Stone Mine Pillar Stability and Design 

G.S. (Essie) Esterhuizen 

Pittsburgh Mining Research Division 

Mining Systems Safety Branch 

Presented at MSHA Roof Control Specialists Seminar 

October 2021 

NIOSH Mining Program 
Outline 

1. Pillars and pillar failure in stone mines 

2. The S-Pillar program to design and assess pillar stability 

2 
1. Pillars and pillar failure in stone mines 

• A look at stone mine pillars 

• Signs of potential overloading 

• Geologic factors that weaken pillars 

3 
Bench mining in stone mines produces tall narrow pillars 

First mining 28 ft high Floor benching to 60 ft 

40 ft 

28 ft 

60 ft 

40 ft 

During benching pillar height is increased but width remains the same 

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Tall narrow pillars: 

1) Weaker because lack of confined core 

Too tall to develop confined core Outer material resists and strengthens inner core 

5 
Tall narrow pillars: 

2) High impact of slips and joints 

Roof 

Floor 

Slip has little impact on pillar strength 

Coal 

Slip has major impact on pillar strength 

6 
Tall narrow pillars: 

3) Can fail suddenly with air blast as roof collapses 

Air blasts associated with 

three recent pillar collapse 

events in USA 

Edge of collapsed area near portal – mine in PA 

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1. Pillars and pillar failure in stone mines 

• A look at stone mine pillars 

• Signs of potential pillar overloading 

• Geologic factors that weaken pillars 

8 
 

Signs of potential pillar overloading 

Rib slabbing and spalling continues long 

after initial mining 
Signs of potential pillar overloading 

Overloaded pillar is “hour-glassing” 
Signs of potential pillar overloading 

Open fractures develop 

Spalling of fractured materials 

results in rounded shape 

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1. Pillars and pillar failure in Stone Mines 

• A look at stone mine pillars 

• Signs of potential pillar overloading 

• Geologic factors that weaken pillars 

12 
Geologic factors that weaken pillars 

Through-going angular joints provide sliding surface 
 

Geologic factors that weaken pillars 

Angular joint causes rib collapse and significant reduction in pillar width 
 

Geologic factors that weaken pillars 

Thin soft/clayey weak bands extrude and result in progressive slabbing of pillar ribs 
 

Geologic factors that weaken pillars 

1) Soft floor weakened by flooding – pillar punching, and extrusion produces open tension cracks in pillar 

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Geologic factors that weaken pillars 

2) Pillars sagging into soft floor – roof breaks-up, caving to surface 

17 
 

Summary 

1) Limestone mine pillars are narrow and tall: 

Increased height after benching weakens pillars, sensitive to 

joints & slips, potential violent failure 

2) Signs of pillar overloading are: 

Continued rib spalling, hourglass, rounded shape 

3) Geologic factors that weaken pillars: 

Large angled joints, soft clayey bands, weak floor with moisture 

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2. The S-Pillar program to assess or design pillars 

• S-Pillar program what does it do? 

• S-Pillar Inputs and results 

• Safety factor and design limits 

• Design example 

• When is S-Pillar applicable? 

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2. The S-Pillar program to assess or design pillars 

• The S-Pillar program what does it do? 

• S-Pillar Inputs and results 

• Safety factor and design limits 

• Design example 

• When is S-Pillar applicable? 

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The S-Pillar program application 

• Calculates average loading, strength and 

safety factor of pillars in stone mines 

• Used to assess stability of existing or 

planned stone mine pillars 

Developed by NIOSH – free download: 

https://www.cdc.gov/niosh/mining/works/coversheet1817.html 
2. The S-Pillar program to design and evaluate stone pillars 

• The S-Pillar program – what does it do? 

• S-Pillar inputs and results 

• Safety factor and design limits 

• Design example 

• When is S-Pillar applicable? 

22 
S-Pillar –Program Calculated Plotted 

Pillar 

dimensions 

Benching 

data 

Depth of 

cover 

Messages 

results results Geology data 

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2. The S-Pillar program to design and evaluate stone pillars 

• The S-Pillar program – what does it do? 

• S-Pillar inputs and results 

• Safety factor and design limits 

• Design example 

• When is S-Pillar applicable? 

24 
Pillar strength, load and safety factor calculation 

Strength 

• Pillar strength calculated from rock strength, W:H ratio, 

presence of large slips/joints in pillars 

Loading 

• Pillars carry full overburden weight to surface: tributary 

load 

Safety Factor 

Strength • SF = Tributary Load

• If strength is twice the tributary load: SF = 2.0 

25 
 

7.0

Safety factor and design limits 

0.0 

1.0 

2.0 

3.0 

4.0 

5.0 

6.0 

8.0 

9.0 

10.0 

Factor of Safety

Recommended area for design 

Safety factor limit = 1.8 

Width to height ratio limit = 0.8 

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 

Width-to-height ratio 
S-Pillar run through and help system 

27 
Safety factor and design limits 

Recent collapsed cases - based on provisional information on pillar as-mined dimensions and geology 

0.0 

1.0 

2.0 

3.0 

4.0 

5.0 

6.0 

7.0 

8.0 

9.0 

10.0 

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 

Width-to-height ratio 

Factor of Safety 

Mine B 

Mine A – average pillars 

Mine C 

Mine A undersized pillars 

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2. The S-Pillar program to assess or design pillars 

• Pillar strength, load, and safety factor 

• S-Pillar Layout and Inputs 

• Safety factor and design limits 

• Design example 

• When is S-Pillar applicable? 

29 
 

Hypothetical mine – planning to mine and bench under 300 ft cover 

Heading centers: 80 x 80 ft 

Entry widths: 40 ft 

Development height: 25 ft 

Bench 1 floor-cut; 15 ft 

Bench 2 floor-cut: 15 ft 

Rock formation: Vanport 

No strength or other geologic data Portals 

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Observation of existing pillars 

Average dip: ? Average frequency per pillar: ? 

31 
 

S-Pillar Analysis: 

Heading centers: 80 x 80 ft 

Heading widths: 40 ft 

Development height: 25 ft 

Bench 1 floor cut; 15 ft 

Bench 2 floor cut: 15 ft 

Benching total heights: 

1: 40 ft 

2: 55 ft 

Depth of cover: 300 ft 

Rock formation: Vanport 

Rock strength: 

Large discontinuities: 

Dip 45-75 deg Spacing: 2 to 4 per pillar 

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2. The S-Pillar program to design and evaluate stone pillars 

• Pillar strength, load, and safety factor 

• S-Pillar Layout and Inputs 

• Safety factor and design limits 

• Design example 

• When is S-Pillar applicable? 

33 
 

When is S-Pillar applicable? 

• Eastern/Midwestern US limestone mines 

• Pillars are near horizontal (< 5°dip) 

• Single level mining 

• Strong limestone strata (6,000 psi) 

• Weak clayey bands not be present in pillars 

• Strong floor and roof, no punching 
 

 

 

 

Summary 

1) S-Pillar can be used to assess stability conditions of existing pillar 

layouts and to design new layouts 

2) Careful observation needed to identify presence and spacing of 

large joints/slips/faults that can compromise pillar strength 

3) Thin weak bands in pillars and soft floor not accounted for in S-Pillar 

but can have detrimental impact on pillar strength 

35 
 

 

 

 

This presentation presents only the highlights of stone pillar stability and design using S-Pillar. 

For full information regarding this topic please review the help-system in S-Pillar and visit: 

https://www.cdc.gov/niosh/mining/works/coversheet1817.html 

NIOSH contacts: Essie Esterhuizen eee5@cdc.gov Michael Murphy mmu5@cdc.gov Nicole Evanek kri5@cdc.gov 

Disclaimer: The findings and conclusions in this report are those of the author(s) and 

do not necessarily represent the official position of the National Institute for 

Occupational Safety and Health, Centers for Disease Control and Prevention. Mention 

of any company or product does not constitute endorsement by NIOSH, CDC. 

NIOSH Mining Program 

www.cdc.gov/niosh/mining 

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