Numerical investigation of pipe umbrella roof support systems in underground coal mining

Weak roof conditions in underground coal mines are a common occurrence and cause significant problems in delaying production. Thus, mine operators must look into additional support methods that reinforce the commonly used bolting, trussing, and cribbing methods. The work presented in this paper conceptualizes and models two methods of pipe umbrella roof support methods intended for employment in the underground coal mining environment. The first system is a pipe umbrella over a single entry of a development section. Secondly, a double layered pipe umbrella mesh is proposed as a reinforced roof over a longwall recovery room. Boreholes for such a configuration as the second system require precision placement and current state of the art technologies in horizontal directional drilling must be utilized. The design methodology was evaluated by examining a case study of a western U.S. coal mine and its specific geologic conditions. Geotechnical laboratory testing was performed for a weak sandstone channel material that occurs in large extents at the mine for input into numerical models. Two and three-dimensional finite difference models in Fast Lagrangian Analysis of Continua (FLAC) were developed and used as tools in the design of the pipe umbrella roof support methods proposed. One method utilizes beam elements embedded in the continuum model, while the other uses an equivalent modulus approach for modeling the reinforced zone. The effectiveness of a carefully designed pipe umbrella system is controlled by the pipe spacing, strength of the steel, and the structural geometry of the pipe. Numerical modeling of the reinforced roof shows that a reduction in recovery room closure can be achieved for the safe extraction of longwall support shields.