Pillar Area Research Articles

Characteristics of Stress Field and Damage Law of Coal Rock in Residual Pillar of Top Slice and Its Application

This article presents a theoretical analysis and numerical simulation studies to determine the suitable position for the lower slice roadway in a residual pillar area after top slicing a thick coal seam. We considered the load concentration on the coal pillar, the goaf floor, and the stress distribution characteristics of the coal pillar area before and after the top slice. With a mechanical model built of the residual pillar load propagation in the floor and the failure model of the floor surrounding rock, the analytical formula we deduced for the coal pillar floor stress and the expression of the floor failure depth of the roadway provides a basis for the reasonable determination of the position of the lower slicing roadway. Studies proved that after mining the top slice working face, the floor stress field of the residual pillar area presented characteristics of the non-uniform distribution. The stress concentration occurred below the coal pillar. The stress variation area appeared at the edge of the coal pillar, with a stable stress area appearing far away from the coal pillar area. Therefore, the roadway layout should avoid the areas below the coal pillar and the coal pillar edge with high stress levels and a large stress variation gradient. High stress concentration formed on the coal pillar transferred to the lower layered coal and floor strata, and decreased the stress concentrations layer by layer. However, mining of the top-layered working face affected the coal, and rock mass damaged the coal pillar floor area and weakened the mechanical properties, which was not conducive to the control of the roadway surrounding rock. The research results applied to No. 30117 working face of the lower slice of the Shancheng Coal Industry, and the proper position of the return airway of the working face was determined to be 8 m outside the east side of the residual pillar in the top slice, achieving a good surrounding rock control effect.

Mechanism and Prevention Practice of Coal Burst in Deep Gradual Residual Coal Pillar Area: A Case Study

Mechanism and Prevention Practice of Coal Burst in Deep Gradual Residual Coal Pillar Area: A Case Study

Optimization of Barrier Pillar Design in Longwall Mining with Top Coal Caving in Spontaneous Combustion Coal Seam

Determining a reasonable barrier pillar along gob-side entry of spontaneous combustion coal seam is of great significance to the prevention of spontaneous combustion. In this paper, considering the actual situation of 4301 and 4302 working faces of II-class spontaneous combustion coal seam in Changheng Mine, the characteristics of rock mass collapse and mining-induced stress redistribution of barrier pillar and adjacent area were analyzed, the cusp catastrophe model of coal pillar instability and the theoretical model of limit width of coal pillar air leakage spontaneous combustion were established, and a measurement scheme of side abutment pressure in 4301 working face was carried out. The theoretical model of the coal pillar along the gob-side entry shows that its instability is related to density of overlying strata, physical and mechanical parameters of coal body, dip angle of coal seam and weak face, and buried depth and mining width of working face. And the coal pillar in 4302 working face will be unstable if the width a ≤ 6.8 m . Field measured data shows that 2-8 m is the stress-relaxation area of side solid coal. Then, the width of coal pillar was determined as 7 m. The maximum displacement of rib-to-rib and roof-to-floor of 4302 tailentry was 520 mm and 280 mm, respectively. The coal spontaneous combustion (CSC) observation monitoring results showed that the carbon monoxide volume fraction was in the normal range during the entire process of mining, and no other signs of gas were found. The research results can provide a reference for the barrier pillar designing in the mines under similar conditions.

Reasonable Determination of Terminal Mining Lines Using the Stress Field with Seismic Wave Excitation in Deep Coalfaces

Many field observations have shown that rock bursts occur frequently near the terminal mining line (TML) and dip coal pillar area in deep coalfaces. Taking the “7.26” rock burst in coalface 3302 in Xingcun Coal Mine as an example, the rock burst mechanism was investigated based on theoretical analysis and field observations, and a combined evaluation method using the stress field under seismic wave excitation was established to determine the reasonable TML of coalface 3302. Firstly, the static geological data revealed during roadway excavation were used for preevaluation of rock burst risk at the working face. By theoretically analyzing the stress transfer mechanism of the two types of the roof structure, the computational model of abutment pressure was established and the calculation method giving the abutment stress was proposed. Subsequently, a dynamic evaluation method that adopts microseismic and stress online monitoring system to monitor dynamic information, such as mine tremors and stress during coalface mining, was developed to define stress anomaly areas and then dynamically determine the TML. Finally, the proposed model was used to optimize the position of the TML of LW3302 in Xingcun Coal Mine; findings obtained in this study provide theoretical guidance for safe coal mining. Combined with the results of theoretical analysis (255 m), online stress monitoring (200 m), microseismic (MS) monitoring (262 m), and passive seismic velocity tomography (220–250 m), it can be finally determined that the width of the protective coal pillar for the TML of coalface 3302 should be at least 262 m.

Facilitation of oral sensitivity by electrical stimulation of the faucial pillars

Dysphagia is common in neurological disease. However, our understanding of swallowing and its central nervous control is limited. Sensory information plays a vital role in the initiation of the swallowing reflex and is often reduced in stroke patients. We hypothesized that the sensitivity threshold of the anterior faucial pillar could be facilitated by either electrical stimulation (ES) or taste and smell information. The sensitivity threshold was measured by ES in the anterior faucial pillar region. The measurement was repeated 5 min after baseline. Thirty minutes after baseline, the participants underwent a test for taste and smell. Immediately after the test, the ES was repeated. Thirty healthy volunteers with a mean age of 27 ± 5.1 participated in the trial. Mean sensitivity threshold at baseline was 1.9 ± 0.59 mA. The values 5 min after baseline (1.74 ± 0.56 mA, p = 0.027) and 30 min after baseline (1.67 ± 0.58 mA, p = 0.011) were significantly lower compared to the baseline, but there was no difference between the latter (p = 0.321). After 5 min, a potentially facilitating effect was found on oral sensitivity by ES of the faucial pillar area. Thirty minutes later, this effect was still present.Trial registration Clinicaltrials.gov, NCT03240965. Registered 7th August 2017—https://clinicaltrials.gov/ct2/show/NCT03240965.

Design of remnant pillar in mechanized depillaring using continuous miner

Ribs and snooks are the critical natural support at the goaf edge in the mechanized depillaring operation of the bord and pillar mining system. The pillar extraction has been carried out by taking the slices and leaving ribs and snooks during the depillaring operation. Remnants are the remaining portion of the extracted pillar. The depillaring operation leads to an unsupported roof, and the immediate unsupported roof imposes its weight on the pillar (remnant) under extraction. The remnant’s purpose is to provide a necessary reaction to the overhang to restrict roof failure until the pillar’s final slice. The remnant’s stability during depillaring operation has been accessed in the study using three-dimensional numerical simulations. A scheme has also been proposed in the study to evaluate the factor of safety (FOS*) of the remnant pillar in the residual phase at different stages of slicing operation. A case of an Indian coal mine using the fish-bone method has been chosen for the study. A typical depillaring stage has been selected for the extraction of the pillar using the fish-bone method. The numerical simulation of the considered panel provides the vertical stress and yielding profile on the pillars at different stages of depillaring. The simulation results show the influence zone up to one pillar from the goaf edge. The immediate intact pillar shows considerable yielding of about 60% of the pillar area. The remnants have completely yielded during the slicing operation but provide a reaction to the immediate strata. The remnant should provide the reaction to the immediate roof till taking the final slice from the pillar. The remnant’s FOS* is calculated by taking the ratio of reaction offered by the remnant (numerical simulations) and the weight of the overhang (estimation). The area’s borehole section shows two layers of medium to coarse-grained sandstone as an immediate stratum. The weight of the immediate strata has been estimated in the study considering the immediate strata’s thickness. Two different scenarios of immediate strata thickness (i.e., 4.75 m and 9 m) have been considered in the study to evaluate the remnant’s FOS at different depillaring stages.

Mechanism of Coal Burst and Prevention Practice in Deep Asymmetric Isolated Coal Pillar: A Case Study from YaoQiao Coal Mine

Coal pillar bursts continue to be a severe dynamic hazard. Understanding its mechanism is of paramount importance and crucial in preventing and controlling its occurrence. The extreme roadway deformations from the asymmetric isolated coal pillars in the central mining district of YaoQiao Coal Mine have responded with frequent intense tremors, with risky isolated coal pillar bursts. The theoretical analysis, numerical simulation, and field measurements were done to research the impact of spatial overburden structure and stress distribution characteristics on the isolated coal pillar area, aiming to reveal the mechanism of coal pillar burst leading to the practice of prevention and control in the asymmetric isolated coal pillar area. The study shows that the overburden structure of the asymmetric is an asymmetric “T” structure in the strike‐profile, and the stress in the coal pillar is mostly asymmetric “saddle‐shaped” distribution, with the peak stress in the east side of the coal pillar, and the coal pillar is a “high stress serrated isolated coal pillar.” Numerical simulation results showed that the support pressure in the isolated coal pillar area on the strike profile was asymmetrically “saddle‐shaped” distribution. The peak vertical stress in the coal pillar area continued to rise and gradually shifted to the mining district′s deep part. As a result, the response of the roadway sides to the dynamic load disturbance was more pronounced. They developed a coal burst prevention and control program of deep‐hole blasting in the roof of asymmetrical isolated coal pillar roof and unloading pressure from coal seam borehole. Monitored data confirmed that the stress concentration was influential in the roadway’s surrounding rock in the asymmetric isolated coal pillar area, circumventing coal pillar burst accidents. The research outcomes reference the prevention and control of coal bursts at isolated working faces of coal pillars under similar conditions.

Detection of stress redistribution in a complex isolated coal pillar with active SVT technology

The detection of stress redistribution in a mining area is essential for forecasting and preventing coal burst disasters in underground mining practices. Active seismic velocity tomography (SVT) is an effective way of investigating large-scale regional stress distribution. Active SVT technology was applied to a complex isolated coal pillar twice to assess coal burst hazard level and to guide the design of de-stress measures as well as to verify their de-stress effect. Test results show that high stress concentration existed in scattered zones along the boundary of the coal pillar rather than in the whole coal pillar. Qualitative comparison of the two SVT tests conducted before and after implementing de-stress measures indicates (1) if high-density wave sources and receivers are appropriately arranged, the spatio-temporal detection precision of active SVT technology could be sufficiently high to identify the de-stress effect of a single deep-hole roof blasting or a group of coal seam blasting upon their completion; (2) The implementation of de-stress measures in the isolated coal pillar area helped drive the stress concentration away from the de-stressed area to the adjacent area instead of fully eliminating the stress concentration. This stress-driving effect indicates that a possibly advisable strategy of coal burst control in the isolated coal pillar area could be driving stress concentration to the area further away from the area of high production activities rather than attempting to eliminate stress concentration.

Analisis fungsi ganda plaza pada atribut jamaah salat di Masjid Agung Jawa Tengah

The Great Mosque of Central Java has an area called the plaza, namely Zone I, the Pillar area, and Zone II, the Payung Hidrolis Plaza Area. Preliminary observations by researchers showed that there was a suspicion related to the architectural settings of the plaza relating to the attributes of the congregation prayers, which were in the Great Mosque of Central Java. The method that will use in this research is the quantitative method using a descriptive approach. In this study, the independent variable is the perception of visitors in using the Central Java Grand Mosque plaza space with observational indicators of visitors' motivation, interests, and expectations. The dependent variable in this study is the hall attribute as a means of visitors to the main prayer room, which divided into two zones, namely: Zone I (alqonatir Pillar area) and Zone II (Hydro Payung Plaza Area). Field data collection was carried out on 300 respondents. Data collection was carried out for one week with division in 2 zones and two different times, namely afternoon and evening. Based on the results of research that has done that, there is a relationship between the function of the plaza of the Great Mosque of Central Java to the attributes of worshipers in Zone 1, Zone 2, Zone 1 and Zone 2.

The effect of the necrotic area on the biomechanics of the femoral head - a finite element study

BackgroundFemoral head collapse is the key to the progress of osteonecrosis of the femoral head (ONFH), but the causes of collapse are not completely clear. The better understanding of the progress of femoral head collapse will guide the treatment strategy for ONFH patients. The purpose of this study was to evaluate the biomechanical influence of necrosis area on the collapse of the femoral head by finite element analysis.MethodsCT and MRI data from the hip joint of a healthy volunteer were collected to establish a finite element (FE) model of a normal hip. Subsequently, five categories of osteonecrosis FE models were established by using the normal model and computer software according to China-Japan Friendship Hospital (CJFH) classification for ONFH. The CJFH system includes five types based on the size and location of necrosis lesions in the femoral head (type M, C, L1, L2, and L3) and the stage of ONFH. The collapse indices of each model were analyzed by FE method, including the displacement, peak von Mises stress and stress index of the simulated necrotic area as well as the lateral pillar contact area of the femoral head to acetabular.Results(1) The displacement increments in the simulated necrotic areas of type M, C, L1, L2, and L3 models were 3.75 μm, 8.24 μm, 8.47 μm, 18.42 μm, and 20.44 μm respectively; the peak von Mises stress decrements were 1.50 MPa, 3.74 MPa, 3.73 MPa, 4.91 MPa, and 4.92 MPa respectively; and the stress indices were 0.04, 0.08, 0.08, 0.27, and 0.27 respectively. (2) The displacement increments in the lateral pillar contact areas of five type models were significantly different (P < 0.001) and increased in sequence as follows: 1.93 ± 0.15 μm, 5.74 ± 0.92 μm, 5.84 ± 1.42 μm, 14.50 ± 3.00 μm, and 16.43 ± 3.05 μm. The peak von Mises stress decrements were also significantly different (P < 0.001) and increased in sequence as follows: 0.52 ± 0.30 MPa, 0.55 ± 0.12 MPa, 0.67 ± 0.33 MPa, 4.17 ± 0.59 MPa, and 4.19 ± 0.60 MPa. (3) The collapse indices including the displacement increments and peak von Mises stress decrements of type L2 and L3 models were markedly higher than those of type M, C, and L1 models (P < 0.001).ConclusionsThe collapse indices of the femoral heads of type L2 and L3 FE models were significantly higher than those of type M, C, and L1. Different areas of necrosis result in varied impact on the femoral head collapse.

Study on Stability Prediction of Pillars Based on Bieniawski Pillar Strength Formula: a Case of a Phosphate Mine

In order to obtain the stability of pillars in Daqiao Phosphate Mine, the stability of pillars in goaf was studied by theoretical calculation. Based on the results of investigation and the calculation formula of pillar strength, a mathematical formula for calculating pillar stability was established to predict the stability of uninvestigated pillars. The effect of various factors on the stability of the pillar was analyzed by orthogonal test. The research results show that the pillar area, rock column area, and mining depth have the most significant effect on pillar stability. A mathematical formula for predicting pillar stability is constructed by means of dimensional grading method. The critical value of pillar factor is 0.06 in Daqiao Phosphate Mine. When the pillar factor is greater than 0.06, the pillar is stable. When the pillar factor is less than 0.06, the pillar is unstable. The reliability of theoretical calculation method of pillar factor is verified by numerical simulation. A mathematical formula for predicting the stability of pillars is proposed, which can be used as a reference for the stability analysis of goafs in similar mines.

Study on Reasonable Size of Coal and Rock Pillar in Dynamic Pressure Roadway Segment of Fully Mechanized Face in Deep Shaft

The deformation mechanism of the protective coal and rock pillar area outside a stope is an important parameter for setting a reasonable size. In this paper, based on the geological condition of working face 1231(1) in a mine in Huainan, a method that combines the use of a borehole and Brillouin optical time domain reflectometry (BOTDR) was proposed to analyze the stress variation laws of coal and rock pillar areas, and the parameters of the monitoring borehole and installation technique of the sensing optical cables were designed. Based on the monitoring data, the strain distribution characteristics of the sensing optical cables and their relationship with the rock strata were analyzed, the development law of coal and rock strata deformation during the mining process was revealed, and the transverse influence range of the coal and rock pillar affected by mining was reasonably divided. According to the results, the sensing optical cables show an overall trend of tensile strain, with a maximum value of 1800 με, and the main areas of rock strata deformation occur near the interface of rock strata. The range of rock strata disturbance along the borehole direction was approximately 38 m, and the maximum deformation of rock strata after the disturbance, namely, the displacement, was 24.87 mm. A numerical model was constructed to acquire the strain variation characteristic within 100 m in the outer floor of the working face. The transverse range of the floor disturbance was analyzed to be 30–36 m. The field test had good correspondence with the numerical simulation results, which indicates that the optical fiber testing technology can effectively describe the stress variation in the coal and rock strata. The test results can provide technical support for the rational setting of coal and rock pillars and disaster prevention and control. The research direction of deep rock mass testing is discussed, and optical fiber testing in boreholes is considered an effective method for studying deep dynamic disaster control.

Design Optimization of Pillar Bump Structure for Minimizing the Stress in Brittle Low K Dielectric Material Layer

Cu pillar bump offers a number of advantages for flip chip packaging, compared to the conventional solder bump. However, due to its rigidity structure, Cu pillar bump introduces a lot of stress to the chip, which causes the failure of packaging structures, especially for the advanced node devices which typically have brittle low K dielectric material. In this paper, for the first time we propose two types of Cu pillar structures to reduce the stress. The first Cu pillar structure has bigger Cu dimensions at the base. The other one is designed to add an additional Cu pad under the Cu pillar bump. Finite element analysis is used to study the stress of the both structures, and it is found that with the increase in pillar bump contact area over the chip surface, the stress decreases in both structures. Results also indicate that the Cu pillar bump undercut induces higher stress, and thin Cu6Sn5 intermetallic compound has less impact on the stress during flip chip mount reflow. The study provides a novel way to improve the reliability by reducing the stress in the Cu pillar bump related packaging.

Delamination Mechanics of Carbon Nanotube Micropillars.

The adhesion of carbon nanotube (CNT) forests to their growth substrate is a critical concern for many applications. Here, we measured the delamination force of CNT forest micropillars using in situ scanning electron microscopy (SEM) tensile testing. A flat tip with epoxy adhesive first established contact with the top surface of freestanding CNT pillars and then pulled the pillars in displacement-controlled tension until delamination was observed. An average delamination stress of 6.1 MPa was measured, based on the full pillar cross-sectional area, and detachment was observed to occur between catalyst particles and the growth substrate. Finite element simulations of CNT forest delamination show that force and strain are heterogeneously distributed among CNTs during tensile loading and that CNTs progressively lose adhesion with increased displacement. Based on combined experiments and simulations, an adhesion strength of approximately 350 MPa was estimated between each CNT and the substrate. These findings provide important insight into CNT applications such as thermal interfaces, mechanical sensors, and structural composites while also suggesting a potential upper limit of tensile forces allowed during CNT forest synthesis.

Model test study on overburden settlement law in coal seam backfill mining based on fiber Bragg grating technology

Taking section 3600 of Pingzhuangxi coal mine as the prototype, we made the test model based on the theory of similarity, used the fiber Bragg grating (FBG) to study the deformation characteristics of overburden in the process of coal seam backfill mining and analyze the correlation between filling material and overburden settlement, and obtained the deformation law of overburden in the seam filling area and the coal pillar area. The results show that due to the impact of compression characteristics of filling materials, the intermediate roof, and the breakage of key strata, the overburden deformation in the seam filling area can be divided into 4 stages, and the coal pillar overburden deformation can be divided into 3 stages according to the slope of strain distribution curve of overburden during backfill mining. Based on the slope of the strain curve of mining overburden and the strain value measured by FBG sensors, this paper identified the stability of rock stratum, proposed a FBG-based method to determine the height of the water-flowing fractured zone, compared it with the existing calculation methods to verify its rationality, and came to a conclusion that this new method can be promoted for future calculation.

Sensitivity analysis and estimation method of natural frequency for large cooling tower based on field measurement

The natural frequency is a key factor for estimation of dynamic deformation and mechanical performance. As simple and effective estimation equation for natural frequency of cooling tower is absent, current investigations of natural frequency of cooling towers are basically based on finite element analyses. In this study, 38 models of a 179 m cooling tower were established by tuning key structural parameters (e.g., tower height, throat height, throat diameter, inlet height, and pillar sectional area) and dynamic characteristics of these models were analyzed. Also, effects of structural parameters on fundamental frequency and overturning frequency were investigated. The sensitivity analysis of structural natural frequency for cooling towers was executed using the perturbation method and the Latin hypercube sampling method and sensitivity factors of different parameters corresponding to different orders were obtained. Based on that, multi-parameter empirical estimation equations for fundamental frequency and overturning frequency considering weighted sensitivity factor were proposed. Then, the estimation equation of natural frequency is verified by field tests of eight cooling towers with typical tower heights and configurations. Specifically, the measured acceleration signals were pre-treated using random decrement method (RDT) and natural excitation technique (NExT) and the first 10 order natural frequencies of the cooling tower using three time-domain modal identification methods (ARMA, ITD, and STD). Finally, structural parameters of cooling tower obtained by field tests were fitted. The results indicated that the fundamental frequency of cooling towers decreases as tower height and throat height increase and increases as throat diameter, inlet height, and pillar sectional area increase. Although sensitivity factors obtained by the two methods are similar, the LHS method shows higher accuracy. The effect of tower height on natural frequency is most significant among all parameters, which means the sensitivity factor of tower height is higher than those of other parameters. The measured fundamental frequencies of cooling towers were between 0.6 Hz and 1.9 Hz and heights and configurations of cooling towers have significant effects on their dynamic characteristics. The field test results of eight cooling towers demonstrated reasonable effectiveness of the proposed empirical estimation equations for the fundamental frequency and overturning frequency of cooling towers (the maximum goodness of fit of fundamental frequency and overturning frequency were 0.996 and 0.975, respectively). Error analysis indicated that the proposed estimation equation for natural frequency is highly accurate and reliable. This study provides references for determination of structural natural frequency of large cooling towers and future studies on structural natural vibration characteristics.

Nanostructured surface topographies have an effect on bactericidal activity

BackgroundDue to the increased emergence of antimicrobial resistance, alternatives to minimize the usage of antibiotics become attractive solutions. Biophysical manipulation of material surface topography to prevent bacterial adhesion is one promising approach. To this end, it is essential to understand the relationship between surface topographical features and bactericidal properties in order to develop antibacterial surfaces.ResultsIn this work a systematic study of topographical effects on bactericidal activity of nanostructured surfaces is presented. Nanostructured Ormostamp polymer surfaces are fabricated by nano-replication technology using nanoporous templates resulting in 80-nm diameter nanopillars. Six Ormostamp surfaces with nanopillar arrays of various nanopillar densities and heights are obtained by modifying the nanoporous template. The surface roughness ranges from 3.1 to 39.1 nm for the different pillar area parameters. A Gram-positive bacterium, Staphylococcus aureus, is used as the model bacterial strain. An average pillar density at ~ 40 pillars μm−2 with surface roughness of 39.1 nm possesses the highest bactericidal efficiency being close to 100% compared with 20% of the flat control samples. High density structures at ~ 70 pillars μm−2 and low density structures at < 20 pillars μm−2 with surface roughness smaller than 20 nm reduce the bactericidal efficiency to almost the level of the control samples.ConclusionThe results obtained here suggests that the topographical effects including pillar density and pillar height inhomogeneity may have significant impacts on adhering pattern and stretching degree of bacterial cell membrane. A biophysical model is prepared to interpret the morphological changes of bacteria on these nanostructures.

Evidence-Based Practice Beliefs and Implementation in Doctor of Nursing Practice Students.

Doctors of Nursing Practice focus on leadership in evidence-based practice (EBP). EBP is influenced by one's beliefs in and implementation of EBP. Little is known to date about the EBP beliefs and implementation of Doctor of Nursing Practice students and outcomes of Doctor of Nursing Practice education. Guided by the Advancing Research and Clinical practice through close Collaboration (ARCC) Model, the Evidence-Based Practice Beliefs (EBPB) and Implementation (EBPI) tools were used to assess the impact of EBP as a program pillar, curricular thread, and content area on EBPB and EBPI of Doctor of Nursing Practice-Family Nurse Practitioner students. Five cohorts who completed the same curriculum were studied. Fifty-four of the 89 students across the five cohorts began and completed the study. Paired t-test for group effects showed statistical significance from pre- to post-measure in students overall EBPB, t = 4.4 (52), p < .001, and EBPI, t = 8.4 (52), p < .001. A large effect size of .75 standard deviation (SD) gain above the mean for EBPB, and a very large effect size of 1 SD gain above the mean for EBPI were observed. Repeated measures ANOVA showed that all cohorts made gains across the curriculum. Effect sizes for EBPB ranged from .25 to 1 SD above the mean, and .75 to 1.5 for EBPI. DNP students who are educated to be EBP leaders must have a curriculum that supports them in the knowledge and skill-set needed to translate evidence into practice. The ARCC Model can guide faculty in EBP curriculum development. EBPB and EBPI are valid and reliable measures to assess for gains across a curriculum. Through educational outcomes, educators can assess desired student outcomes for EBP across a curriculum and can build an evidence base for ongoing curriculum development.

Stress Changes and Deformation Monitoring of Longwall Coal Pillars Located in Weak Ground

Coal pillar stability is strongly influenced by the site-specific geological and geotechnical conditions. Many geological structures such as faults, joints, or rock intrusions can be detrimental to mining operations. In order to evaluate the performance of coal pillars under weak roof degraded by igneous rock intrusion, stress and deformation monitoring was conducted in the affected tailgate areas of Nos. 8208 and 8210 longwalls in Tashan coal mine, Shanxi Province, China. The measurements in the 8208 longwall tailgate showed that the mining-induced stresses in 38-m-wide coal chain pillars under the overburden depth of 300–500 m started to increase at about 100 m ahead of the 8208 longwall working face and reached its peak level at approximately 50 m ahead of the longwall face. The peak stress of 9.16 MPa occurred at the depth of 8–9 m into the pillar from the tailgate side wall. In comparison, disturbance of the headgate block pillar area was negligible, indicating the difference of abutment pressure distribution between the tailgate and headgate sites where the adjacent unmined longwall block carried most of the overburden load. However, when the longwall face passed the headgate monitoring site by 360–379 m, the pillar stress increased to a peak value of 21.4 MPa at the pillar depth of 13 m from the gob side mainly due to stress redistribution in the chain pillar. In contrast to the headgate, at the tailgate side, the adjacent goaf was the dominant triggering factor for high stress concentrations in the chain pillar. Convergence measurements in the tailgate during longwall mining further indicated the evolution characteristics of coal pillar deformation, clearly showing that the gateroad deformation is mainly induced by the longwall extraction it serves. When predicting the future pillar loads from the monitored data, two stress peaks appeared across the 38-m-wide tailgate coal pillar, which are separated by the lower stress area within the pillar center. This 10-m-wide elastic pillar core area indicates that the coal pillar may be narrowed to 30 m to improve coal recovery. The measurements further indicate that, if the headgate of the next panel can be developed after the adjacent gob becomes stable, the coal pillar width may be further reduced to 20 m. This study is applicable for the chain pillar design, the gateroad secondary support design ahead of the longwall mining face, and the gateroad preparation of the next longwall panel under similar geological and geotechnical conditions.

Countermeasure Method for Stope Instability in Crown Pillar Area of Cut and Fill Underground Mine

Maintaining stability as well as optimizing recovery of crown pillar, a pillar separating surface area with the uppermost stope in overhand cut and fill underground mining method, is important. Failures in stope may lead to crown pillar failures and cause surface subsidence. Increasing crown pillar thickness will increase crown pillar stability yet reduce mining recovery because part of crown pillar is formed by ore body. Preventing stope failure is the key to maintain stability and optimize recovery of crown pillar. Therefore, it is important to study countermeasure method for stope failure especially in crown pillar area. An attempt has been made to investigate the effectiveness of various countermeasures for stope failure in crown pillar area by means of parametric study. The result shows active type support system is effective for supporting stope in high vertical stress condition while the passive one needs to be installed if the stope is opened in high horizontal stress condition. In general, more supporting capacity from both type support systems is needed if the stope is opened in more severe geological condition. Another countermeasures, sill pillar and surface pile, are introduced for stope instability in crown pillar and non-crown pillar area. Sill pillar is an abandoned slice of unstable stope based on stability analysis. Sill pillar is very effective to stabilize stope both in crown pillar and non-crown pillar area, especially for stope in high horizontal stress condition. Sill pillar application in model with stress ratio 2 can optimize 20 meter thickness of crown pillar into 5 meter. Another proposed countermeasure is surface pile. Surface pile can be installed from the surface to improve stability of crown pillar and stope. The most effective use of surface pile is found in simulation of model with stress ratio 0.75 where surface pile can optimize 15 meter thickness of crown pillar into 5 meter.