Cleats & Joints
Endogenetic cleats are normal to the bedding plane of coal and generally
occur in pairs. There are at least two sets of near perpendicular fractures
that intersect the coal to form an interconnected network throughout a
coal-bed. These two fracture systems are known as face and butt cleats.
The shorter butt cleat normally terminates at a face cleat, which is the
prominent type of cleat as can be seen in the adjoining figure .
The angle between the face and butt cleat is around 90 degrees.The spacing
between cleats varies according to factors such as the coal maturity, the
mineral matter and the carbon content, but normally is less than 25 mm
but possibly more in dimension. The approximate width of the aperture and
the length of the face and butt cleat spacing in some Australian coal samples
are given in the table below.
Cleat |
Spacing |
Face cleat spacing |
10 - 25mm |
Butt cleat spacing |
10 - 22mm |
Aperture |
0.1 - 2mm |
Coals with bright lithotype layers,
with a high percentage of vitrinite macerals, have greater amount of cleats
than dull coals. Common understanding is that cleats are formed due to
the effects of the intrinsic tensile force, fluid pressure, and tectonic
stress. The intrinsic tensile force arises from matrix shrinkage of coal,
and the fluid pressure arises from hydrocarbons and other fluids within
the coal. These two factors are considered to be the reasons for endogenetic
cleat formation. On the other hand, the tectonic stress is regarded as
extrinsic to cleat formation and is the major factor that controls the
geometric pattern of cleats. Face cleats extend in the direction of maximum
in situ stress, and butt cleats extend in the direction of minimum in situ
stress which existed at the time of their formation. This is why regular
cleats are formed in face and butt pairs. In general three sets of cleats
are present in coal: face, butt and sometimes curviplanar cleat direction,
which intersect both face and butt cleat as shown in the figure.
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