct result of applying empirical design methods to coal pillar design problems that are inconsistent with the founding database. This paper explores the reasons why empirically derived coal pillar strength equations tend to be problem-specific and should be considered as providing no more than a pillar strength “index.” These include the non-consideration of overburden horizontal stress within the mine stability problem, an inadequate definition of super-critical overburden behavior as it applies to standing coal pillars, and the non-consideration of overburden displacement and coal pillar strain limits. All of which combine to potentially complicate and confuse the back-analysis of coal pillar strength from failed cases. A modified coal pillar design representation and model are presented based on coal pillars acting to reinforce a horizontally stressed overburden, rather than suspend an otherwise unstable self-loaded overburden or section, the latter having been at the core of historical empirical studies into coal pillar strength and stability.

Analysis of coal pillar stability (ACPS): A new generation of pillar design software

Thirty years ago, the analysis of longwall pillar stability (ALPS) inaugurated a new era in coal pillar design. ALPS was the first empirical pillar design technique to consider the abutment loads that arise from full extraction, and the first to be calibrated using an extensive database of longwall mining case histories. ALPS was followed by the analysis of retreat mining stability (ARMPS) and the analysis of multiple seam stability (AMSS). These methods incorporated other innovations, including the coal mine roof rating (CMRR), the Mark-Bieniawski pillar strength formula, and the pressure arch loading model. They also built upon ever larger case history databases and employed more sophisticated statistical methods. Today, these empirical methods are used in nearly every underground coal mine in the US. However, the piecemeal manner in which these methods have evolved resulted in some weaknesses. For example, in certain situations, it may not be obvious which program is the best to use. Other times the results from the different programs are not entirely consistent with each other. The programs have also not been updated for several years, and some changes were necessary to keep pace with new developments in mining practice. The analysis of coal pillar stability (ACPS) now integrates all three of the older software packages into a single pillar design framework. ACPS also incorporates the latest research findings in the field of pillar design, including an expanded multiple seam case history data base and a new method to evaluate room and pillar panels containing multiple rows of pillars left in place during pillar recovery. ACPS also includes updated guidance and warnings for users and features upgraded help files and graphics.

Subsidence over room and pillar retreat mining in a low coal seam

The objective of this paper is to study the behavior of a low thick and low depth coal seam and the overburden rock mass. The mining method is room and pillar in retreat and partial pillar recovery. The excavation method is conventional drill and blast because of the small production. The partial pillar recovery is about 30% of the previous pillar size, 7 m × 7 m. The roof displacement was monitored during retreat operation; the surface movement was also monitored. The effect of the blasting vibration on the final pillar strength had been considered. Due to blasting, the pillar reduced about 20%. The consequence is more pillar deformation and roof vertical displacement. The pillar retreat and ground movement were simulated in a three-dimensional numerical model. This model was created to predict the surface subsidence and compare to the subsidence measured. This study showed that the remaining pillar and low seam reduce the subsidence that was predicted with conventional methods.

Research on Catastrophic Pillar Instability in Room and Pillar Gypsum Mining

Gypsum mines in China are mostly exploited through room and pillar mining. Due to backward mining technology and a long history of mining, a great number of pillars were left in gypsum mines. Many serious work safety accidents occurred as the result of goaf instability in history, which posed severe threats to the security of people’s lives and property. Based on the characteristics of surrounding rock damage, this research improved the constitutive equation of gypsum rock mass damage by establishing a damage evolution model and introducing a shape parameter. Meanwhile, the cusp catastrophe equation was deduced based on the catastrophe theory and the constitutive equation of gypsum rock mass damage, thus summarizing the criteria for pillar instability; the pillar safety factor was obtained by means of the interrelation between pillar load and pillar strength. Based on the criteria for pillar instability and the pillar safety factor obtained, the necessary and sufficient conditions for pillar stability were concluded. These conclusions are of significance in that they provide theoretic reference for the treatment of gypsum goaf, as well as for further mining.

Performance of inclined pillars with a major discontinuity

Discontinuities are an inherent part of the rock mass and majorly affect the stability of the excavation skin and pillars. The dip of the discontinuities and their properties also have a significant effect on the strength of the pillars. Empirical approaches are commonly used to determine the pillar strength but can overestimate the strength and don’t consider the inclination of the pillars and the strength reduction caused by discontinuities. Numerical modeling is a powerful tool and if calibrated can be used to evaluate the strength of the pillars with discontinuities having a range of properties. The effect of a discontinuity on inclined pillars was conducted which has been seldom considered in evaluating the pillar strength. Three-dimensional vertical pillars were simulated, and the pillar strength was calibrated to accepted theoretical results and then the discontinuities were introduced in different pillar inclinations with distinct width to height ratios to gain an insight into the effective pillar strength reduction. Based upon the results, it was found that the discontinuities have a significant effect with the increase in the inclination of the pillars even at a higher width to height ratios.