Roof Fall Research Articles

Disaster Control of Roof Falling in Deep Coal Mine Roadway Subjected to High Abutment Pressure

The roof falling accident is a serious threat to the lives of miners in deep coal mining, especially when the coal mine is more than 1000 meters deep. In regard to the 5306 coalface in the Tangkou coal mine, Shandong, China, the depth of coal seam is 992.8 m and the stress concentration coefficient of the roadway surrounding rock is 3.33. This leads to a serious deformation of the roadway roof, thereby producing a high risk of the roof falling disaster. In this pursuit, based on the mechanical analysis of roadway roof subjected to a high abutment pressure, the mathematical expressions of the setting load and movable column length of supports were introduced. Furthermore, the stability control mechanism of the roadway roof was analyzed and the optimized support parameters of supports are provided. The results showed that the longtime effective support of the roadway roof required the strength and deformation coupling of supports and anchored surrounding rock. The support length of the belt roadway should be at least 57.7 m, with 0-30 m away from the coalface supported by hydraulic supports and 32-57.7 m supported by single props. In addition, the maximum setting load and movable column length of hydraulic supports were 21.67 MPa and 280.3 mm and 12.44 MPa and 177.1 mm for single props, respectively. By applying the optimized support parameters of supports to the belt roadway of the 5306 coalface, the effective control of the roadway roof and the disaster control of roof falling were realized.

A Bayesian network approach for geotechnical risk assessment in underground mines

SYNOPSIS Underground mining gives rise to geotechnical hazards. A formal geotechnical risk assessment can help to forecast and mitigate these hazards. Frequentist probability methods can be used when the hazard does not have many variables and a lot of data is available. However, often there is not enough data for probability distributions, such as in the case of new projects. The risk assessment is often subjective and qualitative, based on expert judgement. The purpose of this research is to present the use of Bayesian networks (BNs) as an alternative to existing risk assessment methods in underground mines by combining expert knowledge with data as it becomes available. Roof fall frequency forecasting using parameter learning is demonstrated with 1141 sets of roof fall data across 12 coal mines in the USA. The prediction is nearly identical for individual mines, but when multiple mines are evaluated it is difficult to find a single best fit distribution for annual roof fall frequency. The BN approach with TNormal distribution was twice as likely to fit the observed data compared to the Poisson distribution assumed in the past. A hybrid approach using BN combining multiple probability distribution curves from historical data to predict annual roof fall is proposed. The BN models can account for variability for multiple parameters without increasing the complexity of the calculation. BNs can work with varying amounts of data, which makes them a good tool for real-time risk assessment in mines. Keywords: Bayesian network; expert opinion models; geotechnical risk; incident forecasting; parameter learning; roof fall risk.

Surrounding Rock Stresses on a Working Face‐End Roof under Mining Influence

The evolution process of the surrounding rock failure mechanism is studied because of spalling and roof fall accidents at the top corner of longwall top coal caving faces affected by mining and the difficulty of moving the advanced end support. Methods are proposed to improve the stability of surrounding rocks at the top corner of the end including cutting at the top corner of the end, reinforcing the anchor cable, changing the stress distribution of surrounding rocks at the top corner of the end, and transferring the stress concentration area of surrounding rocks to the deeper rock. Field observations of the surrounding rocks at the top corner of the 15107 fully mechanized caving face show that the stress value of the surrounding rocks at the corner between the roof of the return airway and the coal wall of the working face is 28.9 MPa when the surrounding rocks are in a stable state without mining. The stress value of surrounding rocks at the top corner of the end is 32.3 MPa when it is affected by mining, which results in spalling and roof fall. The surrounding rocks are in a stable state when the maximum stress of the surrounding rocks at the top corner of the reinforced anchor cable’s back‐end is 26.1 MPa. The results show that cutting of the surrounding rocks at the top corner of the end and the reinforcement of the anchor cable can avoid the spalling and roof fall when the top corner of the end is affected by mining and can ensure that the end support advances and working face moves forward.

Quantifying Roof Falling Potential based on CMRR Method by Incorporating DEMATEL-MABAC Method; A Case Study

This work incorporates the DEMATEL-MABAC method for quantifying the potential of roof fall in coal mines by means of the coal mine roof rating (CMRR) parameters. For this purpose, considering the roof weighting interval as a quantitative criterion for the stability of the roof, the immediate roof falling potential was quantified and ranked in 15 stopes of Eastern Alborz Coal Mines Company. In this regard, on the basis of the experts’ judgments, the fuzzy DEMATEL method was used for designation weights of the parameters, and the MABAC method was incorporated to quantify and rank the stopes (alternatives). “UCS of roof” and “joint spacing” in the immediate roof were found to be the most important parameters that controlled roof falling in stopes; and “joint persistence” was also found to be a quite significant parameter. Finding confirms that overall strength of rood rock mass plays a main role in the falling potential. Comparison of the coefficients of determination (R2) between the weighting interval and proposed model with that and original CMRR indicated more than 15% increase, which represented that the new proposed model was more accurate to quantify roof quality. The findings of this work show that using this combined method and specializing the CMRR method for a given mine geo-condition to assess the quality of the roof and its potential of collapse possesses a higher performance when compared with the original CMRR method.

Study on the evolution law of physical properties of soft coal seams under different water immersion time

Aiming at the problems of easy spalling and roof fall in the soft coal seam under the influence of strong deep mining, the 31020 working face of Pingdingshan No.12 coal mine is used as the engineering background, combined with nuclear magnetic resonance, etc. Various technical methods have carried out research on the water absorption characteristics, composition, micro-structure, and fracture morphology of coal at different immersion time. The experimental results show that the coal sample in this mining area belongs to coking coal under the bituminous coal category. The main inorganic substances are kaolinite and calcite.

Influence Factors’ Analysis of the Face‐End Roof Leaks Exposed to Repeated Mining Based on Multiple Linear Regression

In view of the fact that the face‐end roof fall under repeated mining of close‐distance coal seams seriously affects the normal production of the working face, this paper takes working face 17101 as the background, different influencing factors of face‐end roof caving exposed to repeated mining are analyzed through field observation of mine pressure data, different calculation schemes are obtained by using the orthogonal experiment, the subsidence of the face‐end roof is taken as the judgment index, UDEC simulation software is used to calculate the subsidence of the face‐end roof when different influencing factors change, and the application of SPSS statistical software is used for various parameters of multivariate linear regression analysis. The research results show that the influence degree of different factors from large to small is, respectively, mining height > tip‐to‐face distance > advancing speed > distance of coal seams > surrounding rock strength > support setting load. It is necessary to strengthen the coordination of all influencing factors and comprehensively control the stability of the face‐end roof exposed to repeated mining. Through the analysis of the regression model, it is found that there is no collinearity among the influencing factors, which has a significant influence on the regression equation and regression coefficient, and the multiple linear regression equation has a good fitting effect. The model can predict the stability of the face‐end roof exposed to repeated mining, which provides a basis for controlling the face‐end roof exposed to repeated mining.

ОЦЕНКА ВЛИЯНИЯ СОСТОЯНИЯ ПОДГОТОВИТЕЛЬНЫХ ВЫРАБОТОК НА ПОКАЗАТЕЛИ РАБОТЫ КОРОТКОГО ЗАБОЯ

In this paper concept description of the operating short wall within the bounds of the justified risk approach is described. Analysis of the results of working process modeling are carried out. Engineering-and-economical performance of longwall panels at various values of probability of entry roof fall is studied.

Assessing support alternatives for longwall gateroads subject to changing stress

Longwall gateroad entries are subject to changing horizontal and vertical stress induced by redistribution of loads around the extracted panel. The stress changes can result in significant deformation of the entries that may include roof sag, rib dilation, and floor heave. Mine operators install different types of supports to control the ground response and maintain safe access and ventilation of the longwall face. This paper describes recent research aimed at quantifying the effect of longwall-induced stress changes on ground stability and using the information to assess support alternatives. The research included monitoring of ground and support interaction at several operating longwall mines in the U.S., analysis and calibration of numerical models that adequately represent the bedded rock mass, and observation of the support systems and their response to changes in stress. The models were then used to investigate the impact of geology and stress conditions on ground deformation and support response for various depths of cover and geologic scenarios. The research results were summarized in two regression equations that can be used to estimate the likely roof deformation and height of roof yield due to longwall-induced stress changes. This information is then used to assess the ability of support systems to maintain the stability of the roof. The application of the method is demonstrated with a retrospective analysis of the support performance at an operating longwall mine that experienced a headgate roof fall. The method is shown to produce realistic estimates of gateroad entry stability and support performance, allowing alternative support systems to be assessed during the design and planning stage of longwall operations.

Assessment and Quantification of Risks Associated with Small Scale Mining, Khyber Pakhtunkhwa, Pakistan

Small scale mining industry is considered more hazardous than other industries worldwide. Large number of workers receive minor and major injuries leading to disabilities or loss of lives due to frequent accidents in mines. Main causes of accidents in mines are fall of roof, improper ventilation system, gases, fires and mine explosions. Beside these hazards, violation of rules and regulations for mine workers are common, which also cause accidents. This paper is focused on issues associated with the health and safety of workers of Cherat Coal Mines (CCM), Abbottabad Coal Mine (ACM) and Abbottabad Soapstone Mine (ASM), Pakistan. The collected data were analyzed with SPSS computer statistics software. The data analyses indicated that the lack of education and violation of safety laws cause accidents in mines. Results show that problems that were rated higher by more than 60% of workers included slide and fall, dust, roof fall and explosive related hazards. In survey more than 50% of the workers admitted the existence of gases, fire and low height mines are common hazards in their workplace. The results also indicated that not only workers but management are also affected by accidents. More than 17% of worker in CCM faced serious accidents up to 3 times during one year. Up to 26% of workers in CCM, 13% in ACM and 15% in ASM suffered accidents for which they had 3 workdays off. It has been concluded that training should be arranged, especially the safety related training on regular basis to reduce the risk of accidents.

Numerical Simulation Study on Mining Effect of Surrounding Rock When Inclined Mining Face Passing through Empty Roadway

In the coal stopping process, it often happens that the mining face crossing empty roadway often causes increase of the advanced support pressure, and leads to problems such as roof fall and rib spalling, in response to which, taking a recovery mining face in Xipang Mine as the engineering background, and using software FLAC3D to establish the numerical analysis model of the mining face crossing empty roadway, the study on the influence of incline adjustment of the mining face crossing gob on the surrounding rock near the empty roadway. The result shows that with the decrease of the width of coal pillar between mining face and empty roadway, the vertical stress appears three shapes in turn: “saddle shape”, “platform shape” and “solitary peak shape”; stress concentration occurs at the intersection of two roadways, and the stress concentration in acute angle area is more obvious. Adjusting the inclination of the mining face and increasing the angle between the mining face and the empty roadway are beneficial to reduce the stress concentration phenomenon of the coal pillar in the triangle area, and the maximum supporting stress is reduced from 25.3 MPa to 23.5 MPa, which is more conducive to the safe passage of the mining face through the empty roadway.

Research on Failure Characteristics and Zoning Control Technology of Thick-Soft Surrounding Rock for Deep Gob-Side Entry Retaining

Controlling the large deformation caused by bed separation failure of thick and soft surrounding rock in gob-side entry retaining is difficult. The deformation and failure modes of thick and soft surrounding rock are summarized and classified based on field research, theoretical analysis, laboratory tests, and actual measurements. Systematic research is conducted on the lithologic characteristics, failure characteristics, and control methods of the surrounding rock. The research findings are as follows: (1) The low strength, softening, and water swelling of thick mudstone, as well as its cataclasis, dilatancy, and long-term creep under strong mining and high stress are the objective reasons for large deformation of the surrounding rock. (2) Due to the weak stability of the surrounding rock-support structure and low collaborative roof side bearing capacity, no complete supporting structure is formed with the supporting system, causing the support body in each area to be crushed one by one, which is the subjective reason for the deformation and failure of the surrounding rock. (3) The deformation and failure characteristics of thick and soft surrounding rock in gob-side entry retaining are primarily divided into eight types: roof bending and sinking type, roof cutting along filling body, rib spalling type, roof fall type, filling body rotation type, filling body crushing type, roof step type, and roof cutting along the coal side. The initial points and key points for a chain reaction of each failure type are determined. (4) The surrounding rock is divided into 10 support zones at four levels, and control technology for “zonal support and overall reinforcement” is put forward. The mechanical effect of the support body in each zone and its role in maintaining the stability of surrounding rock are analyzed. This technology can ensure the integrity of the surrounding rock structure and improve the roof side collaborative bearing capacity.

Roof bolt identification in underground coal mines from 3D point cloud data using local point descriptors and artificial neural network

ABSTRACT Roof bolts are commonly used to provide structural support in underground mines. Frequent and automated assessment of roof bolt is critical to closely monitor any change in the roof conditions while preventing major hazards such as roof fall. However, due to challenging conditions at mine sites such as sub-optimal lighting and restrictive access, it is difficult to routinely assess roof bolts by visual inspection or traditional surveying. To overcome these challenges, this study presents an automated method of roof bolt identification from 3D point cloud data, to assist in spatio-temporal monitoring efforts at mine sites. An artificial neural network was used to classify roof bolts and extract them from 3D point cloud using local point descriptors such as the proportion of variance (POV) over multiple scales, radial surface descriptor (RSD) over multiple scales and fast point feature histogram (FPFH). Accuracy was evaluated in terms ofprecision, recall and quality metric generally used in classification studies. The generated results were compared against other machine learning algorithms such as weighted k-nearest neighbours (k-NN), ensemble subspace k-NN, support vector machine (SVM) and random forest (RF), and was found to be superior by up to 8% in terms of the achieved quality metric.

Estimation of rock load for junctions based on roof failure cases for safe mining operation

Coal mine roadway junctions in bord and pillar mining, in particular, are more vulnerable to roof fall due to the excavation induced stresses as well as general stress loading pattern inherent to the typical cross section achieved in practice. Roof fall incidences are generally more in junctions of development galleries and a cause of concern as they provide access to faces in different directions (dip/rise). The CMRI-ISM rock mass classification system, popularly known as Rock Mass Rating (RMR), is being used for the design of support system in underground coal mines for the last 30 years in India. In spite of the well-defined guidelines, it has been observed that roof failures are still a major concern in many underground coal mines. In this research, an attempt has been made to develop a new rock load prediction model for junctions of bord and pillar workings based on the geoengineering investigations carried out in 63 coal mines. After analysis of failure and stable cases, a best suited rock load predictor equation was developed. Analysis indicated that the maximum roof failure cases occurred for RMR values in between 30 and 45, and this zone is considered as highly unstable zone. Assessment of stable and unstable class is also done with respect to rock load for varied RMR. A general support design has also been suggested as per the RMR and severity of the roof fall. The suggested approach can help designing a rational support system for underestimated and overestimated zones thus minimising the roof fall cases leading to improved safety and productivity in underground coal mines.

Study on the Stage Failure Mechanism and Stability Control of Surrounding Rock of Repeated Mining Roadway

China is one of the leading countries in the mining and utilization of coal resources, and the problems of coal-mining technology and safety have been concerned by the world, while the serious deformation and destruction of surrounding rock and the difficulty of support have brought inconvenience to the mining of coal resources due to repeated mining. This paper takes the actual engineering 22205 mining roadway in Buertai mine as the research background, through the combination of numerical simulation and field measurement. In this paper, the stress environment, plastic zone, and surrounding rock deformation in the advancing process of coal-mining face are studied, and the stress evolution law of surrounding rock in repeated mining roadway is obtained. It is clarified that the surrounding rock deformation is the failure mechanism under the combined action of principal stress difference and stress direction deflection. As a result, the surrounding rock of the roadway is asymmetrically deformed and destroyed, and the corresponding surrounding rock control scheme is put forward. The results show that the influence of repeated mining on roadway stress environment can be divided into four stages with the mining process: the stability stage of mining influence, the expansion stage of primary mining, the stable stage after primary mining, and the expansion stage of second mining. At the same time, the shape changes of the plastic zone and the displacement monitoring results of the monitoring are analyzed, and the results are obtained; the stage of stress change is suitable, and combined with the failure characteristics of surrounding rock in each stage, it is put forward that reinforcement measures should be taken in the stable stage after mining; the specific reinforcement scheme is determined according to the expansion form of plastic zone and field measurement. The on-site monitoring shows that there is no roof fall accident during the use of the roadway, which ensures the safety in production.

Analysis of Mine Roof Support (AMRS) for US Coal Mines

During the past 15 years, roof fall rates have fallen dramatically in US coal mines, particularly in regions where the roof is weakest. The remarkable reduction in the number of roof falls has been accomplished with more effective roof support systems. The purpose of this paper is to present a design methodology that builds on and quantifies the basic roof support concepts that have been successful in the USA. The methodology starts by defining three modes of roof support, based on the roof strength relative to the stress level: (1) suspension, where roof bolts mainly provide skin control for strong roof; (2) beam building, where moderate strength roof can be supported by roof bolts alone; and (3) supplemental support for weak roof. Next, a large database of roof fall histories at a number of mines is used to define the approximate boundaries of these three regimes based on the coal mine roof rating and the depth of cover. Finally, guidelines are presented for site-specific design of support systems within each regime. The new computer package, analysis of mine roof support (AMRS), implements the design methodology.

Discrete Element Modeling of Delamination in Laboratory Scale Laminated Rock

Laminated rock can delaminate and fail under certain stress states. Delamination-caused roof fall and cutter roof failure is a common failure mechanism in roadways in Pittsburgh coal seam due to the existence of laminated shale roof. Two-dimensional numerical models were developed at laboratory scale based on the bonded particle method in order to simulate the delamination with particle flow code. In these models, an assembly of bonded particles and parallel weak planes represents the laminated rock. This model calibrated the micro-parameters of the bonded particle material and weak planes with laboratory data. The mechanism of fracturing and delamination in a laminated specimen was investigated using unconfined and confined compression test and unsupported roof compression test. The numerical results show that, under a constant horizontal stress, delamination occurs when the stress is much lower than its compressive strength. The confined compressive test indicates that delamination is restrained by applying confining stress since delamination-caused failure does not occur during any stage of the confined compressive test. Unsupported roof compression tests showed propagation of cutter roof failure with initiation of cracks and delamination at the intersections of the roof and ribs. An outer failed zone and inner failed zone form separately. The separated zones connect and form an opening in the roof. The presented numerical results provide additional insight into process of delamination.

Key Technique Study of Stability Control of Surrounding Rock in Deep Chamber with Large Cross-Section: A Case Study of the Zhangji Coal Mine in China

This paper is a case study on the key technique of stability control of surrounding rock in the deep chamber with large cross-section in Zhangji Coal Mine of China. Many investigations and field experiments were performed to reveal the surrounding rock failure mechanisms of the chamber. It was found that the large cross-section, high in situ stresses, large rock zone and construction disturbance were the main causes of the chamber instabilities. For the original supporting scheme, the numerical simulation was adopted to analyze the surrounding rock stress and displacement during excavation and support. There are stress concentrations and large displacement at the junction of the chamber, shaft lining, and machine roadway. The maximum displacements are 74.7 mm in horizontal, 105.9 mm roof fall and 29.2 mm floor heave in vertical. Based on the simulation results, the supporting structure optimization design of the chamber has been completed. The primary supporting structure is composed of the bolt, steel mesh, shotcrete and anchor. The secondary supporting is concrete lining with double-layer reinforcement and an inverted arch. The shallow and deep lagging grouting technique was adopted for reinforcing rock. The three-dimensional steel grid inverted arch lining was developed independently. The indoor model test showed that the inverted arch lining can take advantages and characteristics of various materials through unique spatial network design, which improved the whole bearing capacity. A field experiment of convergence and reinforcement stress monitoring was conducted. The field measurement data showed that the maximum displacement of two sides and roof were 60 mm and 42 mm at 200 days, respectively. The reinforcement stresses of supporting structure were far less than the yield strength, which had a higher safety stock. This successful case study demonstrated that the optimized supporting structure is reasonable and the deformation of surrounding rock has been effectively controlled.

Application of CMRI-ISM RMR for Stability Analysis of Development Workings for Ballarpur Underground Coal Mine - An Empirical and Numerical Approach

Abstract Roof bolt failures in the mine generally occur due to the poor identification of the weak rock layers and considered as a major concern for underground coal mines in terms of its productivity and safety. The triggering factor responsible for underground roof failure is the presence of weak bedding planes and various geological discontinuities. Geological disruptions present in the mine roof seriously affect the continuity of the rock layers resulting in block failure. However, numerous improved methods have been suggested by a number of researchers but still, the upholding of the accidents due to roof fall is considered as a major challenge for underground mine workings. Thus, it is necessary to give proper emphasis on characterization of the roof fall so that stability of the mine workings could be increased and movement of the weak and layered strata can be arrested by applying an adequate roof bolting system. This mechanism of roof support gives the composite binding between the rock layers by restricting the roof fall. The stability of mine workings can be also increased by proper application of rock mass classifications system and adequate design of support system and finally, by extensive simulation methods and statistical methods like FLAC 3D numerical and statistical modelling to ascertain the stability of the workings. CMRI-ISM RMR (Central Mining Research Institute – Indian School of Mines Rock Mass Rating System) classification system is the most popular and practiced method for roof support design in Indian coal mines which is being successfully used from last several years for roof support design in Indian geo-mining conditions. This forms the basis to estimate the rock load and support design. In this paper attempt has been made to assess the stability of workings and pillars below the railway track by using empirical and numerical approach. After both the studies it can be said that, the galleries and pillars underlying the railways track are stable, as factor of safety in each case is more than 2. Later on, the study was also extended to see the effects of subsidence on the surface which reveals that, there shall be no such effect on the surface due to underground workings as the value of NEW (non-effective width) is less than 0.4 for both the cases.

Analysis of Underground Mining Accidents at Cherat Coalfield, Pakistan

Coal is the most extensively used nonrenewable energy source worldwide. In Pakistan coal is mostly mined by underground mining methods under poor and unsafe working environment. In this study, accident records of underground coal mines of Cherat coal field were collected and analysis was carried out to understand the consequences, time of occurrence and causes of accidents. Besides, occupation of a victim, responsible party and effects on production were also considered in present study. The relationship between workers’ age and severity rate was examined using regression analysis. The main causes of accidents were the fall of the roof, mine gases, electrocution, oxygen deficiency and slip fall. At Cherat coalfield, for every two mines, there is one fatality and most of the accidents occur due to roof fall between 10:00 am to 12:00 noon. It is due to the stress of production efficiency. Moreover, there is a negligible impact of accidents on coal production at Cherat coalfield from 1994 to 2015. Keywords

Analysis of Underground Mining Accidents at Cherat Coalfield, Pakistan

Coal is the most extensively used nonrenewable energy source worldwide. In Pakistan coal is mostly mined by underground mining methods under poor and unsafe working environment. In this study, accident records of underground coal mines of Cherat coal field were collected and analysis was carried out to understand the consequences, time of occurrence and causes of accidents. Besides, occupation of a victim, responsible party and effects on production were also considered in present study. The relationship between workers’ age and severity rate was examined using regression analysis. The main causes of accidents were the fall of the roof, mine gases, electrocution, oxygen deficiency and slip fall. At Cherat coalfield, for every two mines, there is one fatality and most of the accidents occur due to roof fall between 10:00 am to 12:00 noon. It is due to the stress of production efficiency. Moreover, there is a negligible impact of accidents on coal production at Cherat coalfield from 1994 to 2015. Keywords