Determination of highwall mining panel dimensions for Mae Moh Mine using numerical simulations

Abstract

Mae Moh mine will be discontinued in Year 2051 because it is not economically viable. However, there are 130 million tonnes of coal under the southern end wall. Many mining methods have been studied in order to excavate out the remaining coal. This study focused on highwall mining method. Highwall mining is a mining method that equipment is utilized to excavate coal seam without excavating overburden. The highwall mining system delves rectangular entries in coal seam and left parts of the coal seam as pillars to stabilize the mining working faces, then the mined coal is transported out of the seam by its conveyor system. The significant advantage of highwall mining is able to obtain the coal which is normally unmineable through pit walls of an open-pit mine. Therefore, highwall mining method can be an economical option and possibly save the overburden excavation cost by obtaining the additional coal and also maintains the stability of the mine.

The objective of this study looked for highwall mining layout for Mae Moh Mine using numerical simulations. The simulation models calculated the dimensions of entries and pillars using burden and coal engineering properties of Mae Moh mine to find the relationships between the height-width of pillar and the height-width of entry along with safety factor. The coal properties in this study were from 2 sources, which were the laboratory test results and the in-situ direct shear test results from the previous studies of the mine. At Mae Moh mine, the coal seams are expected approximately 30 meters thick. Consequently, the highwall mining system in the current market is incapable to excavate the seam at once. Therefore, multi-lift highwall mining is suitable for Mae Moh coal seams to increasing the recovery ratio.

The study found that highwall mining method can be practiced in Mae Moh mine. While using material property data from laboratory test, the suitable panel shape for producing maximum coal is the panel with 48.46% recovery and the factor of safety is 1.55, with 4.5 meters entry width, 3 meters pillar height and 2 meters pillar width. If this panel shape is applied in Mae Moh mine, 8.1 million tonnes of coal will be extracted. However, when using material property data from the in-situ test in multi-lift excavating floor by floor and backfilling, the suitable panel shape for producing maximum coal is the panel with 33% recovery and the factor of safety on the first floor before backfilling is 1.33, with 3 meters entry width, 2 meters pillar height and 3 meters pillar width. If this panel shape is applied in Mae Moh mine, 5.4 million tonnes of coal will be extracted.