Numerical Simulation of Overburden and Surface Movements for Wongawilli Strip Pillar Mining

"INTRODUCTIONThere is a great amount of coal (about 140 billions tons) left unmined under the surface structures, water bodies, railways (referred to as the “three-body”). Coal mining under “threebody” especially that under surface structures has become a major problem in the mining area. The key problem of mining under surface structures is to control overburden strata and surface movements. At present, the major surface subsidence control methods are strip pillar mining, backfilling mining method, room and pillar mining, grouting of bed separations, harmonic mining, and so on. Wongawilli mining method is a high-efficient shortwall mining method developed based on the room and pillar mining, and named after the first successful trial in Australia “Wongawilli” coal seam. The room and pillar mining equipment such as continuous miner is employed in this method. Its major advantage is flexible face layout. It can be used to mine the small wedge coal blocks and those that are difficult to be mined with retreat longwall mining method. It requires less capital investment, short lead time for production, flexible equipment operation and face move and high productivity. It has been applied in some China’s coal mines.The “Wongawilli strip coal mining technology” is a new coal mining method for mining under surface structures. It combines the strip mining longwall layout and Wongawilli high efficient mining technology. This technology can fully utilize the advantages of both the strip pillar and Wongawilli mining methods, and overcomes the disadvantages such as frequent face move, and low mining efficiency in strip pillar mining, and poor ventilation condition and poor long-term stability problems of coal pillars. In this paper, the Wongawilli strip mining method was designed for Wangtaipu mine and its feasibility was studied using numerical modeling and surface subsidence prediction.Geological and mining conditionsAt present, most of the No.15 coal seam reserves totaling 377.07 million tons in Wangtaipu mine are located under surface structures. In order to mine the coal under the surface structures, the Wongawilli strip mining method was employed. An appropriate panel was selected for trial and the surface movement monitoring lines were set up. The trial area is located in panel XV2214 (East) ( re 1). In this area, the floor elevation of coal seam is +634~646m, the surface elevation is +810~817.5m, and the average depth of coal seam is about 174m.The average panel length is 282m and average panel width is 73.5m. The seam dips 1~3° and is 1.8~2.5m thick with an average of 2.15m. The roof and floor rock characteristics are shown in Table 1."