Ground control for highwall mining in the United States

Abstract

Highwall mining is an important coal mining method in the USA and may account for approximately 4% of total USA coal production. Highwall stability is the major ground-control-related safety concern in highwall mining. Engineering away the safety risk by decreasing the highwall slope angle may be the best solution to the hazard posed by vertical joints in highwalls. The Mine Safety and Health Administration (MSHA) requires a ground control plan that usually specifies the hole width, maximum hole depth, maximum overburden depth, seam thickness, web pillar width, barrier pillar width and number of holes between barriers. Design charts for these parameters are given. Web pillars containing pre-existing auger holes are analysed and a design chart for estimating their minimum width is also presented. Close-proximity multiple-split highwall mining, which caused several serious highwall failures, is analysed and recommendations are made. Finally, this study examined records from 5289 highwall miner holes with a total completed hole length of 780 300 m to understand the reasons for early pull out. Average loss was almost 20% of planned hole length, and only 35% of the holes reached the planned depth.