2013/09/03 On 21 January 1960, the Coalbrook North Mine collapsed. - - PDF document

2013/09/03 1

Lessons From Coalbrook

Presentation to SACMA 22 August 2013 Prof Nielen van der Merwe On 21 January 1960, the Coalbrook North Mine collapsed. 437 miners perished in the event. It still is the single most tragic accident in South African mining. It resulted in a research drive that put South Africa at the forefront of mining science for several decades. On 21 January 1960, the Coalbrook North Mine collapsed. 437 miners perished in the event. It still is the single most tragic accident in South African mining. It resulted in a research drive that put South Africa at the forefront of mining science for several decades. 53 years later - what did we learn?

Base evaluation on Coalbrook disaster

• 1. What happened at Coalbrook?
• 2. What were the unknowns at the time?
• 3. How were they addressed?
• 4. Would the new technology have prevented

the accident?

• 1. What is the current situation?
• 2. How do we match up for the future?

Lava 0 – 3 m Coal No 1 Seam 2 m Coal No 3 Seam 6 m Coal No 2 Seam Dolerite Shale and Sandstone Shale and Sandstone Shale and Sandstone

40 m 140 m 60 m

Geology and Dimensions

Coalbrook

Production build-up

500000 1000000 1500000 2000000 2500000 3000000 1905 1915 1925 1935 1945 1955 1965 Year Tonnes Produced

Power Station Contract

Coalbrook

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The Pre-1932 Period

Shaft complex

Development West Panels South Panels North

6.7 m Bords 24.4 m Centres 2.4 m High 24.4 m Barriers Coalbrook

The period 1932 - 1950

Coalbrook 6.7 m Bords 19.8 m Centres 2.4 m High 18.3 m Barriers

The Post 1950 Period

Coalbrook 6.7 m Bords 19.8 m Centres 4.3 m to 5.5 m High 13.1 m Barriers

Coalbrook by the end of 1959

Coalbrook

1905 - 1932

6.7 m Bords 24.4 m Centres 2.4 m High 24.4 m Barriers

1932 - 1950

6.7 m Bords 19.8 m Centres 2.4 m High 18.3 m Barriers

1950 - 1959

6.7 m Bords 19.8 m Centres 4.3 m to 5.5 m High 13.1 m Barriers

28 Years 10 Years 18 Years

Coalbrook by the end of 1959

Coalbrook

Dimension Changes Over Time

5 10 15 20 25 30 Bords Centres Height Barriers 1905 - 1932 1932 - 1950 1950 - 1960

+/- 1955, Steart:

• Competent planning, etc
• ….poor strength of no 2 Seam…
• Extraction < 40%
• Height 2.9 m
• “Adequate barriers…”

1957, Inspector of Mines:

• Bord width – 6.1 m
• Height 4.3 m

Coalbrook

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The 1959 Section 10 Experiment

Coalbrook 6.7 m Boards 19.8 m Centres 4.3 to 6.1 m High 4 m by 2 m Dummies cut in pillars Visual monitoring

Nothing Happened

Following the successful Experiment……

• Section 10 continued top coaling southward
• Elsewhere top coaling on advance to 4.3 m
• Mining either side of experiment
• Centres 18.3 m
• Barriers 12.2 m

This continued to December 1959

Coalbrook

28 December, 1959……

Pillar collapse, old Section 10 Wind blast, 1 injury Afterwards, nothing…..

Coalbrook Shaft

11 January, 1960……

Coalbrook

Inspector visited mine No signs of instability Collapse not reported Everything back to normal

Shaft

21 January, 1960, afternoon shift……

Men working Coalbrook Men working Men working Shaft Coalbrook Men working

21 January, 1960, 16:00……

Cracking noises from Section 10 Two sections withdrew Windblast

Shaft

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21 January, 1960, +/- 17:00……

Coalbrook

Mine Overseer, Acting Manager inspect workings Noises only from Section 10 No CO “Weight had come off…..safe to replace seals….”

Men working Shaft

21 January, 1960, +/- 19:00……

Coalbrook

Increasing noise from Section 10, methane emissions Repair crews left “at the double” Overtaken by hurricane General exodus

Men working Shaft

21 January, 1960, +/- 19:00……

Coalbrook Men working Shaft

21 January, 1960, by 19:20……

Coalbrook Men working Shaft Coalbrook Coalbrook

The Collapsed Area

Area 1 Area 2

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The Unknowns

Small pillars, no top coaling, INTACT Small pillars, no top coaling, COLLAPSED Influence of barriers?

Area 1

Small pillars, top coaling, INTACT

The Unknowns

Reduced span? Dolerite bridging?

Small pillars, top coaling, COLLAPSED

Area 2

The Unknowns Some barriers failed, others survived… How big is big enough?

Barriers

The Unknowns

The Four Main Unknowns

• 1. Pillar strength
• 2. Barrier strength
• 3. Loading
• 4. Role of overburden(dolerite?)

Major research effort

• ver next few decades

Several scientists migrated to South Africa Birth of COMRO

Addressing the Unknowns: Pillar Strength

Bieniawski: Large scale tests h w

s

52 . 1 76 . 2 + = σ Salamon and Munro: Back analysis

66 . 46 .

176 . 7 h w

s =

σ The Pioneering Work

Addressing the Unknowns: Pillar Strength

The Refinements

Wagner: C A we 4 =

1980

Salamon:

• +
• −
• =

1 1

ε

ε σ R R b V R k

a b s

1982

Madden:

• +
• −
• =

1 1 5 5 . 2 5933 . 5

5 . 2 0667 . 5933 .

R V k

s

σ 1991 Madden:

78 . 63 .

24 . 5 h w

s =

σ 1991

Van der Merwe:

66 . 46 .

5 . 4 h w

s =

σ 1993

Van der Merwe: h w

s

5 . 3 = σ 2003

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Shift Toward Time

Addressing the Unknowns: Pillar Strength

Shift Toward Time

x x

mh d T

−

• =

1 1

[ ]

333 , 2

00714 , HhC S w d

m

− = Van der Merwe:

Frequency of Predicted vs Actual Life

5 10 15 20 25 30 20 40 60 80 100 120

Life (Years) Number of Failures

Predicted Actual 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10 20 30 40 50 60 Average Scaling Rate (m/year) Age of Pillar (Years)

Rate vs Time

Measured Data Predicted Data

Addressing the Unknowns: Pillar Load

Tributary Area Theory

2 2

) ( 025 . w B w H

L

+ = σ

2 1 2 2 1 1

) )( ( 025 . w w B w B w H

L

+ + = σ

( ) [ ]

T T H w w B w B w

L

03 . 025 . ) )( (

2 1 2 2 1 1

+ − + + = σ

Roberts et al (2001)

• TAT not strictly valid
• Several unquantifiables
• Can of worms
• Stick to TAT

Addressing the Unknowns: Overburden

Galvin (1983)

Elastic Plate Theory

Van der Merwe (1995)

Modified Key-block ( ) φ β tan ) 2 2 D H D k T Lc − + + = 2 tan λ θ γ γ β k b c

m d −

− =

Dolerite only addressed

Addressing the Unknowns: Barriers

Esterhuizen (1992)

Suggested using Wilson (1983) and 2-D models

Roberts and van der Merwe (2005)

Hybrid ELFEN code

No easy to use formula yet

Assessing Progress: Coalbrook

Pillar Stability SF = 2.3 SF = 1.4 SF = 2.1 SF = 1.3 SF = 1.7 SF = 1.0 SF = 1.1 SF = 0.7 SF = 1.0 SF = 0.6 SF = 1.0 SF = 0.6 SF = 1.6 SF = 1.0 Blue: Salamon and Munro Red: van der Merwe

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Assessing Progress: Coalbrook

?

Barrier Strength

Assessing Progress: Coalbrook

?

Overburden Behaviour

Assessing Progress: Coalbrook

?

Pillar Loading

Anomalies can be explained conceptually

Assessing Progress: Coalbrook

Not easily quantified………………

Assessing Progress: Pillar Strength

Disaster not repeated – could it have been? Failures continue but at reduced rate

Before 1967: 27 failures Expected (mined after 1967): 114 Actual (mined after 1967): 23

Sobering Statistics – Failed Pillars:

Age prior to 1967: 8.2 years Age after 1967: 21.3 years SF of failures mined before 1967 = 1.0 (0.9) SF of failures mined after 1967 = 1.7 (1.8)

Are we getting less or is it just taking longer?

Springlake Colliery 2001

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Welgedacht Colliery 1998 Welgedacht Colliery 1998 Collapsed pillars, Welgedacht

Shuttle car extraction, Welgedacht

Assessing Progress: Deeper Issues

Nature of the experiment

• born in reaction to restrictions
• desperate for coal
• no measurement, ears and eyes only
• “success” after 3 months
• BUT: trigger only, small pillars, greater height for 11 years

Advice not heeded

+/- 1955, Steart:

• Competent planning, etc
• ….poor strength of no 2 Seam…
• Extraction < 40%
• Height 2.9 m
• “Adequate barriers…”

SF would have been 1.6 (2.5)

In quest for coal, Coalbrook moved beyond knowledge Disturbing Parallels Between Coalbrook 1960 and Industry 2010

• Increasing coal demand for electricity
• New pitroom not ready (Waterberg)
• Technology lagging
• Researchers not available
• Funding decreasing

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Urgent Coal Mining Research Needs

Extraction of small, shallow pillars – increase life of Witbank field Time effect on pillar stability – confirm and refine existing Pillar loading, overburden role

Current mining

Barrier strength and spacing

Waterberg preparation

How do we mine thick, deep seams? What are the host rock characteristics? How do we mine using less water? What equipment will we need? How will we support the roof?

South Africa needs a strong mining industry: safe, productive, profitable

“What saves a man is to take a step. Then another step. It is always the same step, but you have to take it.”

Antoine de Saint-Exupéry

We need to do the research

Current Coal RE Doctoral Research at Wits

Extraction of small, shallow pillars – Sandor Petho (part time) Time effect on pillar stability – Self Squat pillar review – Markus Matthey (full time) Vaal Basin strength – Gift Makusha (part time) Waterberg characterisation – Lelani Prinsloo (part time) Probability of failure – Self

Just this once, let us learn from history…………

Remember Coalbrook