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...
