C. Drebenstedt and R. Singhal (eds.), Mine Planning and Equipment Selection, 501 DOI: 10.1007/978-3-319-02678-7_48, © Springer International Publishing Switzerland 2014

Analysis of Roof Caving Characteristics at a Coal Mine by Using Full Scale 3D Numerical Modeling

Bahtiyar Ünver, Mehmet Ali Hindistan, Erhan Tercan, Rohola Hasanpour, Hamid Chakeri, and Güneş Ertunç

Hacettepe University, Department of Mining Engineering, Beytepe Ankara 06800 Turkey

Abstract. Regular and efficient caving of roof strata behind is essential in maintaining a trouble free operation in underground longwall mining when especially mined area is left for caving. As the face advances, roof strata should be regularly caved forming a goaf as homogenous as possible. In case of having an uncaved roof behind the coal face, load on the face increases dramatically leading to serious fall of roof conditions. Therefore it is of paramout importance to have the roof regularly caved behind a longwall face. This paper presents the problems encountered at a coal mine in Turkey due to high face pressures and subsequent flow of roof at the face roof junction. The height of the fully mechanised longwall face is 4.5 m. Sliding of face coal and later the fall of roof strata in front of the shields created serious stability and safety problems in the mine. Stopes opened in the roof had to be filled by usign forepoling, foam and concrete. This rescue operation had to be completed securely before starting of the cutting operation at the longwall face. Obviously rate of production of the longwall face has been severly declined during this period. There were a couple of reasons for having such a difficult condition in the mine. The longwall panel was located near to a syncline axis leading to high tectonic stresses. There were lots of small faults through the working face. Longwall face was extremely loaded by a very strong limestone layer having a thickness of up to 80 m located at 120 m above the coal seam. Although the strata between the limestone layer and the coal seam has a readily caving characteristics, the limestone caved at long intervals causing high face pressures due to its cantilever beam effect. Moreover during caving of the limestone, severe dynamic loads are experienced in the vicinity of longwall face deteriorating stability conditions. Therefore it was decided to model the effect of limestone layer’s behaviour by means of numerical modelling. A full scale model was created in accordance with all geometrical conditions and operational parameters by using FLAC3D software. The face advance is also simulated on the model. Stress and deformation state of the coal face, surrounding rock and especially the problematic limestone layer are analysed. To solve the problem, a blasting pattern is selected to weaken the limestone layer by using drill

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holes opened from the surface. This paper presents the numerical modelling results in relation to selection of the best blasthole geometry to decrease loading on the face and hence maintain a safe, efficient and stable longwall operation.

Keywords: modeling, roof caving, stability, FLAC3D, blasting.

1 Introduction

In the mechanized underground coal production systems, one of the major problems is high stress levels on the units. This may occur when the hanging wall formation doesn’t fail as the longwall face advancing. Hanging wall strata, which is not fail in time, acts like a beam and as a result, cantilever condition causes increase in stresses.

Recent developments in computer technology and release of mining related softwares have pushed the researchers to study mining related problems in more details by using these softwares in numerical modeling together with physical models and analytical solutions. Yaşitli and Ünver [1, 2] studied top coal caving behavior of a thick coal mine in Turkey by using FLAC3D in their models. They suggested a special pre-fracture blasting strategy just sufficient enough to form cracks in the top coal by means of comparing with the results of numerical modeling. Noroozi et al. [3] compared the results of numerical and analytical models to predict the breaking characteristics of immediate roof in longwall mines. They tested their models with reference to three coal mines of Iran and India and found that the proposed model can be used to determine the periodic roof weighing interval in coal mines. Alehossein and Poulsen [4] developed a yield and caveability criterion on the basis of in situ conditions in the top coal in advance of the mining face and behind the supports. They combined yielding and caving effects in a single number called caving number. Gale and Nemick [5] used computer simulation method to understand strata caving characteristics under various geological conditions and its impact on longwall mining operations. Their system also allows a prediction of the monitoring data to use for early warning of weighting events. Singh and Singh [6] proposed a numerical approach to predict the progressive caving behavior of strata and performance of powered roof support in a given strata condition. They tested the model for 23 different strata conditions from different longwall panels and verified with their field observations.

This paper presents only a part of the numerical modeling study. To compare the results of numerical model with field observations, a system of borehole extensometer is built at the field to monitor the downward movements of each layer. Additionally a set of strain gauges are installed in underground to predict how stresses change as the face advance. These results are not presented here since the measurements are still carried out at the field.

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2 Çayırhan Lignite Mine and Sector G

Çayırhan lignite seams, two seams separated from each by 0.5-0.7 m thick parting, are located in Boyalı formation (fig. 1). Since the thickness of parting is not thick, top coal (1.6 m) and bottom coal (1.7 m) are produced in single cut by a double drum shearer.

Fig. 1 A generalized stratigraphy of Çayırhan Lignite Mine

Fig. 2 Lay out of longwall panels in Sector G in Çayırhan Lignite Mine

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In the longwall panel “G-03” in Çayırhan high stress levels are observed because of the tectonic characteristics of the field and syncline axis (fig. 2). For this reason, production rate decreased dramatically. Longwall panels in the mine site are planned and oriented with respect to the surrounding faults. Experimental and numerical model analysis shows that the problem is related to the caving mechanism of the hanging wall.

One of the solutions is pre-blasting of roof in order to achieve better conditions in terms of stress distributions. The full scale of the mine is numerically modeled with FLAC3D software. With models, the stress distributions during production and blasting stages can be interpreted. Input parameters of the model (geometry, geomechanic) are defined according to the characteristics of Çayirhan mine.

3 Numerical Modeling

Geotechnical and structural geology data are obtained from field studies and measurements. Geomechanical properties of rock formations are collected from drillhole cores. The modeling philosophy of the full scale model of G sector is as follows:

− face advance interval is set as 2, 5, and 10 m respectively and there is no significant diferrence between the results. Thus, face advance interval is set as 10 m for the solutions

− for initial loading conditions, hydrostatic stress condition is set (σh = σv)

Since the model is constructed as full scale, all the formations are introduced into the model. Also, the gob material is taken into account as “loose zone”.

In order to define blasting pattern, various patterns are modelled and results are analysed. These blasting patterns are classified as “front of the face” and “back of the face” respectively. The aim of the blasting is not the breaking of the rock, but the weaken the limestone layer. In order to reflect blasting effect, region having 20 m diameter around blasthole is modeled as weak rock.

3D modeling studies are carried out real-time with the production schedule of G03 panel. For this reason, all of the operations regarding G03 panel is introduced into model (fig. 3). These operations were as follows:

− blasting behind the face − blasting in front of the face (line) − blasting in front of the face (V type)

3.1 Results

In this section, stress distributions around G03 panel are given. Two cases, with blasting and without blasting, are compared with respect to the stress distributions in Figure 4. These results are given for various face advance distances from the start point of face (0 m).

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Fig. 3 Different loosening blasting models; line behind face (top), line pattern in front of the face (middle) and V pattern in front of the face (bottom)

In production schedule, limestone layer is weakened by the V type blasting pattern applied in front of the face. The main reason why line type blasting pattern is not applied is to prevent the interaction with the face. Figure 5 shows the stress distributions obtained from the blasting model.

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Fig. 4 Vertical stress distribution for blasted and unblasted analysis

Fig. 5 Vertical stress distribution along face for V pattern blasting

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Results show that when the face is in 490 m, blasting operation has positive effect on the stress distribution. On contrary, in 550 m position, blasting causes increase in stresses acting on face.

In general, most of the blasting operations in front of the face have reduced the stresses, but in particular distances blasting have negative effect as stress increases on the face.

Since there is no general agreement on front of the face blasting operation has positive effect on stress reduction, different types of blasting patterns are carried out in modeling. These patterns are staggered and I-IL type patterns.

3.1.1 Staggered Pattern

In front of the face, staggered pattern blast holes apart from each other with a distance 50-60 m are modeled and results are shown in Figure 6.

The results show that, although the limestone layer is weakened by the effect of blasting, there is no regular reduction in distribution which is not desirable.

Fig. 6 Vertical stress distribution along face for staggered blasting pattern

3.1.2 I-IL Pattern

In previous models, most of the different “front of the face” blasting patterns do not give the desirable stress distributions. As an alternative, blast holes located “back of the face” option is modeled and results are interpreted in detail.

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In this type blasting operation, blasting is not only done in the back of the face but also sides of the panel are assumed to be fractured by the blasting and introduced to the model. The stress distribution of this pattern when longwall face is at 230 m distance is shown in Figure 7.

I-IL blasting pattern results show that there is a significant decrease in stress distribution acting on the longwall face.

Fig. 7 Vertical stress distribution along face for I-IL pattern blasting

4 Conclusions

In the study, it can be concluded that the main reason of problems is the high tectonic stresses that is caused by the syncline axis. These problems are listed as flow of roof at the face roof junction and sliding of face coal and later the fall of roof strata in front of the shields.

Thick limestone layer (80-100 m thick) does not easily cave. It is known from exploration drills that this massive layer contains water and also has few joints.

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Besides, pores inside the layer allows deformation, and thus high stress concentrations do not occur. This phenomenon has direct effect on caving behaviour of limestone layer. Another factor effecting its caving is horizontal stresses in which layer is acting like a beam.

Recommendations for the problems regarding caving of the layer are listed as follows:

− monitoring of the roof movements, − investigation of forces acting on longwall face caused by caving, and − weakening the limestone layer by blasting of drill holes opened from the

surface.

References

[1] Yasitli, N.E., Ünver, B.: 3D numerical modeling of longwall mining with top-coal cav-ing. Int. J. of Rock Mechanics and Mining Sciences 42, 219–235 (2005), doi:10.1016/j.ijrmms.2004.08.007

[2] Ünver, B., Yasitli, N.E.: Modelling of strata movement with a special reference to cav-ing mechanism in thick seam coal mining. Int. J. of Coal Geology 66, 227–252 (2005), doi:0.1016j.coal.2005.05.008

[3] Noroozi, A., et al.: A modelfor determining the breaking characteristics of immediate roof in longwall mining. Yerbilimleri Bulletin for Earth Sciences 66(2), 193–204 (2012)

[4] Alehossein, H., Poulsen, B.A.: Stress analysis of longwall top coal caving. Int. J. of Rock Mechanics and Mining Sciences 47, 30–41 (2010), doi:10.1016/j.ijrmms.2009.07.004

[5] Gale, W.J., Nemick, J.A.: Prediction of strata caving characteristics and its impact on longwall operation. In: Aziz, N. (ed.) Coal 1998: Coal Operator’s Conference, Univ. of Wollongong & the Australian Institute of Mining and Metallurgy (1998)

[6] Singh, G.S.P., Singh, U.K.: A numerical modeling approach for assessment of progres-sive caving of strata and performance of hydraulic support in longwall workings. Com-puters and Geotechnics 36, 1142–1156 (2009), doi:10.1016/j.compgeo.2009.05.001