Coal Bumps Research Articles

An Innovative Longwall Mining Technology in Tangshan Coal Mine, China

In mining of inclined coal seams in Tangshan coal mine of Kailuan group, gateways on either end of a panel were both typically located along the floor and a gateway pillar between adjoining panels was left unmined between adjacent panels to ensure stability, thus forming a planar mining system. According to the practice, however, it turned out that this conventional mining system has long-standing problems, such as face end support problems, coal bumps, sliding of mining equipment downhill, spontaneous combustions, support problems in development entries, etc. In view of this situation and based on the No. Y294 panel, this paper analyzes an innovative mining technology in which the gateways on either end of a panel are located at different heights within the coal seam. For the adjacent panel, the gate development may be superposed on the development entry of the previous panel or may be offset with respect to it. Field data shows that the split-level layout of the longwall panel plays an effective role in control of overall stability of mining equipment in inclined coal seams. Physical modeling demonstrates that the new technology has many advantages in ground control. Under the condition without a pillar, development entry adjacent to the new panel is located in the de-stressed zone and stress concentration is significantly reduced with associated reduction in coal bumps, bursts and support problems which means less support and maintenance requirement and cost. Compared with the conventional rectangular pillar, the gateway pillar width in this new technology is effectively reduced when pillars have to be left unmined. Roof strata behavior and features are analyzed. Corresponding operations in the field are introduced in detail.

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.

Investigation of Intrinsic and External Factors Contributing to the Occurrence of Coal Bumps in the Mining Area of Western Beijing, China

An investigation has been made to relate the occurrence of coal bumps to specific geological and mining conditions to the mining area of western Beijing. This investigation demonstrates that the high frequency of coal bumps in this area is due to four localized conditions, namely intrinsic coal properties, the presence of overturned strata and thrust faults, high in situ stress and the extraction of coal from island mining faces. Laboratory tests of coal samples indicated that the coals have a short duration of dynamic fracture, high bursting energy and high elastic strain energy, indicating that the coal is intrinsically prone to the occurrence of coal bumps. This investigation has also revealed that there are overturned strata and well-developed large- and medium-scale thrust faults in this area, and the presence of these structures results in plastic flow, severe discontinuities, rapid changes in overburden thickness and dipping of the coal seams. Well-developed secondary fold structures are also present in the axes and limbs of the primary folds. The instability of thrust faults, in combination with large-scale intrusion of igneous rocks, is closely associated with sudden roof breaking and induces sharp variations in electromagnetic radiation (EMR) and micro-seismic signals, which could be used to help predict coal bumps. In situ stress tests in the mining area demonstrate that the maximum and minimum principal stresses are nearly horizontal and that the intermediate principal stress is approximately vertical. The in situ stress level in the area is higher than the average in the Beijing area, North China and mainland China. In addition to the presence of overturned strata and thrust faults and high in situ stress levels, another external factor contributing to the frequency of coal bumps is coal extraction from island mining faces in this area. Island mining faces experience intermittent mining-induced abutment stress when a fault exists at one side of the island mining face due to reactivation of the fault, and this stress redistribution increases the likelihood of coal bumps during coal extraction from island mining faces.

Dynamic failure in coal seams: Implications of coal composition for bump susceptibility

As a contributing factor in the dynamic failure (bumping) of coal pillars, a bump-prone coal seam has been described as one that is “uncleated or poorly cleated, strong…that sustains high stresses.” Despite extensive research regarding engineering controls to help reduce the risk for coal bumps, there is a paucity of research related to the properties of coal itself and how those properties might contribute to the mechanics of failures. Geographic distribution of reportable dynamic failure events reveals a highly localized clustering of incidents despite widespread mining activities. This suggests that unique, contributing geologic characteristics exist within these regions that are less prevalent elsewhere. To investigate a new approach for identifying coal characteristics that might lead to bumping, a principal component analysis (PCA) was performed on 306 coal records from the Pennsylvania State Coal Sample database to determine which characteristics were most closely linked with a positive history of reportable bumping. Selected material properties from the data records for coal samples were chosen as variables for the PCA and included petrographic, elemental, and molecular properties. Results of the PCA suggest a clear correlation between low organic sulfur content and the occurrence of dynamic failure, and a secondary correlation between volatile matter and dynamic failure phenomena. The ratio of volatile matter to sulfur in the samples shows strong correlation with bump-prone regions, with a minimum threshold value of approximately 20, while correlations determined for other petrographic and elemental variables were more ambiguous. Results suggest that the composition of the coal itself is directly linked to how likely a coal is to have experienced a reportable dynamic failure event. These compositional controls are distinct from other previously established engineering and geologic criteria and represent a missing piece to the bump prediction puzzle.

Evaluating the coal bump potential for gateroad design in multiple-seam longwall mining: A case study

Evaluating the coal bump potential for gateroad design in multiple-seam longwall mining: A case study

Failure mechanism and control of deep gob-side entry

Gob-side entry (GSE) is defined as caving that is developed along the gob with a yield pillar. Compared to other entry layouts, GSE has many advantages, such as the advantageous stress environment, high recovery rate, and prevention of coal bumps. Recently, many Chinese coal mines have reached deep mining depths. As a result of the high stress in deep mining conditions, severe bulk and dilatant deformation has occurred in deep GSE (DGSE). The direct problems in this situation, such as interruptions to production, increasing rehabilitation cost, and high labor consumption, affect the safety and mining efficiency of deep coal seams. In this paper, based on analysis of the deformation characteristics of a typical DGSE, numerical modeling was used to study the causes and mechanism of DGSE failure. In addition, a series of strategies are proposed for DGSE deformation control. The case study presented indicates that the proposed control strategies can prevent severe deformation of the DGSE. Thus, the results of this research can be used as a reference for DGSE control in other deep mining settings.

Topical areas of research needs in ground control – A state of the art review on coal mine ground control

Ground control is one of the four subsystems of underground mining. It covers not only roof control, but also rib control, floor control, pillar design, shield design, overburden failures and subsidence. In the past three decades, ground control has made a tremendous advancement and many case studies have demonstrated its important role in the daily mining operations. However, there are plenty of room for improvements. This paper discusses the research needs in 12 subject areas including research approach, rock property, geology, computer modeling, in-situ stresses, roof bolting, coal pillars, field instrumentation, failures, surface subsidence, shield supports and coal bumps.

Numerical Simulation on Controlling Factor of Energy Distribution in Coal Bump

Elastic energy that cause coal bump will be stored in coal and rocks when excavating, which has a relationship with time and space. Based on analysis of "7.16 coal bump" accident in Cheng Shan coal mine, 3 impact factors including stability-time, goaf and roof strength were confirmed, then the distribution of energy under different conditions was simulated with Flac3D, the result showed that, 3 mentioned factors all have an impact on energy, and the impact descending order of these factors is: goaf, roof strength and time, while the controlling factor of this accident is goaf.

Mechanism of Coal Mining and Stowing with Upper Entry for Preventing Coal Bump

The material failure induced coal bump and structure failure induced coal bump were paid attention to in this paper. Mechanism of coal mining and stowing with upper entry for preventing coal bump was analyzed. The mechanism of mining with upper entry for preventing material failure induced coal bump is that the stowing material can recover the integrity of the cracked roof, and for preventing structure failure induced coal bump is that water infusion to the roof could release the energy in the roof, and the backfilling could support the roof, both of them could change the stress condition of the roof. It hopes that coal mining and stowing with upper entry for preventing coal bump is a rational way to realize safety mining, but there still are some questions should be the further studied.

Study of the Trellis Support of Working Face Roadway in Impact Coal Seam

Coal bumps happened many times in mining at No.5 seam of Tangshan coal mine. Strengthen the roadway’s support of working face can effectively reduce disaster losses. With the research background of the 3654 working face, the mine pressure monitoring for the existing support form of roadway has been carried on. Perform a numerical simulation for the original roadway support, base on the in-situ stress and physical and mechanical characteristics of surrounding rock in experimental; study the impact of the stability of roadway’s surrounding rock, while the space change of trellis and change of supporting intensity; optimizing the original support form, so as to maximum reducing the impact of the coal bumps.

RISKGATE y operaciones en minas de carbón en Australia

The major Australian Coal Association Research Program (ACARP) project, RISKGATE has now completed three years of knowledge capture and system development. The body of knowledge for risk management of tyres, collisions, fires, isolation, strata underground, ground control open cut, explosions, explosives underground, explosives open cut, manual tasks and slips/trips/falls was launched in December 2012. Recently, the project added knowledge about outbursts, coal bumps and bursts, human-machine interface, tailings dams, occupational hygiene and inrush to the original 11 topics. In 2014, the project plans (pending ACARP funding approval) to focus on issues around Fitness for Work. RISKGATE provides an environment for knowledge capture and knowledge exchange to drive innovation and cross industry sharing of current practice in the identification, assessment and management of risk. By capturing operational knowledge from industry experts, RISKGATE provides a cumulative corporate memory at a time of high personnel turnover in the coal industry. RISKGATE is the largest single ACARP Occupational Health and Safety (OHS) initiative to date. This paper presents an overview of the first seventeen topics, topic structures, and contrasts and inter-relationships between topics. The second part of the paper discusses some early steps that companies are taking to integrate RISKGATE into their operations; and conclude with some thoughts on where RISKGATE can go in the future.

Failure characteristics of three-body model composed of rock and coal with different strength and stiffness

Four different types of three-body model composed of rock and coal with different strength and stiffness were established in order to study the failure characteristics of compound model such as roof-coal-floor. Through stress analysis of the element with variable strength and stiffness extracted from the strong-weak interface, the tri-axial compressive strength of the weak body and strong body near the interface as well as the areas away from the contact surface was found. Then, on the basis of three-dimensional fast Lagrangian method of continua and strain softening constitutive model composed of Coulomb-Mohr shear failure with tensile cut-off, stress and strain relationship of the four three-body combined models were analyzed under different confining pressures by numerical simulation. Finally, the different features of local shear zones and plastic failure areas of the four different models and their development trend with increasing confining pressure were discussed. The results show that additional stresses are derived due to the lateral deformation constraints near the strong-weak interface area, which results in the strength increasing in weak body and strength decreasing in strong body. The weakly consolidated soft rock and coal cementation exhibit significant strain softening behavior and bear compound tension-shear failure under uni-axial compression. With the increase of confining pressure, the tensile failure disappears from the model, and the failure type of composed model changes to local shear failure with different number of shearing bands and plastic failure zones. This work shows important guiding significance for the mechanism study of seismic, rock burst, and coal bump.

Frictional sliding tests on combined coal-rock samples

A test system was developed to understand the sliding mechanism of coal-rock structure. The test system was composed by a double-shear testing model and an acousto-optic monitoring system in association with a digital camera and an acoustic emission (AE) instrument. The tests can simulate the movement of activated faults and the sliding in coal-rock structure. In this regard, instable sliding conditions of coal-rock samples, sliding types under different conditions, displacement evolution law, and AE characteristics during sliding process were investigated. Several sliding types were monitored in the tests, including unstable continuous sliding, unstable discontinuous sliding, and stable sliding. The sliding types have close relation with the axial loads and loading rates. Larger axial load and smaller loading rate mean that unstable sliding is less likely to occur. The peak shear stress was positively correlated with the axial load when sliding occurred, whereas the displacement induced by unstable sliding was uncorrelated with the axial load. A large number of AE events occurred before sliding, and the AE rate decreased after stable sliding. The results show that the tests can well simulate the process of structural instability in a coal bump, and are helpful in the understanding of fault activation and the physical processes during squeezing process of roof and floor.

Numerical Modeling for Yield Pillar Design: A Case Study

Two single-entry gateroad systems employing a yield pillar for bump control in a Chinese coal mine were introduced. The overburden depth of the longwall panels was approximately 390 m. When the width/height (W/H) ratio of the yield pillar was 2.67, coal bumps in the tailgate occurred in front of the longwall retreating face. However, in another panel, the coal bump was eliminated because the W/H ratio was reduced to 1.67. Under this condition, instrumentation results indicated that the roof-to-floor and rib-to-rib convergences reached 1,050 and 790 mm, respectively, during longwall retreat. The numerical model was used to back-analyze the two cases of yield pillar application in the hope to find the principle for yield pillar design. In order to improve the reliability of the numerical model, the strain-hardening gob and strain-softening pillar materials were meticulously calibrated, and the coal/rock interface strength was determined by laboratory direct shear tests. The results of the validated model indicate that if the W/H ratio of the yield pillar equals 1.67, the peak vertical stress in the panel rib (37.7 MPa) is much larger than that in the yield pillar (21.1 MPa); however, the peak vertical stress in the panel rib (30.87 MPa) is smaller than that in the yield pillar (36 MPa) when the W/H ratio of yield pillar is 2.67. These findings may be helpful to the design of yield pillars for bump control.

Experimental study on the mechanisms of fault reactivation and coal bumps induced by mining

Experiments simulating the effect of coal mine stopping through a fault zone were designed based on a working face of the Qianqiu coal mine in Yima, China. Through simulation of the physical process of fault reactivation and coal bumps, the displacement of the surrounding strata and evolution characteristics of fault stress under the effect of mining were studied. The mechanism of fault reactivation induced by coal mining was analyzed. The results show that shortly before fault reactivation, the normal stress and shear stress increased rapidly and the risk of a fault slip occurring was also increased. The fault reactivation, caused by the mining activity, occurred when the working face was 25–35 m from the fault along the hanging wall. The influence of mining increased the possibility of fault reactivation, while the local failure of the bearing capacity of the working face was the direct cause of the fault slip. Our results indicate that the influence of fault slip on the coal of the working face had a transient impact and acted as a loading-unloading function.

Failure behavior of a rock-coal-rock combined body with a weak coal interlayer

Using an MTS 815 testing machine, the deformation and failure behavior of a rock-coal-rock combined body containing a weak coal interlayer has been investigated and described in this paper. Uniaxial loading leads to the appearance of mixed cracks in the coal body which induce instability and lead to bursts in coal. If the mixed crack propagates at a sufficiently high speed to carry enough energy to damage the roof rock, then coal and rock bursts may occur – this is the main mechanism whereby coal bumps or coal and rock bursts occur after excavation unloading. With increasing confining pressure, the failure strength of a rock-coal-rock combined body gradually increases, and the failure mechanism of the coal interlayer also changes, from mixed crack damage under low confining pressures, to parallel crack damage under medium confining pressures, and finally to single shear crack damage or integral mixed section damage under high confining pressures. In general, it is shown that a weak coal interlayer changes the form of overall coal damage in a rock-coal-rock combined body and reduces the overall stability of a coal body. Therefore, the whole failure behavior of a rock-coal-rock combined body in large cutting height working faces is controlled by these mechanisms.

Numerical investigation on the assessment and mitigation of coal bump in an island longwall panel

This study presents a numerical investigation to assess the risk of coal bumps and produces a stress–relief technology using boreholes to mitigate risk during the extraction of an island longwall panel. Based on the geological condition in an island longwall panel in the Tangshan Coal Mine, Tangshan, China, a numerical FLAC3D (Fast Lagrangian Analysis of Continua in 3 Dimensions) model was established to determine and to map the zones in the panel with a high risk for coal bumps. The results of the numerical modeling show that the roof deformation starts to occur at more than 30m ahead of the longwall face and the deformation starts to accelerate after a distance of 10m in front of the longwall face. Large and rapid roof deformation is considered to be an important precursor of coal bump occurrence during the extraction of an island longwall panel. Based on the numerical results, a stress–relief technology using boreholes, which was employed to release abutment pressure, was investigated through numerical methods. The modeled results suggest that the peak stress concentration could be released by drilling boreholes in the zones prone to coal bumps. The effectiveness of the stress release increased with the borehole length and decreased with the borehole spacing.

Study on Characteristics of Energy for Hard Roof Fracture in Island Workface

According to the characteristics of the island workface with hard roof, various mechanical models of hard roof in island workface are established, to analyze energy characteristics of hard roof before and after the first fracture in island workface with all round gobs, derived their simplified formulas of bending strain energy for hard roof before and after the first fracture, and using these formulas to estimate the bending strain energy for the typical case of coal bump, reveals the energy cause of coal bump and verify the validity of these formulas.

Numerical Investigation of the Dynamic Mechanical State of a Coal Pillar During Longwall Mining Panel Extraction

This study presents a numerical investigation on the dynamic mechanical state of a coal pillar and the assessment of the coal bump risk during extraction using the longwall mining method. The present research indicates that there is an intact core, even when the peak pillar strength has been exceeded under uniaxial compression. This central portion of the coal pillar plays a significant role in its loading capacity. In this study, the intact core of the coal pillar is defined as an elastic core. Based on the geological conditions of a typical longwall panel from the Tangshan coal mine in the City of Tangshan, China, a numerical fast Lagrangian analysis of continua in three dimensions (FLAC3D) model was created to understand the relationship between the volume of the elastic core in a coal pillar and the vertical stress, which is considered to be an important precursor to the development of a coal bump. The numerical results suggest that, the wider the coal pillar, the greater the volume of the elastic core. Therefore, a coal pillar with large width may form a large elastic core as the panel is mined, and the vertical stress is expected to be greater in magnitude. Because of the high stresses and the associated stored elastic energy, the risk of coal bumps in a coal pillar with large width is greater than for a coal pillar with small width. The results of the model also predict that the peak abutment stress occurs near the intersection between the mining face and the roadways at a distance of 7.5 m from the mining face. It is revealed that the bump-prone zones around the longwall panel are within 7–10 m ahead of the mining face and near the edge of the roadway during panel extraction.

Assessment and mitigation of coal bump risk during extraction of an island longwall panel

Abstract This study presents an integrated approach for field tests and numerical investigations to assess the risk of coal bumps. This approach produces a stress-relief technology using boreholes to mitigate risk during the extraction of an island longwall panel. The field tests were conducted in an island longwall panel in the Tangshan coal mine in the city of Tangshan, China. In these tests, roadway roof displacement and electromagnetic radiation (EMR) of roadways in the panel were investigated to determine the zones of intensive roof deformation. A numerical model FLAC 3D (Fast Lagrangian Analysis of Continua in 3 Dimensions) was established to understand the results of the field tests and to map the zones in the panel with a high risk for coal bumps. The results of the field tests and the numerical modeling show that the roof deformation starts to occur at more than 30 m ahead of the longwall face and the deformation starts to accelerate after a distance of 10 m in front of the longwall face. Large and rapid roof deformation is considered to be an important precursor of coal bump occurrence during the extraction of an island longwall panel. Based on these results, a stress-relief technology using boreholes was investigated through numerical methods. The modeled results suggest that the abutment stress could be released by drilling boreholes in the zones prone to coal bumps. The effectiveness of the stress release increased with the borehole length and decreased with the borehole spacing.