Estimation of limit angle using laminated displacement discontinuity analysis in the Soma coal field, Western Turkey

doi:10.1016/j.ijrmms.2003.01.002

International Journal of Rock Mechanics and Mining Sciences

Volume 41, Issue 4, June 2004, Pages 547-556

https://doi.org/10.1016/j.ijrmms.2003.01.002 Get rights and content

Abstract

Determination of the limit angle at the Manisa-Soma-Eynez coal field, a major coal production area in Turkey, is difficult because of the presence of a landslide caused by the waste from a neighboring open-pit site being dumped over the underground mine at the Soma-Eynez basin. In fact, it is not possible to determine the effect of extraction on the surface from field studies. Thus, in order to determine the limit angle, in-situ subsidence measurements, geophysical measurements, rock-based tests in the laboratory and the data obtained from a physical model have supported numerical modeling studies. The software, LAMODEL, used for the numerical modeling was developed by National Institute for Occupational Safety and Health (NIOSH), and is based on the Boundary Element Method. The results obtained from modeling have been found to be compatible with other studies and as a result the limit angle has been determined.

Introduction

Manisa-Soma-Eynez coal field is in a major coal production area in Turkey and suffers from subsidence problems due to underground mining. Coal mining has been practiced in this region for a period of at least 100 years by mostly the non-mechanized longwall method of extraction. The area lies about 18 km south of the Soma district in western Anatolia. The distance between Soma and the provincial center of Manisa is about 93 km. The lignite deposit of south-western Turkey contains a series of north-east/south-west trending Tertiary basins. The Miocene deposits rest unconformably on Mesozoic base rocks. Pliocene deposits rest conformably on the Miocene and contain two poorly developed lignite seams designated as KP1 and KP2. The stratigraphy of the Soma region is summarized in the general stratigraphic column shown in Fig. 1 [1].

Underground coal production is conducted in Neogene aged sedimentary rocks to the west of Eynez village. The Soma-Eynez basin contains two thick Miocene lignite seams designated as KM2 and KM3 in Fig. 1. As a result of the coal production at a depth of 200 m, large-scale subsidence cracks observed as 2–2.5 m opening widths on the surface during the studies. In the KM2 coal bed, the seam ranges in thicknesses from 15 to 25 m, with an average extractable thickness of about 18 m. A manual vertical double-slice-caving longwall mining method is employed for the main production at the mine. In this method, the face area is maintained at about 2 m. high using hydraulic steel props and wooden posts. The coal is extracted from the face by drilling and blasting and the additional lignite thickness above the supports is recovered by caving behind the face.

Numerical techniques offer some possibilities for subsidence modeling, however, these are invariably fraught with some difficulties. Because of these difficulties, the influence function method has been resorted to as a more convenient and simpler alternative [2]. However, it is necessary to comment on the limits of the affected zone in order to use this method. Since the waste damp area of the open pits is also located in the same region, exact determination of affected area could not be performed. Therefore, a numerical technique has been adopted. The goal of this study is to investigate the subsidence amount, affected area due to subsidence and limit angle during recovery of the safety pillars for the shaft and the main drift. A displacement discontinuity variation of the boundary element technique has frequently been the method of choice. For this purpose, various models of the extraction of the safety pillars are developed and analyzed using the LAMODEL program. This displacement-discontinuity program incorporating a laminated media has recently been developed by National Institute of Occupational Safety and Health, USA (NIOSH).

In this study, the location of the subsidence area has been determined by laboratory and in-situ studies. It was not possible to determine the limit angle due to the existence of dumped waste from the open pits on around the studied field. The value of break angle, however, was ascertained from subsidence cracks and the end of production border by using topographical measurements, geophysical studies, physical modeling at different locations of the Eynez-Soma Coal Mine, and the limit angle has been determined by calibrated Laminated Displacement Discontinuity Analysis.

Section snippets

Subsidence theory

The removal of material from the earth's crust by underground mining creates potential for ground movement and consequential deformation of the surface. The circumstances under which this may arise varies widely, the main parameters being as follows.

•
The geometry of the mineral deposit.
•
The method of mining.
•
The nature of the mineral deposit and the overlying strata.

The general surface movement pattern associated with a wide underground mining such as the longwall mining method is shown in Fig. 2

Laminated displacement discontinuity analysis for estimating the limit angle at the Soma coal field

The numerical modeling has been carried out using the LAMODEL code based on the Laminated Displacement Discontinuity method. To analyze the displacements and stresses associated with the extraction of large tabular deposits such as coal, potash and other thin vein-type deposits, the displacement-discontinuity version of the boundary-element technique has frequently been the method of choice. In the displacement-discontinuity approach, the mining horizon is treated mathematically as a

Field studies

There are several waste dump areas and a resulting landslide due to various open pit mines within the field studied (Fig. 6). Therefore, the determination of the area affected by subsidence is not possible by field studies. The limit angle, however, has been determined by modeling of the field data, which is calibrated according to previous field studies [1], [3], [4], [5], [6], [7], [8], [14], [15], [18], [19], [20], [21], [22].

Physical model study

Physical models employing sand, clay, coal fragments and plaster materials of carefully determined strength and deformation properties have been used at the Department of Mining Engineering, Dokuz Eylul University, in Izmir, to investigate caving and subsidence behavior of longwall extractions in coal seams. The models caved and subsided under their own weight and consequently reflected the natural behavior of the longwall mining process. Fig. 10 shows the physical model for Eynez-Soma Coal

Conclusions

The boundaries of the area influenced by the active subsidence are usually drawn after the determination of the locations of surface subsidence fractures. In this research, field observations, topographical studies and physical model study have been applied to determine the break angle of the field under investigation.

According to the results of the Displacement Discontinuity Analyses on the shaft grid line, the model indicated that the surface did not subside until the third mining step. On

Acknowledgements

The authors would like to express their gratitude to Dr. C. Mark for his valuable contributions and reviewing, to Dr. K. Heasly, for his valuable contributions and reviewing, to Dr. A.T. Iannacchione for providing the research facilities in National Institute for Occupational Safety and Health (NIOSH), to TUBITAK (The Scientific and Technical Research Council of Turkey) for their financial support, and to A. Guney (Wisconsin University, Department of Mining Engineering) for editing as a native

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