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Numerical modelling and analysis of the influence of local variation in the thickness of a coal seam on surrounding stresses: Application to a practical case
International Journal of Coal Geology 79 (2009) 157–166
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International Journal of Coal Geology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / i j c o a l g e o
Numerical modelling and analysis of the influence of local variation in the thickness of a coal seam on surrounding stresses: Application to a practical case M.I. Álvarez-Fernández a, C. González-Nicieza a,⁎, A.E. Álvarez-Vigil b, G. Herrera García c, S. Torno a a b c
Department of Mining Exploitation, Mining Engineering School, University of Oviedo, Independencia 13, 33004 Asturias, Spain Department of Mathematics, Mining Engineering School, University of Oviedo, Independencia 13, 33004 Asturias, Spain Department of Geoscientific Research and Prospection, Spanish Institute of Geology and Mining, Río Rosas 23, 28003 Madrid, Spain
a r t i c l e
i n f o
Article history: Received 24 February 2009 Received in revised form 25 June 2009 Accepted 25 June 2009 Available online 3 August 2009 Keywords: Thinning Coal seam geometry Numerical modeling Overstress Production works
a b s t r a c t One of the factors with the most influence in the carrying out of underground coal mining work is the stress state, especially when such work is carried out at great depths. One of the mining methods in which the influence of stresses becomes most patent is that of sublevel caving. For that reason, a variety of authors have studied many of the causes which condition the stress state of mining work of this kind at a general level. However, diverse types of conditioning factors exist at a local level that may substantially vary recorded pressures in the surroundings of the workings. Using numerical models, this paper analyses the specific case of a coal seam mined by the sublevel caving method at a depth of 1000 m which suffers significant increases in stress and major deformations in certain advance headings as a result of variations in the geometry and dimensions of the load itself, in particular, thinning. The results of the research carried out were validated by measuring the stresses in the area under study. Furthermore, on the basis of a retrospective analysis of what was observed in the mine, another series of predictive models were analyzed to establish the existing relation between the increase in stress due to thinning of the seam and the depth at which such thinning is situated. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Underground coal mining has been considered a high risk activity worldwide. Innumerable accidents have occurred throughout history in coal mines of this kind, leading to the loss of human lives in many cases. However, if accident statistics are analyzed in more detail (seriousness, frequency, material and personal losses, etc.), it is apparent that the major problems are found in deep coal mines. This is so due to the high mechanical stresses existing at great depths and to their possible interaction with the gas that the coal contains. Furthermore, this interaction may be aggravated by the strength and deformational characteristics of the coal itself and the rock walls (St George, 1997), as well as by surrounding geological structures (Wold and Choi, 2001; Wold et al., 2008). There are two fundamental factors which have a major influence on safety in an underground coal mine. One is the mining method followed (Kaiser et al., 2001; Yuan, 2007), which determines the type of workings employed to extract the coal, while the other is the actual nature of the coal and the surrounding rock: • In the case analyzed here, the mining method employed is that of longwall caving. This method is especially hazardous in its advance phases, when galleries are excavated a few metres below areas that ⁎ Corresponding author. E-mail address: [email protected] (C. González-Nicieza). 0166-5162/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.coal.2009.06.008
are overstressed due to the disequilibrium generated by the caving of preceding sublevels (García de Boto, 1996; Díaz Aguado and González Nicieza, 2007, 2009). • The properties of the coal and rock walls, as well as the location and morphology of the seam within the context of the rock mass have a major influence on how hazardous the mine will be. According to several authors (Cao et al., 2001, 2003; Wold et al., 2008), the majority of accidents are related to faults, shear zones or igneous intrusions. Moreover, local changes in the geometry and/or arrangement of the coal seam appear to influence the increase in the number of rockbursts. These changes may be tectonic (small faults or folds) or sedimentary (thickening and thinning) in origin. In addition, the gas content of the coal must also be taken into account, as well as the moisture content, strength of the coal, etc. In the case analyzed here, the study arose as a result of a series of observations carried out in a mined area in which it was noted that in certain areas the floor of the heading being mined had suffered greater swelling than was normal. At the same time, it was observed that the thickness of the coal seam varied more or less brusquely at other points, giving rise to what are known as “thinning”. In view of the possibility that both phenomena might be related, a back analysis was carried out of the observed phenomena by means of numerical models created with the software application FLAC 2D, version 3.4 (Itasca Consulting Group, 2004), which is based on finite differences and has been widely used by several researchers for
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studying different mining phenomena (Mark et al., 2007; GonzálezNicieza et al., 2008; Wang et al., 2008). A numerical model was thus created which allowed the simulation of the existence of thinning of variable dimensions below the floor of the advance heading. These models, which are described in detail in subsequent sections, were used to calibrate the location and dimensions (height and thickness) that the thinning should have to generate stresses and hence deformations of the magnitude of those observed in the workings. To work with realistic values of the dimensions that the thinning could possibly present, data provided by the company responsible for the mine regarding the thickness of the seam were used as a reference. After reproducing what was actually observed by means of computer simulations and defining the area affected by these geometric anomalies, other models were developed to test whether the degree of influence of the thinning on local stresses is itself influenced by the depth at which said thinning is located. On the basis of the results of all these simulations, it was possible to establish a series of recommendations as regards the location of advance headings in areas which are not seen to be affected by existing thinning of the seam. 2. Description of the area under study The study was carried out to the south of El Entrego, Asturias (Northern Spain) at a depth of approximately 1000 m, under the area known as El Campizu, close to a former opencast mine. The analyzed coal seam is called the “Julia” seam, which is accessed from the Sotón Pit. The area of workings where the observations were made is situated on the first and second sublevels between the Tenth and Sub-tenth levels at a depth of 967 m.
Fig. 1 shows a scheme of the mine and of the detailed situation of the area under study. Coal is mined by the “sublevels caving” method, a stoping method in which relatively thin blocks of ore are caused to cave in by successively undermining small panels. The seam is progressively mined by a series of sublevels spaced at vertical intervals of 20 to 30 m and occasionally more. Normally only one or two sublevels are developed at a time, beginning at the top of the ore body. Caving commences at the ends of the stopes by blasting out cuts and retreating in the same manner toward the raises. Fig. 2 shows a scheme of this method. The Julia seam belongs to the Central Coal Basin, which presents a thickness of around 6000 m and occupies an area of 1400 sq km. Within the Central Coal Basin, the area under study forms part of the Caudal-Nalón Unit, and within this unit it is included in the Sama Group Production Section. This section is also divided into four stretches. Fig. 3 shows the surface geology of the area under study. As can be seen, the area under study belongs to the Third Production Zone, which in turn is part of the Sama Group. This production zone, which belongs chronologically to the Pennsylvanian period and presents predominant slate and sandstone lithologies, is part of the María Luisa and Sotón veins (see Fig. 4), which have a thickness of 750 m and are the most extensively mined beds in the Asturian Coal Basin. The Julia seam forms part of the María Luisa coal bed. In the stratigraphic column, the areas that correspond to marine environments are marked in green, while the white areas represent continental environments (Suárez-Ruiz and Jiménez, 2004; Colmenero et al., 2008).
Fig. 1. Scheme of the mine.
Fig. 2. Sublevel caving scheme.
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Fig. 3. Geology of the area.
Julia seam has three hanging wall veins (San Gaspar, San Luís and María Luisa). The distances of the San Gaspar, San Luís and María Luisa veins to the Julia seam are respectively 30, 56 and 104 m. Their direction and dip have average values of 354/68 according to field work carried out the authors. The Julia seam presents a high dip, of around 68°, and is characterized by its great heterogeneity, as it is made up of three sub-layers of differentiated lithology: a roof vein, a floor vein and between the two a
strip of rock called “earth”. The contact coal–rock wall is filled with clay (around 5 cm in thickness). It is variable in thickness, there being areas in the First Level which may vary between more than 3 m and 0.3 m in the thinnest areas. Fig. 5 shows a plan view of the thickness of the seam along this level. 3. “In situ” phenomena analyzed During a safety study of the mining of the Julia seam, several phenomena related to significant increases in stress levels and deformations in the advance headings were observed. After analyzing numerous parameters and collecting information on similar phenomena that had occurred previously, the conclusion was reached that the main causes of these phenomena were anomalies in the geometry of the coal seam. Specifically, after analyzing in detail the mapping of the seam carried out over the years by those responsible for the mining work, it was seen that when the seam presented thinner stretches than normal, these tended to generated swelling. This swelling was especially significant in the floor of the advance headings as the sublevel caving of the upper level got closer. During the study of the Julia seam, an especially significant event was observed: during the night a crosscutter situated in the advance heading overturned due to sudden swelling of the floor vein. As was discovered later, just under the floor of this level there was a “thinning” of approximately 1 m in width and 2 m in height. After observing this phenomenon, the decision was taken to use biaxial vibrating wire stressmeters (Geokon, vibrating wire stressmeter series 4300 BX) (Geokon Inc., 2004) installed in boreholes to instrument another area of the seam where this type of geometric anomaly existed; specifically a thinning of approximately 1 m in width and 1 m in height. Weekly measurements were taken and it was seen that as the production workings on the upper level approached the area of the thinning, the
Fig. 4. Stratigraphic column.
Fig. 5. Plan view of the First Level of the Julia seam.
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Fig. 6. Horizontal stress variation measured with vibrating wire stressmeters near a thinning of 1 m in width and 1 m in height (C-26) and in a zone without thinning (C-14).
stressmeters recorded an increase in horizontal stress much higher than that due to normal caving workings. Once the area of the thinning had been passed, the stressmeter barely recorded variations, though decreases were not registered either. Blue line in Fig. 6 shows the variation in horizontal stress measured by the stressmeter in the area of the thinning. Pink line in Fig. 6 is a graph of the variations in stress recorded by the other stressmeter in an area without thinning. The increases of up to 4 MPa are due solely to the approach of the workings. The variation in vertical stress in the area surrounding the thinning was also measured, although important changes were not observed, only those due to the approach of the workings. On the basis of this analysis, numerical models were used to verify the influence that a thinning of different geometries in the seam would have in order to establish suitable prior safety measures. 4. Prior work Before starting to develop the numerical models, a series of tasks and tests had to be carried out to characterize both the coal seam and the surrounding rock mass in geotechnical terms.
Table 1 Results of uniaxial compressive strength test in coal. Test
Orientation of the sample
Uniaxial compressive strength (MPa)
Floor vein coal M-1–119 M-1–157 M-1–161
4.9 7.6 8.5
Roof Vein Coal T-1–119
4.5
“Earth” Ti-1–157
4.3
Undifferentiated coal I-1–144 I-1–157 I-1–161
6.4 8.1 4.9
As regards the coal, diverse uniaxial compressive strength tests were carried out with different orientations of the samples. That is, given that coal is characterized by the fact that it presents clearly oriented joints or “cleats”, some tests were carried out in which the load was applied perpendicularly to these cleats, while in others the load was applied in parallel. Table 1 shows the results of the tests carried out, indicating in each case the orientation of the cleats with respect to the applied load. The mean compressive strength for the tests carried out perpendicularly to the cleats is 7.65 MPa, while that of the tests carried out by applying the load parallel to the cleats is 4.65 MPa. Seeing as the models to analyze were developed considering pressure applied perpendicular to the cleats, only the data from the tests thus carried out were taken into account. The standard deviation and variance for these four tests (M-1–157, M-1–161, I-1–144 and I-1–157) are 0.91 and 0.83, respectively. On the other hand, data were available on the cohesion, friction angle and elasticity moduli of the coal, obtained from tests carried out in study surveys in preceding areas. The mean values of these properties for the coal are given in Table 2. To characterize the rock walls of the Julia seam in geotechnical terms, measurements were carried out with a Schmidt hammer type L (ASTM, 2005), obtaining the RMR of these materials by means of field observations. The uniaxial compressive strength, obtained from rebound hardness data, ranged between 64 MPa for the roof and 65.7 MPa for the floor. Note that the low values may be due to the fact that the materials had suffered cracking as a result of blasting work. According to field studies, the RMR of the rock walls is 75. It thus belongs to class II in Bieniawski's classification and may be classified as “good”. To define these rock wall properties, the RMR was calibrated by means of back analysis. That is, starting from the deformational behaviour observed in reality, according to which the cavity opened by sublevel caving in the Julia seam that remains unclosed is around 50 to 60 m in height, the geomechanical properties of the rock wall introduced in the corresponding model were those that reproduce such behaviour.
Table 3 Rock wall properties tested on calibration. Rock walls
Table 2 Properties of the coal. Coal Bulk modulus (K) (Pa)
Shear modulus (G) (Pa)
Cohesion (Pa)
Friction angle (°)
9.5E8
8.6E8
3.9E5
32
RMR 70 RMR 75 RMR 80
Bulk modulus (K) (Pa)
Shear modulus (G) (Pa)
Poisson's coefficient
Cohesion (Pa)
Friction angle (°)
3.3E10 4.1E10 5E10
1.5E10 1.9E10 2.3E10
0.3 0.3 0.3
5.5E6 6.6E6 8E6
33 34 35
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Fig. 7. Horizontal displacements. Model with RMR 75.
This simulation was carried out by means of a FLAC 2D model representing the Julia Seam and its rock walls from the surface to a depth of 1800 m. The calculation is divided into the following phases: 1. Stresses are first initialized in the model to simulate the stress state of the virgin rock mass, prior to the onset of mining work. 2. Subsequently, the Julia Seam is sublevel-caved up to the real height of mining work, allowing the model stabilize; i.e., until the stresses and displacements induced by the opening up of the cavity reach equilibrium. 3. In order for the introduced properties to provide a closer fit to the real ones, the model must stabilize after closing most of the cavity, except for the final lower 50–60 m, which remain semi-open during real mining work. We examined several RMRs for the whole rock mass (though starting off from a minimum value of 70), as both in-situ observations and resistance tests show that it constitutes a rock mass that can be catalogued, at the least, as ‘good’ according to Bieniawski's classification. We thus analysed models with an RMR of 70, 75 and 80. The properties corresponding to each of these are included in Table 3. Note that the cohesion, friction angle and tension properties were calculated according to the Hoek–Brown criterion (Hoek and Brown, 1998), which involves a more realistic modification of the values of these parameters as established in the original classification provided by Bieniawski (1989). The results obtained after evaluating the three models show that the properties that provide the best fit to the actual properties are those corresponding to RMR 75. The model calculated with these properties is the one that best simulates the deformation suffered by the rock mass after sublevel caving, leaving an open cavity of around 60 m after stress re-equilibration. Fig. 7 shows the horizontal displacements suffered by the model with RMR 75. Although examination of the Fig. 7 suggests that the sublevel caving cavity has been completely closed, the displacement values in
the colour caption of the figure show that RMR 75 constitutes the case in which the rock walls are in the most similar position to that found in reality, since the final 60 sublevel-caved metres remain unclosed. In addition to the geotechnical characterization of the coal and the rock walls, direct shear lab tests were also carried out to obtain the cohesion and friction angle of the clay in the contact coal–rock (around 5 cm in thickness), which must also be taken into account in the numerical model. The results of the five direct shear tests carried out are shown in Table 4. The standard deviation and variance of these tests are 18.2 and 334.5 for cohesion, and 2 and 4.3 for the friction angle. 5. Development and discussion of results Having obtained the strength properties of the coal and the rock walls, we shall now describe the working method followed in analyzing the influence of thinning on the local stress state. First, a 2-dimensional model was generated using the programme FLAC 2D which represents the advance heading of a sublevel and its nearest surroundings (some 40 m thick and 40 m high), as shown in Fig. 8. To simulate the existing stresses at the depth at which the sublevel is actually located, principal stresses are applied at the top and sides.
Table 4 Results of the direct shear tests. Test
Cohesion (kPa)
Friction angle (°)
M1 M2 M3 M4 M5
87 115 78 96 124
22 19 23 20 18
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Fig. 8. Scheme of the model created to simulate thinning in the Julia seam.
Maximum principal stress has been applied perpendicular to roof and floor. To simulate the sublevel caving workings in the upper level, the top of the coal seam is not subjected to any applied vertical loads. The values of these principal stresses were taken from models that simulate a greater portion of ground, developed to analyze the behaviour of the ground versus sublevel caving. The stress factor estimated from these models for the case under study is 1.5. Different parameters may be varied in this model, such as the properties of the coal seam, of the rock walls and of the coal–rock wall interface, the dimensions of the heading and especially those of the thinning, which may be varied both in height and in its angles with respect to the horizontal (see Fig. 8). The properties of the coal and the rock walls considered for the model are those described previously, while the properties employed for the coal–rock wall contact zone were those corresponding to direct shear test results of the clay in that zone. 5.1. Influence of the geometry of the thinning on the local stress-deformation state The first part of the research study focussed on the general analysis of the influence that a thinning of different geometries situated below an advance heading may have in order to assess to what extent it may give rise to phenomena of swelling and overstress in the floor of the sublevel. Seven different models were calculated, using the dimensions of the thinning given in Table 5. Model 7 is used as the standard for comparison with the other cases, as it is assumed that in this model the sublevel advances in an area in which the Julia seam presents a constant thickness of 3 m. To assess the degree to which thinning affects the surrounding of the advance heading, stresses as well as deformations were analyzed. 5.1.1. Analysis of stresses The stress analysis was carried out by measuring the value of the horizontal and vertical stresses below the floor of the advance heading in each model.
Fig. 9 shows an example of the graphical output of horizontal stresses generated by the programme FLAC 2D for Model 3. The stresses are represented by colours which indicate intervals of values, as specified in the legend on the left of the figures. Stress data are given in international system units. Likewise, the results for the other parameters, such as displacements, are also represented by means of iso-values, and are expressed in international system units. Fig. 10 shows the evolution of vertical and horizontal stresses evaluated at the centre of the coal seam. The origin of the x axis corresponds with the floor of the advance heading, evaluating from this point the stress that exists with the increasing depth of said floor. Fig. 11 shows how the axes of the graphs are considered. It can also be seen in Fig. 10 that, due to the existence of thinning, a substantial peak in horizontal stresses is produced just below the advance heading, whereas the vertical stresses remain practically constant after the decompression of the first few metres induced by the actual mining of the heading. We now discuss the results of the comparison of the evolution of stresses for each model, including Model 7, which has no thinning. Fig. 12 shows the comparison of horizontal stresses recorded for each model below the floor of the advance heading. As can be seen, three models present a substantial stress peak 3 m from the advance heading floor.
Table 5 Models for analyzing the influence of thinning on the Julia seam. Model
Angle of the thinning (°)
Thinning in the width (m)
Thinning in the height (m)
Model 1 Model 2 Model 3 Model 4 Model 5 Model 6 Model 7
30 30 30 30 30 30
1 2 1 2 1 2 Without thinning
1 1 2 2 3 3
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Fig. 9. Horizontal stresses for Model 3.
The overstress recorded in these three cases with respect to the model without thinning varies between 30 and 50 MPa; i.e. stress increases of between 200 and 300% are produced. From the Fig. 12 legend, it can be seen that the three models with the greatest overstress correspond to thinning of 1 m in width and 1 to 3 m in height. This implies that the greater the difference in width between the thinning and the seam itself, the greater the accumulation of tensions produced. Furthermore, considering a constant width, the increasing height of the thinning accentuates the increase in stresses. As can be seen in Fig. 12, the model with a thinning of 1 m in width and 1 m in height presents a stress increase of 30 MPa with respect to the
model without thinning. This result is similar to that obtained with vibrating wire stressmeter measurements, in which the difference between the stress increase in a zone near a thinning and the increase in stress in a zone without thinning was about 22 MPa. When the thinning is wider (Models with a width of 2 m), the recorded stress barely varies with respect to that of the model without thinning. Only when the height increases is a slight increase observed, in the order of 10 MPa, which supposes an increase of barely 40% with respect to the model without thinning (see Fig. 12). Fig. 13 shows the same type of representation for vertical stresses. The first thing to notice is that the values of the vertical stresses are much lower than those of the horizontal stresses in all the models. Moreover, the existence of thinning of different geometries does not suppose a substantial change in the magnitude of these stresses. Nevertheless, the thinning of less width (1 m) can be observed once more to have the greatest influence, not only in terms of magnitude, but also in the change in location of the stress peak, which is now some 4 m from the floor of the advance heading. 5.1.2. Analysis of displacements The overstress described in the previous section gives rise to displacements of a different type within each model. As the greater stresses are horizontal, more or less marked swelling of the floor tends to occur in all cases, in accordance with the effect shown in Fig. 14. Highly pronounced swelling of more than 30 cm is produced in all cases. This implies that these displacements are due not only to the overstress caused by the thinning, but also to the properties of the coal–rock wall contact zone, which are the same in all the models. 5.2. Influence of the depth of the workings
Fig. 10. Evolution of the stresses below the floor of the advance heading for Model 3.
The following stage of the research study was to generate and analyze a series of models which assessed the influence of the depth of the workings on the overstress generated by the actual geometry of the thinning. To test this influence, the aforementioned models presenting extreme results were chosen; that is, the model without thinning and
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Fig. 11. Axes considered in the graphs.
the model with a thinning of 1 m in width and 3 m in height, which was the one in which the greatest overstress was produced. These two models, initially calculated for a depth of around 950 m (the height above sea level of the current workings), were therefore recalculated for depths of 400 m and 1200 m. To analyze the influence of the depth, the evolution of the horizontal stresses below the floor of the advance heading was newly represented. Fig. 15 shows the results obtained for all the models at different depths. In view of the above stress graphs, there is no doubt that, in absolute terms, depth notably affects the value of the stress peak below the floor of the advance heading. However, if the models with and without thinning
are compared for each depth, it can be seen that the increase in stresses in the former with respect to the latter is always in the order of 250–300%. It therefore seems that the depth at which the thinning of the seam is located always produces the same relative increase in horizontal stress, although the starting out value of the model without thinning logically increases with increasing depth. 6. Conclusions The research carried out has allowed us to reconstruct and explain by means of numerical models the phenomena of swelling of the floor of an advance heading in coal observed in a mine.
Fig. 12. Horizontal stresses below the floor of the advance heading for each model.
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Fig. 13. Vertical stresses below the floor of the advance heading for each model.
The conclusion drawn was that these excessive swellings are the result of the existence of anomalies in the dimensions of the seam being mined, specifically “thinning”, which lead to a major concentration of horizontal stresses that tend to “thrust” the coal in the floor up into the open cavity. The phenomenon of swelling and overstress observed in the mine was reproduced by means of a numerical model, obtaining a magnitude of displacements and stresses similar to those actually measured. This fact corroborates the initial hypothesis that the existence of a thinning of the seam is what conditions phenomena of this type. The relation existing between the generated overstress and the geometry of the thinning were likewise assessed, it being shown that
the greater the variation in the thickness of the seam (more thinning), the greater the stresses generated in that area. Furthermore, for the same degree of thinning of the seam, the greater the height of the thinning, the greater the stresses. The vertical stresses barely vary in magnitude, regardless of the geometry of the thinning, although their maximum values tend to concentrate closer to the floor than in the case of the model without thinning. Finally, it was also shown that, regardless of the depth at which the thinning is located, for any given geometry, the increase in horizontal stresses with respect to the standard model is always of the same
Fig. 14. Vertical displacements in the area of the thinning.
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Fig. 15. Horizontal stresses in the models at different depths.
order. This implies that it constitutes an overstress phenomenon that is exclusively related to the geometry of the seam and not to general mining conditions, such as the depth of the working. The results reported in this paper may serve as the basis for establishing safety measures in the mining of coal seams of the kind analyzed, as the major stress concentrations created in the surroundings of a geometrical anomaly such as that described here mean that any workings carried out in that area suppose a risk. Acknowledgements The information and technical and logistic support provided by those responsible for the Sotón Pit (HUNOSA), where mining work takes place in the area under study, was of inestimable help in the carrying out of this research study. References ASTM, 2005. ASTM, standard test method for determination of rock hardness by rebound hammer method. ASTM Stand (2005). p. D 5873-05. Bieniawski, Z.T., 1989. Engineering Rock Mass Classifications. Wiley, Chichester, p. 272. Cao, Y., He, D., Glick, D., 2001. Coal and gas outbursts in footwalls of reverse faults. International Journal of Coal Geology 48 (1–2), 47–63. Cao, Y., Davis, A., Liu, R., Liu, X., Zhang, Y., 2003. The influence of tectonic deformation on some geochemical properties of coals: a possible indicator of outburst potential. International Journal of Coal Geology 53, 69–79. Colmenero, J.R., Suárez-Ruiz, I., Fernández-Suárez, J., Barba, P., Llorens, T., 2008. Genesis and rank distribution of Upper Carboniferous coal basins in the Cantabrian Mountains, Northern Spain. International Journal of Coal Geology 76, 187–204. Díaz Aguado, M.B., González Nicieza, C., 2007. Control and prevention of gas outbursts in coal mines, Riosa–Olloniego coalfield, Spain. International Journal of Coal Geology 69, 253–266.
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International Journal of Coal Geology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / i j c o a l g e o
Numerical modelling and analysis of the influence of local variation in the thickness of a coal seam on surrounding stresses: Application to a practical case M.I. Álvarez-Fernández a, C. González-Nicieza a,⁎, A.E. Álvarez-Vigil b, G. Herrera García c, S. Torno a a b c
Department of Mining Exploitation, Mining Engineering School, University of Oviedo, Independencia 13, 33004 Asturias, Spain Department of Mathematics, Mining Engineering School, University of Oviedo, Independencia 13, 33004 Asturias, Spain Department of Geoscientific Research and Prospection, Spanish Institute of Geology and Mining, Río Rosas 23, 28003 Madrid, Spain
a r t i c l e
i n f o
Article history: Received 24 February 2009 Received in revised form 25 June 2009 Accepted 25 June 2009 Available online 3 August 2009 Keywords: Thinning Coal seam geometry Numerical modeling Overstress Production works
a b s t r a c t One of the factors with the most influence in the carrying out of underground coal mining work is the stress state, especially when such work is carried out at great depths. One of the mining methods in which the influence of stresses becomes most patent is that of sublevel caving. For that reason, a variety of authors have studied many of the causes which condition the stress state of mining work of this kind at a general level. However, diverse types of conditioning factors exist at a local level that may substantially vary recorded pressures in the surroundings of the workings. Using numerical models, this paper analyses the specific case of a coal seam mined by the sublevel caving method at a depth of 1000 m which suffers significant increases in stress and major deformations in certain advance headings as a result of variations in the geometry and dimensions of the load itself, in particular, thinning. The results of the research carried out were validated by measuring the stresses in the area under study. Furthermore, on the basis of a retrospective analysis of what was observed in the mine, another series of predictive models were analyzed to establish the existing relation between the increase in stress due to thinning of the seam and the depth at which such thinning is situated. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Underground coal mining has been considered a high risk activity worldwide. Innumerable accidents have occurred throughout history in coal mines of this kind, leading to the loss of human lives in many cases. However, if accident statistics are analyzed in more detail (seriousness, frequency, material and personal losses, etc.), it is apparent that the major problems are found in deep coal mines. This is so due to the high mechanical stresses existing at great depths and to their possible interaction with the gas that the coal contains. Furthermore, this interaction may be aggravated by the strength and deformational characteristics of the coal itself and the rock walls (St George, 1997), as well as by surrounding geological structures (Wold and Choi, 2001; Wold et al., 2008). There are two fundamental factors which have a major influence on safety in an underground coal mine. One is the mining method followed (Kaiser et al., 2001; Yuan, 2007), which determines the type of workings employed to extract the coal, while the other is the actual nature of the coal and the surrounding rock: • In the case analyzed here, the mining method employed is that of longwall caving. This method is especially hazardous in its advance phases, when galleries are excavated a few metres below areas that ⁎ Corresponding author. E-mail address: [email protected] (C. González-Nicieza). 0166-5162/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.coal.2009.06.008
are overstressed due to the disequilibrium generated by the caving of preceding sublevels (García de Boto, 1996; Díaz Aguado and González Nicieza, 2007, 2009). • The properties of the coal and rock walls, as well as the location and morphology of the seam within the context of the rock mass have a major influence on how hazardous the mine will be. According to several authors (Cao et al., 2001, 2003; Wold et al., 2008), the majority of accidents are related to faults, shear zones or igneous intrusions. Moreover, local changes in the geometry and/or arrangement of the coal seam appear to influence the increase in the number of rockbursts. These changes may be tectonic (small faults or folds) or sedimentary (thickening and thinning) in origin. In addition, the gas content of the coal must also be taken into account, as well as the moisture content, strength of the coal, etc. In the case analyzed here, the study arose as a result of a series of observations carried out in a mined area in which it was noted that in certain areas the floor of the heading being mined had suffered greater swelling than was normal. At the same time, it was observed that the thickness of the coal seam varied more or less brusquely at other points, giving rise to what are known as “thinning”. In view of the possibility that both phenomena might be related, a back analysis was carried out of the observed phenomena by means of numerical models created with the software application FLAC 2D, version 3.4 (Itasca Consulting Group, 2004), which is based on finite differences and has been widely used by several researchers for
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studying different mining phenomena (Mark et al., 2007; GonzálezNicieza et al., 2008; Wang et al., 2008). A numerical model was thus created which allowed the simulation of the existence of thinning of variable dimensions below the floor of the advance heading. These models, which are described in detail in subsequent sections, were used to calibrate the location and dimensions (height and thickness) that the thinning should have to generate stresses and hence deformations of the magnitude of those observed in the workings. To work with realistic values of the dimensions that the thinning could possibly present, data provided by the company responsible for the mine regarding the thickness of the seam were used as a reference. After reproducing what was actually observed by means of computer simulations and defining the area affected by these geometric anomalies, other models were developed to test whether the degree of influence of the thinning on local stresses is itself influenced by the depth at which said thinning is located. On the basis of the results of all these simulations, it was possible to establish a series of recommendations as regards the location of advance headings in areas which are not seen to be affected by existing thinning of the seam. 2. Description of the area under study The study was carried out to the south of El Entrego, Asturias (Northern Spain) at a depth of approximately 1000 m, under the area known as El Campizu, close to a former opencast mine. The analyzed coal seam is called the “Julia” seam, which is accessed from the Sotón Pit. The area of workings where the observations were made is situated on the first and second sublevels between the Tenth and Sub-tenth levels at a depth of 967 m.
Fig. 1 shows a scheme of the mine and of the detailed situation of the area under study. Coal is mined by the “sublevels caving” method, a stoping method in which relatively thin blocks of ore are caused to cave in by successively undermining small panels. The seam is progressively mined by a series of sublevels spaced at vertical intervals of 20 to 30 m and occasionally more. Normally only one or two sublevels are developed at a time, beginning at the top of the ore body. Caving commences at the ends of the stopes by blasting out cuts and retreating in the same manner toward the raises. Fig. 2 shows a scheme of this method. The Julia seam belongs to the Central Coal Basin, which presents a thickness of around 6000 m and occupies an area of 1400 sq km. Within the Central Coal Basin, the area under study forms part of the Caudal-Nalón Unit, and within this unit it is included in the Sama Group Production Section. This section is also divided into four stretches. Fig. 3 shows the surface geology of the area under study. As can be seen, the area under study belongs to the Third Production Zone, which in turn is part of the Sama Group. This production zone, which belongs chronologically to the Pennsylvanian period and presents predominant slate and sandstone lithologies, is part of the María Luisa and Sotón veins (see Fig. 4), which have a thickness of 750 m and are the most extensively mined beds in the Asturian Coal Basin. The Julia seam forms part of the María Luisa coal bed. In the stratigraphic column, the areas that correspond to marine environments are marked in green, while the white areas represent continental environments (Suárez-Ruiz and Jiménez, 2004; Colmenero et al., 2008).
Fig. 1. Scheme of the mine.
Fig. 2. Sublevel caving scheme.
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Fig. 3. Geology of the area.
Julia seam has three hanging wall veins (San Gaspar, San Luís and María Luisa). The distances of the San Gaspar, San Luís and María Luisa veins to the Julia seam are respectively 30, 56 and 104 m. Their direction and dip have average values of 354/68 according to field work carried out the authors. The Julia seam presents a high dip, of around 68°, and is characterized by its great heterogeneity, as it is made up of three sub-layers of differentiated lithology: a roof vein, a floor vein and between the two a
strip of rock called “earth”. The contact coal–rock wall is filled with clay (around 5 cm in thickness). It is variable in thickness, there being areas in the First Level which may vary between more than 3 m and 0.3 m in the thinnest areas. Fig. 5 shows a plan view of the thickness of the seam along this level. 3. “In situ” phenomena analyzed During a safety study of the mining of the Julia seam, several phenomena related to significant increases in stress levels and deformations in the advance headings were observed. After analyzing numerous parameters and collecting information on similar phenomena that had occurred previously, the conclusion was reached that the main causes of these phenomena were anomalies in the geometry of the coal seam. Specifically, after analyzing in detail the mapping of the seam carried out over the years by those responsible for the mining work, it was seen that when the seam presented thinner stretches than normal, these tended to generated swelling. This swelling was especially significant in the floor of the advance headings as the sublevel caving of the upper level got closer. During the study of the Julia seam, an especially significant event was observed: during the night a crosscutter situated in the advance heading overturned due to sudden swelling of the floor vein. As was discovered later, just under the floor of this level there was a “thinning” of approximately 1 m in width and 2 m in height. After observing this phenomenon, the decision was taken to use biaxial vibrating wire stressmeters (Geokon, vibrating wire stressmeter series 4300 BX) (Geokon Inc., 2004) installed in boreholes to instrument another area of the seam where this type of geometric anomaly existed; specifically a thinning of approximately 1 m in width and 1 m in height. Weekly measurements were taken and it was seen that as the production workings on the upper level approached the area of the thinning, the
Fig. 4. Stratigraphic column.
Fig. 5. Plan view of the First Level of the Julia seam.
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Fig. 6. Horizontal stress variation measured with vibrating wire stressmeters near a thinning of 1 m in width and 1 m in height (C-26) and in a zone without thinning (C-14).
stressmeters recorded an increase in horizontal stress much higher than that due to normal caving workings. Once the area of the thinning had been passed, the stressmeter barely recorded variations, though decreases were not registered either. Blue line in Fig. 6 shows the variation in horizontal stress measured by the stressmeter in the area of the thinning. Pink line in Fig. 6 is a graph of the variations in stress recorded by the other stressmeter in an area without thinning. The increases of up to 4 MPa are due solely to the approach of the workings. The variation in vertical stress in the area surrounding the thinning was also measured, although important changes were not observed, only those due to the approach of the workings. On the basis of this analysis, numerical models were used to verify the influence that a thinning of different geometries in the seam would have in order to establish suitable prior safety measures. 4. Prior work Before starting to develop the numerical models, a series of tasks and tests had to be carried out to characterize both the coal seam and the surrounding rock mass in geotechnical terms.
Table 1 Results of uniaxial compressive strength test in coal. Test
Orientation of the sample
Uniaxial compressive strength (MPa)
Floor vein coal M-1–119 M-1–157 M-1–161
4.9 7.6 8.5
Roof Vein Coal T-1–119
4.5
“Earth” Ti-1–157
4.3
Undifferentiated coal I-1–144 I-1–157 I-1–161
6.4 8.1 4.9
As regards the coal, diverse uniaxial compressive strength tests were carried out with different orientations of the samples. That is, given that coal is characterized by the fact that it presents clearly oriented joints or “cleats”, some tests were carried out in which the load was applied perpendicularly to these cleats, while in others the load was applied in parallel. Table 1 shows the results of the tests carried out, indicating in each case the orientation of the cleats with respect to the applied load. The mean compressive strength for the tests carried out perpendicularly to the cleats is 7.65 MPa, while that of the tests carried out by applying the load parallel to the cleats is 4.65 MPa. Seeing as the models to analyze were developed considering pressure applied perpendicular to the cleats, only the data from the tests thus carried out were taken into account. The standard deviation and variance for these four tests (M-1–157, M-1–161, I-1–144 and I-1–157) are 0.91 and 0.83, respectively. On the other hand, data were available on the cohesion, friction angle and elasticity moduli of the coal, obtained from tests carried out in study surveys in preceding areas. The mean values of these properties for the coal are given in Table 2. To characterize the rock walls of the Julia seam in geotechnical terms, measurements were carried out with a Schmidt hammer type L (ASTM, 2005), obtaining the RMR of these materials by means of field observations. The uniaxial compressive strength, obtained from rebound hardness data, ranged between 64 MPa for the roof and 65.7 MPa for the floor. Note that the low values may be due to the fact that the materials had suffered cracking as a result of blasting work. According to field studies, the RMR of the rock walls is 75. It thus belongs to class II in Bieniawski's classification and may be classified as “good”. To define these rock wall properties, the RMR was calibrated by means of back analysis. That is, starting from the deformational behaviour observed in reality, according to which the cavity opened by sublevel caving in the Julia seam that remains unclosed is around 50 to 60 m in height, the geomechanical properties of the rock wall introduced in the corresponding model were those that reproduce such behaviour.
Table 3 Rock wall properties tested on calibration. Rock walls
Table 2 Properties of the coal. Coal Bulk modulus (K) (Pa)
Shear modulus (G) (Pa)
Cohesion (Pa)
Friction angle (°)
9.5E8
8.6E8
3.9E5
32
RMR 70 RMR 75 RMR 80
Bulk modulus (K) (Pa)
Shear modulus (G) (Pa)
Poisson's coefficient
Cohesion (Pa)
Friction angle (°)
3.3E10 4.1E10 5E10
1.5E10 1.9E10 2.3E10
0.3 0.3 0.3
5.5E6 6.6E6 8E6
33 34 35
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Fig. 7. Horizontal displacements. Model with RMR 75.
This simulation was carried out by means of a FLAC 2D model representing the Julia Seam and its rock walls from the surface to a depth of 1800 m. The calculation is divided into the following phases: 1. Stresses are first initialized in the model to simulate the stress state of the virgin rock mass, prior to the onset of mining work. 2. Subsequently, the Julia Seam is sublevel-caved up to the real height of mining work, allowing the model stabilize; i.e., until the stresses and displacements induced by the opening up of the cavity reach equilibrium. 3. In order for the introduced properties to provide a closer fit to the real ones, the model must stabilize after closing most of the cavity, except for the final lower 50–60 m, which remain semi-open during real mining work. We examined several RMRs for the whole rock mass (though starting off from a minimum value of 70), as both in-situ observations and resistance tests show that it constitutes a rock mass that can be catalogued, at the least, as ‘good’ according to Bieniawski's classification. We thus analysed models with an RMR of 70, 75 and 80. The properties corresponding to each of these are included in Table 3. Note that the cohesion, friction angle and tension properties were calculated according to the Hoek–Brown criterion (Hoek and Brown, 1998), which involves a more realistic modification of the values of these parameters as established in the original classification provided by Bieniawski (1989). The results obtained after evaluating the three models show that the properties that provide the best fit to the actual properties are those corresponding to RMR 75. The model calculated with these properties is the one that best simulates the deformation suffered by the rock mass after sublevel caving, leaving an open cavity of around 60 m after stress re-equilibration. Fig. 7 shows the horizontal displacements suffered by the model with RMR 75. Although examination of the Fig. 7 suggests that the sublevel caving cavity has been completely closed, the displacement values in
the colour caption of the figure show that RMR 75 constitutes the case in which the rock walls are in the most similar position to that found in reality, since the final 60 sublevel-caved metres remain unclosed. In addition to the geotechnical characterization of the coal and the rock walls, direct shear lab tests were also carried out to obtain the cohesion and friction angle of the clay in the contact coal–rock (around 5 cm in thickness), which must also be taken into account in the numerical model. The results of the five direct shear tests carried out are shown in Table 4. The standard deviation and variance of these tests are 18.2 and 334.5 for cohesion, and 2 and 4.3 for the friction angle. 5. Development and discussion of results Having obtained the strength properties of the coal and the rock walls, we shall now describe the working method followed in analyzing the influence of thinning on the local stress state. First, a 2-dimensional model was generated using the programme FLAC 2D which represents the advance heading of a sublevel and its nearest surroundings (some 40 m thick and 40 m high), as shown in Fig. 8. To simulate the existing stresses at the depth at which the sublevel is actually located, principal stresses are applied at the top and sides.
Table 4 Results of the direct shear tests. Test
Cohesion (kPa)
Friction angle (°)
M1 M2 M3 M4 M5
87 115 78 96 124
22 19 23 20 18
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Fig. 8. Scheme of the model created to simulate thinning in the Julia seam.
Maximum principal stress has been applied perpendicular to roof and floor. To simulate the sublevel caving workings in the upper level, the top of the coal seam is not subjected to any applied vertical loads. The values of these principal stresses were taken from models that simulate a greater portion of ground, developed to analyze the behaviour of the ground versus sublevel caving. The stress factor estimated from these models for the case under study is 1.5. Different parameters may be varied in this model, such as the properties of the coal seam, of the rock walls and of the coal–rock wall interface, the dimensions of the heading and especially those of the thinning, which may be varied both in height and in its angles with respect to the horizontal (see Fig. 8). The properties of the coal and the rock walls considered for the model are those described previously, while the properties employed for the coal–rock wall contact zone were those corresponding to direct shear test results of the clay in that zone. 5.1. Influence of the geometry of the thinning on the local stress-deformation state The first part of the research study focussed on the general analysis of the influence that a thinning of different geometries situated below an advance heading may have in order to assess to what extent it may give rise to phenomena of swelling and overstress in the floor of the sublevel. Seven different models were calculated, using the dimensions of the thinning given in Table 5. Model 7 is used as the standard for comparison with the other cases, as it is assumed that in this model the sublevel advances in an area in which the Julia seam presents a constant thickness of 3 m. To assess the degree to which thinning affects the surrounding of the advance heading, stresses as well as deformations were analyzed. 5.1.1. Analysis of stresses The stress analysis was carried out by measuring the value of the horizontal and vertical stresses below the floor of the advance heading in each model.
Fig. 9 shows an example of the graphical output of horizontal stresses generated by the programme FLAC 2D for Model 3. The stresses are represented by colours which indicate intervals of values, as specified in the legend on the left of the figures. Stress data are given in international system units. Likewise, the results for the other parameters, such as displacements, are also represented by means of iso-values, and are expressed in international system units. Fig. 10 shows the evolution of vertical and horizontal stresses evaluated at the centre of the coal seam. The origin of the x axis corresponds with the floor of the advance heading, evaluating from this point the stress that exists with the increasing depth of said floor. Fig. 11 shows how the axes of the graphs are considered. It can also be seen in Fig. 10 that, due to the existence of thinning, a substantial peak in horizontal stresses is produced just below the advance heading, whereas the vertical stresses remain practically constant after the decompression of the first few metres induced by the actual mining of the heading. We now discuss the results of the comparison of the evolution of stresses for each model, including Model 7, which has no thinning. Fig. 12 shows the comparison of horizontal stresses recorded for each model below the floor of the advance heading. As can be seen, three models present a substantial stress peak 3 m from the advance heading floor.
Table 5 Models for analyzing the influence of thinning on the Julia seam. Model
Angle of the thinning (°)
Thinning in the width (m)
Thinning in the height (m)
Model 1 Model 2 Model 3 Model 4 Model 5 Model 6 Model 7
30 30 30 30 30 30
1 2 1 2 1 2 Without thinning
1 1 2 2 3 3
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Fig. 9. Horizontal stresses for Model 3.
The overstress recorded in these three cases with respect to the model without thinning varies between 30 and 50 MPa; i.e. stress increases of between 200 and 300% are produced. From the Fig. 12 legend, it can be seen that the three models with the greatest overstress correspond to thinning of 1 m in width and 1 to 3 m in height. This implies that the greater the difference in width between the thinning and the seam itself, the greater the accumulation of tensions produced. Furthermore, considering a constant width, the increasing height of the thinning accentuates the increase in stresses. As can be seen in Fig. 12, the model with a thinning of 1 m in width and 1 m in height presents a stress increase of 30 MPa with respect to the
model without thinning. This result is similar to that obtained with vibrating wire stressmeter measurements, in which the difference between the stress increase in a zone near a thinning and the increase in stress in a zone without thinning was about 22 MPa. When the thinning is wider (Models with a width of 2 m), the recorded stress barely varies with respect to that of the model without thinning. Only when the height increases is a slight increase observed, in the order of 10 MPa, which supposes an increase of barely 40% with respect to the model without thinning (see Fig. 12). Fig. 13 shows the same type of representation for vertical stresses. The first thing to notice is that the values of the vertical stresses are much lower than those of the horizontal stresses in all the models. Moreover, the existence of thinning of different geometries does not suppose a substantial change in the magnitude of these stresses. Nevertheless, the thinning of less width (1 m) can be observed once more to have the greatest influence, not only in terms of magnitude, but also in the change in location of the stress peak, which is now some 4 m from the floor of the advance heading. 5.1.2. Analysis of displacements The overstress described in the previous section gives rise to displacements of a different type within each model. As the greater stresses are horizontal, more or less marked swelling of the floor tends to occur in all cases, in accordance with the effect shown in Fig. 14. Highly pronounced swelling of more than 30 cm is produced in all cases. This implies that these displacements are due not only to the overstress caused by the thinning, but also to the properties of the coal–rock wall contact zone, which are the same in all the models. 5.2. Influence of the depth of the workings
Fig. 10. Evolution of the stresses below the floor of the advance heading for Model 3.
The following stage of the research study was to generate and analyze a series of models which assessed the influence of the depth of the workings on the overstress generated by the actual geometry of the thinning. To test this influence, the aforementioned models presenting extreme results were chosen; that is, the model without thinning and
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Fig. 11. Axes considered in the graphs.
the model with a thinning of 1 m in width and 3 m in height, which was the one in which the greatest overstress was produced. These two models, initially calculated for a depth of around 950 m (the height above sea level of the current workings), were therefore recalculated for depths of 400 m and 1200 m. To analyze the influence of the depth, the evolution of the horizontal stresses below the floor of the advance heading was newly represented. Fig. 15 shows the results obtained for all the models at different depths. In view of the above stress graphs, there is no doubt that, in absolute terms, depth notably affects the value of the stress peak below the floor of the advance heading. However, if the models with and without thinning
are compared for each depth, it can be seen that the increase in stresses in the former with respect to the latter is always in the order of 250–300%. It therefore seems that the depth at which the thinning of the seam is located always produces the same relative increase in horizontal stress, although the starting out value of the model without thinning logically increases with increasing depth. 6. Conclusions The research carried out has allowed us to reconstruct and explain by means of numerical models the phenomena of swelling of the floor of an advance heading in coal observed in a mine.
Fig. 12. Horizontal stresses below the floor of the advance heading for each model.
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Fig. 13. Vertical stresses below the floor of the advance heading for each model.
The conclusion drawn was that these excessive swellings are the result of the existence of anomalies in the dimensions of the seam being mined, specifically “thinning”, which lead to a major concentration of horizontal stresses that tend to “thrust” the coal in the floor up into the open cavity. The phenomenon of swelling and overstress observed in the mine was reproduced by means of a numerical model, obtaining a magnitude of displacements and stresses similar to those actually measured. This fact corroborates the initial hypothesis that the existence of a thinning of the seam is what conditions phenomena of this type. The relation existing between the generated overstress and the geometry of the thinning were likewise assessed, it being shown that
the greater the variation in the thickness of the seam (more thinning), the greater the stresses generated in that area. Furthermore, for the same degree of thinning of the seam, the greater the height of the thinning, the greater the stresses. The vertical stresses barely vary in magnitude, regardless of the geometry of the thinning, although their maximum values tend to concentrate closer to the floor than in the case of the model without thinning. Finally, it was also shown that, regardless of the depth at which the thinning is located, for any given geometry, the increase in horizontal stresses with respect to the standard model is always of the same
Fig. 14. Vertical displacements in the area of the thinning.
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Fig. 15. Horizontal stresses in the models at different depths.
order. This implies that it constitutes an overstress phenomenon that is exclusively related to the geometry of the seam and not to general mining conditions, such as the depth of the working. The results reported in this paper may serve as the basis for establishing safety measures in the mining of coal seams of the kind analyzed, as the major stress concentrations created in the surroundings of a geometrical anomaly such as that described here mean that any workings carried out in that area suppose a risk. Acknowledgements The information and technical and logistic support provided by those responsible for the Sotón Pit (HUNOSA), where mining work takes place in the area under study, was of inestimable help in the carrying out of this research study. References ASTM, 2005. ASTM, standard test method for determination of rock hardness by rebound hammer method. ASTM Stand (2005). p. D 5873-05. Bieniawski, Z.T., 1989. Engineering Rock Mass Classifications. Wiley, Chichester, p. 272. Cao, Y., He, D., Glick, D., 2001. Coal and gas outbursts in footwalls of reverse faults. International Journal of Coal Geology 48 (1–2), 47–63. Cao, Y., Davis, A., Liu, R., Liu, X., Zhang, Y., 2003. The influence of tectonic deformation on some geochemical properties of coals: a possible indicator of outburst potential. International Journal of Coal Geology 53, 69–79. Colmenero, J.R., Suárez-Ruiz, I., Fernández-Suárez, J., Barba, P., Llorens, T., 2008. Genesis and rank distribution of Upper Carboniferous coal basins in the Cantabrian Mountains, Northern Spain. International Journal of Coal Geology 76, 187–204. Díaz Aguado, M.B., González Nicieza, C., 2007. Control and prevention of gas outbursts in coal mines, Riosa–Olloniego coalfield, Spain. International Journal of Coal Geology 69, 253–266.
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