Origin and Spacing of Cleats in Coal Beds

Cleats are natural fractures in coal beds resulting from dehydration, devolatilization, and stresses in the earth’s upper crust during coalification. The orientation of these fractures usually parallels that of the fractures (joints) in the associated rocks except that the former is better developed. Spacing of cleats ranges from less than one millimeter to over one meter. The frequency of cleating in coal beds affects not only mining but also the flow of gases in the coal, and the strength of pillars used for roof support. The variation in spacing in controlled primarily by two parameters, namely rank and petrographic composition of the coal. The cleat frequency increases with increasing rank and reaches a maximum at the low-volatile bituminous coal rank. Within the same coal bed at the same mine face, dull coal layers tend to have fewer cleats than bright coal layers. The nature of the cleats is further complicated by local disturbances such as faults, folds, and fissility of bedding planes in the coal seams.