Analysis of Roof Failure at the Intersections on a Coal Mining

Abstract

The objective of this paper is to understand the behavior of the roof and the cause of roof failure of a coal mining. Eleven roof falls occurred in the mine section in sequence. All the roof falls were surveyed, and the geological features were determined which include rock type, mechanical properties, and thickness. The immediate roof thickness was interpolated to determine the cause of the roof failures which was the inappropriate roof support where the immediate roof is thicker. The average thickness of the immediate roof was determined by the drilling log in 1.2 m (3.9 ft.). The roof support was design based on the average thickness, and the bolt length is 1.5 m (4.9 ft.) in suspension. The challenge was to estimate where the immediate roof gets thicker and to design the support considered to maintain the regular bolt length. This is because the cost and productivity of support operation could increase if the bolt length changes as the roof thickness changes. The idea was to adjust the support for beam building with a fully grouted bolt and rearrange the bolt geometry to keep the roof stable. It means, with the same bolt length the type of roof support would change from suspension to beam building depending on when the roof gets thicker. Two empirical approaches were considered to design the roof support: 1) CMRR and 2) RMR. The entry width is 5 m and the roof support was designed with four bolts per row. Where the roof support must change to beam building, the number of bolts per row changes to five or six. The results of the design and application on underground showed that the approach keeps stable.