Pillar Strength Research Articles

Mechanical mechanism of dip effect on bearing capacity of the pillar strength

Abstract The dip effect on pillar strength in underground ore-body mining is well established, but the variation in stress path (magnitude and direction of stress) due to changing inclination angles requires further study. Using elasticity theory, the Euclidean mean stress tensor characterizes the stress state in pillar zones. Numerical simulations provided the second-order tensor of peak stress for each pillar unit. Through tensor statistical analysis, the Euclidean mean stress tensor matrix was calculated, and its eigenvalues and eigenvectors, representing the magnitude and direction of the principal stress, were derived. This analysis explained the intrinsic dip effect on pillar strength through principal stress characteristics. Finally, the pillar strength envelope function for varying width-height ratios at any dip angle was obtained using the random gradient descent algorithm. Results indicate that in the peak stress state, the average principal stress directions of the pillar change with orebody dip angle, affecting the stress path. The average principal stress increases with pillar size due to increased constraints. These findings offer theoretical insights for pillar design and stability analysis.

Determination of strength of UG2 chromitite pillars at Impala Platinum from laboratory tests and FLAC3D

The results of FLAC3D modelling on chromitite Upper Group 2 Reef pillars from the Bushveld Complex of South Africa are described. The model input parameters were determined from laboratory triaxial tests with post-failure measurements. These geomechanical tests were performed on rock materials within the pillars and the immediate pillar foundations. In the models, post-failure behaviour was simulated using the concept of cohesion softening. Models were built to determine the strength and behaviour of pillars with a width-to-height ratio of approximately two. The original research aimed to find a suitable depth below the surface where Impala Platinum Ltd could safely introduce crush pillars; however, the paper only describes the model results: the ideal depth of the crush pillar introduction is not discussed. The results of the modelling are compared with the PlatMine formula for peak pillar strength and with an underground instrumented pillar located elsewhere on the same reef. Some insight into the effects of grid size on strength is also provided. Further underground measurements are recommended to verify the model results.

Beyond the empirical pillar design method: The strain criterion and the pillar load inversion concepts

Pillar design is a crucial aspect of underground mining engineering, as it directly impacts the safety, stability, and overall effectiveness of mining operations. In this paper we present a method to calculate pillar stability based on the strain criterion and pillar load inversion concept, which complements the empirical pillar strength formulae. Those formulae do not account for certain parameters that can affect the stability of the pillars, e.g. weak layers, changes in stress orientations due to mining, orebody dip, mining layout complexity, and the influence of regional stabilizing pillars. The approach presented here is not intended to replace the empirical method; rather, we suggest using it as the initial step in the pillar design process. This should be followed by iterative numerical modelling to study pillar behaviour, define pillar stability, and optimize the pillar design by considering site-specific and representative rock mass properties and criteria.

The influence of cracks on pillar strength based on SRM and DFN models

This research focuses on the examination of natural fractures within underground mines, emphasizing their substantial impact on the strength and stability of ore pillars. The study adopts the Strength Reduction Method (SRM) theory and employs the Discrete Fracture Network (DFN) model, offering a novel approach to investigating the behavior of fractured rock masses. The objective of this article is to analyze the influence of natural fractures on the strength of ore pillars by employing SRM and DFN methods. The research begins by establishing a multi-level amplification program that incorporates a homogenization process. The findings reveal that, for a W/H ratio of 0.5, the strength reduction aligns consistently with empirical equations. A notable observation is that when W/H is less than or equal to 1.0, there is good agreement, but when W/H exceeds 1.0, there is a tendency to overestimate pillar strength. Subsequent investigations emphasize the significance of considering pillar development in the overall assessment of pillar forces. The study underscores the importance of integrating pillar development into the analysis, aligning with previously established research results. Therefore, by evaluating the strength and failure mechanism of columns under different aspect ratios, we studied the influence of discrete discontinuous bodies on column stability, revealed the influence of natural cracks on column strength, and provided theoretical basis and reference for the design and support of underground mines.

Strength and Deformation of Pillars during Mining in the Shaft Pillar

This study of the strength and deformation of coal samples was triggered by the need to define the stress–strain characteristics of pillars during room and pillar mining in the shaft protective pillar at the ČSM Mine. It was probably the world’s deepest deployment of this mining method in a coal mine. In order to solve the bearing capacity of pillars, the dependence of coal strength on the slenderness ratio is used. For this reason, coal samples with different slenderness ratios were investigated. After considering the purpose of this research, slenderness ratios (width/height) of 1 to 7.7 were chosen. At the same time, the modulus of deformation as a function of the slenderness ratio was determined, and the vertical deformation of the pillars and the safety factor were calculated. Attention is also paid to the influence of sampling on the results of measured coal strengths.

Analysis and Prevention of Rock Burst Risk of Working Face under the Influence of Continuous Irregular Triangular Coal Pillar Stress Concentration Area.

Irregular coal pillars inevitably appear in the layout of the current long-wall mining method, which easily forms stress concentrations and becomes a heavy disaster area of rock burst. In order to solve the impact risk of irregular coal pillar working face, it is necessary to study the instability mechanism of the coal pillar and put forward effective prevention and control measures. Based on the research background of 14320 working face of the Dongtan Coal Mine in the Yanzhou mining area of China, this paper studies the prediction and prevention of rock bursts in this kind of coal pillar by means of theoretical calculation, numerical simulation, engineering analogy, and field monitoring. The results show that (1) the absolute stability of coal pillar is that the width of coal pillar B reaches twice the support pressure of 2L, and the possibility of instability from large to small is coal pillars 2, 5, 3, 1, and 4. (2) The ratio of coal pillar strength to its average load determines the stability coefficient of the coal pillar, and it is judged that coal pillars 1 and 4 are in a stable state, coal pillars 3 and 5 are in a limit equilibrium state, and coal pillar 2 is in an unstable state. The numerical simulation shows that the maximum stress value inside the coal pillar during the mining process is basically consistent with the theoretical calculation of the bearing strength of the coal pillar. (3) The new evaluation method is used to evaluate the rock burst risk degree of the working face roadway: 156.75 m is a strong rock burst risk zone, 728.18 m is a medium rock burst risk zone, and 176.88 m is a weak rock burst risk zone. (4) Regional prevention and local prevention measures are proposed for the risk of rock burst in the roadway, which reduces the stress concentration of the coal pillar. It is verified that the pressure relief effect is remarkable, and the safe mining of such an irregular coal pillar working face is completed, which provides a solution for studying and solving such rock burst risk.

Study of the Critical Safe Height of Goaf in Underground Metal Mines

The empty-space subsequent filling mining method is the main mining scheme for underground metal mines to achieve large-scale mechanized mining. The stage height, one of the main parameters of this method, affects the various production process aspects of the mine and influences the stability of the goaf. In order to determine the stage height scientifically and rationally in the empty-space subsequent filling mining method, a formula for the stabilized critical safe height of a high goaf in an underground metal mine was derived based on Pu’s arch equilibrium theory, Bieniawski’s pillar strength limit theory, and the Kastner equation and combined with the results of an orthogonal analysis to rank the importance of the main factors in the formula. A copper mine in Jiangxi Province was used as a case study, with the reliability of the formula verified by numerical simulation and industrial testing. The factors in the formula influencing the critical stabilized safe height of the goaf were, in descending order, the compressive strength of the rock body, the width of the two-step mining pillar, the width of the one-step mining room, the mining height, and the depth of mining. Based on the calculation results, the recommended stage heights are 30 m (−378 m middle section) and 25 m (−478 m middle section) in the area of poor rock body stability and 50 m in the area of better rock body stability. The simulation results show that the goaf is significantly affected by the compressive stress under the condition of a certain rock body stability and that the compressive stress increases with increasing goaf height. The minimum recommended values of the sidewall safety coefficients in areas of poor and better rock stability are 1.04 and 1.06, respectively. The volume deviation coefficients of the three industrial test mines were all controlled within 3%, indicating that no obvious collapse and destabilization phenomenon occurred in the goaf. This paper provides some theoretical and applied guidance for the stage height design of similar underground metal mines using the empty-space subsequent filling mining method.

Multiscale study on coal pillar strength and rational size under variable width working face

The reasonable size of the coal pillar in the working face is usually the most critical aspect in coal mining, which is related to the deformation of the surrounding rock of the roadway and the degree of damage to the coal pillar during the coal resource extraction process. The reasonable-size design of coal pillars usually adopts methods such as strength and elastic core zone calculation. However, for the remaining coal resources, the width of the working face is often unequal, and widening or narrowing the working face can significantly change the reasonable size of the coal pillar. In the laboratory, uniaxial compression tests were conducted on coal samples with different aspect ratios. Based on the possible sizes of coal pillars in coal mines, four three-dimensional numerical models of coal pillar compression with different aspect ratios were established. Obtained the failure characteristics and strength of coal pillars with different aspect ratios and provided the strength formula and aspect ratio calculation formula for coal pillars. A mechanical roof model for widening the working face was established, and the relationship between coal pillar strength and working face width was proposed. The strength of coal pillars increases with the increase of aspect ratio. The length of the working face and the aspect ratio of the coal pillar were calculated using the coal pillar strength formula. The width of the working face has increased from 63 m to 160 m, and the size of the coal pillar has increased from 3.6 m to 13.4 m, which has improved the resource recovery rate of the coal pillar. According to the deformation monitoring of the A503 working face roadway that there is no evidence of roof caving or sheeting, and the roadway’s maximum deformation is 147.3 mm, which proves that the width of the coal pillar is suitable for the mining requirements of uneven working faces. This provides important theoretical support for reasonably determining the size of coal pillars and improving the utilisation rate of irregular coal resources.

Assessment of coal pillar strength under the influence of sand stowing in deep coal mines

Assessment of coal pillar strength under the influence of sand stowing in deep coal mines

Deformation behavior of TiB reinforced Ti and Ti6Al4V composite particles under in-situ microcompression

To reveal the role of nano-TiB whiskers in micromechanical behaviors of two common titanium composite particles (TCPs), this paper presents experimental investigation on the elastic and plastic deformation of micron-sized Ti-TiBw and Ti6Al4V-TiBw particles. Scanning electron microscope with in-situ uniaxial compression experiment of single TCP was performed within a diameter range of ∼1–5 μm. This study enables detailed insights into the elastic and plastic deformation and geometrical shape evolution of TiBw reinforced TCPs. It is found that, when the particle size decreases, the flow stress and contact yield stress in microcompression for both TCPs exceed the yield strength of bulk composite pillars with identical content of TiB. The contact yield stress is as high as ∼1.5 GPa and ∼1.3 GPa for the smallest Ti-TiBw (D∼1.45μm) and Ti6Al4V-TiBw (D∼1.78μm) particles measured, respectively. In elastic-plastic and fully plastic regime, the steady plastic flow for both particles can be maintained over ∼2 GPa till the engineering compressive strain of εE ∼ -0.4. Compared with bulk composites, higher strain hardening rate of ∼34.2 and ∼37.5 GPa for Ti-TiBw and Ti6Al4V-TiBw particles demonstrate remarkable pinning capacity induced by nano-TiBw network. No structural damage and microcracks was observed in surface morphology of severely deformed Ti6Al4V-TiBw particles, showing distinctive microscale plasticity. The deformation and strain hardening mechanism for such composite particles have been discussed in terms of grain size, aspect ratio of TiBw and the network architecture.

Graphene-Material-Modified PMMA Coated with 1,3,5,7-Tetranitro-1,3,5,7-tetraazacyclooctane

Since the energetic material 1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane (HMX) has potential safety hazards during its application, it was chosen to solve this problem by coating the surface of HMX through the self-polymerization reaction of methyl methacrylate (MMA). However, its mechanical properties were poor for further application, so graphene oxide (GO), hydroxylated graphene (GO-OH), and reduced graphene oxide (rGO) were chosen to be doped into PMMA for coating modification. The properties were also investigated. The composite microspheres were regular in shape. Furthermore, it was observed that graphene materials were present on the surface of the microspheres, and no crystal transformation of HMX occurred during the process. The thermal stability of the composite microspheres was improved, and the activation energies of the HMX/PMMA/GO, HMX/PMMA/rGO and HMX/PMMA/GO-OH composite microspheres were increased compared with those of the HMX/PMMA microspheres. At the same time, the high-energy dropout characteristics of the composite microspheres were improved, and the impact sensitivity of all microspheres was reduced, compared with that of the HMX/PMMA microspheres. The compressive strength of pillars pressed with composite microspheres increased by 1.91, 0.92 and 3.13 MPa, respectively. The mechanical properties of the composite microspheres were improved. As a result, HMX/PMMA composite microspheres have better properties.

Effect of specimen size and crystallographic orientation on the nano/microscale mechanical properties and deformation behavior of CrCoNi medium-entropy alloy

CrCoNi medium-entropy alloy (MEA) shows an exceptional combination of macroscale mechanical properties, however, knowledge of its mechanical properties at nano/microscale is lacking. In this paper, nano/microscale single crystal MEA pillars with orientation of 〈116〉, 〈014〉, 〈102〉 were fabricated using focused ion beam milling, and the compressive strength was determined with micropillar compression testing using a flat-end tip in an indentation machine. Results showed that the yield and flow strength of these MEA pillars were orientation-dependent, i.e., the lower the Schmid factor, the higher the strength. At the same time the strength of the pillars was dependent on the pillar’s size, the strength increasing as the pillar diameter reduced. From the results of the micropillar compression testing, the bulk critical resolved shear stress (CRSS) was determined to be 92 MPa, which is the highest value of the group of CrCoNi-based medium and high-entropy alloys. Analysis of transmission electron microscope images and large-scale molecular dynamics simulations revealed a group of complimentary deformation mechanisms, including dislocation slides, dislocation annihilation, dislocation-slip band interaction, formation of nanoscale stacking-faulting networks, and deformation nanotwins.

Study of the Impact of Hard Rock Pillar Behavior on the Raise Caving Mining System

Raise caving is a novel mining method with an active stress control approach, which is suitable for deep mining environments. The application of raise caving is investigated for LKAB’s Kiruna mine. This mining method consists of two different phases. In the de-stressing phase, inclined de-stressing slots are created with a minimum amount of infrastructure and long-raise bore holes. The de-stressing slots provide stress shadows for subsequent large scale mining activities. These slots are separated by massive pillars, with width to height ratios of up to 10 and a length of several hundreds of meters, which are required to control stresses and seismicity and prevent hangingwall caving. Due to the stress redistribution in the mine, pillars and abutment areas become highly stressed. In the so-called “production phase”, which follows the de-stressing phase, large scale mineral extraction is conducted. As pillars are highly stressed after the de-stressing phase, it is important to de-stress them in the production phase to be able to extract them in a safe manner. Overall, pillars play a decisive role for the success of the raise caving mining method. Hence, studies on pillars and their impact on the raise caving mining method are of great importance. The first part of the contribution outlines the importance of pillars in the raise caving mining method on a conceptual basis. Numerical simulations for different mine layouts and different pillar behavior are then conducted to study the impact of pillar behavior on the system. The stress redistribution in the mining system and the infrastructure stability, with the help of the RCF-value, are analyzed. For this purpose, simulations with a linear elastic material behavior are a well-suited method. Consequently, the de-stressing of the pillar due to crushing cannot be analyzed with a linear elastic model. To do so, a method for de-stressing the pillar has to be implemented into the simulation. The behavior for pillars with a different width to height ratio is implemented with the help of pre-calculated stress-strain curves from available knowledge on pillar strength and behavior. Since mining is a progressive process, the sequence of mining plays an important role for the stress development in pillars and abutment areas and is also included in the simulations. Different ways for pillar de-stressing are outlined and discussed. Furthermore, the influence of stoping in the production phase is discussed in this contribution.

Innovative design of slender rock pillar formed within large span road tunnels and cavern Y-junction in Hawkesbury Sandstone

This paper presents an innovative design method for a slender rock pillar formed in a road tunnel Y-junction. This method has been successfully implemented in two of Sydney's widest-span road tunnel projects. The pillar strength is estimated based on synthetic or virtual uniaxial compression testing results on pillar models with anticipated adverse geological features and conditions encountered in Hawkesbury Sandstone of Sydney using distinct element numerical modelling. The proposed pillar design comprises two main steps. Firstly, the Factor of Safety against rock pillar failure is calculated based on the estimated peak pillar strength with and without rock bolt stitching and induced stresses. If the target Factor of Safety is not achieved, a set of longer rock/cable bolts to install adjacent to the rock pillar as a new type of pillar treatment is proposed to minimise the support contribution of the rock pillar. The required minimum support pressure the rock pillar provides is calculated using a ground reaction curve concept. Then the safety factor against the Y-junction's global failure is calculated based on the rock pillar's minimum required support pressure and residual strength. The proposed design method is demonstrated in the context of a recent large road tunnel project in Sydney, Australia. The proposed design method can provide confidence in rock pillar stability, potentially preventing costly and carbon-intensive full pillar replacement with concrete.

Numerical modelling of post-failure behaviors of coal specimens

A modelling approach consisting of best-fit relations to estimate the post-yield strength parameters is presented for simulating post-peak behavior beyond the point of residual strength of coal pillars having different w/h ratios. The model was developed based on back-analysis of the complete stress-strain behavior of specimens belonging to six different Indian coal seams with different w/h ratios of 0.5–13.5. It was found that the simultaneous degradation of the cohesion and friction angle of the Mohr-Coulomb rock material characterizes the post-peak strength behavior of the rock. The resulting expressions are simplistic as they require parameters that can be easily determined using uniaxial and triaxial compression results. Eventually, the developed model was validated by simulating the triaxial tests of coal specimens with different sizes under varying confining stresses and comparing its findings with the published test results. The study showed that its implementation in the numerical model could reproduce laboratory-observed mechanical response, deformation behavior, and failure mechanism very closely.

Estimating shear strength of high-level pillars supported with cemented backfilling using the Hoek–Brown strength criterion

Deep metal mines are often mined using the high-level pillars with subsequent cementation backfilling (HLSCB) mining method. At the design stage, it is therefore important to have a reasonable method for determining the shear strength of the high-level pillars (i.e. cohesion and internal friction angle) when they are supported by cemented backfilling. In this study, a formula was derived for the upper limit of the confining pressure σ3max on a high-level pillar supported by cemented backfilling in a deep metal mine. A new method of estimating the shear strength of such pillars was then proposed based on the Hoek–Brown failure criterion. Our analysis indicates that the horizontal stress σhh acting on the cemented backfill pillar can be simplified by expressing it as a constant value. A reasonable and effective value for σ3max can then be determined. The value of σ3max predicted using the proposed method is generally less than 3 MPa. Within this range, the shear strength of the high-level pillar is accurately calculated using the equivalent Mohr–Coulomb theory. The proposed method can effectively avoid the calculation of inaccurate shear strength values for the high-level pillars when the original Hoek–Brown criterion is used in the presence of large confining pressures, i.e. the situation in which the cohesion value is too large and the friction angle is too small can effectively be avoided. The proposed method is applied to a deep metal mine in China that is being excavated using the HLSCB method. The shear strength parameters of the high-level pillars obtained using the proposed method were input in the numerical simulations. The numerical results show that the recommended level heights and sizes of the high-level pillars and rooms in the mine are rational.

A proposed method for optimizing coal pillar design using coalfield-specific uniaxial compressive strength

The research described considers whether the variability in coal material strength, as derived through a series of uniaxial compressive strength (UCS) tests, could be used to indicate the variability in coal pillar strength. The aim is to be able to use a distribution of UCS tests as input into the coal pillar strength calculation. This will allow the pillar design to be expressed in terms of a probability of failure rather than as the commonly used safety factor. To achieve this, the bulk strength factor associated with commonly used pillar strength formulae was replaced with a distribution of UCS results divided by an adjustment factor. The factor was determined so as to ensure that the resulting bulk strength does not deviate from the statistically determined bulk strength published in the original formulae. This approach enabled pillar strength distributions to be obtained using industry-accepted strength formulae, subsequently allowing for a probability of failure to be calculated for a specific pillar design. Using a regional coal material strength curve as a baseline, coalfields in which the coal is stronger than the regional mean can be identified and the pillar designs optimized. This is based on the stronger coals achieving lower probabilities of failure at similar safety factors. The research has considered actual UCS data from multiple mines in the Mpumalanga coalfields of South Africa, and has proved that the variability in material strength between coalfields could allow for some optimization using the proposed approach. Based on the data used in the study, a 2.78% increase in extraction could be achieved. However, further research will be required to validate the results of the study in an underground environment.

A study of the effect of pillar shape on pillar strength

Pillar strength is affected by pillar shape, but this has largely been ignored in past research studies. Bord-and-pillar layouts are typically designed using empirical strength equations developed for square pillars. Owing to the poor quality of pillar cutting, many hard-rock pillars have an irregular shape and it is not clear how this affects pillar strength. Furthermore, the strength of rectangular pillars in comparison with square pillars is also difficult to quantify. The 'perimeter rule' is widely adopted for rectangular pillars, but its applicability for pillars with irregular shapes has never been tested. We used numerical modelling in this study to investigate the effect of pillar shape on strength. An analytical limit equilibrium model of a square and a strip pillar also provided useful insights. For slender pillars, the strength of a long rib pillar is essentially similar to that of a square pillar. In contrast, for rib pillars with a large width to height ratio, there is a substantial increase in strength. The study found that the perimeter rule should not be used for irregularly shaped pillars. Displacement discontinuity modelling, using a limit equilibrium approach, is proposed as an alternative to determine the strength of these pillars.

A study of backfill confinement to reinforce pillars in bord-and-pillar layouts

This study explores the use of backfill in hard rock bord-and-pillar mines to increase the pillar strength and extraction ratio at depth. The use of backfill will also minimize the requirement for tailings storage on surface and the risk of environmental damage. A literature survey indicated that backfill is extensively used in coal mines, but rarely in hard rock bord-and-pillar mines. To simulate the effect of backfill confinement on pillar strength, an extension of the limit equilibrium model is proposed. Numerical modelling of an actual platinum mine layout is used to illustrate the beneficial effect of backfill on pillar stability at greater depths. The magnitude of confinement exerted by the backfill on the pillar sidewalls is unknown, however, and this needs to be quantified using experimental backfill mining sections equipped with suitable instrumentation.

A study of UG2 pillar strength using a new pillar database

A recent experimental pillar extraction project at a UG2 bord-and-pillar mine presented a unique opportunity to compile a new pillar database. Currently, the South African hard rock bord-and-pillar mines are designed using the Hedley and Grant formula with a modified K-value. This empirically derived formula was developed for uranium mines in the Elliot Lake district of Canada. The use of this formula for the design of pillars in South Africa is questionable. Very few pillar failures have nevertheless been observed and its current calibrations for the various reef types are possibly too conservative. A new UG2 pillar database of 66 pillars, of which seven are classified as failed, was compiled by the authors. This enabled a revised 'first-order' calibration of the K-value for the Hedley and Grant formula. The new estimated value for the UG2 is K = 75 MPa. This gives a pillar strength that is more conservative than the PlatMine formula. This work should nevertheless be considered as only a preliminary calibration as the database was small. Further work is also required to determine whether the exponents in the formula for the width and height parameters are appropriate for UG2 pillars.