Longwall Retreat Research Articles

UWB Radar Imaging Method for the Longwall Retreat and Creep Displacements Measurement in Underground Coal Mine

UWB Radar Imaging Method for the Longwall Retreat and Creep Displacements Measurement in Underground Coal Mine

Investigation of the Evolution Characteristic of Abutment Pressure during the Longwall Retreat Mining Process: A Case Study Using Field Tests and Numerical Simulation Method

Investigation of the Evolution Characteristic of Abutment Pressure during the Longwall Retreat Mining Process: A Case Study Using Field Tests and Numerical Simulation Method

Longwall Mining Automation—The Shearer Positioning Methods between the Longwall Automation Steering Committee and China University of Mining and Technology

The shearer positioning method is of great significance to the automation of longwall mining. The research teams in the Longwall Automation Steering Committee (LASC) of Australia and China University of Mining and Technology (CUMT) have focused on shearer positioning and identified the shearer inertial navigation system, the measurement of longwall retreat and creep displacement, and the backward calibration of the shearer trajectory as three key technologies to obtain accurate shearer positioning information. In underground environments without GPS, due to the characteristics of inertial navigation system (INS) autonomous full-parameter navigation, shearer positioning based on INS is adopted by the LASC and CUMT, and error reduction algorithms are proposed to inhibit the rapid error accumulation of INS. In order to obtain the periodic calibration information when the shearer reaches the end of the longwall face, it is necessary to measure the retreat and creep displacements in order to back-correct the shearer trajectory. Finding a suitable measurement method for this task is challenging, especially in the presence of dust and moisture. The LASC used a scanning laser and FMR 250 microwave radar to measure these two displacements, while CUMT adopted an ultra-wideband (UWB) radar. In terms of the backward calibration method, minimum-variance fixed-interval smoothing (MFS) proposed by LASC and the global optimization model (GOM) for the shearer trajectory from CUMT are described in detail. The experiment demonstrates that the GOM outperforms MFS in terms of accuracy but requires more computational resources. Therefore, our next research objective is to develop an efficient and accurate algorithm for performing backward calibration on the shearer trajectory.

Responses Of Free-Standing Railway Embankments As A Consequence Of Mine Subsidence In The NSW Southern Coalfield

To support our civilisation’s requirement for carbon steel, longwall mining of the Bulli Seam at Appin and Tahmoor Collieries has occurred to recover high quality coking/metallurgical coal, being for steelmaking. The Main Southern Railway crosses the footprint of both mines, and it was therefore important to manage the risk to infrastructure and public safety during longwall retreat in this strain-driven environment. One technical issue is the response to subsidence-induced ‘valley closure’. This has occurred in both gently undulating Wianamatta Group Ashfield Shale and the steeper upper Hawkesbury Sandstone valleys. This paper covers the observed responses of embankments on the Main Southern Railway and the heritage railway Picton to Mittagong Loop Line, and illustrates the responses of the embankments to valley closure that produced up to 11% strain, as well as illustrating displacement field development and derived principal strain vectors. The responses of four embankments are presented beneath which longwalls have been successfully extracted, and done so without adverse impact upon public safety. An understanding of the strain-driven responses of the embankments is presented.

Stability assessment of long gateroad pillar in ultra-thick coal seam: an extensive field and numerical study

Sustainability of an underground longwall operation is highly dependent on stability of the pillars during the panels extractions to ensure the continuous serviceability of gateroads. In Chinese underground longwall mining, the gateroads are typically driven as a single roadway with a “long gateroad pillar” which is different to a common practice where a gateroad consists of a number of pillars known as chain pillars. Such a unique practice has been proven to be more economical with maximum recovery while the safety remains at its highest level. In this study, based on the data obtained from Tongxin coal mine in China, the mechanical stability of the driven long gateroad pillar was investigated. The focus was on two nearby longwall top coal caving panels and their impacts on the mechanical behaviour of long gateroad pillar during the longwall retreat. To do so, initially an area of the gateroad was selected for the field instrumentation and data collection. These included vertical stress measurement and depth of damage assessment within the gateroad pillar and the longwall panel. Hence, the convergence level of gateroad was quantified to be used for the numerical modelling and assessing the performance of the designed long gateroad pillar based on the finite difference modelling technique using FLAC3D. Double-yield and strain-softening ubiquitous-joint constitutive models were used to simulate goaf material and strata, respectively. Finally, an extensive sensitivity analysis was conducted to compare the mechanical behaviour of a range of wide and narrow long gateroad pillars. It was concluded that the 50 m wide pillar is an ideal dimension for the future panels of Tongxin coal mine to achieve the maximum productivity and safety.Article highlightsThe validated numerical model was used to study on stability of “long gateroad pillar”.Premature yielding of the 38 m pillar would lead to severe geotechnical issues.Deep roof blasting of goaf edge or 50 m pillar can improve pillar conditions

Numerical Investigation on Influence of Two Combined Faults and Its Structure Features on Rock Burst Mechanism

With the increase in coal mining depth, engineering geological conditions and the stress environment become more complex. Many rock bursts triggered by two combined faults have been observed in China, but the mechanism is not understood clearly. The focus of this research aims at investigating the influence of two combined faults on rock burst mechanisms. The six types of two combined faults were first introduced, and two cases were utilized to show the effects of two combined faults types on coal mining. The mechanical response of the numerical model with or without combined faults was compared, and a conceptual model was set up to explain the rock burst mechanism triggered by two combined faults. The influence of fault throw, dip, fault pillar width, and mining height on rock burst potential was analyzed. The main control factors of rock burst in six models that combined two faults were identified by an orthogonal experiment. Results show that six combinations of two faults can be identified, including stair-stepping fault, imbricate fault, graben fault, horst fault, back thrust fault, and ramp fault. The particular roof structure near the two combined faults mining preventing longwall face lateral abutment pressure from transferring to deep rock mass leads to stress concentration near the fault areas. Otherwise, a special roof structure causing the lower system stiffness of mining gives rise to the easier gathering of elastic energy in the coal pillars, which makes it easier to trigger a rock burst. There is a nonlinear relationship between fault parameters and static or dynamic load for graben faults mining. The longwall face has the highest rock burst risk when the fault throw is between 6 and 8 m, the fault dip is larger than 65°, the mining height is greater than 6 m, and the coal pillar width is less than 50 m. The stair-stepping, imbricate, horst, and ramp fault compared to the other fault types will produce higher dynamic load stress during longwall retreat. Fault pillar width is the most significant factor for different two combined faults, leading to the rise of static load stress and dynamic proneness.

Numerical investigation into impacts of major fault on coal burst in longwall mining – A case study

The presence of geological faults is a significant contributing factor to coal burst in underground mining. In this study, a large-scale discontinuum model was established based on geological and geotechnical conditions in a mine site, where a longwall face passed through a major fault. Four model indicators were calculated during the longwall retreat: model seismic energy (energy released by the model system), fault seismic energy (energy released by fault-slip only), peak abutment stress and maximum strain energy density in coal seam. The change pattern of model seismic energy in the discontinuum model was calibrated against the seismic energy recorded in the mine site. It aims to verify that the model seismic energy can properly demonstrate the seismic energy observed in practice. The model results showed that the peak abutment stress and maximum strain energy density in coal seam increased when the longwall face approached the major fault, and then both of them experienced a most significant reduction when the largest fault-slip event occurred. A coal burst risk classification was then developed using multiple model indicators for the case study. A parametric study was conducted based on the calibrated model, which aims to investigate the impacts of various conditions on both fault-slip behaviour and stress and strain energy distributions around longwall excavations. Four factors were studied: the static friction angle of fault, the cover depth of fault, the dip angle of the coal seam and the mining direction. This study and the proposed model indicators can provide an improved understanding of fault-slip burst mechanisms and insights into the associated coal burst proneness assessment.

Goaf gas drainage and its impact on coal oxidation behaviour: A conceptual model

In gassy underground and spontaneous combustion prone coal mines, goaf management is a challenging task to reduce the risks of gas explosion and spontaneous combustion to acceptable levels. Goaf gas drainage via vertical surface goafholes is a primary control method to manage gas emissions in Australian coal mines. However, the operation of vertical surface goafholes maybe significantly impacted by insufficient understanding of goaf gas flow and coal self-heating behaviour, especially under different goaf gas drainage operating conditions. This paper focuses on analysing the field monitoring data collected from 161 goaf drainage holes at two coal mines in Australia. The data formed the basis to develop a conceptual model, which represents the changes of goaf atmosphere during the longwall retreat along with goaf drainage operations. The results indicate that the goaf drainage operating conditions may significantly affect goaf gas composition, which contributes to a different goaf atmosphere in comparison to existing models without goaf drainage. Within the studied longwall goaf, the ‘oxidation zone’ and ‘inert zone’ are identified with different carbon monoxide (CO) and oxygen (O2) trends, and the ‘oxidation zone’ is conceptually further divided into the ‘heat accumulation zone’, ‘elevated oxidation zone’ and ‘slow oxidation zone’ based on the variation of gas composition and potential degree of oxidation of residual coal. The coal oxidation potential risk and corresponding goaf gas drainage strategies at these conceptual zones are also evaluated in this paper. The results of this study are expected to act as a guideline for future goaf gas drainage operations.

Study on Rib Sloughage Prevention Based on Geological Structure Exploration and Deep Borehole Grouting in Front Abutment Zones

This research presents the grouting method of preventing rib sloughage which severely impacts mine safety and longwall retreat speed in thick coal seam with numerical simulation and laboratory tests. Based on the analysis of the plastic failure mode of five types of coal seam, roof strata ahead of the longwall face, and fractures developed in the coal seam, the following results are drawn, the range and degree of plastic failure generated in the coal seam and roof strata ahead of the longwall face gradually decreased as the coal mass strength increased; the grouting boreholes are essentially laid out within the coal rib instead of the roof. For a particular case of a coal mine in Shanxi province, a novel cement-based material was grouted, which fulfilled the reinforcement requirements under the tectonic stress regions and front abutment zones. Besides, the grouting borehole construction requested predrilled boreholes, full borehole intubation, lengthened hole sealing, and multiple-step drilling and grouting. This study can provide a theoretical framework of a design overview and practical basis for similar mining conditions in other coalfields.

Longwall Retreat and Creep Measurement Based on UWB Radar Imaging Method

The measurement of longwall retreat and creep displacements is of great significance for the safety production of a coal mine. In order to reliably and accurately obtain the longwall retreat and creep displacements, this paper proposes an ultrawideband (UWB) radar imaging method. The bolt plates fixed in the roadway coal wall are imaged as the target points. First, a signal model is built, and a modified nonlinear chirp scaling (NLCS) imaging algorithm is designed to obtain the high-resolution image. Then, the retreat and creep displacements are estimated based on the imaging results. Finally, simulation and experiment are performed. The simulation results show that the radar imaging method is theoretically feasible to measure the retreat and creep displacements. The measurement errors in the experiment are 0.058 m and 0.040 m, respectively. In the experiment, the radar velocity fluctuation and signal attenuation cause the target azimuth and range errors in the imaging results, which makes the measurement errors in the experiment larger than that in the simulation. This method requires no additional roadway information except for the bolt plates fixed in the coal wall. In addition, the signal propagation of UWB radar is rarely affected by the dust and moisture in the harsh environment.

Field study on the load transfer mechanics associated with longwall coal retreat mining

The two-entry longwall panel system is frequently used in underground coal mines in China. One of these entries serves only the current longwall panel while the other serves both current and subsequent longwall panels. The latter entry is built to survive the continuously high mining-induced stress generated by mining the current panel throughout the longwall mining process. The failure of the entry is controlled by the load transfer mechanics associated with the advancement of the longwall face. This paper presents a field study of the response of the entry during driving and longwall retreat phases. Extensive field data including entry convergence and changes in principal stresses are obtained for the Pingshu coal mine, China. A careful analysis of these data shows that the influence zone of the front abutment load on the trail entry can reach as far as 120 m outby the longwall face during panel mining. Bed separation can occur in competent fine-grained sandstone, which is 7.4 m deep into the trail entry roof. It is also found that the periodic collapse of the main roof above the mined-out area can continue when the longwall face has advanced 120 m away.

Mechanism of Dynamic Failure in Roadways with Thick and Competent Roof Strata: A Case Study

Rock and coal dynamic failures have a higher chance to occur when thick and competent roof strata are presented, which can build up a high level of energy. This paper studied the fracturing characteristics of the competent roof strata during longwall face retreat and their effects on roadways’ stability to gain an improved understanding of the rock dynamic failure occurred in roadways as well as the associated ground controls. The results indicated that the main reason of dynamic failure in roadways was the structural change of lateral main roof over a chain pillar, which transformed from the cantilever beam in the period of primary mining impact (PMI) to the triangular arch in the period of secondary mining impact (SMI). Based on corresponding mechanical models developed for the lateral main roof over the chain pillar, it was calculated that the force on roadways’ roof in the period of SMI was 3.7 times larger than that in the period of PMI. Then, the dynamic failure control method of increasing the chain pillar width was proposed. The stress distribution and deformation of roadways with different chain pillar widths were simulated using FLAC3D. The results indicate that as the pillar width increases, the influence of secondary mining on roadways’ stability decreases, and a threshold of not less than 30 m pillar width was suggested. The outcomes of this study would provide guidance for the design of chain pillars where thick and competent roof strata are presented.

Challenges of mining the first right-handed longwall panel in a new reserve block in Pittsburgh Seam

In this paper, the mining experience and challenges for the first right-handed longwall panel in the Pittsburgh Seam are introduced. The longwall headgate T-junction experienced very high face convergence (up to 61 cm), accompanied by roof sag, floor heave, and rib loading. The headgate convergence was so large that, in a few places, it threatened longwall retreat and ultimately required the bottom to be re-graded. Different underground instruments, such as a roof scope, de-gas drill, tell-tale, laser meter, and borehole pressure Cell (BPC), were employed to explore the roof geology and to monitor the entry convergence and the stress changes in the pillar. In addition, the impact of other geologic factors, such as large overburden depth, laminated sandstone roof geology, soft floor, and large headgate equipment, were also analyzed. Subsequently, geotechnical solutions were provided to avoid or mitigate the impact of these challenging geologic factors.

Mechanics of rib deformation—Observations and monitoring in Australian coal mines

The risk of fatalities from rib failure is still prevalent in the coal mining industry. This risk has prompted further research to be conducted on rib deformation in order to understand the mechanisms of rib failure, with the long-term objective being to improve rib support design. This paper presents the results of ACARP research project C25057, which investigated the mechanics and drivers of rib failure. The results of rib deformation monitoring at three different mines in Australia provide rib deformation characteristics for overburden depths ranging from 160 to 530 m. Monitoring includes deformation during development drivage conditions and during the longwall retreat abutment stress environment. The rib deformation monitoring covered three different seams: the Goonyella Middle Seam, Ulan Seam, and Bulli Seam in the Bowen Basin, Western Coalfield, and Southern Coalfield, respectively. The observed mechanisms driving the rib deformation ranged from bedding shear failure along weak claystone bands to vertical shear fractures to kinematic failures driven by shear failure dilation. The variation in mechanisms of rib failure, together with the seemingly consistent method of rib support design, prompts the question: what exactly is the role of rib support?

Ground response to high horizontal stresses during longwall retreat and its implications for longwall headgate support

Roof falls in longwall headgate can occur when weak roof and high horizontal stress are present. To prevent roof falls in the headgate under high horizontal stress, it is important to understand the ground response to high horizontal stress in the longwall headgate and the requirements for supplemental roof support. In this study, a longwall headgate under high horizontal stress was instrumented to monitor stress change in the pillars, deformations in the roof, and load in the cable bolts. The conditions in the headgate were monitored for about six months as the longwall face passed by the instrumented site. The roof behavior in the headgate near the face was carefully observed during longwall retreat. Numerical modeling was performed to correlate the modeling results with underground observation and instrumentation data and to quantify the effect of high horizontal stress on roof stability in the longwall headgate. This paper discusses roof support requirements in the longwall headgate under high horizontal stress in regard to the pattern of supplemental cable bolts and the critical locations where additional supplemental support is necessary.

Failure Propagation of Pillars and Roof in a Room and Pillar Mine Induced by Longwall Mining in the Lower Seam

Underground mining using the room and pillar method leaves pillars to support the overlying strata. When longwall mining is used to extract a lower coal seam beneath the room and pillar mine, the bending of the interburden strata can induce pillar failure and roof caving. In this study, two physical model experiments with different pillar width/height ratios were performed, and a non-contact displacement measurement technique was used to record and analyse the deformation and failure processes. The failure mechanism of pillars and the roof in a room and pillar mine induced by longwall mining in a lower coal seam was investigated. The results show that, during the longwall retreat, the immediate roof of the longwall panel collapses and the main roof bends resulting in load redistribution among a series of overlying pillars. Eventually pillar failure and unsteady strata movement are induced. Physical model tests indicate the entire deformation and failure process can be divided into two stages, i.e. steady deformation and failure propagation. In addition, it is observed that different width/height ratios of pillars lead to different failure patterns in the second stage. For model #1 with higher pillar width/height ratios, load redistribution is induced by the bending of the main roof of the longwall mine, but most panel pillars remain stable. The unsteady movement of the main roof of the longwall causes an upward failure propagation in the overburden strata. For model #2 with lower pillar width/height ratios, the failure propagates downward and it is triggered by the progressive collapse of the panel pillars during the load redistribution process.

A semi-quantitative coal burst risk classification system

Safety is the highest priority in the mining industry as underground mining in particular poses high safety risks to its workers. In underground coal mines, coal bursts are one of the most catastrophic hazards, which involves sudden and violent dynamic coal mass failure with rapid ejection of the broken material into the mine workings. Despite decades of research, the contributing mechanisms of coal bursts are still not completely understood. Hence, it remains challenging to forecast coal bursts and quantify their likelihood of occurrence. However, a range of geological and geotechnical factors are associated with coal bursts and can increase the coal burst proneness. This paper introduces a semi-quantitative coal burst risk classification system, namely, BurstRisk. Based on back-analysis of case histories from Australia, China and the United States, BurstRisk classifies the coal burst risk into three categories: low, medium and high risk. In addition, it allows mining engineers to modify the weighting of the selected factors based on specific conditions. The risk classification charts introduced are for both longwall retreat and development sections of longwall mining operations. This paper also provides a set of risk management strategies and control measures for effective coal burst mitigation.

Energy Dissipation and Storage in Underground Mining Operations

In this paper, we focus on the energy alteration during longwall mining in an attempt to mimic the conditions of a coal mine in Western Turkey. We verify the proposed model using existing analytical and numerical solutions in terms of stress components. Based on the verified numerical model, the energy balance during longwall retreat is studied rigorously. It is found that excavation-induced increment of external work increases linearly with time, while the stored strain energy increment is quadratic. Meanwhile, the strain energy increment rate gradually decreases with longwall progress because of excavation-induced higher stored energy within the adjacent coal block. The energy dissipation process during lonwall mining, corresponding to crack propagation, is divided into four stages, namely initiation stage, steady growth stage, sharp increment stage, and stabilisation stage. Our results provide new insights into energy evolution during longwall mining both from the reversible and irreversible points of view. The current paper shows, for the first time, that the extended finite element method is suitable to describe the crack propagation during longwall mining. The excavation induced crack propagation in the roof strata predicted by the model is in agreement with the “arch-shaped” patterns obtained using laboratory tests and Discrete Element numerical simulations.

Extraction of the remnant coal pillar in regular and irregular shapes: A case study

With the increasing coal mining rate, many coal mines in China are facing the coal resource exhaustion problem. This paper presents a case study on extracting a coal pillar which was left in a longwall retreat mine, Wangzhuang mine, in China. The left coal pillars usually have boundaries of regular and irregular shapes, which result in complex stress distributions within the coal pillar. It is of great importance to study the stress distributions and to determine the roadway layout prior to extracting the left coal pillar. This study numerically analyzes the stress distributions in the left coal pillar, based on which the roadway layout and support techniques are proposed. The roadway stability were well maintained throughout the mining process and the mining safety and productivity were ensured. The left coal pillar in Wangzhuang mine was successfully extracted. This case study would be helpful for extracting the left coal pillar in the longwall retreat mines.

Rock mechanical investigation of strata loading characteristics to assess caving and requirement of support resistance in a mechanized powered support longwall face

Longwall mining is one of the most acclaimed and widely used in underground method for coal extraction. The interaction of powered supports with the roof is the key issue in strata mechanics of longwall mining. Controlled caving of rock mass is a prerequisite pro thriving exploitation of coal deposits by longwall retreat with caving technique and support resistance has evolved as the most promising and effective scientific tool to predict various aspects related to strata mechanics of such workings. Load density, height of caving block, distance of fractured zone ahead of the face, overhang of goaf and mechanical strength of the debris above and below the support base have been found to influence the magnitude of load on supports. Designing powered support has been attempted at the different countries in different methods. This paper reviews the mechanism of roof caving and the conventional approaches of caving behaviour and support resistance requirement in the context of major strata control experiences gained worldwide. The theoretical explanation of the mechanism of roof caving is still continuing with consistently improved understanding through growing field experiences in the larger domain of geo-mining conditions and state-of-art strata mechanics analysis and monitoring techniques.