Assessment of roof fall risk in longwall coal mines

Transcription

1 International Journal of Mining, Reclamation and Environment ISSN: (Print) (Online) Journal homepage: Assessment of roof fall risk in longwall coal mines Stanisław Prusek, Sylwester Rajwa, Aleksander Wrana & Alicja Krzemień To cite this article: Stanisław Prusek, Sylwester Rajwa, Aleksander Wrana & Alicja Krzemień (2017) Assessment of roof fall risk in longwall coal mines, International Journal of Mining, Reclamation and Environment, 31:8, , DOI: / To link to this article: The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group Published online: 24 Jun Submit your article to this journal Article views: 1617 View Crossmark data Citing articles: 4 View citing articles Full Terms & Conditions of access and use can be found at

2 INTERNATIONAL JOURNAL OF MINING, RECLAMATION AND ENVIRONMENT, 2017 VOL. 31, NO. 8, Assessment of roof fall risk in longwall coal mines OPEN ACCESS Stanisław Prusek a, Sylwester Rajwa a, Aleksander Wrana a and Alicja Krzemień b a Department of Extraction Technologies and Mining Support, Central Mining Institute (Główny Instytut Górnictwa), Katowice, Poland; b Department Risk Assessment in Industry, Central Mining Institute (Główny Instytut Górnictwa), Katowice, Poland ABSTRACT Based on extensive experience in shield support design and the results of underground measurements and observations, the major factors influencing the stability of the roof in retreat longwall panels were determined. A practical method for assessing the risk of roof falls is presented. It is a combination of empirical methods and expert techniques that enable investigators to determine the probability and potential consequences of roof falls. This report includes an example of applying the method to one longwall panel. The method is a practical decision tool to support the mining management when planning the roof fall preventive measures. ARTICLE HISTORY Received 10 January 2016 Accepted 9 June 2016 KEYWORDS Underground coal mine; longwall; stability of the roof; roof fall; risk assessment 1. Introduction Roof fall hazards exist in virtually all underground mines independent of the mineral the mines produce. Roof falls are also one of the more prominent hazards in underground hard coal mines, which has been confirmed by statistical data from hard coal mines in the USA, India and Poland [1 3]. The longwall mining system is the main system used in Poland s hard coal mines. Retreat longwalls with natural roof caving in the gob are the most common. Difficult mining conditions (e.g. significant depth, influence of previous mining activities, rock mass tremors, geological conditions) result in minor and massive roof falls in the longwalls. The falls cause accidents and, when massive roof falls occur, as a consequence stoppages in mining operations adversely affects the economic outcomes for coal mines. To limit roof fall hazards in longwall panels, prior to the start of extraction, the conditions of maintaining a roof with shield support are assessed in this report. The conditions are estimated with an empirical method based on an approach formulated by Biliński [4]. In recent years, employees of the Central Mining Institute s (CMI), Poland, have been conducting research and observations aimed at analysing the phenomena that influence roof fall hazards in caving longwalls. Based on results of this research and extensive experience in selecting shields for specific geo-mining conditions, a method for assessing the risk of roof falls in retreated longwalls with roof caving was developed. The method, described in this article, is a combination of empirical and expert methods. 2. Background In the underground mining of hard coal deposits, various methods of assessing the risk of roof falls in mines have been developed. A method for assessing the risk of roof fall in underground hard coal CONTACT Alicja Krzemień akrzemien@gig.eu, akrzemien@gig.katowice.pl 2016 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group. This is an Open Access article distributed under the terms of the Creative Commons Attribution-NonCommercial-NoDerivatives License ( creativecommons.org/licenses/by-nc-nd/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properly cited, and is not altered, transformed, or built upon in any way.

3 International Journal of Mining, Reclamation and Environment 559 mines in South Africa is presented in [5]. The method was introduced in mines in the Republic of South Africa in 1997 to assess the risk of roof falls in board and pillar mining systems. Based on the statistical data analysis of 1141 roof falls, which occurred in 12 underground hard coal mines in the Appalachian region of the USA, Duzgun and Einstein (2004) developed a method for assessing the risk of roof falls by employing an objective method to assess roof fall probability [5,6]. Based on the data for the years describing accidents in an underground hard coal mine in Iran (Kerman Coal Mine), Hossaini and Behraftar (2009) created a method called Geotechnical Risk Assessment [5,7]. Similar to Duzgun and Einstein, they employed statistical analysis of the data for the years to assess the probability of an accident. Experiences in implementing risk assessment and management in mines in Australia were described by Joy and by Evans and Brereton [8,9]. These investigators highlighted the results of risk assessment using Sustainability Opportunity and Threat Analysis in three hard coal mines in Central Queensland that employ a strip mining system. In each of the methods for assessing the risk of roof failure, it is necessary to determine the factors that influence the stability of the roof. The factors largely depend on the mining system (room and pillar or longwall). This addresses the stability of the roof in hard coal seams mined with a longwall system with roof caving. Research into the assessment of roof stability in longwalls equipped with shields has been conducted since the 1950s, when shields were first implemented. This kind of research was undertaken by Biliński [4]. The outcome of this research is shield support selection, which is more broadly described in Section 3 of this article. In recent years, numerical modelling is commonly used to assess the stability of the roof in longwall workings. To optimise selection of a shield for specific conditions in mining operations, the concept of Ground Response Curve is also used [10,11]. An interesting concept for advanced warning of the formation of roof cavities in longwalls, which is a method that employs recording pressure values in the shield legs, was presented in [12]. In this method, the Cavity Risk Index was determined, based on trends in pressure recorded in the shield legs. Based on numerous observations and underground measurements in mines in Germany, the factors influencing the stability of the roof in longwall panels were determined, and then a method of assessing the frequency of roof falls in a longwall was presented [13]. To summarise, it can be stated that the risk of roof failure is one of the more prominent hazards in underground mines. Many methods of assessing this hazard have been developed. These methods allow mine managers to take certain preventive steps, if necessary. Numerous research studies show that the stability of the roof in longwall depends on a number of geo-mining and technical factors. Based on these factors, a method for risk assessment has been proposed in this article. 3. Method for selecting shields for mining conditions In underground hard coal mines in Poland, the selection of a shield, as well as the assessment of the stability of the roof in longwalls, is based on the calculations of the roof bearing capacity index g [14]. The index is determined according to an empirical method, based on many years of underground tests, developed by Biliński [4]. The value of index g is dimensionless, and it is calculated with Equation (1). 1 Roof bearing capacity (g) = UCS M P M Q (1) where UCS: uniaxial compressive strength, determined basing on tests in given longwall panel, MPa; M P : support capacity moment, MNm; M Q : rockmass load moment, MNm. The method considers the influence of a number of factors on the shield capacity (M P ) and the load of the support (M Q ) in a longwall, which can be divided into natural factors independent of human activity and technical factors resulting from human activity (Figure 1). Table 1 presents the state of maintaining the roof in a longwall with a specific support, which is assessed according to the assumed criteria for the determined values of index g.

4 560 S. Prusek et al. Figure 1. Factors influencing on value of roof bearing capacity index g. Table 1. State of maintaining the roof in a longwall depending on the value of index g. Value of roof bearing capacity index g Roof maintenance in the longwall working g < 0.7 Very bad roof maintenance 0.7 g < 0.8 Difficult conditions for roof maintenance g 0.8 Good or very good roof maintenance The value of the roof bearing capacity index g for the designed longwall panel is presented in graphic form in Figure 2 [15]. 4. Factors influencing increase in roof fall risk in longwall CMI s employees conduct various types of underground tests and observations aimed at assessing the stability of the roof in longwall [16,17]. In recent years, there have been a few hundred analyses concerning the selection of shields for the specific conditions of mining operations in hard coal mines in Poland. Extensive experience resulting from these underground observations and calculations allowed researchers to isolate seven significant factors influencing the stability of roof rocks in longwalls. To date, the factors have not been considered in the method for calculating index g, or the influence of these factors was underestimated. The selected factors (marked FC Fall Cause) belong to the following groups: geological factors, mining factors and technical factors. All of the factors are described further in this article Low self-supporting ability of the first roof strata (FC1) The self-supporting ability of the first roof strata has a significant influence on providing stability in longwall. Even a minor roof fall into a working area may eventually cause a roof failure and significant difficulties in longwall advance [18,19]. Roof falls are dangerous because they can increase the height of a working beyond the operating height of the shield. To avoid this situation, the operating height of the support should exceed the planned height of a longwall by 0.4 m. It is also worth noting that the stability of the roof in a longwall is tightly associated with the thickness of the first roof strata in a mining seam. If the strata thickness does not exceed 0.3 m after cutting the coal face with the shearer, there is high probability that it will fall independently of its strength parameters [11,13].

5 International Journal of Mining, Reclamation and Environment 561 Figure 2. Examples of the results of calculations of the roof bearing capacity index g for the designed longwall panel [15]. Taking these facts into consideration, it was determined that the probability of roof failure increases in cases described in Table Massive roof layers overhanging behind shields (FC2) The phenomenon of roof beds overhanging (cantilever) behind the shield line is associated with the occurrence of strong and thick layers, which are most often sandstones or conglomerates, in the roof of a mined seam. The overhanging roof units behave as cantilever beams and cause significant increase in the load exerted on shields, which can lead to: yielding events, roof convergence and degradation, coal face spalling, and either roof falls or failures in the longwalls [18 22] (Figure 3). Overhanging occurs less frequently when the longwall face is roughly parallel to the dominant direction of the joints occurring in the roof strata [23]. Underground tests and observations concerning abutment pressure, which have been conducted for years by CMI, indicate that the increase in load associated with overhanging rocks behind shields is influenced by the thickness and distance of the strong layer from the immediate roof of a longwall and the orientation of the longwall face in relation to the direction of the main joints in the roof strata. It was assumed that there are four unfavourable cases associated with possibility of roof strata overhanging behind the shields (Table 2) Distribution of support capacity along the canopy length (FC3) Based on the analyses of numerous underground situations, it was concluded that there is an influence of the distribution of shield support along the canopy on the roof conditions in a longwall. In properly designed shields, the leg loads should be applied to the roof as close to the face as possible. The shield legs should also be able to set the roof along all or most of their canopy length [24 26]. Otherwise, the real tip to face distance could start to grow, which has a negative influence on the roof stability in a longwall. In some cases, (e.g. incorrect shield construction), a lack of capacity at the front edge of the canopy can occur. The unsupported distance on a canopy can lead to, in certain situations, the loss of roof stability in a longwall. To calculate the distribution of support capacity along the canopy

6 562 S. Prusek et al. Table 2. Risk assessment of a roof fall in longwall based on different cause factors (FC). Symbol Factors Risk assessment criteria FC1 FC2 FC3 FC4 FC5 FC6 Low self-supporting ability of the first roof strata Massive roof layers overhanging behind shield line Distribution of support capacity along the canopy length Low bearing capacity of immediate floor strata Orientation of the longwall face in relation to the directions of the joints in the roof strata Longwall face line orientation to faults Occurrence of a layer in the immediate roof with a thickness less than 0.3 m and the operating height of shield matches planned mining height Occurrence of a layer in the immediate roof with a thickness less than 0.3 m and the operating height of shield is greater than the planned mining height by less than 0.3 m Occurrence of a layer in the immediate roof with a thickness less than 0.3 m and the operating height of shield is greater than the planned mining height by more than 0.3 m The thickness of a massive layer is greater than 20 m, the distance between the layer and the longwall roof is greater than the longwall height (x > h), the longwall face line is oriented at an angle between 30 and 150 to the direction of the main joints in the roof strata; or the thickness of a massive layer is greater than 10 m and the distance to the longwall roof is shorter than or equal to the longwall height (x h) The thickness of a massive layer is greater than 20 m, the distance between the layer and the longwall roof is greater than the longwall height (x > h), the longwall face line is oriented at an angle greater than 150 and smaller than 30 to the direction of the main joints in the roof strata The thickness of a massive layer is between 5 m and 10 m, the distance to the longwall roof is shorter than or equal to the longwall height (x h) The length of unsupported roof in the front edge of the canopy is over 0.7 m The length of unsupported roof in the front edge of the canopy is between 0.4 and 0.7 m The length of unsupported roof in the front edge of the canopy is between 0.1 and 0.4 m (less than 0.4 m) Occurrence in the immediate floor of a layer with a high content of clay minerals (e.g. clay shale, fireclay and mudstone) and a steady flow of water into the longwall working is observed Occurrence in the immediate floor of a layer with a high content of clay minerals (e.g. clay shale, fireclay and mudstone) and water dripping from the roof Occurrence in the immediate floor of a layer with a high content of clay minerals (e.g. clay shale, fireclay and mudstone), damp roof and sidewalls The angle between the longwall face line and the direction of the main crack in the roof strata is α < 30 or α > 150 Figure 6, there is a crack run into the goaf Figure 7, and UCS of the roof strata is less than 40 MPa The angle between the longwall face line and the direction of the main crack in the roof strata is α < 30 or α > 150 Fig. 6, and the UCS of roof strata is less than 40 MPa The angle between the longwall face line and the direction of the main crack in the roof strata is α < 30 or α > 150 Figure 6, there is crack run into the goaf Figure 7, and the UCS of roof strata is greater than 40 MPa The fault is almost parallel (oriented at less than 30 ) to the face line and the retreated longwall is climbing from the hanging wall to the footwall, the fault is dipping at 60 to 90 towards the longwall face The fault is almost parallel (oriented at less than 30 ) to the face line and the retreated longwall is running down from the hanging wall to the footwall, the fault dipping at 60 to 90 towards the longwall face Assessment value Weight α The fault is perpendicular in relation to the longwall face line 1 FC7 Longwall inclination Longitudinal inclination over Transverse inclination over ±10 2 2

7 International Journal of Mining, Reclamation and Environment 563 Figure 3. Strong overhanging layers in the roof behind the longwall shields: m thickness of the strong roof layer [m], x distance between the floor of the strong layer and the longwall roof [m], h longwall height [m]. Figure 4. An example of calculating the distribution of the support capacity along the canopy length. length in given shield construction, a mathematical model of a shield and then a computer program were developed at CMI (Figure 4) [27]. For assessment of factor FC3 on the roof maintenance of the longwall, the distribution of support capacity along the canopy length should be calculated. Depending on the length of unsupported roof, the appropriate number of points for FC3 is selected (Table 2) Water sensitive rocks within the immediate floor of the longwall working (FC4) Laboratory tests clearly show that water decreases the strength parameters of coal and rocks surrounding hard coal seams [28,29]. Tests and observations conducted by CMI personnel in hard coal mines in Poland confirmed that if the longwall floor contains clay minerals exposed to water, then the shields, both at setting and under significant load, tend to penetrate into the floor [30,31]. In this method for assessing roof fall risk, it was assumed that the probability of an event in a longwall increases if there is

8 564 S. Prusek et al. Figure 5. An example of the joint rose of the discontinuity pattern a longwall face line situated parallel to the direction of the joints in the roof layers and in the coal seam. Figure 6. The angle between the direction of joints in the roof layers and the longwall face line. a layer of rocks containing clay minerals (e.g. clay shale, fireclay or mudstone) in the immediate floor and if water flows into the longwall working. The intensity of water flow into a longwall is determined descriptively with three cases: steady flow, dripping and damp, which are similar to cases included in Innaecionne s article [32] Orientation of the longwall face in relation to the directions of joints in the roof strata (FC5) Joints in the rock mass, which most often result from the influence of tectonic forces, play an important role in the mining operations conducted in hard coal seams with the longwall method. The density of the joints in the roof and the orientation of joints planes in relation to the longwall face line (Figures 5 7) influence the stability of a roof [23]. It is recommended for rocks with high strength

9 International Journal of Mining, Reclamation and Environment 565 Figure 7. The longwall-parallel joints in the roof strata run into the goaf. parameters and low caving ability that the longwall face line be oriented parallel to or at an angle lower than 30 or greater than 150 in relation to the dominant direction of the joints in the roof strata (angle α in Figure 6) [23]. If there are layers with low strength parameters (uniaxial compression strength lower than 40 MPa) in the roof, the longwall face line should be oriented at an angle of (α) between 30 and 150 in relation to the dominant direction of the joints in the roof strata. When the longwall face line is parallel to the dominant direction of the joints in the roof strata in the longwall, two different cases may occur. First, the longwall and parallel joints run into the goaf (Figure 7), and second, the longwall and parallel joints run into the coal face [13]. Underground tests showed that more difficulties in maintaining the roof in a longwall arise when the joints run into the goaf [4,13] Orientation of the longwall face line in relation to a fault (FC6) Observations made by CMI indicate that the probability of roof fall depends on, among other factors, the orientation of the longwall face line in relation to the fault zone [16]. The three most common cases of such a situation have been identified and presented on Figures In each case the longwall face needs to be specially oriented to follow the coal seam. These operations could have negative influence on interaction of support with roof rocks. Based on experiences in Polish hard coal mines, an assessment of the influence factor FC6 on roof conditions in a longwall was conducted and presented in point scale in Table Significant transverse and longitudinal inclination of a longwall (FC7) Observations showed that significant inclination of a longwall, both a transverse one (greater than ±10 ), and a longitudinal one (greater than 15 ) [33], can cause an increase in roof fall probability. In a longwall with longitudinal inclination, there are often problems with the stability of shield sections. Sections working at a steep longitudinal inclination tend to set only with the edge of the canopy, which can damage the structure of the roof rocks and lead to a roof fall (Figure 11). In a longwall with a transverse inclination, especially ascending, the probability of roof falls increases, which is associated with lower stability of the longwall face and, as a consequence, an increase in the width of face to tip area. 5. Assessment of the roof fall risk One of the most powerful tools to cope with hazards during the production process is a risk assessment. An assessment is a systematic approach that can be applied when key decisions concerning production, work management, the environment, machine maintenance and, most importantly, occupational

10 566 S. Prusek et al. Figure 8. The fault is almost parallel (oriented at less than 30 ) to the face line and a retreated longwall is climbing from the hanging wall to the footwall. Figure 9. The fault is almost parallel (oriented at less than 30 ) to the face line and a retreated longwall is running down from the hanging wall to the footwall. health and safety are being made so the mineral mining can be as successful as possible. According to the European Standard on Risk Management [34], methods used in analysing risk can be qualitative, semi-qualitative or quantitative. The selection of a method should be based on local knowledge and experience combined with external expertise to meet the formal requirements and to solve specific problems. In the case of roof falls, it is very difficult to choose a method because different geological, mining and technical factors influence the occurrence of each event. Risk estimation for a roof fall consists of determining the consequences and the probability of an event. The consequences and its probabilities are combined to determine a level of risk. Therefore, the roof fall risk can be defined as: R RF = P RF C RF (2) where R RF : risk associated with a mine roof fall in a longwall with caving; P RF : probability that a roof fall occurs; C RF : consequences understood as a quantification of different effects of a roof fall, which mostly cause human, tangible or economic loss.

11 International Journal of Mining, Reclamation and Environment 567 Figure 10. The fault is perpendicular in relation to the face line. Figure 11. Possible destruction of the roof strata by the shield canopy in a longwall with significant longitudinal inclination. To calculate the probability, different methods can be applied: a probabilistic approach (based on statistical data), an empirical method (based on experience) or expert judgements (based on specialists knowledge). In Upper Silesian Region the conditions between mines vary, that is why the use of probabilistic method would not be representative, and even useless for some mines. That is why a semi-qualitative approach based on empirical and expert methods, and in the line with Ghasemi et al. s contribution [35], was selected for testing. The key issue was to make the method accessible and useful for stakeholders so that each value for probability and consequences could be analysed in order to provide an answer about which solutions should be applied Probability The probability assessment of a roof fall has been determined based on the value of the index g (Section 3) and the value of other roof fall risk factors FC (Section 4). Each factor was given an assessment value (Table 2), which is a qualitative expression of the probability of a roof fall (empirical method). Because

12 568 S. Prusek et al. Table 3. Matrix for the roof fall probability assessment. Assessment of roof fall Assessment of the state of the roof bearing capacity according to index g (Table 1) causes RFC (Table 2) g g < 0.8 g < Low Medium High Medium Medium High High High High each factor affects the performance of an underground coal mine environment to different degrees, it was necessary to independently weigh them using a judgement assessment. The Expert Panel Method has been selected for this purpose. This method is a repetitive interview with a group of specialists, who represent specific areas of knowledge and practice, to capture on-going changes and assess the impact of the factors that cause changes. This approach is explained in detail in the research of Krause and Krzemień [36]. To facilitate the use of this method in the mining industry, a risk assessment criterion for each FC factor was established using values between 1 and 3. A value of 1 indicates a minor roof fall probability, and 3 reflects a major possibility. This work has been done based on data from all Silesian mines in which roof falls occurred during the last few years and where CMI performed the index g calculations. The weights for each FC factor may be changed by the management of the mine if the experts find it appropriate due to various geological and mining conditions. Nevertheless, the proposed values seem to adequately reflect the experience gained while analysing the causes and consequences of roof falls in Polish coal mines. The final value for all FC factors taking part in the risk assessment process is calculated based on Equation (3). RFC = n α i FC i i=1 (3) where RFC: value of Risk considering all roof Fall Cause factors; n: number of factors taken into consideration in a single assessment (i.e. factors reflecting geological, mining and technical conditions at the site); α: weight of each factor FC; FC: roof fall cause factors. The calculated value together with the index g for the longwall enables investigators to assess the probability of roof fall occurrence (Table 3). Interpretation of results: Low low probability of roof falls, good conditions for mining operations. Medium possible roof falls, medium conditions for mining operations. High high probability of roof falls, bad conditions for mining operations Consequences Evaluation of the consequences causing both tangible and intangible losses [37], or quantitative and qualitative losses [38], is a very important step in the risk assessment. The consequences of the risk are most often associated with the uncontrollable influence of energy-causing accidents, work-related diseases and material damage in the working environment [39]. In other words, the losses caused by roof falls can refer to personnel injuries and fatal injuries as well as material losses that can be expressed in terms of money. For roof falls in longwalls, most of the losses can be identified as financial consequences incurred by an entrepreneur due to the costs of repairing damaged equipment and the costs of unmined coal. The potential losses caused by roof falls in longwalls are as follows: injuries, including fatalities, losses due to the amount of unmined coal during stoppage,

13 International Journal of Mining, Reclamation and Environment 569 the cost of shifts necessary to remove roof fall, the cost of materials used to remove roof fall, and the cost of repairing longwall equipment and cost of leasing a shearer and other equipment in the longwall. To determine financial losses, analyses were conducted in four mines in which roof failures in longwalls occurred. The average cost of roof falls in the mines was determined and their variability within the group of mines was tested, which provided homogenous conditions and similar conditions in comparative tests. Because most of the losses were included in the tangible effects group, which could be expressed in terms of money (except injuries), further analyses considered financial losses in an additive way. The analyses enabled investigators to estimate each of the above-mentioned potential losses and attribute a pecuniary value to them, according to coal prices, as well as the 2014 cost of materials and services in Poland. The average cost of a financial loss due to a roof fall and removing the roof fall is as follows (at the price of $120 per ton for coking coal): loss due to the amount of unmined coal (per shift) during longwall stoppage ~$250, ,000; cost of a shift necessary to remove a roof fall ~$2900; cost of materials used to remove a roof fall, per shift ~$1700; cost of leasing and repairing the equipment in the area of a roof fall per shift ~$600. The analysed cases of roof falls included events that lasted between one and 20 days. The time of a roof fall is meant to be the time from emergence of the event to the time at which consequences of the fall are removed. The cost analysis of roof falls showed that the most important category is the loss due to the amount of unmined coal, and that loss accounts for 93% of all of the losses resulting from roof failure. Therefore, it was proposed the consequences of a roof fall could be determined based only on the potential loss due to unmined coal to simplify the method of risk assessment (Equation (4)). L(RF) = Q CP S P C where L(RF): loss of production due to a roof fall; Q CP : quantity of coal production tones (planned longwall production per shift); S: number of shifts; P C : price of 1 ton of coal [$]. Of course, the human aspect cannot be ignored in evaluating the consequences of roof fall. Injuries and other human consequences, unlike in Duzgun and Einstein s [6] approach, are treated as a separate category (Table 4) and evaluated with the methodology described in Polish Standard PN-N [40], which is a standard commonly applied in the Polish mining industry. The standard, based on European standards OHSAS and 18002, requires the assessment of the consequences of hazardous events for all work positions. This approach assumes three categories of human loss. The application of the human losses assessment according to the current standard used in the mining industry facilitates the assessment procedure and shortens the time necessary to apply it. Interpretation of results: Low removal of a roof fall should not exceed one shift, no injuries or minor injuries may occur. Medium average time for removing a roof fall for this category is approximately five shifts, possible injuries are expected, but no fatalities. (4) Table 4. Matrix for assessing the consequences of a roof fall in a longwall. Consequences for the personnel due to a roof Financial loss due to a roof fall (Equation (4)) fall (according to PN-N-18002) <$360,000 $360, ,000 >$600,000 Few harmful consequences, e.g. minor injuries Low Medium High Moderately harmful consequences, e.g. possible Medium Medium High injuries but no fatalities Highly harmful consequences, e.g. extensive injuries or fatalities High High High

14 570 S. Prusek et al. Figure 12. Algorithm for assessing the roof fall risk in caving longwalls. Table 5. Matrix of the assessment of roof fall risk in a longwall working space. Assessment of the roof fall consequences (Table 4) Assessment of the roof fall probability in a longwall (Table 3) Low Medium High Low Low risk Low risk Medium risk Medium Medium risk Medium risk High risk High High risk High risk High risk High the time for removing a roof fall might be 15 shifts or more, it should be assumed that extensive injuries or fatalities may occur. The proposed method for assessing consequences (Table 4) emphasises the problem of personnel in the area of roof fall hazard Risk assessment method The proposed method was presented in the form of an algorithm (Figure 12). It is a combination of empirical methods and expert techniques that enable investigators to determine both the probability and potential consequences of a roof fall in a longwall. The level of risk of a roof fall is determined with a matrix (Table 5). The matrix is a result of analyses of over 150 support designs for over 150 longwall panels that the CMI has analysed in recent years for the Polish mining industry. The results of the analyses were compared with the real conditions of mining operations. The following criteria for accepting the risk of roof falls in longwalls were determined: Low risk acceptable risk. No preventive actions are necessary. Constant monitoring of the level of risk is required to provide work-related safety in a given area.

15 International Journal of Mining, Reclamation and Environment 571 Medium risk acceptable risk. Actions are required to reduce the level of risk and to improve safety of mining operations in a given longwall area while also considering the rules of optimising costs. High risk unacceptable risk. Mining operations cannot start before actions to reduce risk to the acceptable level are taken. 6. Application of method This chapter presents a practical application of the method for assessing the risk of roof fall in longwall panel. Example of retreat longwall with roof caving in to the gob, which has been operated recently in hard coal mine in Poland, is described. In the longwall, there were significant difficulties in maintaining the roof stability. The longwall panel width was approximately 300 m. The cutting height was approximately 2.2 m. The longwall was retreated with roof caving into the gob at a depth of approximately 320 m. In the immediate roof of the seam, there was a shale layer m thick (UCS = 5 10 MPa) with low self-supporting ability. Above the shale there was a sandstone layer approximately 50 m thick, (UCS = MPa). In the floor, there was a shale layer with a compressive strength of UCS = 6 7 MPa with low water resistance. In the longwall, there were many places where water was dripping from the roof. For roof support, two-legged shields with a width of 1.5 m and an operating height between 1.4 and 2.3 m were employed. The diameter of each shield leg was 320 mm. The shield capacity at setting was 4.02 MN, whereas the shield capacity at yield was 6.92 MN. Immediately after the longwall start-up, and shield setting at the right geometry (canopy parallel to base), the longwall retreat was stopped due to a malfunction of the Armoured Face Conveyor (AFC). As a result, there was an increase in the load exerted on shields. In addition, the water inflow changed the structure of floor strata, which negatively affected the floor bearing capacity. The shield bases started to penetrate the weak floor (mud). As a consequence of the load increase, the coal face started to spall, and the tip to face distance significantly increased to approximately 2 m. Then, a cavity approximately 3 m in height occurred in the roof (Figure 13). In the following days, due to low longwall advance (approximately m/day), the situation in the longwall continued to deteriorate. As a consequence, after reaching approximately 30 m of panel length, it was decided to stop mining operations and abandon the longwall panel. The calculated values of the roof bearing capacity index g for longwall panel were between 0.92 and The values Figure 13. Sketch of the roof cavity in longwall panel.

16 572 S. Prusek et al. Table 6. Assessment of the roof fall risk in longwall panel using the algorithm (Figure 12). No. Steps of the algorithm Assessment method Assessment results 1 Calculating the roof bearing capacity index g Equation (1) for longwall advance rate of 0.8 m/day 2 Assessing probability of a roof fall based on index g Table 1 g Analysis of roof fall causes (FC) Table 2 FC1, FC2, FC3, FC4 4 Assessing probability of a roof fall based on FC factors Equation (3) 28 5 Assessing the probability of a roof fall Table 3 Medium 6 Assessing losses due to unmined coal Equation (4) $1,000,000 7 Assessing consequences for personnel due to the emergence PN-N Low and removal of a roof fall 8 Assessing consequences of roof falls Table 4 High 9 Potential time of a roof fall removal Interpretation of ~5 shifts results for Table 4 10 Risk of roof fall Table 5 High did not show possible difficulties in longwall panel. Because of water inflow, the floor strata structure changed from rock to mud. Under these conditions, it was not possible to set the shields properly. An increased roof convergence in the longwall appeared, and then coal face spalling, shield penetration in the mud floor and a cavity in the roof occurred. Employing the new method, and using the algorithm presented in Figure 12, the risk of roof fall was assessed for the case study mine. The results of the assessment are encrypted in Table Conclusions The start of underground mining operations in a hard coal seam using a longwall method is associated with significant investments incurred by a coal companies. Apart from the necessary mining work associated with longwall panel development, a mine invests in expensive longwall equipment. To achieve a return on the invested capital, the assumed production plan (extraction) needs to proceed without problems. Various types of difficulties resulting from geological and mining conditions, including roof falls in the longwall, pose a significant threat to meeting the production targets of a mine. Roof falls may also cause work-related accidents, including fatal ones. This paper presents a new method that enables the risk of roof falls in retreat longwall panels with natural roof caving to be assessed. The method is a combination of empirical methods and expert techniques and was developed based on practical experience from implementing hundreds of research tests at the CMI concerning the selection of the supports for longwall panels in underground hard coal mines in Poland. To develop the method, the authors used the results of measurements and observations of roof stability in longwall panels obtained in recent years by CMI and combined it with the results of research performed in other countries, which were obtained through examination of the literature. In the present method, the probability of roof fall in a longwall is assessed based on the value of the roof bearing capacity index g and seven geological, mining and technical factors (FC) selected during research to create an empirical-expert method. The proposed method for assessing risk enables investigators to consider only the factors FC, which may have a significant influence on the increase in the risk of roof fall in a longwall. It is a universal approach because it is possible to incorporate further FC factors identified during research into the risk assessment method. The consequences of roof falls were assessed in two areas (i.e. human and financial). Three categories of losses were established for each of the areas by considering the experiences of the mining industry in Poland with accidents caused by roof falls and additionally considering tangible losses, which were expressed in terms of the costs incurred by coal mines as a consequence of roof falls.

17 International Journal of Mining, Reclamation and Environment 573 This paper presents verification of the method by applying it to assess the roof fall risk in longwall panel operated in hard coal mine in Poland. In this longwall panel, stoppages in production caused by roof falls have been occurred despite prognosis of good roof conditions with the application of g index. The application of new method indicated the high roof fall risk what was confirmed during longwall operation. At the design stage for underground mining operations in hard coal seams, the use of this method allows for mining companies to obtain information about the risk of roof fall in designed longwall panels. When the risk level is unacceptable, this assessment gives the companies the opportunity to take steps to reduce the risk. Disclosure statement No potential conflict of interest was reported by the authors. Funding This research was supported by the Polish Ministry of Science and Higher Education [contract no. 3989/E-263/S/2014, task: ]. ORCID Sylwester Rajwa Aleksander Wrana Alicja Krzemień References [1] C. Mark, D.M. Pappas, and T.M. Barczak, Current trends in reducing groundfall accidents in U.S. coal mines, Min. Eng. 63(1) (2011), pp [2] S.K. Palei and S.K. Das, Sensitivity analysis of support safety factor for predicting the effects of contributing parameters on roof falls in underground coal mines, Int. J. Coal Geol. 75 (2008), pp [3] J. Martyka and P. Hetmańczyk, Annual report: The state of natural and technical hazards in Polish hard coal mines in 2013, Report under Prof. Kabiesz lidership, GIG, Katowice, 2013, pp [In Polish]. [4] A. Biliński, The symptoms of rock mass pressure in longwall panels located in hard coal seams, Zeszyt Naukowy nr 221, Górnictwo z.31, Politechnika Śląska, Gliwice, 1968 [In Polish]. [5] M. Pappas and C.Mark, Roof and rib fall incident trends: A 10-year profile. Trans. Soc. Min. Metall. and Explor. 330 (2012), pp [6] H.S.B. Duzgun and H.H. Einstein, Assessment and management of roof fall risks in underground coal mines, Safety Science 42(1) (2004), pp [7] M. Hossaini and S. Behraftar, Geotechnical risk assessment in Kerman coal mine-central Iran, Proceedings of the 10th Coal Operators Conference, University of Wollongong & the Australasian Institute of Mining and Metallurgy, Wollongong, New South Wales, 2009, pp [8] R. Evans, D. Brereton, and J. Joy, Risk assessment as a tool to explore sustainable development issues; lessons from the Australian coal industry, Int. J. Risk Assess. Manage. 7(5) (2007), pp [9] J. Joy, Occupational safety risk management in Australian mining, Occup. Med. 54(5) (2004), pp [10] T.M. Barczak and S.C. Tadolini, Longwall shield and standing gateroad support designs is bigger better?, Proceedings of the Proceedings of Longwall USA, Pittsburgh, PA, 5 7 June, 2007, pp [11] T.P. Medhurst, Practical considerations in longwall support behaviour and ground response, Proceedings of the 5th Coal Operators Conference, Wollongong, New South Wales, 2005, pp [12] D. Hoyer, Early warning of longwall of cavities using LVA software, Proceedings of the 12th Coal Operators Conference, Wollongong, New South Wales, 2012, pp [13] U. Langosch, U. Ruppel, and U. Wyink, Longwall roof control by calculation of the shield support requirements, Proceedings of the Coal Operators Conference, Wollongong, New South Wales, 2003, pp [14] S. Rajwa, M. Płonka, Zb. Lubosik, A. Walentek, and W. Masny. Principles of safe use of powered supports, Proceedings of the School of Underground Mining, Ukraina, Jałta, [15] M. Płonka, S. Prusek, and K. Rułka, 3D strata model application for the selection method of the support for longwall excavation, III International Conference Mining Techniques, Kraków Krynica, 2003, pp

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