Applying the ground reaction curve concept to the assessment of shield support performance in longwall faces

The longwall method is used in many countries around the world in the underground extraction of coal seams. This method enables significantly improved production results to be achieved when compared to the bord and pillar mining system. However, this mining method requires higher capital investment compared to bord and pillar mining. One of the essential elements required to achieve the anticipated level of production from longwall panels is good shield-strata interaction. This means that the shields used in the longwall faces should have an adequate capacity to ensure the maintenance of roof stability in the longwall working. The issue of determining shield capacity has been the goal of research in many countries resulting in a number of different methods for calculating the required capacity of shields. In recent years, numerical modeling and ground reaction curves (GRCs) have been used to determine adequate shield capacity. An important factor to be considered in analyses using the concept of GRC for shield support selection for ground and mining conditions is roof convergence. This paper presents an analysis of shield-roof strata interaction in two longwall panels with natural roof caving in the gob using the concept of GRC. The GRCs for the specific mining conditions in the two longwall faces were determined by means of numerical modeling using Phase2 software. Performance characteristics of two-leg shields were obtained from underground measurements conducted continuously during the retreat of the longwall panels. In a specially prepared measuring shield, the changes in the leg pressures were measured. In addition, the changes in shield geometry were assessed by means of inclinometers. For the two longwall panels studied, the selected variations of leg pressures and changes of shield height in time are presented for a single shield's cycle during the longwall operations for shield advance, setting, loading, and lowering. An analysis of the interaction between the shield and the roof strata rock mass was performed based on a comparison of the GRC and the operating characteristics of the shield. The values of the roof convergence, which occurred in the longwall faces during the single shield's cycle, are presented. It is strongly recommended that a system enabling the characterization and mining conditions appropriate for shield capacity determination and selection be developed.