Multiple Seam Mining Research Articles

Deformation and Failure Mechanisms and Support Structure Technologies for Goaf-Side Entries in Steep Multiple Seam Mining Disturbances

Deformation and Failure Mechanisms and Support Structure Technologies for Goaf-Side Entries in Steep Multiple Seam Mining Disturbances

Loading of rib pillars in multiple seam mining at the Upper Kama salt deposit

Loading of rib pillars in multiple seam mining at the Upper Kama salt deposit

Investigation of Overburden Movement and Ground Stress Behaviour in Multiseam Mining

Abstract In multiple-seam mining, severe deformation of roadways and coal bursts caused by overlying coal pillars can occur in the lower seams. Overburden movement and ground pressure behaviour when mining thick seam below the strong roof and upper four extracted coal seams were investigated in the Tongxin coal mine, China. The numerical model indicated that the abutment stress in the lower seam is affected by overlying coal pillars as suggested by the bubble and the cantilever beam models. The microseismic monitoring indicated that high-energy seismic events caused by large overburden movement under the overlying pillars were the source of the dynamic loads. The influence of upper pillar on lower longwall mining conditions can be broken down into three cases: (a) under no pillar area, no excessive deformation or dynamic failure was observed in the tailgate with medium static loads and low dynamic loads; (b) under single pillars or two overlapping pillars, severe deformation occurred in the tailgate with low to medium static loads and medium dynamic loads; and (c) under the three to four overlapping pillars, extreme deformation was observed with the high static loads and high dynamic loads. These findings can provide guidance for the roadway layout in the multiple-seam operations.

Integrated green mining technology of "coal mining-gangue washing-backfilling-strata control-system monitoring"-taking Tangshan Mine as a case study.

In Tangshan Mine, there are four engineering problems, including the layout of multiple seam mining system, serious environmental damages caused by gangue accumulation and surface collapse, nearly 100 million tons of three under coal (coal trapped under buildings, water bodies, and railways) to be released, and the control accuracy of stratum formations to be improved. The work developed the integrated green mining technology of coal mining, gangue washing-backfilling, strata control, and system monitoring. First, a production system was designed for surface-underground transportation of backfilling materials, underground gangue separation, multiple-seam combined backfilling, and mining. Second, key backfilling equipment was developed at mining heights of 2.2 and 3.5 m by backfilling system layout and precise stratum control methods in Tangshan Mine. After real-time monitoring of stope pressure, backfilling effect, and surface deformation, we evaluated the implementation effect of mining, washing, and backfilling technology. The integrated coal mining in seams 5, 8, and 9 of Tangshan Mine showed that the four backfilling surfaces of T3281, T3292, F5001, and F5002 recovered 946,000 t of raw coal and 1.18 million tons of filled gangue, with a net profit of 363.20 million yuan. The maximum land subsidence values (18, 119, 64, and 47mm) were far lower than the deformation extremes based on the requirements of surface building protection.

Measurement of overburden failure zones in close-multiple coal seams mining

In the Kaiping Coal field, mining of five coal seams, located within 80 m in the Kailuan Group, #5, #7, #8, #9 and #12 coal seam, is difficult due to small interburden thickness, concentrated stress distributions, high coal seam metamorphism, and complex geological conditions. By using the ZTR12 geological penetration radar (GPR) survey combined with borehole observations, the overburden caving due to mining of the five coals seams was measured. The development characteristics of full-cover rock fractures after mining were obtained from the GPR scan, which provides a measurement basis for the control of rock strata in close multiple coal seam mining. For the first time, it was found that the overburden caving pattern shows a periodic triangular caved characteristic. Furthermore, it is proposed that an upright triangular collapsed pile masonry and an inverted triangular with larger fragments piled up alternately appear in the lower gob. The research results show that the roof structure formed in the gob area can support the key overlying strata, which is beneficial to ensure the integrity and stability of the upper coal seams in multiple-seam mining of close coal seams.

Time Series Effect on Surface Deformation above Goaf Area with Multiple-Seam Mining

The surface subsidence caused by coal mining is a large area, and computer simulation is a fast and intuitive method, which can help us understand the macroscopic subsidence law. The mined-out area left over by coal mining is not disposed of appropriately for a long time. Thus, it can easily cause ground subsidence, collapse, or spot cracking, especially when mining multiple coal seams, which seriously restricts the construction and safety of the near-surface rock and soil layers. Based on the engineering background of five-layer coal mining in the Beibu Coal Mine of Laiwu City, a “Fast Lagrangian Analysis of Continua in 3D” numerical calculation model was established. The model was used to analyze the surface deformation indexes of four groups with different mining sequences in multiple coal seams, revealing the sequence effects of mining time on the surface deformation law in the goaf collapse areas, hence obtaining optimal mining sequences. The results showed that the four groups of mining sequences (including vertical settlement and horizontal deformation) have stable surface deformation centers, but the deformation ranges and amounts are quite different. The settlement deformation is the main difference. Mining sequence I has the largest deformation of 62.7 cm, followed by mining sequence III. Mining sequences II and IV are basically the same, at only 22% of the value of mining sequence 1. A multi-index analysis of the surface deformation curve including the inflection point, stagnation point, and slope showed that the larger the surface deformation, the more evident the change of the curve (concave or convex) and slope, the more uneven the foundation stress, the more severe the damage to the surface structures, and the less suitable the surface construction. Finally, upon analyzing the indicators of surface stability and adaptability, mining sequence IV was indicated as the optimal scheme. It is suggested that an optimal mining sequence should be appropriately selected before the mining of multiple coal seams. The research results can provide effective guidance for addressing surface deformations under similar geological conditions, and can provide scientific evaluations for the safety and stability of surface buildings and structures, leading to considerable economic and social benefits.

Mining Failure Response Characteristics of Stope Floor: A Case of Renlou Coal Mine

Currently, shallow coal resources are being exhausted gradually, mining depth is continuing to extend downward, and hydrogeological conditions are becoming increasingly complex. Therefore, accurate determination of the failure floor position is necessary to perform multiple‐seam mining. In this study, the 7255 working face of the Renlou coal mine is regarded as the research object. Through a comprehensive measurement of ground penetrating radar detection and fixed‐point grating optical fibers, the law of floor deformation and failure is analyzed dynamically, and the characteristics of the floor rock deformation response are discussed. The results of on‐site monitoring indicate that the mining effect of the working face is greater than that of the tectonic stress. With the advance of the working face, the deformation of the shallow area (0–8 m) first increases gradually, then increases rapidly, and finally increases gradually again; the middle area (8–19 m) experiences three stages, from a gentle increase to temporary stability and then a rapid increase; the deep area (19–29 m) undergoes three stages, from being stable to increasing and then being stable again. After mining, the floor of the working face can be classified into four areas in the vertical direction: complete failure area (0–5 m), poor severe influence area (5–11 m), failure development area (11–19 m), and elastic deformation area (19–29 m). Mining‐induced stresses cause resistance at the interface of different lithologies and weaken the effect of downward propagation. Coal seams and the interface between different lithologies are more prone to deformation. The results can provide a certain reference basis for the advanced exploration scheme of the underlying seam mining under the multiple‐seam mining method, as well as provide a new approach for floor failure depth measurement.

Fissure evolution of weak-cemented jurassic overburden induced by multiple-seam mining: a case study in Western China

ABSTRACT The coal seams in western China are predominantly formed in the lower Jurassic, with generally low strength and poor cementability, which are markedly different from the coal seams in eastern China. In addition, mining layout has an important impact on the fissure development pattern and height in weak-cemented overburden. Thus, investigating the relationship between fissure development pattern and mining layout is of great significance to mining safety. In this study, the mechanical properties of coal and rock strata in no. 3 mining area of Saier Mine, Xingjiang, China, were tested. And a numerical simulation was performed by using the Universal Distinct Element Code (UDEC) distinct element numerical simulation software. The different working face layout schemes were performed to investigate the variation patterns of overburden fissure development height, and coal pillar stress and deformation. The simulation results indicate that as the δW (ratio of the working face width to coal pillar width) increases, the fissure development height gradually increases and the coal pillar horizontal displacement, vertical stress, and horizontal stress first decrease and then increase. Four empirical equations for computing the fissure development height and the horizontal displacement, vertical stress, and horizontal stress of coal pillars were obtained by using the regression analysis method. What is more, all the correlations between the above independent variables and δW can be expressed as quadratic functions. The equations can not only be used as reference criteria for the working face layout design of A3 coal seam in no. 3 mining area of Saier Mine, but also are of great guiding significance to the safety of multiple-seam mining actions under confined water in western China.

Experimental Investigation on Crack Development Characteristics in Shallow Coal Seam Mining in China

The development of cracks in mining is the scientific basis for the safety and environmental exploitation of shallow multiple-seam. According to the “thickness of coal seam, interactive distance, and buried depth,” four mining coal mines are selected in Shen Fu-Dong Sheng coalfield (SFDSC). To research the mining conditions of shallow coal seam under different base-load ratio mining conditions and different working faces by the physics simulation and in-sit measurement, the key roof caves are sketched by different colors. This study shows that the typical shallow coal seams in the thin overlying bedrock and thick loose sand layer (LSL) as well as the development of the setup entry cracks (SEC) is dominated by LSL arch damage. The surface cracks are almost directly above the setup entry. The flat seam mining and the SEC development is dominated by parabolic type. The surface cracks are located inside the setup entry. With the mining height increased typically in a shallow coal seam, the rate of crack development and the extent of damaged area increased significantly. The SEC and boundary cracks are fixed. The dynamic periodic cracks (DPC) show the ability of the strata to self-repair. During the multiple-seam mining, the above three kinds of cracks have the phenomenon of activation and development. Through the reasonable coal pillar distance arrangement, the development of boundary cracks can be effectively controlled and the relatively uniform surface settlement and crack closure can be achieved. The purpose of reducing damage mining can also be achieved. Furthermore, it provides scientific support for the green mining in the shallow coal seam.

Experimental Investigations on the Progressive Failure Characteristics of a Sandwiched Coal-Rock System Under Uniaxial Compression

Overlapped residual coal pillars, together with the surrounding rock strata, play a combined bearing role in ultra-close multiple seam mining. Global stability of the whole bearing system is significant for the mining design, construction, and operation. Laboratory uniaxial compressive experiments for different kinds of sandwiched coal-rock specimens are carried out to investigate the progressive failure characteristics and mechanisms. Results show that: (1) The mechanical behavior of the sandwiched coal-rock specimen is mainly divided into four stages during the failure process. The response of the electrical resistivity and the evolution of acoustic emission (AE) energy are in good agreement with the mechanical behaviors at different stages, which are a reflection of the global failure characteristics of sandwiched specimens. (2) The distribution of AE events and the development of local strain can provide further insight into the local failure characteristics of coal elements or rock elements in sandwiched specimens. AE events are more likely to be generated in coal elements, which can propagate across coal-rock interfaces and induce damage to rock elements in a certain area. Similarly, the unbalanced deformation characteristics of coal elements and rock elements are apparently revealed in the progressive failure process. (3) Progressive failure of a sandwiched coal-rock specimen is closely associated with the interactions between the coal elements and rock elements. Initial failure usually appears in the coal elements. At this process, the recovery of elastic deformation and the output of strain energy are observed in the rock elements, which can accelerate the rupture of coal elements. In turn, the dynamic fracture energy generated in the rupture process of coal elements can propagate into rock elements and induce damage to rock elements a certain area. (4) The experimental results are helpful for maintaining the long-term stability of a sandwiched coal-rock system in ultra-close multiple seam mining.

Risk assessment and prevention of surface subsidence in deep multiple coal seam mining under dense above-ground buildings: Case study

ABSTRACTRecently, solid waste backfill mining has rapidly developed, finding application in the control of surface subsidence and reduction of accumulated solid wastes. However, with the mining depth, the technique faces a number of other problems, in regard to the source of backfill materials, transportation costs, and control of the surface subsidence in deep multiple coal mining. In this article, an integrated system comprising deep mining, coal-gangue dressing, and underground backfill mining is proposed on the basis field-work. Analysis of the type, construction, and protection standards of the buildings resulted in the elaboration of numerical model aimed at studying the surface subsidence and deformation. Research results demonstrate that backfill mining of the coal seam group leads to a gradual increase in maximum surface subsidence, horizontal movement, inclination, and horizontal deformation, whereas the key index to control is the compression ratio of backfill materials. The results of surface subsidence prediction and measurements in the Tangshan Coal Mine show that the solid waste backfill mining technique can effectively control the surface subsidence, deformation, and safety of buildings. This study is highly instrumental in sustained deep mining and environmental protection.

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.

In situ investigations into mining-induced overburden failures in close multiple-seam longwall mining: A case study

Preventing water seepage and inrush into mines where close multiple-seam longwall mining is practiced is a challenging issue in the coal-rich Ordos region, China. To better protect surface (or ground) water and safely extract coal from seams beneath an aquifer, it is necessary to determine the height of the mining-induced fractured zone in the overburden strata. In situ investigations were carried out in panels 20107 (seam No. 2-2upper) and 20307 (seam No. 2-2middle) in the Gaojialiang colliery, Shendong Coalfield, China. Longwall mining-induced strata movement and overburden failure were monitored in boreholes using digital panoramic imaging and a deep hole multi-position extensometer. Our results indicate that after mining of the 20107 working face, the overburden of the failure zone can be divided into seven rock groups. The first group lies above the immediate roof (12.9 m above the top of the coal seam), and falls into the gob after the mining. The strata of the second group to the fifth group form the fractured zone (12.9-102.04 m above the coal seam) and the continuous deformation zone extends from the fifth group to the ground surface. After mining Panel 20307, a gap forms between the fifth rock group and the continuous deformation zone, widening rapidly. Then, the lower portion of the continuous deformation zone cracks and collapses into the fractured zone, extending the height of the failure zone to 87.1 m. Based on field data, a statistical formula for predicting the maximum height of overburden failure induced by close multiple seam mining is presented.

The Impact of Multiple Seam Mining Exploitations on Seismic Activity and State of Stress

Abstract The paper presents an analysis of seismic activity for selected areas of hard coal mine executing exploitation in a rock mass with a variable degree of rock disturbance, i.e., also with a varied number of previous mined-out seams. A distribution of vertical stress and a value of vertical stress concentration coefficient were also determined in the strata of immediate roof of the seams planned for mining. In the analyzed case, despite the lack of thick and solid strata of sandstones in the roof, the rock mass emits seismic activity, where the energy largely depends upon an impact of exploitation edges and tectonic disturbance.

Analysis of Geomechanical Factors Affecting Rock Bursts in Sedimentary Rock Formations

Rock bursts are defined as sudden, violent failures of rock mass that are of such a magnitude that they expel large amounts of coal and rock into the face area during longwall or pillar extraction in sedimentary rocks. In an attempt to develop tools for assessing stress bump potential, the first author initiated a comprehensive study using site specific information from 25 case studies undertaken in U.S. mines. This effort builds on an initial study while expanding the data base and including additional variables and analyses. Multiple linear regression and numerical modeling analyses of geological and mining conditions were used to identify the most significant factors contributing to stress bumps in coal mines. Twenty-five factors were considered initially (mechanical properties of strata, stress fields, face and pillar factors of safety, joint spacings, mining methods, and stress gradients, among others). Allowances were made for favorable local yielding characteristics of mine roof and floor in reducing damage severity. Pillar and face factors of safety were calculated using displacement-discontinuity methods for specific geometries in case studies having experienced both violent and nonviolent failures. This work identified the most important variables contributing to coal bumps and violent failure of near seam strata. These are [1] mechanical properties of strata, including local yield characteristics of a mine roof and floor, [2] gateroad geometry and/or gate pillar factors of safety, [3] stress gradients associated with the approach of mining to areas of higher stress concentrations such as abutment stresses from multiple seam mining, and [4] roof beam thickness, joint spacing, and stiffness characteristics, which influence cave conditions and dynamic loading. The latter variables, combined together to form a new variable called “strata rigidity-cavability”, reflect some of the most important aspects of violent failure, i.e. having massive and stiff near-seam stratigraphic units capable of absorbing high strain energy, forming high stress on mine structures and poor cave conditions, and thus the potential for dynamic loads.

A case study of multi-seam coal mine entry stability analysis with strength reduction method

In this paper, the advantage of using numerical models with the strength reduction method (SRM) to evaluate entry stability in complex multiple-seam conditions is demonstrated. A coal mine under variable topography from the Central Appalachian region is used as a case study. At this mine, unexpected roof conditions were encountered during development below previously mined panels. Stress mapping and observation of ground conditions were used to quantify the success of entry support systems in three room-and-pillar panels. Numerical model analyses were initially conducted to estimate the stresses induced by the multiple-seam mining at the locations of the affected entries. The SRM was used to quantify the stability factor of the supported roof of the entries at selected locations. The SRM-calculated stability factors were compared with observations made during the site visits, and the results demonstrate that the SRM adequately identifies the unexpected roof conditions in this complex case. It is concluded that the SRM can be used to effectively evaluate the likely success of roof supports and the stability condition of entries in coal mines.

Interactions of overburden failure zones due to multiple-seam mining using longwall caving

This paper presents an investigation on the interactions of overburden failure zones induced by the mining of adjacent coal seams using the longwall caving method. Overburden failure is an important factor in safety assessments in the fields of mining engineering geology and safety geology, especially when mining under water bodies. In this study, the influence of the thickness and properties of the interburden between seams on the development and interactions of caving and fractured water flow zones are investigated by using in situ measurements, scale model testing and numerical simulations. The height of the fractured water flow zones in the scale model tests and numerical simulations are basically in good agreement with measurements after mining of the upper and lower seams of Seam No. 3 in the Cuizhuang Coalmine. Therefore, the scale tests and numerical simulations in the study are verified. The results show that interaction and superposition between two close distance seams cannot be ignored when the ratio (h/M) of the interburden thickness (h) to the cutting height of the lower seam (M) is less than a defined critical value. A dividing line, Line D, has been proposed to judge whether the interactions exist. When the (M, h/M) points are located above Line D, the caving zone induced by excavation of the lower seam will not propagate to the caving zone induced by the upper seam. Otherwise, for the (M, h/M) points below Line D, the interactions and superposition of the overburden failure must be considered when predicting the heights of the caving and fractured water flow zones.

Ultra-close multiple-seam mining : Zhou, Y; Haycocks, C Proc 8th Annual Workshop Generic Mineral Technology Center Mine Systems Design and Ground Control, Reno, 5–6 November 1990P67–74. Publ Blacksburg: Virginia Polytechnic Institute and State University, 1990

Ultra-close multiple-seam mining : Zhou, Y; Haycocks, C Proc 8th Annual Workshop Generic Mineral Technology Center Mine Systems Design and Ground Control, Reno, 5–6 November 1990P67–74. Publ Blacksburg: Virginia Polytechnic Institute and State University, 1990

Evaluation of success probability in multiple seam room-and-pillar mining

A complete evaluation of multiple seam mining potential for any coal property requires the analysis of historical data and the controlling mining conditions. The major problem affecting multiple seam mining is interaction between workings in the contiguous seams. The success or failure of multiple scam mining can be evaluated using statistical techniques. A computer-based stepwise multiple regression algorithm is used to develop an equation for predicting the probability of success in two seam-mining situations. The model provides a measure of risk involved in mining multiple seams. The data used to develop the model are from a historical record of attempted multiple seam mining in the Eastern United States. The variables significantly influencing success or failure of multiple seam mining are percent sandstone in the interburden, ratio of interburden thickness to lower seam thickness, time delay, interburden thickness, thickness of both seams, depth of cover to upper seam, columnated pillars, columnated entries and the leading seam. The model developed was found to be quite satisfactory as a predictor of the probability of success within the range for which it was developed.

Room-and-pillar mine design in multiple seams : Matetic, R J; Chekan, G J; Galek, J A Coal MinV24, N12, Dec 1987, P36–40

The author discusses two case studies of multiple-seam mining. These studies offer practical solutions for subsidence and pillar load transfer. Critical parameters for understanding these interactions fall into two groups: fixed and design. Each case study includes a description of the instrumentation, results, conclusions, and design considerations.