Pittsburgh, PA: U.S. Department of the Interior, Bureau of Mines, TN 427, 1994 Jan; :1-2
Objective: Provide longwall operators with practical information and guidelines concerning mine design to reduce problems associated with the interaction of adjacent workings in multiple-seam longwall mines. Background: The high productivity achieved by longwall mining demonstrates its potential to provide a substantial segment of underground coal production. Longwall mines produce more than 30 pct of underground coal, up from 5 pct just 15 years ago. However, the continued growth of this mining method without appropriate multiple· seam planning may increase the cost and risk of mining. Research by the U.S. Bureau of Mines (USBM) indicates that 25 longwall mines (32 pct of the total number in the United States) have mining in adjacent coalbeds either above or below. About half of these longwall mines report some type of interaction problem with the adjacent workings, such as caving due to subsidence. Optimization of mine design factors is the best means for controlling interactions between operations in adjacent seams. To avoid higher mining costs, operators should focus on adopting practices and procedures that prevent and control interactions in multiple seams. The USBM, in an effort to improve longwall mine planning, has been investigating multiple-seam longwall design and development. Approach: There are three primary design factors to consider for longwall mining of multiple seams, which are very closely related and should be weighed equally for effective mine planning. These factors are as follows: first, the sequence or order in which the seams will be mined, which will determine the type of interaction; second, the design of the gate road pillars, which will define the magnitude of the interaction; and third, the layout of the gate roads and longwall panels, which will define the location of the interaction. Other parameters fixed by the geologic environment such as depth and interburden thickness will influence interaction magnitude and location and therefore must also be considered in the design process. The successful design of multiple-seam longwall layouts depends primarily on the operator's intrinsic knowledge of local geology and strata behavior. Empirical investigations involving case study documentation and analysis have constituted most of the USBM research and have provided important information in the development of design procedures. However, computer-based numerical models are gaining more research attention because they can provide insight into relative stress transfer and distribution in multiple seams. The analysis of different designs using numerical models has considerable potential in helping operators find solutions to complex multiple-seam interactive problems. The USBM's MULSIM/NL model is a boundary element model for calculating stresses and displacements in tabular deposits. The MULSIM/NL model has been used to evaluate stress distribution and transfer for design problems that are commonly encountered in multiple-seam longwall layouts. Accomplishments: Some primary findings from these investigations are summarized as follows: 1. To ensure optimum ground conditions, coalbeds should be longwall mined in descending order. This extraction order prevents coalbeds from being damaged by caving and other strata displacements caused by subsidence. 2. Two basic approaches are available for gate road pillar design: yield pillars and conventional pillars. For the most part, further study is required to assess the performance of yield pillars under multiple-seam conditions. However, there are several conventional pillar design approaches available to the operator that have demonstrated their success in the field. 3. Gate roads and panels can be oriented in either a superpositioned or an offset arrangement, depending on mining conditions. Superpositioned gate roads will usually produce high stress concentrations in gate road pillars, and special design considerations may be required. Offset arrangements will improve gate road conditions, but may create stress concentrations on the longwall face. 4. Common high-stress zones encountered in multiple-seam longwall operations are gob-solid coal boundaries, where gob meets solid coal or a pillar line. To reduce stress concentrations on the longwall face and gate road pillars, the longwall panel should be mined from the gob to the solid side of the boundary, keeping the approach angle under 30 degrees.
Pittsburgh, PA: U.S. Department of the Interior, Bureau of Mines, TN 427