Estimating shear strength of high-level pillars supported with cemented backfilling using the HoekeBrown 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 eBrown 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 s3max 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 MohreCoulomb theory.The proposed method can effectively avoid the calculation of inaccurate shear strength values for the high-level pillars when the original HoekeBrown 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.

Investigation of stress-induced progressive failure of mine pillars using a Voronoi grain-based breakable block model

The Voronoi grain-based breakable block model(VGBBM)based on the combined finite-discrete element method(FDEM)was proposed to explicitly characterize the failure mechanism and predict the deformation behavior of hard-rock mine pillars.The influence of the microscopic parameters on the macroscopic mechanical behavior was investigated using laboratory-scale models.The field-scale pillar models(width-to-height,W/H=1,2 and 3)were calibrated based on the empirically predicted stress-strain curves of Creighton mine pillars.The results indicated that as the W/H ratios increased,the VGBBM effectively predicted the transition from strain-softening to pseudo-ductile behavior in pillars,and explicitly captured the separated rock slabs and the V-shaped damage zones on both sides of pillars and conjugate shear bands in core zones of pillars.The volumetric strain field revealed significant compressional deformation in core zones of pillars.While the peak strains of W/H=1 and 2 pillars were relatively consistent,there were significant differences in the strain energy storage and release mechanism.W/H was the primary factor influencing the deformation and strain energy in the pillar core.The friction coefficient of the structural plane was also an important factor affecting the pillar strength and the weakest discontinuity angle.The fracture surface was controlled by the discontinuity angle and the friction coefficient.This study demonstrated the capability of the VGBBM in predicting the strengths and deformation behavior of hard-rock pillars in deep mine design.

Pillar effect induced by ultrahigh phosphorous/nitrogen doping enables graphene/MXene film with excellent cycling stability for alkali metal ion storage

Graphene's large theoretical surface area and high conductivity make it an attractive anode material for potassium-ion batteries(PIBs).However,its practical application is hindered by small interlayer distance and long ion transfer distance.Herein,this paper aims to address the issue by introducing MXene through a simple and scalable method for assembling graphene and realizing ultrahigh P doping content.The findings reveal that MXene and P-C bonds have a "pillar effect" on the structure of graphene,and the P-C bond plays a primary role.In addition,N/P co-doping introduces abundant defects,providing more active sites for K^(+) storage and facilitating K^(+) adsorption.As expected,the developed ultrahigh phosphorous/nitrogen co-doped flexible reduced graphene oxide/MXene(NPrGM) electrode exhibits remarkable reversible discharge capacity(554 mA hg^(-1) at 0.05 A g^(-1)),impressive rate capability(178 mA h g^(-1) at 2 A g^(-1)),and robust cyclic stability(0.0005% decay per cycle after 10,000 cycles at 2 A g^(-1)).Furthermore,the assembled activated carbon‖NPrGM potassium-ion hybrid capacitor(PIHC) can deliver an impressive energy density of 131 W h kg^(-1) and stable cycling performance with 98.1% capacitance retention after5000 cycles at 1 A g^(-1).Such a new strategy will effectively promote the practical application of graphene materials in PIBs/PIHCs and open new avenues for the scalable development of flexible films based on two-dimensional materials for potential applications in energy storage,thermal interface,and electromagnetic shielding.

Numerical and theoretical study of load transfer behavior during cascading pillar failure

To further study the load transfer mechanism of roofemulti-pillarefloor system during cascading pillar failure(CPF),numerical simulation and theoretical analysis were carried out to study the three CPF modes according to the previous experimental study on treble-pillar specimens,e.g.successive failure mode(SFM),domino failure mode(DFM)and compound failure mode(CFM).Based on the finite element code rock failure process analysis(RFPA^(2D)),numerical models of treble-pillar specimen with different mechanical properties were established to reproduce and verify the experimental results of the three CPF modes.Numerical results show that the elastic rebound of roofefloor system induced by pillar instability causes dynamic disturbance to adjacent pillars,resulting in sudden load increases and sudden jump displacement of adjacent pillars.The phenomena of load transfer in the roofemulti-pillarefloor system,as well as the induced accelerated damage behavior in adjacent pillars,were discovered and studied.In addition,based on the catastrophe theory and the proposed mechanical model of treble-pillar specimen edisc spring group system,a potential function that characterizes the evolution characteristics of roof emulti-pillarefloor system was established.The analytical expressions of sudden jump and energy release of treble-pillar specimenedisc spring group system of the three CPF modes were derived according to the potential function.The numerical and theoretical results show good agreement with the experimental results.This study further reveals the physical essence of load transfer during CPF of roof emulti-pillarefloor system,which provides references for mine design,construction and disaster prevention.

防隔水煤柱定向爆破应力转移机理及参数优化设计

目的针对内部存在断层的防隔水煤柱在采动影响下应力场持续变化对其稳定性和工作面安全开采造成极大威胁的问题,方法以鹤壁中泰矿业有限公司3309工作面为工程地质背景,采用理论分析、正交试验和FLAC3D数值模拟试验,研究定向爆破应力转移保护防隔水煤柱的影响因素以及不同切顶深度和切顶角度下防隔水煤柱内部应力分布特征。结果结果表明:随着切顶角度和切顶深度增加,垂直应力集中区逐渐向深部转移,但当切顶深度和切顶角度达到一定数值后,继续增加切顶角度或切顶深度对应力集中区位置及垂直应力峰值影响不再明显,对比不同方案防隔水煤柱内部测线数据可知,当切顶深度为15 m、切顶角度为15°时,应力集中区距回风巷距离最远,最远距离为19.76 m,应力峰值最小,最小值为15.65 MPa。结论采用定向爆破技术可以实现切顶卸压应力转移,能够阻断回风巷上覆岩层与防隔水煤柱周边岩层的联系,将防隔水煤柱靠近回风巷一侧应力向深部转移,以减少采动对防隔水煤柱的影响;现场工业性试验结果验证了选定方案切顶深度15 m、切顶角度15°的合理性,防隔水煤柱内部实现了应力转移,选定方案的成功应用有效提高了防隔水煤柱的稳定性,能够满足安全生产要求。研究结果可为类似地质条件下内部存在断层的防隔水煤柱的应力转移保护提供参考依据。

Pillar植入系统联合鼻部手术治疗OSAHS 38例疗效分析

目的探讨Pillar植入系统联合鼻部手术治疗阻塞性睡眠呼吸暂停低通气综合征(OSAHS)的效果。方法选取2006年7至2008年11月本科OSAHS患者38例,38例均同期行Pillar植入系统治疗和鼻部手术,术前及术后6个月均行多导睡眠呼吸监测(polysomnography,PSG),对术前及术后的PSG结果(包括AHI、LSaO2)进行统计学分析。结果术后6个月进行PSG监测,治愈10例,好转17例,减轻7例,无效4例,总有效率达89.5%;所有患者无手术并发症发生。结论Pillar植入系统联合鼻部手术治疗单纯型鼾症(primary snoring,PS)及轻-中度OSAHS可取得良好效果。

Experimental studies on pillar failure characteristics based on acoustic emission location technique

Acoustic emission （AE） technique is a useful tool for investigating rock damage mechanism, and is used to study the temporal-spatial evolution process of microcracks during the similar pillar material experiment. A combined AE location algorithm was developed based on the Least square algorithm and Geiger location algorithm. The pencil break test results show that the location precision can meet the demand of microcrack monitoring. The 3D location of AE events can directly reflect the process of initiation, propagation and evolutionary of microcracks. During the loading process, stress is much likely concentrated on the area between pillar and roof of the specimen, where belongs to danger zone of macroscopic failure. When rock reaches its plastic deformation stage, AE events begin to decrease, which indicates that AE quiet period can be seen as precursor characteristic of rock failure.

Predicting pillar stability for underground mine using Fisher discriminant analysis and SVM methods

The purpose of this study is to apply some statistical and soft computing methods such as Fisher discriminant analysis （FDA） and support vector machines （SVMs） methodology to the determination of pillar stability for underground mines selected from various coal and stone mines by using some index and mechanical properties, including the width, the height, the ratio of the pillar width to its height, the uniaxial compressive strength of the rock and pillar stress. The study includes four main stages： sampling, testing, modeling and assessment of the model performances. During the modeling stage, two pillar stability prediction models were investigated with FDA and SVMs methodology based on the statistical learning theory. After using 40 sets of measured data in various mines in the world for training and testing, the model was applied to other 6 data for validating the trained proposed models. The prediction results of SVMs were compared with those of FDA as well as the measured field values. The general performance of models developed in this study is close; however, the SVMs exhibit the best performance considering the performance index with the correct classification rate Prs by re-substitution method and Pcv by cross validation method. The results show that the SVMs approach has the potential to be a reliable and practical tool for determination of pillar stability for underground mines.

微创经椎弓根Pillar植入治疗胸腰椎压缩性骨折疗效及相关并发症分析

目的 ：探讨经椎弓根pillar植入治疗胸腰椎压缩性骨折的临床效果及相关并发症。方法 ：回顾分析2010年1月～2013年4月采用微创经椎弓根pillar置入治疗胸腰椎压缩性骨折患者27例的临床疗效。手术前后进行疼痛强度视觉模拟评分（visual analogue scale,VAS）、Oswestry功能障碍指数（Oswestry disability index,ODI）、伤椎前缘高度比值、矢状面指数（伤椎前缘高度/伤椎后缘高度×100%）、伤椎Cobb′s角。结果：27例患者共植入54枚椎体支柱块,均获得6～26个月的随访,平均手术时间为（47.6±8.5）min,平均术中失血量（28.4±12.3）ml。术后CT证实17例患者椎弓根皮质有破裂,其中外侧皮质破裂15例,内侧皮质破裂2例,其中1例患者因内侧壁破裂引起下肢神经症状,经治疗后缓解。术后1周及末次随访时VAS评分、ODI值、椎体前缘高度比值、矢状面指数值及伤椎Cobb′s角与术前比较差异均具有统计学意义（P〈0.05）,而术后1周与末次随访时比较差异无统计学意义（P〉0.05）,椎体支柱块未发生移位或塌陷。结论：微创经椎弓根pillar置入是治疗胸腰椎压缩骨折一种安全、有效、可行的方法,严格掌握手术适应证、术中规范操作可降低并发症的发生。

Pillar植入术治疗阻塞性睡眠呼吸暂停综合征病人的观察和护理

[目的]探讨Pillar植入术治疗阻塞性睡眠呼吸暂停综合征(OSAS)的护理及术后随访。[方法]46例鼾症病人经我导睡眠仪(PSG)监测分为轻至中度OSAS(28例)、重度OSAS(6例)和单纯鼾症(12例)3组,行Pillar植入术,并给予全面的身心护理和健康教育,术前及术后1个月和6个月均进行鼾声视觉类比量表(VAS)评分和PSG监测。[结果]术后随访6个月,Pillar植入系统对于轻至中度OSAS病人和单纯鼾症病人打鼾的改善显著,而对于重度OSAS病人疗效较差。28例轻至中度OSAS病人治愈4例,显效10例,有效5例,无效9例,总有效率达67.9%,呼吸紊乱指数(AHI)由术前的每小时17.03次±10.63次到术后6个月降至每小时11.17次±9.52次;6例重度OSAS病人有效1例(1/6),AHI由术前每小时62.93次±9.21次降至每小时55.42次±13.77次,46例病人术后无并发症。[结论]Pillar植入系统对于治疗鼾症可以有效地改善打鼾,对于睡眠呼吸暂停的改善轻至中度OSAS病人的疗效较好,而重度OSAS病人的疗效较差。而做好Pillar手术的护理,可帮助病人顺利完成手术治疗,减轻他们术前的忧虑和术后的痛苦,获得满意效果,提高生活质量。

三角煤柱孤岛工作面冲击地压防治技术研究

为解决近邻三角形煤柱孤岛工作面冲击地压防治问题,分析了国内该条件下导致冲击地压的研究现状;以唐山矿Y392工作面为研究对象,通过理论分析得出了以大埋深、上覆不规则煤柱、本层三角形煤柱、孤岛工作面冲击地压的主控因素;采用FLAC3D建立了数值模型,分析了本层三角形煤柱孤岛工作面的应力分布规律;提出了通过顶板预裂调控冲击地压主控因素的防冲方法;最后通过现场工程实践,建立并实施了以顶板预裂为主的冲击地压防治技术,通过应力法和钻屑法验证了顶板预裂对该条件下的冲击地压防控具有良好的效果。

厚煤层大巷保护煤柱合理尺寸优化

针对红庆梁煤矿12308回采工作面大巷保护煤柱宽度留设过大导致煤炭资源浪费的问题,通过数值仿真分析工作面前方区段煤柱的应力分布规律,得到大巷保护煤柱合理宽度为100~150 m,通过12308回风顺槽顶板锚索支护压力监测,得到大巷保护煤柱宽度不得小于121 m,通过极限强度理论分析煤柱的稳定性,最终将12308回采工作面大巷保护煤柱的宽度从200 m减小到125 m。通过对停采后北翼回风大巷钻孔应力进行监测,进一步验证了125 m宽大巷保护煤柱留设的安全性与合理性,大大减少了煤炭资源的浪费。

小煤柱沿空掘进瓦斯综合防治技术研究

以某矿31018回风顺槽小煤柱沿空掘进为工程背景,对采空区侧煤体卸压范围考察,确定采空区侧煤体卸压范围,并在卸压范围进行残余瓦斯含量测试,划定无突出危险性范围;采用对小煤柱进行注浆加固和巷道全断面喷浆措施防止采空区瓦斯溢出;使用钻屑指标法对掘进工作面进行预测和验证。统计表明:31018回风顺槽小煤柱沿空掘巷过程中,预测指标K_(1max)=0.24,S_(max)=3.4,平均进尺5.8 m/d,回风流平均瓦斯浓度为0.15%,单日绝对瓦斯涌出量平均为1.04 m^(3)/min,实现了巷道安全掘进。

Numerical investigation into the effect of backfilling on coal pillar strength in highwall mining

This paper attempts to quantify the effect of backfilling on pillar strength in highwall mining using numerical modelling. Calibration against the new empirical strength formula for highwall mining was conducted to obtain the material parameters used in the numerical modelling. With the obtained coal strength parameters, three sets of backfill properties were investigated. The results reveal that the behavior of pillars varies with the type and amount of backfill as well as the pillar width to mining height ratio(w/h). In case of cohesive backfill, generally 75% backfill shows a significant increase in peak strength, and the increase in peak strength is more pronounced for the pillars having lower w/h ratios. In case of noncohesive backfill, the changes in both the peak and residual strengths with up to 92% backfill are negligible while the residual strength constantly increases after reaching the peak strength only when 100%backfill is placed. Based on the modelling results, different backfilling strategies should be considered on a case by case basis depending on the type of backfill available and desired pillar dimension.

Heavy rockbursts due to longwall mining near protective pillars:A case study

Rockburst represents a very dangerous phenomenon in deep underground mining in unfavourable conditions such as great depth, high horizontal stress, proximity of important tectonic structures, and unmined pillars. The case study describes a recorded heavy rockburst in the Czech part of the Upper Silesian Coal Basin, which occurred during longwall mining near the protective pillar. The artificial dividing of geological blocks and creation of mining protective pillars(shaft pillars, crosscut pillars etc.) is a dangerous task in light of rockbursts occurring mainly due to overstressing of remaining pillars. A simple model of this situation is presented. Natural and mining conditions are analysed and presented in detail as well as registered seismicity during longwall mining in the area. Recorded rockbursts in the area of interest are described and their causes discussed. Many rockbursts near protective pillars were recorded in this mining region. Methodical instructions for rockburst prevention in proximity of protective pillars as well as for gates driving were devised based on the evaluation of rockburst causes. The paper presents these principles for prevention.

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 supercritical 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.

Stability and control of room mining coal pillars-taking room mining coal pillars of solid backfill recovery as an example

The stability of room mining coal pillars during their secondary mining for recovering coal was analyzed. An analysis was performed for the damage and instability mechanism of coal pillars recovered by the caving mining method. During the damage progression of a single room coal pillar, the shape of the stress distribution in the pillar transformed from the initial stable saddle shape to the final arch-shaped distribution of critical instability. By combining the shapes of stress distribution in the coal pillars with the ultimate strength theory, the safe-stress value of coal pillar was obtained as 11.8 MPa. The mechanism of instability of coal pillar groups recovered by the caving mining method was explained by the domino effect. Since the room coal pillars mined and recovered by the traditional caving mining method were significantly influenced by the secondary mining during recovery, the coal pillars would go through a chain-type instability failure. Because of this limitation, the method of solid backfilling was proposed for mining and recovering room coal pillars, thus changing the transfer mechanism of stress caused by the secondary mining(recovery) of coal pillars. The mechanical model of the stope in the case of backfilling and recovering room coal pillars was built. The peak stress values inside coal pillars varied with the variance of backfilling ratio when the working face was advanced by 150 m. Furthermore, when the critical backfilling ratio was 80.6%, the instability failure of coal pillars would not occur during the solid backfill mining process. By taking Bandingliang Coal Mine as an example, the coal pillars' stability of stope under this backfilling ratio was studied, and a project scheme was designed.

Coupling effects of coal pillars of thick coal seams in large-space stopes and hard stratum on mine pressure

Concerning the issue of mine pressure behaviors occurred in fully mechanized caving mining of thick coal seams beneath hard stratum in Datong Mining Area, combined with thin and thick plate theory, the paper utilizes theoretical analysis, similar experiments, numerical simulations and field tests to study the influence of remaining coal pillars in Jurassic system goaf on hard stratum fractures, as well as mine pressure behaviors under their coupling effects. The paper concludes the solution formula of initial fault displacement in hard stratum caused by remaining coal pillars. Experiments prove that coupling effects can enhance mine pressure behaviors on working faces. When inter-layer inferior key strata fractures, mine pressure phenomenon such as significant roof weighting steps and increasing resistance in support.When inter-layer superior key strata fractures, the scope of overlying strata extends to Jurassic system goaf, dual-system stopes cut through, and remaining coal pillars lose stability. As a result, the bottom inferior key strata also lose stability. It causes huge impacts on working face, and the second mine pressure behaviors. These phenomena provide evidence for research on other similar mine strata pressure behaviors occurred in dual-system mines with remaining coal pillars.

Numerical investigation into pillar failure induced by time-dependent skin degradation

This paper focuses on the instability mechanism of an isolated pillar, caused by time-dependent skin degradation and strength heterogeneity. The time-dependent skin degradation is simulated with a non-linear rheological model capable of simulating tertiary creep, whereby two different pillar failure cases are investigated. The first case is of an isolated pillar in a deep hard rock underground mine and subjected to high stresses. The results show that pillar degradation is limited to the regions near the surface or the skin until two months after ore extraction. Afterwards degradation starts to extend deeper into the pillar, eventually leaving a highly-stressed pillar core due to stress transfer from the failed skin.Rockburst potential indices show that the risk increases exponentially at the core as time goes by. It is then demonstrated that the progressive skin degradation cannot be simulated with conventional strain-softening model assuming brittle failure. The parametric study with respect to the degree of heterogeneity reveals that heterogeneity is key to the occurrence of progressive skin degradation. The second case investigated in this study is pillar failure taking place in a very long period. Such failure becomes significantly important when assessing the risk for ground subsidence caused by pillar collapse in an abandoned mine. The analysis results demonstrate that the employed non-linear rheological model can simulate gradual skin degradation taking place over several hundred years. The percentage of damage zone volume within the pillar is merely 1% after a lapse of one days and increases to 50% after one hundred years, indicating a high risk for pillar collapse in the long term. The vertical displacements within the pillar also indicate the risk of subsidence. The proposed method is suitable for evaluating the risk of ground surface subsidence above an abandoned mine.