Planning of the mining system to be employed in the development of a coal seam should take into account many factors, such as the seam thickness, dip, and depth of overburden. And for mechanized longwall mining, in particular, there are many in-seam criteria that need to be examined. This paper discusses a system of classifying the roof conditions from which a Roof Quality Index may be determined and used as a parameter for deciding the feasibility of longwall mining, also the influence of the roof caving characteristics on the loading parameters at the coal face. A method by which the required face advance for the first cave of the gob is given. Finally, criteria for the coal face support requirements are outlined.2
Traditionally the room and pillar mining method is employed in shallow coal seams with competent roof strata. In Europe, longwall mining is applied in a variety of mining conditions of roof strata, thickness and inclination of coal seams and depth of working. The longwall mining method is generally accepted to be the preferable method in deeper workings and for weak roof conditions.
In the United States the longwall retreat method has been favoured. In Europe, however, especially for deep coal seams the longwall advance method is often employed due to the difficulty in maintaining the panel development roadways. Although a large initial capital investment is required for longwall mining recent experience in the United States has indicated that many advantages are offered, especially that of safety. Justification of the high initial capital outlay for a longwall face demands a close examination of the principal design parameters, such as the length of face, number, type and size (duty) of face supports, panel length, coal production requirements and associated rates of face advance. The outlay of the coal face should be planned for well in advance and be integrated with the overall mine plan in the same coal seams and neighboring seams.
The classification of the coal seam roof strata was developed in Poland and has been used by the Polish mining industry for many years. The system is based upon a simple procedure of gathering roof data that can be classified into a Roof Quality Index (RQI). The RQI may then be used as a guideline to determine the possibility of longwall mining and support requirements for particular geological mining conditions. The determination of the designfactures are based on a standard length of longwall face of between 150 to 200 metres.
CLASSIFICATION OF ROOF STRATA
The working conditions at the longwall coal face is influenced by the correct choice and efficient operation of the hydraulic face supports. Mining practice has shown that the choice of face support primarily depends upon the classification of the immediate roof strata and the coal face area; the classification of the roof strata refers to the ability to cave in the gob area. A strong competent roof will generally require a heavier duty face support to induce caving at the edge of
