(Pr) 5. Geomechanics
The pillars of coal separating longwall panels are designed to be 60 m (200 feet) wide. This width is selected based on sea bed strain and overlap between walls. It was later checked using Wilson's method and also by ALPS. The latter shows a stability factor = 2.3 for the tailgate.
S. Forgeron has reported the average mechanical and physical properties as follows:
Rockmass
UCS
Tensile strength
Young's Modulus
Poisson's Ratio
Density
Sandstone
46.5
4.5
16.5
0.24
2.29
Siltstone
50.7
5.8
12.9
0.18
2.54
Mudstone/ Shale
18.4
-
-
-
2.59
Coal
12.0
-
-
-
1.30
Note: UCS (Unconfined Compressive Strength) and Tensile Strength are measured in MPa, Young's Modulii are in GPa and the densities are in gm/cc.
S. Forgeron reported a Rock Mass Rating of (CSIR) 57 for the sandstones, and 16 for mudstone. Dr. C. Mark reported the overall CMRR as 30 for a roof height of 2.4 m (8 feet) immediately above the Seam.
The in situ stress state at Prince Colliery is similar to the overall panorama of the Sydney coalfield. The major horizontal stress is about 2 ~ 2.5 times the vertical stress along N60ºE. Its adverse effect on mining have become more pronounced as the workings approach the axis of the Mackenzie Syncline. Most of the longwall panels are oriented N70°W.