(SR) 5. Geomechanics
The pillar stability factor is more than 0.7 in Smoky River as per Analysis of Retreat Mining Pillar Stability (ARMPS).
The following strength properties are reported by Dr. C. Mark. However, on many occasions, he has quoted Dr. P. Cain's earlier publications on the strata surrounding the 5B-4 Mine.
Rock type
Test type
Strength (MPa)
Sandstone
Unconfined Compressive Strength
104.1 (Average)
Siltstone
Unconfined Compressive Strength (Lab.)
70.00 ~ 120.00
Grey Siltstone
Diametral Point Load Test
24.60
Grey Siltstone
Axial Point Load Test
103.00
Mudstone
Diametral Point Load Test
0.33
Mudstone
Axial Point Load Test
28.50
The following strength data (in MPa) are provided by the Smoky River Mine:
Rock type
Is(50) diametrical
Is(50) axial
Ax.:Dia. Ratio
UCS(Calc.)
UCS(test)
Carbonaceous Mudstone
0.19
-
-
32.97
-
Fissile Mudstone
0.05
-
-
-
37.72
Mudstone
0.77
1.78
2.31
27.94
-
Grey Mudstone with Siltstone
0.92
-
-
-
-
33% Silt
0.22
3.07
13.95
48.19
-
50% Silt
0.68
4.96
7.29
77.86
15.70
70% Silt
-
-
-
-
-
Grey Siltstone
1.23
5.13
4.17
80.53
-
Iron Nodule
2.57
-
-
-
-
The following rockmass classification ratings for Smoky River Coalfield are reported by Dr. C. Mark (unless otherwise notified) on the 5B-4 Mine:
Rock type
Classification type
Rating (Overall)
Weak Siltstone and thick Mudstone
CMRR
50 (Normal case), 40 (Fracture zone)
Siltstone and Mudstone*
RMR
35 (Roof)
The Smoky River area has clearly been subjected to intense horizontal tectonic pressures, as evidenced by the pervasive thrust faulting. In 1985, Jeremic reported that one underground observation at the #2 Mine had the maximum stress oriented approximately N35°E.
At present, there is little evidence of horizontal stress underground. Dr. C. Mark reports that in one location near the outcrop, a few large roof "pots" are indicative of a N30°E horizontal stress. It is possible that as mining progresses into the ridge towards deeper area, the horizontal stress may increase and rotate towards the regional stress orientation. The estimated in situ vertical stress is proportional to the cover depth and the horizontal stresses are 1/3 to 1 time of the vertical stress.