Abstractcarried out at the mine sites where the rock strengths are medium to strong (Keilich, Seedsman, & Aziz, 2006;Zhang, Mitra, & Hebblewhite, 2006;Xu et al., 2013), only few researches have been carried out at the mine sites where the rocks are weak Sasaoka et al., 2015b). Therefore, the understanding of surface subsidence in poor ground conditions has still been limited.The objective of this research was to study the characteristics of surface subsidence induced by longwall mining under poor ground conditions in Indonesia. The effect of single panel and multi-panel mining at various depths, the influence of longwall panel and pillar widths, and the impact of backfill materials, were studied by means of numerical simulation using a three dimensional finite difference code "FLAC3D". The angle of draw (AoD) and maximum surface subsidence (S max ) were used to describe the characteristics of subsidence at the surface. MethodThe characteristics of surface subsidence were studied numerically using a three dimensional finite difference code "FLAC3D". FLAC3D is a numerical software which is widely used for analyzing stress and deformation around surface and underground openings conducted in both soil and rock. The software utilizes an explicit finite-difference formulation that can model complex behaviors of three-dimensional geomechanical problems (http://www.itascacg.com/software/flac3d, 2017). Effects of various mining conditions were considered in this study. Several models were constructed for numerical simulations. The models were 1330 m in width, 2000 m in length, and with various heights depending upon the mining depths. An example of a 200 m mining depth model is illustrated in Figure 4. The bottom of the model was fixed in the vertical direction, the sides were fixed in the horizontal direction, and the surface was set free in all directions. The vertical stress component was modeled as a function of overburden thickness or mining depth (P v =γH, γ is unit weight of overburden, and H is overburden thickness) (Hoek & Brown, 1980;Hoek, Kaiser, & Bawden, 1993;Hoek, 2006), while the horizontal stress component was assumed to be equal to the vertical stress. The elasto-plastic Mohr-Coulomb criterion was used as a failure criterion throughout the analyses. Table 1. In simulation, a coal panel was extracted step by step. After the excavation face moved forward, the caved area behind the coal face was filled by a very soft goaf material. The excavation steps were repeated until the coal panel was entirely extracted. ResultsIn EffectThe subsid three pane 130 m and increased w As the sec AoD exten because of Figure 8. From the results, it was observed that the less AoD and S max were generated after the first panel was mined, while the more AoD and S max were generated after the second and subsequent panels were mined. However, in case of a 50 m mining depth, the AoD and S max remained unchanged after three panels were mined. The reason of this phenomena can be explained by the ratio of pillar width to mining d...
Abstract:The purpose of this research is to study the effect of longwall mining on the stability of main roadway in the underground coal mine. The PT GDM (Gerbang Daya Mandiri) underground coal mine in Indonesia, where the rocks are weak, was selected as a representative study site. To accomplish the objective of the research, the finite difference code software FLAC3D was used as a tool for the numerical simulations. The longwall mining of several panel and barrier pillar widths at various depths was simulated and discussed. Based on the simulation results, it indicates that the effect of coal panel extraction on the main roadway stability depends on the width of panel and barrier pillar. The greatest effect occurs when the large panel width and the small barrier pillar width are applied, whereas the smallest effect happens when the narrow panel width and the large barrier pillar width are adopted. In this paper, therefore, to maintain the stability of the main roadway with the aim of maximizing the coal recovery, the appropriate size of panel and barrier pillar width is proposed for each mining depth for this underground coal mine.
This paper focuses on the stability analysis and support design of the coal mine tunnel excavated in weak rock mass in an Indonesian underground coal mine through numerical simulations using the FLAC3D software. The PT Gerbang Daya Mandiri (GDM) coal mine situated in Indonesia was selected as a mine site in this study. According to the results of a series of numerical simulations, the stability of the mine tunnel decreases by increasing the depth and stress ratio. Ground control problems, for example falling roof, sidewall collapse, and floor heave are expected unless an appropriate support system is anticipated. Three support systems, including friction rockbolt, steel arch, and shotcrete are discussed as methods to stabilize the roof and sidewalls of the mine tunnel. From the simulated results, the steel arch is considered to be the most effective support method when compared with other support systems. The steel arch which is installed with closer space and larger crosssection delivers a better stability control to the roof and sidewalls of the mine tunnel. Although the stability of the roof and sidewalls of the mine tunnel can be maintained effectively by the steel arch support, the occurrence of floor heave is expected when the mining depth is increased. To control the floor stability of the mine tunnel, three techniques by applying cablebolt, invert-arch floor, and grooving method are therefore investigated and discussed. Based on simulated results, the heaving of the floor is well controlled after the cablebolt, invert-arch floor, and grooving methods are applied. Nevertheless, it is found that controlling the floor heave by cablebolt support could be the most suitable method comparing with other support systems in terms of the installation process, providing flat and safe working conditions of the floor, and economy. Additionally, the cablebolt with closer row space and longer length works more effectively to control the heaving problem of the floor. Keyword
This paper attempts to predict the surface subsidence induced by multi-seam longwall mining in the PT Gerbang Daya Mandiri (GDM) underground coal mine in Indonesia. Several numerical models of multi-seam longwall mining under various depths were built in the finite difference code software “FLAC3D” which was used as a tool for numerical simulations. Effect of mining sequence and influence of lower seam mining were firstly investigated. The angle of draw (AoD) and maximum surface subsidence (Smax) were used to describe characteristics of the surface subsidence. Based on simulated results, it is indicated that the undermining provides a better mining sequence in multi-seam longwall mining compared to the overmining. Mining the coal seam in an undermining order will not cause any difficult mining conditions in a lower seam, whereas some ground control problems in an upper seam are expected when the coal seam is mined in an overmining order. Under all mining depths in the undermining, extracting the lower seam panels significantly influences the magnitude of surface subsidence. The AoD and Smax increase significantly after all panels in the lower seam is mined. This indicates that very large surface subsidence is expected when multi-seam mining is applied at GDM underground coal mine. An application of some countermeasures such as adopting a large pillar width and a small panel width is suggested in this underground coal mine in order to minimize the surface subsidence caused by multi-seam longwall mining. Minimizing the surface subsidence by adopting a large pillar width and a small panel width is therefore numerically investigated in this paper. Based on simulated results, it is found that the AoD and Smax decrease significantly when larger pillar width and narrower panel width are adopted. The use of larger pillar width and narrower panel width result in smaller AoD and Smax.
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