8
softer rocks, such as shale and claystone, with thin lamina-
tions. In stone mines, however, cutters occur in harder rock,
like limestone, with thicker laminations. Consequently,
softer rocks with thinner laminations tend to produce
vertical cutters, as seen in coal mines, while harder rocks
with thicker laminations lead to the zig-zag cutter pattern
observed at the study mine.
According to Mohr Coulomb theory, fracture takes
place through the direction of the intermediate principal
stress and inclined at an angle less than 45 degrees from
the direction of the maximum principal stress (Jaeger et al.,
2007). The fracture angle is the angle between the shear
fault plane and the axis of the maximum principal stress.
The maximum shear stress angle can shift depending on the
horizontal to vertical stress ratio and the magnitude of the
shear stress. The fracture angle increases with increasing σ3,
but decreases with increasing σ2, particularly under lower
σ3 values (Mogi, 2007).
Figure 9. Cutter propagation in a zig-zag pattern between roof beds