In the U.S., it was found that many of the reported ground control problems at longwall face were related to poor geologic conditions such as sandstone channel, faults, fold, and so on. In this study, the effects of sandstone channel on longwall face stability were evaluated using finite element models, which include a 600-ton two-leg real size shield model. It was found that, stress distribution in coal strata was greatly altered due to the existence of large-size sandstone channel in the roof. Cousequently, shield loading, roof sagging, face closure, floor heave, and floor punching were effected significantly as longwall face advanced toward the channel. Analysis reveals that shield leg started to yield from a certain distance (158 ft under specific condition in this study) and continued to yield until the longwall face entered low stress zone, which is below the channel body. Under this case, roof-to-floor convergence increased sharply and shield structure became unstable. Base on analysis results, a method used to determine optimum distance of longwall face from sandstone channel is proposed.
With improvement of equipment reliability, geologic anomalies such as sandstone channel, crevasse splays, mold-and-cast structures, faults, folds etc are gradually found to be important factors that cause ground control problems, safety hazards, and production downtime at longwall face. Study shows that (Christopher, 1991), poor geological conditions are major safety hazard and source of downtime at a high percentage in longwall mines. Although sandstone channels were commonly encountered in underground longwall mine, design of ground control system at work face often ignores the effects of fandstone channel. Severe stability problems may be caused at longwall face by sandstone channel depositing in coal mine roof. Firstly, the margins of channel deposit are usually separated from surrounding strata by slickensided interfaces formed by differential compression (Valois, 1993). Such slicken sided interfaces cause adjacent and underlying beds in the roof to separate and fall after the coal is extracted, resulting in costly cleanups, production delays and losses, injuries, and even fatalities. Also, sandstone channel itself can cause stress distribution problems when it is in direct contact with the coal seam. In addition, the massive sandstone that will not cave behind the shield support may result in stress accumulation and disastrous accidents at longwall face. Due to extreme difficulty in measuring and/or evaluating the effects of sandstone channel On work face in the field, long wall panels usually do not extend near to channel body and under most of the cases stop prematurely once the existence of sandstone channel is determined. In this study, the effects of a large-size sandstone channel on longwall face stability in terms of shield loading, stress distribution, floor heaving, floor punching, and face closure were analyzed and discussed.
Generalized geological condition of the IIIinois Coal Basin, where deposition of sandstone channel is typical, was employed in finite element modeling. A geologic column of coal measure strata is shown in Fig 1.
