Reasonable width of district sublevel coal-rock pillar is the premise of scientific mining for a safe and high efficient mine. they all indicate that, the setting width of forty meters coal pillar is obviously large in Ningdong test mine now, by the analysis of FLAC3Dnumerical simulation based on ideal elastoplastic theory and engineering practice results of many mines. The in situ test under the mine involves real-time monitoring of sixteen hollow inclusions' 3-D strain, dynamic monitoring of surrounding rock stress and deformation in test roadways with different coal pillar width (10m & 20m) before and after the mining-induced influence by the workface. Combination of the above theoretical calculations, simulation and underground test results are considered to determine a reasonable width scope of district sublevel coal pillar design from 8.9 meters to 11.9 meters in the test mine under specific geological condition, which provides some guiding role for width design method of district sublevel coal-rock pillar under similar mining conditions.
Scientific and rational setting of coal-rock pillar width of district sublevel can improve resource recovery, and also favor gob-side entry retained support and maintenance of roadways layout, so as to reduce materials economic input such as bolt, cable with metal stents and so on [1, 2]. Coal pillar design is based on the coal pillar strength theory and the law of coal pillar stress distribution, there has been more than a century since Bunting (1911) firstly proposed the empirical formula to calculate coal pillar strength [3], Many major countries with coal mining gradually carry out laboratory strength testing of coal-rock, in situ coal mass even wide coal pillar strength test, simultaneously a variety of methods of numerical simulation analysis and theoretical derivation, basing on the purpose of mining safety and resource recovery improvement. Constraint theory of two districts put forward by A. H. Wilson (1972) based on the coal pillar 3-D strength characteristics, overcomes the shortcomings of some other methods [4], which currently has been widely used in the coal mining design [1, 5, 6]. Zheng et al. (2012) studied stress distribution of coal pillar with gob-side entry driving in the process of excavation and mining [7]. Xi et al (2008) proposed combined methods of field measurements and numerical simulation to determine the coal pillar width [8]. Bai et al (2004) analyzed the relationship among the stability of narrow coal pillar, the rock deformation of roadway driving along goaf in fully mechanized caving face, the strength of bolt support, the mechanical property of coal seam and the pillar width through numerical simulation [9].
