Assessing Pillar Collapse and Airblast 

Hazards in Underground Stone Mines
Pillars are blocks of rock left to support the overlying strata. A pillar collapse is 

when an array of pillars fail suddenly. Pillar collapses can occur with very little 

warning, and can affect miners far away from the collapse.
Overburden rock 

collapse material Rock shaken down 

from the roof

Rock thrown into 

the roadway
The total collapse of 

the overburden 

typically causes a 

subsidence feature 

(sinkhole) on the 

surface.
An airblast is a rapid 

displacement of large 

quantities of air, often 

under pressure, 

caused by a fall of 

ground in a 

constrained 

underground 

environment. 
Collapse Area

Portals

Miners located in 

high-velocity air 

pathways are at 

most risk. 
During the years 2015-

2021, five major pillar 

collapses occurred at 

four underground stone 

mines. 

These events resulted in 

three injuries and 

numerous near-misses. 

Similar events have 

occurred in previous 

years in marble mines 

and in lead/zinc mines.
It is not possible to predict 

when, or even whether, a 

particular array of pillars will 

collapse. 

However, experience has 

shown that certain factors are 

associated with an increased 

likelihood of a pillar collapse. 
MASSIVE PILLAR COLLAPSES

Large 

collapse 

area

Stopping destroyed by 

airblast
Pillar Design for 

Underground Stone 

Mines
Proper pillar design is a 

necessary first step for new 

workings.

NIOSH developed the S-Pillar 

program in 2010 and it is 

widely known in the industry.
Slender Pillars, with 

width-to-height ratios 

less than 0.8, are at risk 

of collapse.

In this photo, the w/h 

ratio is 30/50 = 0.6. 30 ft

50 ft
Limestone is a very strong rock.

However, it often contains 

joints or other geologic 

features.

These can greatly reduce the 

strength of a slender pillar, 

particularly when they 

“daylight” on both sides of the 

pillar.
Benching

Mining the floor 

increases the 

pillar height.

Development 

height

Benched 

height
Poor mining practices (blasting, surveying) 

can result in undersized pillars

Measured Pillar Widths 
Collapse Area

Many active underground 

stone mines contain 

“legacy” areas where 

pillars, particularly in 

benched areas, may be 

prone to collapse.

Mine operators should 

assess the risk of pillar 

collapse in these areas.
High value Medium value Low Value

High Value High risk

Medium Value Moderate risk

Low Value Low Risk 

Consequence

Likelihood of Occurrence

 

Source: Iannacchione, Varley, and Brady (2008). The application of major hazard risk assessment (MHRA) to eliminate multiple fatality occurrences in the 

US mineral industry. NIOSH IC 9508.

Risk Assessment requires rating the likelihood and the consequences of an unwanted event.

The greatest risk is when both the likelihood and the consequences are high.
Factors contributing to 

the likelihood of pillar 

collapse

A pillar design method, like 

S-Pillar, is the primary tool 

for evaluating the 

likelihood of a pillar 

collapse.

The pillar’s width-to-height

ratio is an essential part of 

the pillar stability analysis. 
Tributary Area Loading Pressure Arch Loading

Many pillar design methods assume the pillars carry tributary area loading. Where 

the panels are deep or narrow, and the overburden strong, the actual loading may be 

less due to pressure arch behavior.
A larger pillar array is more likely to collapse because there is less pressure 

arch potential, and it will also cause a greater airblast. 
Major geologic features, like 

faults or karst, can reduce 

pillar strength or weaken 

the overburden.
Weak bands or soft 

floor can cause rib 

slabbing and reduce 

pillar strength.

Weak Band
Blasted stone, shot in the floor 

(benched), ready to be loaded

Consequences of 

a Pillar Collapse

Miners working in the 

area of a pillar collapse 

would likely be killed. 

In one recent instance, 

miners were engaged 

in benching operations 

just days before a 

pillar collapse.

Collapsed pillars
Miners working 

nearby a collapse, 

or traveling in an 

adjacent haulroad, 

could be killed or 

injured by falling 

rock or extremely 

high air velocities.
Miners located 

at some distance 

from the collapse 

can be at risk if 

they are working 

or traveling in 

high velocity air 

pathways. 
Tech Support Study of Airblasts in Underground 

Mines Worldwide

• 40 documented airblasts

• Coal, trona, potash, copper/gold, lead/zinc

• Pillar Collapses

• Caving Events (longwalls, room and pillar, 

block caving)
Northparkes Mine

Disaster (1999)

• 10 million tons 

fell 500 feet

• 4 Miners killed

• 57 other miners 

unhurt

Muck

500 ft

Air Flowpaths

600 ft

Miners killed 

here

Main working level
Miners can also be harmed if fans 

or other ventilation controls are 

damaged by an airblast, or if 

egress from the mine is blocked. 
Collapse Area

Miners working in front of the portals, or on the surface, could also 

be harmed by a pillar collapse.
Control techniques 

that can reduce the 

risk from pillar 

collapse

1. Relocate 

travelways and other 

mine infrastructure 

to areas of lower risk.
2. Construct bulkheads or 

rock barriers to redirect 

airblast away from miners. 
3. Airblast relief openings can 

provide a direct ventilation 

pathway that does not expose 

miners to the hazard. 
Backfill

Material 4. Backfill?
5. Underground 

observations and 

monitoring 
Low Moderate High

Pillar Stability Meets all applicable design 

criteria

Does not meet applicable 

design criteria

Width-to-height ratio 

(average) w/h>1.0 0.8<w/h<1.0 w/h<0.8

Pillar Dimension 

Variability

All pillars approximately equal 

sized A few pillars smaller than average Many pillars smaller than the average 

Pressure Arch Potential Strong overburden/Deep 

cover/Narrow pillar array

Moderate strength 

overburden/Moderate 

cover/Moderate pillar array width

Weak overburden/Shallow 

cover/Wide pillar array

Size of Benched Area Small Moderate Large

Major Geologic Features 

(Faults, karst) None If a fault, karst, or other major geologic feature is present, assess its 

potential contribution to the collapse likelihood

Soft Floor None Possible, but minimal evidence of 

pillar distress

Thick weak floor causing pillar 

dilation

Weak Bands in the Pillars None Possible, but minimal evidence of 

pillar distress

Thick, weak band causing pillar 

dilation

The Pillar Collapse Likelihood Matrix. The top two parameters are most important.

A “high” value for a parameter correlates with a greater likelihood of collapse.
The Pillar Collapse Consequences Matrix helps evaluate the hazards at 

different locations in the mine, and the potential exposure of miners. 

Location of miners Conditions/Hazards

Consequence 

if miners are 

present

Number of 

miners 

exposed

How often are 

miners 

exposed

Working within collapse area, 

engaged in active mining/benching 

operations

Massive rock fall, no warning Death

Working or traveling in roadways 

directly adjacent to collapse area

High air velocities, flying 

debris, small rock falls

Death or 

severe injury

Working or traveling directly above a 

collapse area

Sudden development of a 

surface sinkhole

Death or 

severe injury

Working or traveling in high velocity 

air pathways leading from collapse 

area to portals

Diminishing air velocities 

depending on number of 

pathways and distance from 

collapse

Injury

Other locations in the mine Damage to ventilation 

controls or egress routes

Indirect 

hazards