Numerical and theoretical study of large-scale failure of strata overlying sublevel caving mines with steeply dipping discontinuities

The deformation and fracture evolution mechanisms of the strata overlying mines mined using sublevel caving were studied via numerical simulations.Moreover,an expression for the normal force acting on the side face of a steeply dipping superimposed cantilever beam in the surrounding rock was deduced based on limit equilibrium theory.The results show the following:(1)surface displacement above metal mines with steeply dipping discontinuities shows significant step characteristics,and(2)the behavior of the strata as they fail exhibits superimposition characteristics.Generally,failure first occurs in certain superimposed strata slightly far from the goaf.Subsequently,with the constant downward excavation of the orebody,the superimposed strata become damaged both upwards away from and downwards toward the goaf.This process continues until the deep part of the steeply dipping superimposed strata forms a large-scale deep fracture plane that connects with the goaf.The deep fracture plane generally makes an angle of 12°-20°with the normal to the steeply dipping discontinuities.The effect of the constant outward transfer of strata movement due to the constant outward failure of the superimposed strata in the metal mines with steeply dipping discontinuities causes the scope of the strata movement in these mines to be larger than expected.The strata in the metal mines with steeply dipping discontinuities mainly show flexural toppling failure.However,the steeply dipping structural strata near the goaf mainly exhibit shear slipping failure,in which case the mechanical model used to describe them can be simplified by treating them as steeply dipping superimposed cantilever beams.By taking the steeply dipping superimposed cantilever beam that first experiences failure as the key stratum,the failure scope of the strata(and criteria for the stability of metal mines with steeply dipping discontinuities mined using sublevel caving)can be obtained via iterative computations from the key stratum,moving downward toward and upwards away from the goaf.

Support-surrounding rock relationship and top-coal movement laws in large dip angle fully-mechanized caving face

When mining the fully-mechanized longwall caving face along strike, the unstable equipment, the low top-coal recovery ratio and the difficulty in controlling surrounding rock may occur due to large dip angle. Considering the effects of strike angle on support stability, the ‘‘support-surrounding rock"mechanical models of support topple and support slip were established in this paper. On the basis, the influencing factors of support stability were analyzed and the technical measures of controlling support and surrounding rock stability were put forward. Then the loose particles simulation experiment was conducted to analyze the impacts of caving directions and methods on the top-coal recovery in large dip angle fully-mechanized caving face. Finally, the ‘‘upward sequence and double-openings doublerounds" caving technology was determined. The research results are of great scientific significance and practical values to improve large dip thick seam mining technology.

Characteristics of Gas Emission at Super-Length Fully-Mechanized Top Coal Caving Face

Characteristics of gas emission at the K8206 working face in the Third mine of the Yangquan Coal Group were investigated. The effects of strata movement,advancing velocity of working face,production capacity of working face and gas extraction capability of strike high-level entry on gas emission at K8206 working face were analyzed. A regression equation,reflecting the relationship between relative gas emission rate and the production capacity of work-ing faces,was established. Another regression equation showing the relationship between the gas emission rate from adjacent layers when the working face was advancing for one metre and advancing velocity was derived. It can be con-cluded that,1) the amount of gas emitted at the K8206 working face is far greater than that of ordinary top coal caving faces with a dip length of 180-190 m; 2) the dynamic process of gas emission from adjacent layers during the initial mining stage is controlled by the movement of key strata; 3) the amount of gas emitted that needs to be forced out by air is greatly affected by the capability of gas extraction; 4) when the advancing velocity is between 3.5-5.5 m/d or when the output is up to 8-12 kt/d,the gas emission from adjacent layers is almost constant.

Patterns governing distribution of surrounding-rock stress and strata behaviors of fully-mechanized caving faces

By employing numerical modeling, similar material simulation and comprehen-sive field observation, investigations were made and patterns were obtained governing surrounding-rock stress distribution and strata behaviors. It shows that patterns governing displacement of FMC roadway surrounding rocks and those governing deformation of supports are basically the same along the strike, but the displacements vary greatly. The front stresses affect greater areas than the lateral stresses and their limit widths of equilib-rium zones and K are almost similar. The stress transmits very deep. Our findings offer scientific basis on which to determine parameters for coal pillar retaining and for roadway out-laying, thus increasing the recovery ratio and improving the maintenance of roadway.

Study on gob-side entry retaining in fully-mechanized longwall with top-coal caving and its application

Based on the engineering background of gob-side entry retaining in fully-mechanized longwall with top coal caving(GER-FLTC) on N2105 working face of Yuwu coal mine, by adopting the methods of theoretical analysis and numerical calculation, the control techniques of surrounding rocks in GER-FLTC working face are studied in this paper. The two main difficulties of stability of surrounding rocks at gob-side retained entry in fully-mechanized longwall working face are the stability control of top coal and control of large deformation of GER-FLTC working face. Interaction mechanical model between roofing and roadside backfilling in GER-FLTC is established and the equations for the calculation of roof-cutting resistance of roadside backfilling are derived. Results of numerical calculation show that the damage zones of top coal can be categorized into the delaminating zone of top coal above the backfilling, tensile damage zone of top coal above the retained roadway and shear damage zone of the upper rib of the solid coal. Stability control of top coal is the critical part to success of GER-FLTC. With consideration of large deformation of surrounding rocks of gob-side retained entry in fully-mechanized longwall, the support technique of‘‘roofing control and wall strengthening'' is proposed where high strength and highly prestressed anchor rods and diagonal tensile anchor cables support are used for top coal, high strength and highly prestressed yielding anchor rod support is used for solid coal and roadside prestressed load-carrying backfilling is constructed by high-water material, in order to maintain the integrity of the top coal, transfer load, high resistance yielding load-carrying of solid coal, roof-cutting of roadside backfilling and support,and to achieve GER-FLTC. Results from this study are successfully applied in engineering practice.

SAFETY PROBLEMS IN FULLY-MECHANIZED TOP-COAL CAVING LONGWALL FACES

The thick-san top-ctal catrig technology has been in use in China for over a decade,and has given rise to siguscant economic efficiendes. Eftorts in reeent years to extend its applica tion to more complex mining conditions, mostly high-gas seams , with or without proneness ofspontaneous combustion. have brought about new safety problems This paper will highlight thefcatures and problems retared with thick-seam top-coal caving systems, compared with conventionai, fully-mecbanized longwall systems , particularly issues retated to methane, spontaneous combustion and dust,and disam the methods and measures to ded with them.

Study on safe thickness of overlying thin bedrock in fully-mechanized top-coal caving face with thick coal seam

To prevent support crush, the overlying strata safe thickness and its influential elements were studied by the adoption of theoretical analysis, numerical simulation and in-situ measurement. According to the production and geological condition of first face in Sima coal mine, the results indicate that the clay contains large permissible bearing ability and has better arching force. After mining destruction, stable structure is formed in bedrock to ensure face safety. The clay thickness ＆ bedrock thickness are the key influential elements to stable structure. The minimal bedrock thickness is about 40 m to ensure safe mining under loose surface soil condition. When surface soil contains mainly thick clay, it forms steady structure with the composition of thin bedrock, so that it can reduce minimal thickness of bedrock and to ensure safe mining. When clay thickness is 40 m, minimal bedrock thickness is 20 m. When clay thickness is 30 m, minimal bedrock thickness is 30 m. Bearing pressure peak ranges from 5 to 15 m in the front face under thin bedrock condition. The bearing pressure distribution range is 15 m. Main roof break distance is small, and initial weighting of main roof is not distinctive, while first periodic weighting of main roof is quite distinctive.

Analysis on distribution law of the abutment pressure of the integrated coal beside the road-in packing for gob-side entry retaining in fully-mechanized caving face

The three-dimensional damage constitutive relationship of coal is established and distribution law of the abutment pressure of the integrated coal beside the road-in packing for gob-side entry retaining in fully-mechanized caving face under the effect of given deformation of the main roof is analyzed by the damage mechanics theory. And the relationship between distribution of the abutment pressure and thickness of coal seam is explored. The presented result is of great theoretical significance and practical value to the study on stability control of the surrounding rock of road-in packing for gob-side entry retaining in fully-mechanized caving face.

Failure of hanging roofs in sublevel caving by shock collision and stress superposition

Hanging roofs or high hang-ups.a common problem in sublevel caving mining,usually result in a large ore loss and undermine mining safety.This paper analyzed the formation of a hanging roof and showed that increased confining pressure and reduced free surface were its main characteristics.In order to break down a hanging roof,a new method based on shock wave collision and stress superposition was developed.In this method,two blastholes containing multi-primer at different positions are simultaneously initiated at first.By doing this,a new free surface and a swell room can be created.After these holes are fired,a long delay time is given to the next blasthole so that the fragments from the first twohole blasting have enough time to fall down.This new method was applied to three hanging roofs in one production area,and all of them were successfully broken down.Field inspection indicated that almost no damage was caused in the nearby drifts/tunnels due to the new method.In addition,the far field vibrations were found to be smaller than the maximum vibrations induced by some other blasts.

Numerical Simulation of Independent Advance of Ore Breaking in the Non-pillar Sublevel Caving Method

The mechanism of stress generation and propagation by detonation loading in five separate independent advance of ore breaking patterns is discussed in the paper. An elastic numerical model was developed using AN- SYS/LS-DYNA 3D Nonlinear Dynamic Finite Element Software. In this package ANSYS is the preprocessor and LS-DYNA is the postprocessor. Numerical models in the paper to actual were l：10 and the element mesh was dissected in scanning mode utilizing the symmetry characteristics of the numerical model. Five different advance rates were studied. Parameters, such as the time required to maximum stress, the action time of the available stress, the maximum velocity of the nodes, the stress penetration time, the magnitude of the stress peak and the time duration for high stress were numerically simulated. The 2.2 m advance appeared optimum from an analysis of the simulation results. The results from numerical simulation have been validated by tests with physical models.

Statistic constitutive equation of top-coal damage for fully mechanized coal face with sublevel caving

Under the action of abutment pressure in front of fully mechanized coal face with sublevel caving(CFSC),top-coal over CFSC deformed.In the process of whole de- formation of top-coal,it changed from continuum elastic mass to non-continuum plastic mass contained fissures,become a loose body.According to its bearing characteristics and mechanical properties,top-coal mass can be divided into four deformation zones along the winning direction of CFSC,i.e.initial stress zone,elastic zone,plastic zone and loose zone.Top-coal in plastic zone located in the post-peak zone of the stress-strain curve for top-coal.With equivalent strain principle of damage mechanics and mathemati- cal theory of statistic,combining the movement law of top-coal,set up a constitutive equa- tion with damage statistics for top-coal in different position in CFSC.The equation illus- trated the mathematical relationship among top-coal bearing capacity,horizontal confining pressure along the winning direction of CFSC and mechanical properties of top-coal mate- rial.The conclusions not only provide a basis for numerical computer simulations on damage deformation and failure mechanism for top-coal,but also further promote the ap- plication of damage mechanics in CFSC.

STUDY ON THE BROKEN-ORE DRAWING DATA MANAGEMENT SYSTEM OF SUBLEVEL CAVING

In order to realize the modernized miningmanagementi the authors. based on the practice of a specified mine, developed the application software of the broken ore drawing data management system for sublevel caving through the study on FOXBASE computer language. This paper elabo-rates the overall conception of this system , indicates the main task which should he completed in this system and introduces its module structure and main functions.

Analysis of factors of top coal caving in fully mechanized sublevel caving face in soft coal and control

According to the analysis of the mechanism of top coal caving, the caving condition was pointed out, and many factors of caving were also determined. Then the relationship between factors and caving was studied. Based on the above research, one effective method by using field monitoring was brought forward to determine the controlling factor. Then some related key technologies were provided, such as keeping the integrality of the top-coal, raising the horizontal resistance of supports and decreasing the real end-face distance etc.. At last one application of this method was presented, and it was proved to be an effective method.

Study on rock burst of fully mechanized sublevel caving mining in deep mine

The theoretical analysis, numerical simulation and field observation were used to study distribution characters of abutment pressure of fully mechanized sublevel caving face in deep mine, fully mechanized sublevel caving and pressure relief in entries along goaf to the influence of rock burst. The results show that： （1） With the increasing of mining depth, the abutment pressure zone is larger, its peak point is transferred to the front of face, the danger area occurring rock burst in the two fully mechanized sublevel caving entries is larger, and its position is far from the face; （2） There is larger failure area in the upper coal in front of the fully mechanized caving face, and strata possibly occurring rock burst transferred to the upwards or far from faces because of the main roof＇s buffer effect to the dynamic pressure burst, then possibility and strength reduced; （3） The position occurring rock burst is stress concentration zones （abutment pressure along the goaf and residual tectonic stress） and zones with geological structure; （4） Strenuous activities of roof is the inducing factor occurring rock burst.

Optimal location of terminal line in sublevel caving face with suspend-shift support in thinner thick-seam

According to the geological and mining technical conditions of No.18128 sub- level caving face with suspend-shift support,the first working face of the working area,the distribution laws of abutment pressure were studied by jointly employing FIAC^(3D) numerical simulations and field tests.With the full consideration of these laws and the service time- limit of the mining district,the optimal location of the terminal mining line,should be 60 m away from the W4-B8 track dip,which was 20 m shorter than the 80 m distance deter- mined according to traditional experiences.The retained coal safety pillar for the protect- ing of dips,already proved by the industrial practice,can not only improve the rate of coal recovery and satisfy the subsequent service requirement for the following working faces, but also decrease the maintaining expenditure and thus increase profits.

Arrangement of anchor reinforcement in roadway for fully mechanized sublevel caving

Bolting of mining roadway for fully mechanized sublevel caving has been practised successfully in Hebi mining area.It provides a new method for roadway support and settles the problem of support difficulty radically for sublevel caving in Hebi mining area.Where anchor reinforcement holds an important station in roadway support.This article brings forward the arrangement project of anchor based on theoretic analysis.Compared with arranged in the middle of the entry, anchor arranged in the vertex of the entry can reduces the length of anchor,shortens the anchor installation time,and heightens the reliability of anchor installation.

STUDY ON PRODUCTION SYSTEM OPERATION REGULARITY OF FULLY MECHANIZED SUBLEVEL CAVING MINING AND SEQUENTIAL OPERATION

According to a lot of practical data in Liujialiang Mine and reliability theory and result of computer simulation, operation regularity of fully mechanized sublevel caving mining production system in the condition of gently inclined complicated geological structure and production shortcomings are found out and reliability of system and output of the working face are predicted finally.

有限回采空间内贫矿资源无底柱分段崩落法低贫损开采方案研究

做好无底柱分段崩落法的贫损控制对于矿山经济效益及环境保护具有重要意义,但当矿床开采条件复杂、矿石品位较低时,矿石贫损控制难度将会大幅增加,在此条件下如何做好矿石贫损控制一直是崩落法开采方向的重要关注点。金川龙首矿西二采区无底柱分段崩落法采场仅可布置4个分段,回采空间十分有限,不利于矿石的贫损控制,加之西二采区矿石品位较低但经济价值较高,因此在开采过程中需要严格控制矿石贫损。针对西二采区的复杂开采条件及贫损控制需求,提出了一种“首采分段总量控制出矿、中间分段低贫化出矿、最末分段低贫化出矿+辅助进路回收残留矿石”的组合式低贫损回采方案。以西二采区为工程背景,设计并制作了一种高仿真度的物理放矿模型并对该方案展开了试验研究。结果表明:即便是在暂时不考虑回收最末分段残留矿石的情况下,该方案可使西二采区崩落法采场的矿石回收率达到75.6%,而废石混入率仅为6.0%,在确保矿石回收率的基础上显著降低了矿石贫化。该方案在现场应用后,实际的矿石回收指标与试验结果基本接近,表明该方案在西二采区有限回采空间内实现了贫矿资源低成本、高质量回收的总体目标。

无底柱分段崩落法结构参数的影响因素及计算

分析无底柱分段崩落法的特征及其优缺点,整理了该采矿方法主要结构参数的影响因素,并归纳了主要结构参数的理论计算公式,同时通过尖山采场的室内放矿实验及现场实际效果,对归纳整理的主要结构参数计算公式进行验证。可为采用无底柱分段崩落法的矿山提供设计依据。

基于降雨量数据的程潮铁矿涌水量时序性预测模型

无底柱分段崩落法开采必然形成大范围的地表塌陷区,所分布的导水裂隙将导致大量的地表降水向下渗透,引起巷道涌水量骤增,程潮铁矿现已进入-500 m开采水平,大气降雨与崩落法开采所引发的裂隙持续动态发展之间的相互作用机制十分复杂。因此,为科学准确地预测井下涌水量,本研究提出了一种以降雨量为输入数据的涌水量灰色GM(1,2)时序性预测模型。以程潮铁矿2019—2021年的降雨量、涌水量实际数据为训练样本,充分考虑崩落法开采对上覆岩体的持续影响,引入时序性系数K对模型进行优化,最终建立的灰色GM(1,2)时序性预测模型与传统的GM(1,2)预测模型相比,预测精度平均提高了7.79%。运用该模型对2022年的涌水量进行预测检验结果表明,其旱、雨两季的预测精度分别为93.51%、93.58%,预测效果较好。研究成果是通过地表降雨量数据直接预测矿山井下涌水量数据的一种有效方法。