Determining the optimal dimensions of the pillars under static and dynamic loads in room and pillar mines (Case study: Tabas central coal mine)

A pillar dimension in room and pillar mining method has been always a technical and economical issue for mining and rock mechanic engineers. The strength of the pillars is usually determined by empirical relationships, which have been determined by experience and the data collected from the coal mines of the United States, South Africa, and China, and which, except in one case, have never been considered seismic loads. This study aims to define the optimum pillar dimension based on the pillar strength derived from a new approach implemented in the numerical modeling by gradually applying an increasing load on the pillar and monitoring its displacement, using the Central section of Tabas coal mine data. The results are compared with the method of Salamon-Munro (1967) which is one of the most commonly used empirical methods. This comparison shows that the strength obtained from the numerical method for pillar widths of less than 15 m is well consistent with the empirical Salamon-Monroe method, whereas the difference between the results of the two methods increases progressively with a pillar width increment. The safety factor of the pillar is then defined by dividing the obtained pillar strength and monitoring critical stresses, under static and dynamic conditions. Tabas (1978) earthquake with 7.4 magnitudes is used for dynamic analyses. The results show that the optimal width of the pillar in the static and dynamic states is 12 and 15 meters, respectively. Moreover, curve fitting with high regression to the results obtained in both static and dynamic states, relations in terms of width to height ratio are presented for use in other areas of the mine with similar geomechanical conditions.