Joints within coal-bearing deposits are important features to be considered during mine planning and design, and when optimizing a CBM project. A thorough understanding of their characteristics including frequencies and orientations, and their relationships to other structures (e.g., cleats, faults, etc.) is important for evaluating rock behavior in underground and surface mines and in determining hydrodynamics and methane concentrations.
Joints are fractures having no significant movement (Pictures 1 and 2). Joints are formed in response to loss in cohesion and loss of load resistance of the rock mass resulting in the release of stored elastic strain energy which subsequently forms breakage planes (Friedman, 1964). Joint formation is believed by many to develop early in the geological history of the coal-bearing strata. Joints occur in parallel series that vary in frequency (Pictures 3 and 4) and they can penetrate all or part of the rock layers. A joint set is unidirectional and many coal deposits contain more than one joint set (Picture 5). Two or more joint sets are referred to as "systems", and the joints are subdivided into a principle, or primary, joint set and a complimentary, or secondary, joint set. In more complex tectonic settings, multiple joint systems can occur in the deposit.