Handbook for Methane Control in Mining - AMMSA

103Figure 7–4.—Methane drainage of a reverse fault from the“hanging” wall side.Joints, cleats, and fractures.Joints, cleats, and fractures areubiquitous features in most coalmeasure rocks and are related tothe confining stress fields actingupon those strata during burial,diagenesis, and uplift. Generally,these features follow asystematic pattern. Joints areclosely spaced with even walls,whereas fractures are morewidely spaced with irregularwalls [Nickelsen and Hough1967]. Joints usually occur inorthogonal pairs (at an approximately90° orientation to eachother), and in coalbeds, thesejoint sets are referred to as“cleats.”Figure 7–5.—Methane drainage of a reverse fault from the“footwall” side.The main joints and cleats of anygiven set are generally morecontinuous and are the dominantmigration pathways for gas[McCulloch et al. 1974]. Theyare referred to as “systematicjoints” and “face cleats,” respectively.The corresponding jointsand cleats at 90° to the mainfeatures are referred to as“nonsystematic joints” and “buttcleats,” respectively. Thesejoints and cleats generally terminateagainst the systematic jointsand face cleats, making themnotably less continuous. Whencoalbeds are mined, the redistribution of stresses allows cleats to expand and facilitates gasmigration through the coal to the face. Similarly, the stress redistribution opens joints andfractures in roof and floor strata, facilitating gas migration from adjacent strata.The ubiquitous nature of joints and the unpredictable spacing of fractures make prediction andremediation of abnormal gas emissions related to these features difficult. It is important foroperators to realize that although joints and fractures may contain free gas at the face, their realhazard potential is as a conduit for unexpected gas flows to the mine workings from within thecoalbed and/or other source beds adjacent to the mined coalbed. These types of conditions aremost often recognized when the continuous miner or longwall shearer is deenergized due to themachine-mounted methane sensors reading concentrations above the allowable limits. The most