### A Stability Factor for Supported Mine Entries Based on Numerical

```OMSHR
Office of Mine Safety and Health Research
A Stability Factor for
Supported Mine Entries
Based on Numerical
Model Analysis
GS Esterhuizen
31st International Conference on Ground Control in Mining
Morgantown, WV
2012
DEPARTMENT OF HEALTH AND HUMAN SERVICES
Centers for Disease Control and Prevention
National Institute for Occupational Safety and Health
Need for improved effectiveness
of support systems in coal mines
• More than 1200 large
unplanned ground falls
reported per year
• Each fall represents failure
of the support system
• NIOSH objective to improve
support design procedures
Need a technique to evaluate
effectiveness of design
• How far is the roof from failing
– what is the margin of safety?
• How does stability change if
support is changed?
• Safety factor approach:
• For entries: What strength?
Obtaining a safety factor
• Strength reduction
technique:
– Slope stability (1975)
– Create model of slope and
reduce strength until failure is
indicated
– FOS = 1/strength reduction
factor at slope failure
SRF = 0.82
FOS = 1.21
Stability factor for entries
• Stability Factor:
– SF = 1/strength reduction
factor at entry failure
• Definition of failure:
– Roof collapse at or above
bolted horizon
– Assume smaller falls between
supports taken care of
• Expect relatively high SF
• Give it a try:
SRF = 0.56
FOS = 1.78
Rock strength parameters
• Systematic procedure for
creating model inputs
• CMRR – coal mine roof
rating
• Unit rating of each bed
– UCS of intact rock
strength
– Bedding strength
– Bedding intensity
Stability factor of three
case histories
• NIOSH experimental sites
• Model inputs from field
measurements and lab testing
• Model output calibrated against
measured and observed response
• Calibrated model used to calculate
the entry stability factor (SF)
1. Pittsburgh seam case history
Low strength immediate roof subject to high horizontal
stress at 600 ft cover (Oyler et al. 2004)
6.0
Extensometer Results
Model Results
Distance above roof (m)
5.0
4.0
3.0
2.0
1.0
0.0
0
10
20
30
40
50
60
Displacement (mm)
Development: Unsupported SF = 1.31
Development: Supported
SF = 2.94
2. Illinois basin case history
Thick-weak roof in room and pillar conditions 300 ft cover
(Spearing et al. 2011)
Development: Unsupported SF = 1.20
Development: Supported SF = 1.98
Moderate to strong roof longwall entries at 2000 ft cover
(Lawson, Zahl & Whyatt, 2012)
Development: Unsupported
Development:
Supported
SF = 1.83
SF = 2.38
SF = 1.45
SF = 1.31
Sample application – effect of roof bolt
length and spacing on entry stability
5 bolts across entry
2.0
2.0
Factor of Safety
Factor of Safety
Shale roof
1.8
1.8
1.6
1.6
1.4
1.4
1.2
1.2
3 bolts across entry
1.0
1.0
0.0
0.0
2.0
2.0
4.0
4.0
6.0
6.0
BoltLength,
Length,ftft
Bolt
5 BOLTS 5 BOLTS
3 BOLTS
8.0
8.0
10.0
10.0
12.0
12.0
Conclusions
• The strength reduction technique
provides realistic SF values for wide
range of case histories
• Relatively high SF values of entries
agrees with observation that very
small proportion of entries fail
• Entry stability factor is a useful tool
for evaluating relative merits of
support systems
DEPARTMENT OF HEALTH AND HUMAN SERVICES
Centers for Disease Control and Prevention
National Institute for Occupational Safety and Health
The findings and conclusions in this presentation have not been formally disseminated by NIOSH and should not
be construed to represent any agency determination or policy.
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