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.

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.

Lateral abutment pressure distribution and evolution in wide pillars under the first mining effect

The wide pillars are generally popular due to the high productivity and efficiency in Northwest China.The distribution of lateral abutment pressure in coal pillars is important for mining safety.To reveal the effect of the first mining on the lateral abutment pressure distribution and evolution in wide pillars,an in-situ experiment,theoretical analysis and numerical simulation were performed.First,the field monitoring of lateral abutment pressure was conducted from the perspective of time and space in the Chahasu Coal Mine,Huangling No.2 Coal Mine and Lingdong Coal Mine during the first mining.Based on the field monitoring stress,a theoretical model was proposed to reveal the lateral abutment pressure distribution.The methodology was demonstrated through a case study.Aiming at the distribution mechanism,a numerical experiment was conducted through the finite-discrete element method(FDEM).Last,field observations of borehole fractures were performed to further study the damage distribution.In addition,two types of lateral abutment pressure evolution with mining advance were discussed.Suggestions on the stress monitoring layout were proposed as well.The results could provide foundations for strata control and disaster prevention in wide pillars in underground coal mines.

Mechanical behaviors of deep pillar sandwiched between strong and weak layers

A variety of coal room and pillar mining methods have been efficiently practiced at depths of up to 500 m with least strata mechanics issues.However,for the first time,this method was trialled at depths of 850 e900 m in CSM mine of Czech Republic.The rhomboid-shaped coal pillars with acute corners of 70,surrounded with 5.2 m wide and 3.5e4.5 m high mine roadways,were used.Pillars were developed in a staggered manner with their size variation in the Panel II from 83 m×25 m to 24 m×20 m(corner to corner)and Panel V from 35 m×30 m to 26 m×16 m.Coal seam inclined at 12was affected by the unusual slippery slickenside roof bands and sometimes in the floor levels with high vertical stress below strong and massive sandstone roof.In order to ensure safety,pillars in both the panels were continuously monitored using various geotechnical instruments measuring the induced stresses,side spalling and roof sagging.Both panels suffered high amounts of mining induced stress and pillar failure with side-spalling up to 5 m from all sides.Heavy fracturing of coal pillar sides was controlled by fully encapsulated steel bolts.Mining induced stress kept increasing with the progress of development of pillars and galleries.Instruments installed in the pillar failed to monitor actual induced stress due to fracturing of coal mass around it which created an apprehension of pillar failure up to its core due to high vertical mining induced stress.This risk was reduced by carrying out scientific studies including the three-dimensional numerical models calibrated with data from the instrumented pillar.An attempt has been made to study the behavior of coal pillars and their yielding characteristics at deeper cover based on field and simulation results.

Stability prediction of hard rock pillar using support vector machine optimized by three metaheuristic algorithms

Hard rock pillar is one of the important structures in engineering design and excavation in underground mines.Accurate and convenient prediction of pillar stability is of great significance for underground space safety.This paper aims to develop hybrid support vector machine(SVM)models improved by three metaheuristic algorithms known as grey wolf optimizer(GWO),whale optimization algorithm(WOA)and sparrow search algorithm(SSA)for predicting the hard rock pillar stability.An integrated dataset containing 306 hard rock pillars was established to generate hybrid SVM models.Five parameters including pillar height,pillar width,ratio of pillar width to height,uniaxial compressive strength and pillar stress were set as input parameters.Two global indices,three local indices and the receiver operating characteristic(ROC)curve with the area under the ROC curve(AUC)were utilized to evaluate all hybrid models’performance.The results confirmed that the SSA-SVM model is the best prediction model with the highest values of all global indices and local indices.Nevertheless,the performance of the SSASVM model for predicting the unstable pillar(AUC:0.899)is not as good as those for stable(AUC:0.975)and failed pillars(AUC:0.990).To verify the effectiveness of the proposed models,5 field cases were investigated in a metal mine and other 5 cases were collected from several published works.The validation results indicated that the SSA-SVM model obtained a considerable accuracy,which means that the combination of SVM and metaheuristic algorithms is a feasible approach to predict the pillar stability.

Study on the disaster caused by the linkage failure of the residual coal pillar and rock stratum during multiple coal seam mining:mechanism of progressive and dynamic failure

Multi-seam mining often leads to the retention of a significant number of coal pillars for purposes such as protection,safety,or water isolation.However,stress concentration beneath these residual coal pillars can significantly impact their strength and stability when mining below them,potentially leading to hydraulic support failure,surface subsidence,and rock bursting.To address this issue,the linkage between the failure and instability of residual coal pillars and rock strata during multi-seam mining is examined in this study.Key controls include residual pillar spalling,safety factor(f.),local mine stiffness(LMS),and the post-peak stiffness(k)of the residual coal pillar.Limits separating the two forms of failure,progressive versus dynamic,are defined.Progressive failure results at lower stresses when the coal pillar transitions from indefinitely stable(f,>1.5)to failing(f,<1.5)when the coal pillar can no longer remain stable for an extended duration,whereas sud-den(unstable)failure results when the strength of the pillar is further degraded and fails.The transition in mode of failure is defined by the LMS/k ratio.Failure transitions from quiescent to dynamic as LMS/k.<1,which can cause chain pillar instability propagating throughout the mine.This study provides theoretical guidance to define this limit to instability of residual coal pillars for multi-seam mining in similar mines.

Experimental study on the movement law of overlying rock non-pillar coal overhead mining

Pillarless coal mining technology is a new practical technology.Based on the compensating mechanical behavior of the Negative Poisson’s Ratio(NPR)anchor cable on the roof,the roadway was successfully retained by the top cutting and pressure relief technology.This study utilizes the Digital Speckle Monitoring(DIC monitoring),stress-strain monitoring,and infrared thermal imaging systems to conduct physical model experiment of similar materials from the displacement,stress-strain,and temperature fields to investigate in depth the fracture change law of the overlying rock.In addition,it uses FLAC3D numerical simulation to invert the surface displacement settlement.The results show that the non-pillar overhead mining under the 110 mining method has little influence on the rock crack in the middle of the coal seam,and the crack development area is mainly concentrated in the overlying rock mass of the upward coal seam.The compensatory mechanical behavior of NPR anchor cable and the dilatation characteristics of rock mass have a good effect of retaining roadway along goaf,and can also reduce surface settlement.The 110 mining method provides a scientific basis for ecological environment protection and the development of other kilometer deep soft rock high ground stress underground projects.

The Potential of Commercial Pension Insurance to Develop into One of the Pillars of Rural Pension Insurance:A Study from the Demand Perspective

The development of a multi-pillar pension insurance system is an effective solution for an aging society.Commercial pension insurance,as the third pillar of pension insurance,is an integral part of this system in China and can play a critical and complementary role in rural areas where support for the elderly is a more pressing concern and a second pillar of pension insurance remains absent.To this end,we first elaborate on the theoretical logic that commercial pension insurance can develop into one of the pillars of rural pension insurance.We then empirically test rural residents’willingness to participate in a commercial pension insurance plan(CPIP)in a probit model with household research data from rural areas in major labor-exporting provinces,such as Sichuan and Henan so as to explore whether commercial pension insurance has the potential to become one of the pillars of rural pension insurance.Our research findings can be synthesized in three points.First,rural residents out of agricultural production for five consecutive years are more willing to participate in a CPIP than other rural residents,indicating that progress in industrialization and urbanization can significantly boost such willingness.Second,the younger rural residents are more inclined to participate in a CPIP than the older generation.Third,income increases can significantly boost rural residents’willingness to participate in a CPIP.Thus,with progress in industrialization and urbanization and an increase in rural disposable income,commercial pension insurance has a promising potential in rural areas and can hopefully develop into one of the pillars of rural pension insurance.

The current perspective of the PA 1957 gas well pillar study and its implications for longwall gas well pillars

Many states rely upon the Pennsylvania 1957 Gas Well Pillar Study to evaluate the coal barrier surrounding gas wells.The study included 77 gas well failure cases that occurred in the Pittsburgh and Freeport coal seams over a 25-year span.At the time,coal was mined using the room-and-pillar mining method with full or partial pillar recovery,and square or rectangle pillars surrounding the gas wells were left to protect the wells.The study provided guidelines for pillar sizes under different overburden depths up to 213 m(700 ft).The 1957 study has also been used to determine gas well pillar sizes in longwall mines since longwall mining began in the 1970 s.The original study was developed for room-and-pillar mining and could be applied to gas wells in longwall chain pillars under shallow cover.However,under deep cover,severe deformations in gas wells have occurred in longwall chain pillars.Presently,with a better understanding of coal pillar mechanics,new insight into subsidence movements induced by retreat mining,and advances in numerical modeling,it has become both critically important and feasible to evaluate the adequacy of the 1957 study for longwall gas well pillars.In this paper,the data from the 1957 study is analyzed from a new perspective by considering various factors,including overburden depth,failure location,failure time,pillar safety factor(SF),and floor pressure.The pillar SF and floor pressure are calculated by considering abutment pressure induced by full pillar recovery.A statistical analysis is performed to find correlations between various factors and helps identify the most significant factors for the stability of gas wells influenced by retreat mining.Through analyzing the data from the 1957 study,the guidelines for gas well pillars in the 1957 study are evaluated for their adequacy for roomand-pillar mining and their applicability to longwall mining.Numerical modeling is used to model the stability of gas wells by quantifying the mining-induced stresses in gas well casings.Results of this study indicate that the guidelines in the 1957 study may be appropriate for pillars protecting conventional gas wells in both room-and-pillar mining and longwall mining under overburden depths up to 213m(700 ft),but may not be sufficient for protective pillars under deep cover.The current evaluation of the 1957 study provides not only insights about potential gas well failures caused by retreat mining but also implications for what critical considerations should be taken into account to protect gas wells in longwall mining.

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。

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。

Construction of TiO_(2)-pillared multilayer graphene nanocomposites as efficient photocatalysts for ciprofloxacin degradation

We successfully constructed TiO_(2)-pillared multilayer graphene nanocomposites(T-MLGs)via a facile method as follows:dodecanediamine pre-pillaring,ion exchange(Ti4+pillaring),and interlayer in-situ formation of TiO_(2) by hydrothermal method.TiO_(2) nanoparticles were distributed uniformly on the graphene interlayer.The special structure combined the advantages of graphene and TiO_(2) nanoparticles.As a result,T-MLGs with 64.3wt%TiO_(2) showed the optimum photodegradation rate and adsorption capabilities toward ciprofloxacin.The photodegradation rate of T-MLGs with 64.3wt%TiO_(2) was 78%under light-emitting diode light irradiation for 150 min.Meanwhile,the pseudofirst-order rate constant of T-MLGs with 64.3wt%TiO_(2) was 3.89 times than that of pristine TiO_(2).The composites also exhibited high stability and reusability after five consecutive photocatalytic tests.This work provides a facile method to synthesize semiconductor-pillared graphene nanocomposites by replacing TiO_(2) nanoparticles with other nanoparticles and a feasible means for sustainable utilization of photocatalysts in wastewater control.

Preparation and Microstructure of Al-pillared Interlayered Montmorillonite

Al-pillared interlayered montmorillonite (Al-PILM) was prepared using the artificial Na-montmorillonite from the Qingfengshan bentonite mine as a starting material mixed with Al-pillaring solutions.The microstructure of the materials was studied by an X-ray powder diffractometer and a Fourier transform infrared (FTIR) spectrometer.The results indicated that the basal spacing [d(001) value] of the materials was increased significantly to 1.9194 nm relative to Na-montmorillonite (1.2182 nm).After calcined for 2 h at 300℃,the basal spacing was stabilized at 1.8394 nm and the layered structure of the materials was not destroyed.Thermal analysis was conducted by a thermal gravimetry and differential thermal analysis (TG-DTA) instrument,it showed that Al-PILM lost physically adsorbed water below 230.6℃ and water formed by dehydroxylation of the pillars at around 497.1℃, with a peak of the phase transformation at 903.0℃.

Synthesis and Characterization of Al-Cr-Pillared Montmorillonite with High Thermal Stability and Adsorption Capacity*

Al-Cr-pillared montmorillonite was synthesized by using bentonite and Al-Cr pillaring solutions as starting materials.The basal spacing and specific surface areas of the materials were significantly increased relative to those of untreated clays.When the Al/Cr molar ratio(R) was 0.10,the d(001) value and specific surface area of pillared montmorillonite were 1.9194 nm and 165.7 m2 g -1,respectively.Thermal stability of the materials was determined using calcined tests and X-ray diffraction (XRD) analysis.The materials formed at different R(0.05;0.10;0.15;0.25) exhibit a high thermal stability at 300℃,especially at initial R=0.10,the basal interlayer spacing of materials is stabilized at 1.7313 nm after calcined at 500℃ for 2 h.Adsorption behavior of the materials was studied by adsorption experiments.The results show that the Al-Cr-pillared montmorillonites exhibit much stronger adsorption capacity on Cr 6+ in aqueous solution than untreated clays do.

Pillar design and coal burst experience in Utah Book Cliffs longwall operations

Longwall mining has existed in Utah for more than half a century.Much of this mining occurred at depths of cover that significantly exceed those encountered by most other US longwall operations.Deep cover causes high ground stress,which can combine with geology to create a coal burst hazard.Nearly every longwall mine operating within the Utah’s Book Cliffs coalfield has been affected by coal bursts.Pillar design has been a key component in the burst control strategies employed by mines in the Book Cliffs.Historically,most longwall mines employed double-use two-entry yield pillar gates.Double-use signifies that the gate system serves first as the headgate,and then later serves as the tailgate for the adjacent panel.After the 1996 burst fatality at the Aberdeen Mine,the inter-panel barrier design was introduced.In this layout,a wide barrier pillar protects each longwall panel from the previously mined panel,and each gate system is used just once.This paper documents the deep cover longwall mining conducted with each type of pillar design,together with the associated coal burst experience.Each of the six longwall mining complexes in the Book Cliffs having a coal burst history is described on a panel-by-panel basis.The analysis shows that where the mining depth exceeded 450 m,each design has been employed for about 38000 total m of longwall panel extraction.The double-use yield pillar design has been used primarily at depths less than 600 m,however,while the inter-panel barrier design has been used mainly at depths exceeding 600 m.Despite its greater depth of use,the inter-panel barrier gate design has been associated with about one-third as much face region burst activity as the double-use yield pillar design.

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.

Geotechnical considerations for concurrent pillar recovery in close-distance multiple seams

Room-and-pillar mining with pillar recovery has historically been associated with more than 25% of all ground fall fatalities in underground coal mines in the United States.The risk of ground falls during pillar recovery increases in multiple-seam mining conditions.The hazards associated with pillar recovery in multiple-seam mining include roof cutters, roof falls, rib rolls, coal outbursts, and floor heave.When pillar recovery is planned in multiple seams, it is critical to properly design the mining sequence and panel layout to minimize potential seam interaction.This paper addresses geotechnical considerations for concurrent pillar recovery in two coal seams with 21 m of interburden under about 305 m of depth of cover.The study finds that, for interburden thickness of 21 m, the multiple-seam mining influence zone in the lower seam is directly under the barrier pillar within about 30 m from the gob edge of the upper seam.The peak stress in the interburden transfers down at an angle of approximately 20°away from the gob, and the entries and crosscuts in the influence zone are subjected to elevated stress during development and retreat.The study also suggests that, for full pillar recovery in close-distance multiple-seam scenarios,it is optimal to superimpose the gobs in both seams, but it is not necessary to superimpose the pillars.If the entries and/or crosscuts in the lower seam are developed outside the gob line of the upper seam,additional roof and rib support needs to be considered to account for the elevated stress in the multiple-seam influence zone.

Preparation and Properties of Pillared Montmorillonite by Polyhydroxyl-aluminum-manganese Cations

Al-Mn-pillared montmorillonite(AMPM) was prepared by using the artificial Na-montmoril-lonite from the Qingfengshan bentonite mine as starting materials mixed with Al-Mn pillaring solutions at different Al/Mn molar ratios (R). The basal spacing and specific surface area of the materials were increased significantly compared with untreated clays. When R = 0.5, the d (001) value and specific surface area of pillared montmoril-lonite were 1.8987 ran and 146.01 m2 g-1, respectively. The thermal stability was determined using calcined tests , X-ray diffraction ( XRD ) analysis, thermal gravimetry and differential thermal analysis (TG - DTA). The materials formed at initial R = 0.5 exhibited a high stability, the basal interlayer spacing was stabilized at 1.7859 nm after calcined for 2 h at 300℃. The adsorption behavior of the materials was studied by adsorption experiments. The results show the AMPM and calcined Al-Mn-pillared montmorillonite ( CAMPM) exhibit a strong capacity of adsorbing the Zn(Ⅱ) in aqueous solution at pH 10.0.

Web pillar stability in open‑pit highwall mining

When highwall mining technology is applied to recover large amounts of residual coal left under the highwall of a big openpit mine,a reasonable coal pillar width is required to ensure the stability of the web pillars.Using numerical simulations,this paper studied the characteristics of the abutment stress distribution in the web pillars under different slope angles and mining depths,and established a relation describing the stress distribution in the web pillar.The relationship between the abutment stress and the ultimate strength of the web pillar under different pillar widths was also analyzed.In combination with the failure characteristics of the pillar yield zone,this relationship was used to explore the instability mechanism of web pillars.Finally,the optimal retaining widths of the web pillars were determined.Based on the modeling results,a mechanical bearing model of the web pillar was established and a cusp catastrophe model of pillar-overburden was constructed.Additionally,the web pillar instability criterion was derived.By analyzing the ultimate strength of the web pillars,a formula for calculating the yield zone width either side of the pillars was established.Using the instability criterion of web pillars in highwall mining,a reasonable pillar width can be deduced theoretically,providing significant guidance on the application of highwall mining technology.