Office of Mine Safety and Health Research

OMSHR

DEPARTMENT OF HEALTH AND HUMAN SERVICESCenters for Disease Control and PreventionNational Institute for Occupational Safety and Health

31st International Conference on Ground Control in MiningMorgantown, WV

2012

A Stability Factor for Supported Mine Entries

Based on Numerical Model Analysis

GS Esterhuizen

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: Strength/Load

• For entries: What strength? What load?

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 = 0.82FOS = 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 = 0.56FOS = 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– Diametral point load

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

0.0

1.0

2.0

3.0

4.0

5.0

6.0

0 10 20 30 40 50 60

Dis

tanc

e ab

ove

roof

(m)

Displacement (mm)

Extensometer Results

Model Results

Low strength immediate roof subject to high horizontal stress at 600 ft cover (Oyler et al. 2004)

Development: Unsupported SF = 1.31Development: Supported SF = 2.94

2. Illinois basin case historyThick-weak roof in room and pillar conditions 300 ft cover (Spearing et al. 2011)

Development: Unsupported SF = 1.20Development: Supported SF = 1.98

3. Colorado deep cover case

Development: Unsupported SF = 1.83Development: Supported SF = 2.38Longwall loading 1: Supported SF = 1.45Longwall loading 2: Supported SF = 1.31

Moderate to strong roof longwall entries at 2000 ft cover (Lawson, Zahl & Whyatt, 2012)

Sample application – effect of roof bolt length and spacing on entry stability

0.0 2.0 4.0 6.0 8.0 10.0 12.01.0

1.2

1.4

1.6

1.8

2.0

5 BOLTS

Bolt Length, ft

Fac

tor

of S

afet

y

0.0 2.0 4.0 6.0 8.0 10.0 12.01.0

1.2

1.4

1.6

1.8

2.0

5 BOLTS 3 BOLTS

Bolt Length, ft

Fac

tor

of S

afet

y

Shal

e ro

of

5 bolts across entry

3 bolts across entry

DEPARTMENT OF HEALTH AND HUMAN SERVICESCenters for Disease Control and PreventionNational Institute for Occupational Safety and Health

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

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.