Monitoring the performance of rock reinforcement

A rock mechanics program has been developed and implemented in order to achieve a safe and economical room and pillar extraction of the shallow dipping orebodies at the McArthur River Mine in Australia. Three closely spaced tabular orebodies have been targeted for extraction over the life of the mine and due to economics the lowermost number 2 orebody is being mined first. This paper formulates the calculation of the 2 Orebody hangingwall spans and the appropriate rock reinforcement design for the long-term stability of the room and pillar excavations. The field trials suggested that a rock beam was formed within the hangingwall of the room and pillar operations, arresting any vertical movement of the roof. The results indicated that the weight of the beam is transferred to the pillars via an arching process and there is no need for deep-anchored reinforcement.     

Prediction of surface crown pillar stability using artificial neural networks

A relatively novel technique, artificial neural networks (ANN), is used in predicting the stability of crown pillars left over large excavations. Data for the training and verification of the networks were obtained from the literature. Four artificial networks, based on two different architectures, were used. The networks used different numbers of input parameters to predict the stability or failure of crown pillars. Multi‐layer perceptron networks using mine type, dip of orebody, overburden thickness, pillar thickness, pillar length, stope height, backfill height, Rock Mass Rating (RMR) of the host rock and RMR of the orebody showed excellent performance in training and verification. Adding three more variables, namely pillar width, rock density and pillar thickness to width ratio, showed symptoms of over‐learning without degrading performance significantly. Radial basis function networks were capable of predicting crown pillar behaviour on the basis of few input functions. It was shown that mine type, dip and pillar thickness to width ratio can be used for a preliminary estimation of stability. Copyright © 2006 John Wiley & Sons, Ltd.     

Research on reasonable coal pillar width of roadway driven along goaf in deep mine

Driving roadway along a goaf is commonly adopted for mining face of thick seam in a deep mine. Determining a reasonable width of coal pillar is a key scientific problem for driving roadway along a goaf in a deep mine. The paper took a roadway driven along a goaf at Zhaolou coal mine which is a typical kilometer-deep mine in China as engineering background. Field monitoring, model test, and numerical experiment are conducted. Stress and displacement evolution mechanism are analyzed with different pillar widths. The test results show that with the increase of coal pillar width, the peak stress value at the coal pillar working slope and integrated coal beside the roadway increases firstly and then tends to be stable, its position is transferred to the side of the roadway, and the deformation of coal pillar decreases gradually during roadway excavation. The coal pillar deformation and roadway vertical displacement increased as the coal pillar width increases under high abutment pressure. In order to reduce the waste of non-renewable resources and meet the requirements of bearing capacity and stability of coal pillars, a method is proposed for setting a reasonable width of coal pillars and the specific width of coal pillars is designed and applied in engineering practices based on the above research. All the tests are significant in the study of driving roadway along a goaf in a deep mine.     

Risk considerations for crown pillar stability assessment for mine closure planning

Evaluation of the long-term surface stability of crown pillars overlying underground mines is an important component of mine closure planning. The definition of a crown pillar, as well as a brief discussion of the assessment of the probability and consequence of crown pillar failure are given in this paper. Techniques for stability assessment using mechanistic, empirical and numerical simulation techniques are discussed. Consequence assessment is discussed, but is still subjective and difficult to quantify. Where crown pillars are suspected to be marginally stable or unstable either at the time of the investigation or over the long term, and where the consequence of failure is medium to high, the closure plan for the site must include proposed rehabilitation alternatives. Selection of the optimum solution depends largely upon financial considerations, but also upon the common public expectation that the result of mine closure planning be a permanent solution that does not restrict public access or future land use on the site.     

Numerical Study of Failure Mechanism of Serial and Parallel Rock Pillars

Using a numerical modelling code, rock failure process analysis, 2D, the progressive failure process and associated acoustic emission behaviour of serial and parallel rock samples were simulated. Both serial- and parallel sample models are presented for investigating the mechanism of rock pillar failure. As expected, the numerical results show that not only the stiffness, but also the uniaxial compressive strength of the rock plays an important role in pillar instability. For serial pillars, the elastic rebound of a rock pillar with higher uniaxial compressive strength can lead to the sudden failure of an adjacent rock pillar with lower uniaxial compressive strength. The failure zone forms and develops in the pillar with lower uniaxial compressive strength; however, the failure zone does not pass across the interface of the two pillars. In comparison, when two pillars have the same uniaxial compressive strengths but different elastic moduli, both serial pillars fail, and the failure zone in the two pillars can interact, passing across the interface and entering the other pillar. For parallel pillars, damage always develops in the pillar having the lower uniaxial compressive strength or lower elastic modulus. Furthermore, in accordance with the Kaiser effect, the stress-induced damage in a rock pillar is irreversible, and only when the previous stress state in the failed rock pillar is exceeded or the subsequent applied energy is larger than the energy released by the external loading will further damage continue to occur. In addition, the homogeneity index of rock also can affect the failure modes of parallel pillars, even though the uniaxial compressive strength and stiffness of each pillar are the same.     

Empirical Approaches for Design of Web Pillars in Highwall Mining: Review and Analysis

Pillar design is of paramount importance to any underground mine design. Oversized pillars may lead to loss of coal while undersized pillars may lead to instability. While underground pillars are mostly square and rectangular, highwall mining pillars are long and narrow, as they are formed after driving parallel entries in the seam from the highwall. These pillars are termed as web pillars. The overall stability of highwall depends upon these pillars as no other supports are provided in the entries. Web pillar differs from usual coal pillars in respect of w/h ratio being <3.0, with an exceptionally longer length compared to its width, to the tune of 50–500 m. Several empirical coal pillar strength equations developed for rectangular pillars are still being used with some modifications to adapt to web pillars. Review and analysis of these empirical approaches for determining web pillar strength along with a numerical approach for web pillar design are discussed in this paper. Their application to some Indian case studies is also discussed.     

大采高综放面区段煤柱合理留设研究

侧向支承压力分布、资源回收率以及煤柱和巷道的稳定性是大采高综放面区段煤柱宽度留设要兼顾的因素,为了确定大采高综放面区段煤柱宽度,以某矿8103面为工程背景,首先,采用理论计算和现场应力监测等方法确定大采高综放工作面倾向支承压力分布规律,得出应力降低区宽度约为8 m,原岩应力区为巷帮侧28 m外。其次,采用工程类比方法确定大采高综放工作面巷帮外侧煤体严重破裂区宽度约为4 m。最后,采用FLAC3D数值软件分析了下区段工作面回采时窄煤柱（6、8 m）和宽煤柱（28、30 m）的应力场、位移场及塑性区特征,获得不同煤柱宽度时巷道和煤柱力学特征。研究表明：当煤柱宽度6 m和8 m时,在采动支承压力下煤柱几乎无承载能力,且巷道变形量较大;当煤柱宽度28 m和30 m时,在采动支承压力下煤柱中央仍有一定的弹性核,煤柱保持稳定且巷道变形量较小。综合考虑资源回收、巷道稳定性、次生灾害控制等因素,确定大采高综放工作面区段煤柱宽度为28 m。     

辛安煤矿1402工作面临空窄煤柱采掘响应及动态加固

随着我国煤炭资源去产能整合煤矿的增多,复采工作面临空窄煤柱采动失稳问题日益凸显,已严重制约矿井安全高效生产。为此,针对辛安煤矿复采1402工作面辅运巷道5号钻场临空窄煤柱稳定性控制的工程难题,运用数值模拟与理论分析相结合的方法,探究5号钻场临空窄煤柱稳定性采掘扰动响应特征,提出5号钻场临空窄煤柱动态注浆加固技术方案并开展现场应用和效果检验。研究结果表明:1402工作面辅运巷道掘进对5号钻场临空窄煤柱稳定性影响较小;在1402工作面回采期间,距5号钻场18~6 m范围,临空窄煤柱集中垂直应力由非对称马鞍形分布逐渐过渡为拱形分布;距5号钻场6 m时,临空窄煤柱承载叠加垂直应力超过煤体强度,塑性区完全贯通,极易破坏失稳;现场采用MP364型注浆材料及专用注浆设备对5号钻场临空窄煤柱前后5 m区域进行加固,动态注浆始终超前工作面10 m,通过深孔窥视和气体监测手段验证临空窄煤柱良好的封堵固化效果,保障了工作面安全回采,为我国整合矿井类似条件下煤柱稳定性控制提供借鉴和参考。移动阅读      

Ultimate bearing capacity and settlement of coal pillar sub-strata

Summary This paper develops a rational approach for design of coal pillars under weak floor strata conditions considering ultimate bearing capacity (UBC) as well as pillar settlement. An approximate solution is presented for estimation of UBC for a shallow foundation on a two-layered rock system with consideration of both cohesion (c) and (ø) for both layers. Similarly, deformability underneath a full-size pillar is estimated from deformability calculated from plate loading tests. The effect of adjacent pillars on UBC and deformability of coal pillars in a panel is considered using foundation engineering analysis techniques. The design of pillars based on limiting settlements considers both differential settlements as well as mean settlement of pillar in a panel. An attempt is made to validate the proposed design approach based on field data and observations at an Illinois mine.     

基于巨厚岩层-煤柱协同变形的煤柱稳定性

采前煤柱稳定性研究是工作面冲击危险性评估和开采方案设计的关键。以山东某矿深井巨厚砾岩条件工作面开采遗留煤柱为背景,采用案例调研、理论分析、数值模拟和工程实践等方法,对巨厚岩层-煤柱协同变形机制及其煤柱稳定性进行了研究,建立了巨厚岩层-煤柱协同变形的简化力学模型,探讨了引起煤柱变形的主要应力来源和变形形式,推导了在协同变形条件下煤柱的应力-应变关系。以此为基础,综合煤柱煤体应力、围岩稳定性和变形特征等条件,提出了煤柱整体失稳的力学判据。研究结果表明：巨厚岩层-煤柱失稳诱发冲击与煤柱的位置、尺寸和上覆岩层运动或变形关系密切,上覆岩层运动或变形是诱发煤柱失稳的动力因素;巨厚岩层-煤柱的变形主要包括受集中力F压迫的协同挠曲压缩变形和受集中力G作用的重力沉降变形,二者保持内在协调性;巨厚岩层下煤柱整体失稳的工程判据为煤柱煤体平均支承应力p超过其平均极限支承强度Rc（p≥Rc）;评估得到遗留的50 m煤柱具有强冲击危险性,并通过优化开采设计,取得了良好的效果。该研究成果对相似条件煤柱留设及其稳定性分析具有参考意义。     

Buckling failures of reserved thin pillars under the combined action of in-plane and lateral hydrostatic compressive forces

The reasons that pillars with small width-to-height ratios fail remain unclear. This study established a mechanical model for the buckling failure of a thin pillar subjected to compressive forces to investigate the stability of reserved thin pillars (RTPs) on both sides of mining units during barrier pillar recovery. The critical buckling stress of the thin pillar was obtained using the energy variational method, and its relationship to the aspect ratio (length-to-width ratio) was investigated. The buckling instability of RTPs can be determined by comparing the RTP stress obtained from numerical simulations with the buckling stress derived from the mechanical model.     

陕北某矿双煤层开采对覆岩影响的模拟对比

多煤层开采条件下煤层覆岩破坏具有独特的特征，影响矿井生产布置。以陕北某矿为例，以该矿地质采矿条件为基础，采用相似材料模拟实验与数值模拟相结合的方法，通过建立模拟模型，开展了双煤层开采对覆岩的破坏影响研究。结果显示:留设不同宽度的煤柱，采用相似材料模拟和数值模拟2种方法得到的煤层覆岩垮落带高度、裂隙带高度都基本一致；在双煤层开采时，留设的煤柱宽度越大，两个煤层的叠置区域就越小，煤层开采对覆岩的破坏程度就越小。在工作面布置时，建议增大两个煤层的开采距离，并尽量增加煤柱宽度，以减缓覆岩移动破坏范围和破坏程度。研究成果为类似双煤层开采工作面的设计及覆岩破坏控制提供技术支撑。      

矿柱稳定性影响因素敏感性分析及其应用研究

为研究矿柱稳定性影响因素的敏感程度,以招金大尹格庄金矿为研究背景,从矿柱载荷、强度、失稳形式及影响因素确定四方面推导出两种矿柱形式的安全系数计算公式,设计采用6因素5水平正交试验对矿柱稳定性影响因素敏感性进行分析,并研究主要影响因素与矿柱安全系数之间的函数关系。研究表明,矿柱宽度、矿体开采深度和矿房宽度3个主要因素对矿柱稳定性影响程度是最为剧烈的;矿柱安全系数随着矿柱宽度地增加呈指数形式递增且递增速率不断加大,随矿体开采深度地增加呈幂函数形式递减且递减速率逐渐变缓,随矿房宽度地增加呈负指数形式递减且降低速率逐渐变慢。利用DPS和Matlab建立的矿柱安全系数回归方程得出保证安全生产的合理矿柱尺寸,矿房宽度应不超过8 m,布置条形矿柱宽度应不小于3.6 m,布置方形矿柱宽度应不小于5.9 m。结合采场矿柱布置实际,应用效果较为理想,可为下中段矿体回采矿块布置提供较好的依据。     

Possibilities of oil shale mining under the Selisoo mire of the Estonia oil shale deposit

The paper presents results of the study on oil shale mining (Estonia underground mine) possibilities under the Selisoo mire. The Selisoo area is 2,051 ha in extent, and most of the mire is in natural state. Peat layer consists of thick (4.4–6.5 m) oligotrophic peat. Mining under Selisoo will go at depths 65–70 m under the surface. The mining field of the Estonia mine was planned between Ahtme and Viivikonna fault zones. The lowest hydraulic conductivity of carbonate rocks 0.11 l/day per m² is found in the Oandu layer and for peat it is 0.35–0.0002 m/day. Therefore, together they form a good aquitard. When the annual rainfall amount is the highest, the difference between horizontal water inflow and runoff is positive with 127,000 m³. Positive water balance is guaranteed in case of precipitation being at least 540 mm/year. The positive water balance is important for preserving the ecological system of Selisoo mire. For guaranteeing long-term stability of mine pillars, a new calculation method has been elaborated, based on the conventional calculation scheme, where the factor of safety is more than 2.3. Rheological processes are out of question, collapse of the pillars is impossible. Stability of the underground constructions and overburden rocks must be “eternal”. The criteria were elaborated for oil shale mining and will guarantee preservation of mires in natural or close to natural state.     

Study on Progressive Instability Characteristics of Coal-Rock Composite Structure with the Different Height Ratios

Geotechnical and Geological Engineering - To ensure the safety of coal mining, lots of coal pillars were reserved in the coal mine. The stability of coal pillar and roof composite structures...     

Sudden surface collapse disasters caused by shallow partial mining in Datong coalfield,China

Numerous sudden surface collapses induced by shallow partial mining in the Datong Jurassic coal seam have caused fatalities, significant property losses and brought about harmful results to the environment. By introducing efficient pillar widths and using the Voronoi diagram, irregular pillar stability can be estimated rationally. Theoretical analysis and numerical simulation demonstrate that the failure of a single pillar increases the load on surrounding pillars. If the magnitude of the transferred load is sufficiently high, the adjoining pillars will also fail in a chain reaction. This can be interpreted by the merger of inner stress arches combined with the external stress arch. In this paper, the evolution mode of sudden surface collapse caused by shallow partial mining is proposed and has been verified by ‘similar material simulation.’ Finally, the potential of sudden surface collapse is determined and an example of collapse prediction and prevention of surface building damage with relocation is given.     

Subsidence prediction method of solid backfilling mining with different filling ratios under thick unconsolidated layers

Solid backfill mining for coal pillar recovery in industrial squares has to ensure that the mine infrastructure, such as the shafts and substations, is not degraded or has its utility impaired by that mining. At the same time, it is important to recover as much coal as possible. As a result, it is necessary to predict mining subsidence during solid backfilling mining of coal pillars in industrial squares and to optimize the design of the working faces. At the Baishan coal mine in Anhiu province, China, there are thick layers of unconsolidated overburden above the coal seam so it is not appropriate to use the surface subsidence prediction method of equivalent mining height to predict subsidence during the mining of the coal pillars there. In order to find a reasonable coal pillar recovery scheme for the Baishan mine, a numerical simulation method is used to determine the relationships between the compression ratio of the backfilling material and the surface subsidence prediction parameters. Research was done to determine the appropriate parameters, and based on the final prediction parameters and taking the mandated protection standards for buildings and structures into account, surface subsidence is predicted and a backfill mining scheme for pillar recovery is proposed. The results show that of the six mining schemes considered, scheme 5 is the best scheme for coal pillar recovery in the industrial square at the Baishan mine. The research results are significant for similar mines with thick unconsolidated overburden anywhere in the world.     

内蒙古高家梁煤矿巷道加固及围岩支护结构的数值模拟分析

高家梁煤矿位于内蒙古鄂尔多斯市东胜区东南8km处,矿井设计规模年产600×104吨。建井期间在埋深50～150m的运输大巷中部,遇到一种低强度软岩,巷道掘进与施工过程中冒顶、塌方时有发生,常规锚杆支护失效,对施工安全和进度造成很大影响。基于巷道地质情况与软岩特征,根据巷道围岩松动圈理论及围岩与支护作用关系,重点分析了软岩巷道的变形、顶板失稳机理,确定了运输大巷的工程加固方法。经过ADINA数值模拟验算以及后期的运输大巷变形监测曲线都证明加固方案满足运输大巷对支护刚度和强度的要求,同时还将加固方案进行了经济评价,得出了这种加固方案对于高家梁矿的特殊软岩巷道段加固在技术上可行,经济上可以接受的结论。     

A Comparative Analysis of Pillar Design Methods and its Application to Marble Mines

Summary. The methods for designing pillars in underground mines are fundamentally based on empirical formulae that do not take into account the quality of the rock mass as an input parameter. This makes them difficult to apply in other types of ground that are different to those used to establish each empirical formula. To avoid this inconvenience, the present paper examines existing empirical formulae to then propose a modification of these formulae adjusting the resistance of the pillars on the basis of the RMR (Bieniawski’s Rock Mass Rating). The compression safety factor of the pillars is analyzed for each modified formula and a study is carried out of shear failure if planes of weakness exist in the pillars. Finally, the safety factors of the pillars in a marble mine situated in Alicante (Southern Spain) were calculated in order to validate the new formulae. From the results obtained, it is concluded that this new formulation determines the safety factor of pillars of the mine with greater reliability, provided that the pillars are isolated. At the same time, the introduction of the RMR in the formulae results in a better fit of the strength of each pillar to the characteristics of the rock mass.     

Improving Short- and Long-term Stability of Underground Gypsum Mine Using Partial and Total Backfill

The stability of underground mines represents a key issue for active and abandoned mines. Over the last few years, several collapses of underground mines in France have affected existing buildings and infrastructures. Many factors are generally identified as the cause of failures: pillar ageing, fractures, and pillars’ height to width ratio, etc. Among the treatment techniques available to prevent instability and reduce the deterioration of pillars, backfill is the most frequently used. A research programme, supported by the French Ministry of the Environment, was developed to study the operability of partial and total backfill using waste material in the Livry-Gargan gypsum mine (near Paris, France), where pillar height is 17 m. The paper focuses on: (1) the characterisation of the gypsum and fill material (laboratory and in situ tests), (2) the in situ measurements, involving 5 pillars equipped with 19 pressure cells, since 1999, (3) and numerical modelling of fractured pillars performed in order to improve understanding of the effects of backfill on the stability of room-and-pillar mines. The study clearly shows the operability and the advantages of partial and total backfill for short-term pillar stability. The induced horizontal pressure generated by backfill can reach 200 kPa. The use of numerical modelling also shows the effect of backfill on fractures and that backfill reduces indicatively the shear displacement and the opening of fractures. Numerical modelling helps in identifying the mechanisms of backfill and in a better understanding of the behaviour of backfilled mines.