Reduction of risk from roof and side fall in Indian coal mines

1.0 Introduction:

Accidents due to movement of strata in underground coal mines had been a

major concern for the mining community from the very beginning. Over the

years, compiled statistics of accidents in Indian coal mines identified “Fall of

Roof” as a major cause of mine accidents. Continuous efforts were made by

all concerned to reduce the hazard of strata movement by mining companies,

research institutions, academicians and DGMS. A number of recommendations

were made in National Conferences on Safety in Mines to reduce accident

caused by movement of strata. As a result of all these efforts, the accidents

caused by fall of roof and fall of sides have shown a downward trend. Still fall

of roof and fall of side are the major causes of accident in underground coal

mines as it contributed 25% and 9% of total fatal accident and 42% and 16%

of the accidents in underground coal mines respectively during 1997-2006.

Hence it is essential to further emphasize on the issue of strata control

mechanism and reduce the accidents due to fall of roof & sides. With the

estimated growth of mining activities in Indian coal industry, the magnitude

and complexity of the problem will be multiplied and needs attention of all

concerned.

2.0 Cause-wise analysis of accident due to fall of roof & fall of side

Table 1 and Figure 1 below shows the details of fatal accidents due to fall of

roof and sides compared to total below ground accidents and total accidents

in coal mines.

Table 1: Cause wise Fatal Accidents in Coal Mines

Year Fall of roof Fall of sides Total BG AccidentsTotal accidents in Coal Mines

1997 38 12 94 1431998 35 15 80 1281999 33 11 74 1272000 27 14 62 1172001 30 9 67 1052002 23 11 48 812003 18 5 46 832004 26 8 49 872005 18 7 49 962006

*13 4 44 79

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Comparison of Accidents in coal mines due to Fall of Roof and Fall of Sides with Belowground Accidents

(1997-2006)

Other B/G Causes

42%

Fall of Sides16%

Fall of Roof42%

* Provisional

Figure 1(a): Comparison of fatal accidents due to fall of roof and

sides and other causes in coal mines since 1997 to 2006.

Comparision of Accident in coal mines due to Fall of Roof & Fall of Side with Total No. of Accidents (1997-2006)

Fall of Side9%Other Causes

66%

Fall of Roof25%

Figure 1(b): Belowground accidents due to fall of roof and fall of

sides

From the above it may be observed that

(i) Fall of Roof contributes 25 % of total accidents and 42 % of total below

ground accidents in last 10 yrs but there is a decreasing trend. The

number of fatal accidents due to fall of roof has come down from 38 to 13.

In the year 2006, Fall of Roof contributed 16 % of total accidents and 30%

of below ground accidents.

(ii) Fall of Side contributes 9 % of total accidents and 16 % of total below

ground accidents in last 10 yrs and this has also a decreasing trend. The

number of fatal accidents due to fall of side has come down from 12 to 04.

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In the year 2006, Fall of Side contributed 5% of total accidents and 9% of

below ground accidents.

(iii) Though there is a general decreasing trend in fatal accidents due to roof

and side fall, there had been sharp increase in the figure in some odd

years which needs special attention.

3.0 In-depth Analysis of the accident due to fall of roof:

As it is observed that fall of roof and side is a major cause of non-disaster fatal

accidents and its contribution in below ground accidents is still very high, it is

essential to analyse these accidents in more details.

3.1 Analysis of accidents due to fall of roof vis-à-vis Method of

work

Table 2: Details of accidents due to roof fall – method wise

Method 199

7

199

8

199

9

200

0

200

1

200

2

200

3

200

4

200

5

200

6

Tota

l

Board &

Pillar

Developmen

t

19 21 16 11 10 13 07 09 10 04 119

Depillaring 18 14 16 16 16 10 11 13 06 06 126

Long wall &

Others01 01 01 03 04 00 00 03 00 01 14

Total 38 36 33 30 30 22 18 25 16 11 259

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Figure 2: Method wise percentage of accidents due to fall of roof.

From Table 2 and Figure 2, it can be observed that accidents due to fall of roof

occurred in almost same proportion in bord and pillar development as well as

depillaring districts in the last ten years. With introduction of roof bolts for

supporting freshly exposed roof in development district, there has been

decreasing trend in accidents due to fall of roof in development districts. The

percentage of roof fall accidents in depillaring district is quite significant

during this period. However, this may be noted that the support system in

depillaring districts is still conventional wooden support with comparatively

less share of roof bolting.

3.2 Analysis of fatal accidents due to fall of roof vis-à-vis framing

of SSR

Table 3: Details of Fatal Accidents due to fall of roof vis-à-vis

Framing of SSR in last five years

Year No. of accidents due to

fall of roof

No. of SSR framed No. of SSR not

framed

2002 22 20 0

2003 18 13 1

2004 26 20 0

2005 18 15 0

2006 13 9 0

Total 97 77 1

Distribution of accidents due to Fall of Roof - Method wise (1997-2006)

Long wall & Others

5%

Depillaring49%

Board & Pillar

Development46%

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From the available data regarding framing of SSR as required under the

statute, it is revealed from Table 3 that in almost all the mines where accident

due to fall of roof has taken place, SSR has been framed. However,

effectiveness of framing of SSR or its implementation needs to be assessed to

identify the weakness in the system.

3.3 Analysis of status of support at accident place

Figure 3: Status of support at place of accidents

From Figure 3, it may be observed that though SSR has been framed in almost

all the mines where accidents due to fall of roof have occurred, in 49% cases

the roof were not kept supported. This is a matter of serious concern because

of the fact that only framing of SSR does not serve any purpose unless the

SSR is implemented in its true spirit. This may further be noted that in 51%

cases, the places of accidents were supported. This necessitates further

examination of the support system to identify the shortcomings in the SSR

and its implementation process.

3.4 Analysis of roof fall accidents by distance from face

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Status of Support at accident place ( Roof Bolt and Conventional support ( 2002-2006)

Not Supported

49%

Supported

51%

Figure 4: Distribution of roof fall accidents by distance from face

While analyzing the accidents, from Figure 4, it may be noted that the area up

to 10 metre from the face is the most critical one. 42% accident occurred

within 5 metres from the face and 58% accident occurred within 10 metres

from the face. If proper attention is given to support the freshly exposed roof,

majority of the roof fall accidents may be controlled.

3.5 Analysis of Roof fall accidents by thickness of fall

One of the critical parameter of accidents due to fall of roof is the thickness of

fall or the location of the plane of weakness above the working section. From

Figure 5, it is revealed that 59% accident occurred where thickness of fall

were up to 0.30 m and 86% accident occurred where thickness of fall were up

to 1.0 m. This clearly indicates that in Indian coal measure rock, the roof rock

up to 1 metre above the working section is the most critical one and steps are

to be taken to take care of the roof up to this horizon. However, the location of

this plane of weakness varies from mines to mines and from place to place.

Hence it is essential to identify this horizon by suitable scientific method and

design the support system accordingly.

Distribution of roof fall accidents by Distance from Face

10.01 to 20.00 m

9%

20.01 m & Above

11%

5.01 - 10.00 m

16%

Other places22%

0.00 - 5.00 m

42%

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Figure 5: Distribution of accidents due to fall of roof by thickness of

fall

3.6 Analysis of Roof fall accidents by nature of fallen strata

Nature of roof rock is also a very critical parameter of stability of roof rock.

Hence it is also essential to analyse the roof fall accidents according to the

nature of roof rock. Figure 6 shows the details of roof fall accidents and the

nature of the strata.

Figure 6: Distribution of Fatal Roof Fall Accidents by nature of Fallen

Strata

From the above it is observed that in 40% roof fall accident cases nature of

fallen strata was sandstone. It is contrary to the common belief or

Distribution of Fall of Roof accidents by Thickness of Fall

0.31 - 1.00 m

27%

0.16 - 0.30 m

32%

Not Applicable

4%

1.01 m & Above

10% 0.00 - 0.15 m

27%

Distribution of Fatal Roof Fall Accidents by nature of Fallen Strata

Data Not Available4%

Coal/Shale/Sandstone

2%

Shale

17%

Coal & Shale

8%

Coal & Sandstone

0%

Sandstone

40%

Shale & Sandstone

9% Coal

20%

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understanding that shale roof is the most dangerous one, which has caused

relatively less (17%) accidents due to fall of roof. Reasons behind this may be

that in case of sand stone roof, either the roof condition is underestimated or

supporting the roof by bolts are not being implemented properly because of

unavailability of suitable drilling machines in these mines.

3.7 Analysis of Roof fall accidents by time elapsed after blasting:

Effect of blasting on the condition of roof rock is quite apparent and many roof

fall accidents take place within a short duration after blasting. An analysis of

the accidents due to fall of roof has been done and the result is shown in

Figure 10.

Figure 7: Distribution of roof fall accidents by time elapsed after

blasting since

1997.

From Figure 7, it may be observed that 30% accident occurred within ½ hour

after blasting and 61% accident occurred within 2 hours after blasting. Hence

this period of two hours is very critical and no persons except supporting crew

should be allowed to enter into the face after blasting unless it is supported

properly.

3.8 Analysis of Roof fall accidents by operation

To identify the operations which are critical from the point of roof fall

accidents, an analysis of roof fall accidents vis-à-vis the operations being

carried out during the accidents has been done and the results are shown in

Figure 8.

From Figure 8, it is observed that in 45% accidents, the operations being

carried out at the time of accidents were supporting (conventional), dressing,

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Distribution of Roof Fall accidents by Time (in hours) Elapsed after Blasting1997-2006

2.01 & Above39%

1.01 - 2.0019%

0.51 - 1.0012%

0.00 - 0.5030%

drilling/roof bolting and in 31% accidents loading/shoveling/cleaning,

operations were being done. These are the critical operations during which

people are exposed to the hazard of roof fall and steps are to be taken to

evolve suitable mechanism for either reducing the exposure of such persons

or to provide effective support to protect from roof fall hazards.

Figure 8: Distribution of accidents due to fall of roof (Operation wise)

3.9 Designation wise analysis of persons killed in roof fall accidents

Figure 9: Distribution of roof fall accidents ( Category wise)

Distribution of fall of roof accidents (Designation wise)

Contractor Worker

1%

Supervisory Staff6%

Others4%

Roof Bolter/Driller8%

Trammer2%

SDL/LHD/RH Operator

5%

Dresser 7%

Loader/Mazdoor/Miner42%

Support Person

25%

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Distribution of fall of roof accidents (Operation wise)

Face Drilling3%

Drilling/Roof Bolting11% Dressing

10%

Reduction of Rib3%

Tramming/Travelling3%

Inspection6%

Repairing & Maintenance

1% Others8%

Loading/Shoveling/Cleaning

31%

Supporting (Conventional)

24%

From Figure 9, it is observed that in 42% cases loader/mazdoor/miner were

involved and in 40% cases support personnel including dresser and roof

bolter/driller were involved. Another critical observation is that in 6%

accidents the supervisors themselves were also getting involved. This

highlights the fact that the support personnel and the supervisors getting

involved in such accidents because either suitable temporary supports are not

provided before dressing or setting any support or due precautions are not

being taken for their own safety.

3.10 Analysis of roof fall accidents by type of support

Figure 10: Distribution of accidents due to fall of roof by type of

supports during 1997-2006

From Figure 10, it is revealed that in 41% cases, accident took place where

the place was supported by conventional supports, which is quite high. It is

further revealed that even though roof bolting is a very effective method of

support, in 31% cases accident took place where support system was roof

bolt. This shows that though roof bolting as a primary support system is being

practiced, the efficacy of the system is not as per the desired standard.

3.11 Analysis of Roof Fall accidents by depth of cover

Depth of cover is also a critical parameter affecting the stability of roof. An

analysis of roof fall accidents vis-à-vis depth of cover in Figure 14 shows that

44% accidents due to fall of roof have taken place in the working places within

100m of depth followed by 30% in the range of 100 to 200 meter depth.

Though load on the roof increases with increase in depth of cover and thereby

affecting the stability, it is observed that maximum accidents occurred in the

low depth workings. This may be due to the fact that most of our underground

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Distribution of Fall of Roof Accidents by Type of Support 1997-2006

Roof Bolt31%

Mixed/Others28% Conventional

41%

workings are within the depth of cover range of 0-200m. Hence influence of

depth on load on strata is not very prominent in this range.

Figure 11: Distribution of roof fall accidents by depth of cover

3.12

A n a l y s i s o f r o o f f a l l a c c i d e n t s i n s e m i - m e c h a n i s e d w o r k i n g s w i t h

SDL/ LHD

Table 4: Roof fall accidents vis-à-vis involvement of SDL / LHD operator

YearTotal

Roof fall accident

SDL/LHD Accidents/Fatality

Size of Fall (m)

Type of support

Remark

2002 23 2 (2) (i)1.8*1.6*0.2, (ii)0.6*0.4*0.4

Roof bolt

Canopy could protect operator

2003 17 1 (1) 18*4.5*2.25 Mixed support

2004 26 1 (2) Main fall extended into

working

Mixed support

2005 16 1 (1) 5.0*4.5*1.2-1.5

Mixed support

2006 11 1 (1) 0.8*0.75*0.37 Roof bolt

Canopy could protect operator

Total 93 6(7)

From Table 4, it is observed that during the period of 2002 – 2006, in 50% of

the six accidents due to fall of roof in semi-mechanised workings with SDL /

LHD, the thickness of the fall was only up to 0.4m. Though the work place was

supported with roof bolts, such small thickness of fall has caused fatal injury

to the operators as these machines were not provided with any canopy. Hence

it is essential to provide substantially strong canopy in such machines to

protect the operators.

4.0 In-depth Analysis of the accident due to fall of side:

From Figure 1 (a and 3(b) (Para 2.0) it is observed that 9% of the total

accidents in coal mines are caused due to side fall. Figure 1(b) further shows

that 16% of the below ground accidents are due to side fall during the same

period of 1997-2006, which is quite substantial. Hence analysis of the

accidents due to fall of sides have also been done and the results are depicted

below.

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Distribution of Roof Fall accidents by depth of cover (2002-2006)

301-400 m6%

201-300 m19%

101-200 m30%

0-100 m44%

400 m & above1%

4.1 Analysis of accidents due to fall of side vis-à-vis Method of

work

From Figure 12, it is observed that in 42% cases accident due to fall of side

occurred in bord and pillar development districts and in 58% cases accident

due to fall of side occurred in depillaring district. This reveals the fact that

stability of the pillars are quite vulnerable in depillaring districts and attention

is needed to maintain proper manner of extraction to reduce the problems of

instability of the pillars or ribs or support of the working areas in depillaring

district.

Figure 12: Distribution of side fall accidents and method of working

Distribution of Side Fall Accidents by Method of Working (2002-2006)

Longwall & Others0%

Depillaring58%

Board & Pillar Development

42%

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4.2 Distribution of Side fall accidents by distance from face (2002-

06)

Figure 13: Distribution of accidents due to fall of sides by distance

from face

Distribution of side fall accidents by Distance from Face (2002-2006)

More than 10m37%

Upto 10m52%

At Face11%

Figure 13 reveals that 11% accidents occurred at face and 63% accidents

occurred within 10 metres from the face. Hence the distance of 10m is very

critical from side fall point of view compared to the distance of more than 10m

from the face.

4.3 Analysis of side fall accidents by thickness of fall

Figure 14: Distribution of side fall accidents by thickness of fall

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From Figure 14 it is observed that 60% accidents occurred where thickness of

fall were up to 0.30 metre and 100% accidents occurred where thickness of

fall were up to 1.0 metre. This highlights the fact that outer core of the pillars

are not very stable due to various factors like weathering, formation of cracks

due to blasting etc. and this outer layer has a tendency of spalling and

causing side fall. Hence stability of the sides of the pillars is very important

and if needed, sides of the pillars should be reinforced by side bolts with or

without wire mesh and plastering or shotcreting. Sometimes the sides may be

strengthened by brick walls also.

4.4 Analysis of Side fall accidents by time elapsed after blasting

Figure 15: Distribution of Side fall accidents by time elapsed after

blasting

From the above it is revealed that 11% accident occurred within ½ hour after

blasting, 22% accidents occurred within 2 hours after blasting and 78%

accidents occurred beyond 2 hours after blasting. Hence this may be noted

Distribution of side fall accidents by Thickness of Fall(2002-2006)

0.31 - 1.00 m40%

0.16 - 0.30 m44%

1.01 m & Above0%

0.00 - 0.15 m16%

Distribution of Side Fall accidents by Time Elapsed in hours after blasting

2.01 & Above

(78%)

1.01 - 2.00

(0%)

0.51 - 1.00

(11%)

0.00 - 0.50

(11%)

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that occurrence of side fall is a time dependant phenomena. It is also a fact

that supporting of sides are not given due attention in most of the cases and

with time, the condition of sides further deteriorates; whereas comparatively

more attention is paid for supporting the exposed roof.

4.5 Analysis of side fall accidents by operation at the time of

accident

Figure 16: Distribution of side fall accidents – operation wise

From

Figure

16, it is

revealed that 84% accidents occurred during loading/shoveling/cleaning,

dressing/support (conventional) operations. However, only loading / shoveling

accounts for 61% of the accidents due to fall of sides, which is very high

figure. This may be due to the fact that the manual loaders are exposed to the

danger of side fall while cleaning or shoveling coal from the sides of gallery

which are not properly dressed or supported beforehand.

4.6 Analysis of side fall accidents as per designation of persons

killed

From Figure 17, it is observed that in 72% cases loader/mazdoor/miners were

involved and in 20% cases support personnel including dresser and roof

bolter/driller were involved.

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Distribution of Side Fall accidents (Operation wise)

Face Drilling4%

Drilling/Roof Bolting0%

Dressing12%

Reduction of Rib0%

Tramming/Travelling8%

Inspection0%

Repairing & Maintenance

0% Others4%

Loading/Shoveling/ Cleaning

61%

Supporting (Conventional)

11%

Figure 17: Distribution of side fall accidents – designation wise

4.7 Analysis of Side Fall accidents by depth of cover

Figure 18: Distribution of side fall accidents by depth of cover

From Figure 18 no specific trend is available. 33 % accidents have occurred in

the depth range of 0-100m and 200-300 m. The number of mines at greater

depth is very few and hence the influence of depth on the stability of sides of

pillars is not well established in the current analysis, though the influence of

depth of cover on the stability of sides is a well established fact.

Distribution of side fall accidents (Designation wise)

Contractor Worker

4%

Supervisory Staff0%

Roof Bolter/Driller4%

Trammer0%

SDL/LHD/RH

Operator4%

Dresser 8%

Loader/Mazdoor/Miner72%

Support Person8%

Distribution of Side Fall accidents by depth of cover

(2002-2006)

301-400 m

0%

201-300 m

33%

101-200 m

24%

0-100 m

33%

400 m & above

10%

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5.0 Summary of Analysis of Accidents due to Fall of Roof and Fall of Side

General

(i) Total number of accidents has come down from 143 to 79 during the

period of 1997 to 2006.

(ii) Reduction in number of accidents in below ground mines is more than

50%, i.e. from 94 to present 44 whereas there have been ups and down in

the figure in opencast mines during the same period.

(iii) However, accidents in belowground mines contributed 59% of total

accidents for the last ten years whereas belowground mine contributed

18% of total production during the same period.

(iv)Though there is a general decreasing trend in fatal accidents due to roof

and side fall, there had been sharp increase in the figure in some odd

years which needs special attention.

Fall of roof

(i) Fall of Roof contributes 25 % of total accidents and 42 % of total below

ground accidents in last 10 yrs but there is decreasing trend. The number of

fatal accidents due to fall of roof has come down from 38 to 13. In the year

2006, Fall of Roof contributed 16 % of total accidents and 30% of below

ground accidents.

(ii) Accident due to fall of roof occurred in almost same proportion in bord and

pillar development as well as depillaring districts.

(iii) With the introduction of roof bolts for supporting freshly exposed roof in

development district, there has been decreasing trend in accidents due to fall

of roof in development districts.

(iv)The percentage of roof fall accidents in depillaring district is quite

significant during this period. However, this may be noted that the support

system in depillaring districts is still conventional wooden support with

comparatively less share of roof bolting.

(v) Though SSR has been framed in almost all the mines where accidents due

to fall of roof have occurred, in 49% cases the roof were not kept supported.

This is a matter of serious concern because of the fact that only framing of

SSR does not serve any purpose unless the SSR is implemented in its true

spirit.

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(vi) This may further be noted that in 51% cases, the places of accidents were

supported. This necessitates further examination of the support system to

identify the shortcomings in the SSR and its implementation process.

(vii) The area up to 10 metre from the face is the most critical one. 42%

accident occurred within 5 metres from the face and 58% accident occurred

within 10 metres from the face. If proper attention is given to support the

freshly exposed roof, majority of the roof fall accidents may be controlled.

(viii) 59% of the roof fall accidents occurred where thickness of fall were up

to 0.30 m and 86% accidents occurred where thickness of fall were up to 1.0

m. This clearly indicates that in Indian coal measure rock, the roof rock up to 1

metre above the working section is the most critical one and steps are to be

taken to take care of the roof up to this horizon.

(ix)However, the location of this plane of weakness varies from mines to

mines and from place to place. Hence it is essential to identify this horizon by

suitable scientific method and design the support system accordingly.

(x) In 40% roof fall accident cases nature of fallen strata was sandstone. It is

contrary to the common belief or understanding that shale roof is the most

dangerous one, which has caused relatively less (17%) accidents due to fall of

roof. Reasons behind this may be that in case of sand stone roof, either the

roof condition is underestimated or supporting the roof by bolts are not being

implemented properly because of unavailability of suitable drilling machines

in these mines.

(xi)30% accident occurred within ½ hour after blasting and 61% accident

occurred within 2 hours after blasting. Hence this period of two hours is very

critical and no persons except crew should be allowed to enter into the face

after blasting unless it is supported properly.

(xii) In 45% accidents the operations being carried out at the time of

accidents are supporting (conventional), dressing, drilling/roof bolting and in

31% accidents loading/shoveling/cleaning, operations were being done. These

are the critical operations during which people are exposed to the hazard of

roof fall and steps are to be taken to evolve suitable mechanism for either

reducing the exposure of such persons or to provide effective support to

protect from roof fall hazards.

(xiii) In 42% cases loader/mazdoor/miner were involved and in 40% cases

support personnel including dresser and roof bolter/driller were involved.

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(xiv) Another critical observation is that in 6% accidents the supervisors

themselves are also getting involved. This highlights the fact that the support

personnel and the supervisors getting involved in such accidents because

either suitable temporary supports are not provided before dressing or setting

any support or due precautions are not being taken for their own support.

(xv) In 41% cases, accident took place where the place was supported by

conventional supports, which is quite high.

(xvi) It is further revealed that even though roof bolting is a very effective

method of support, in 31% cases accident took place where support system

was roof bolt. This shows that though roof bolting as a primary support system

is being practiced, the efficacy of the system is not as per the desired

standard.

(xvii) During the period of 2002 – 2006, in 50% of the six accidents due to

fall of roof in semi-mechanised workings with SDL / LHD, the thickness of the

fall was only up to 0.4m. Though the work place was supported with rock

bolts, such small thickness of fall has caused fatal injury to the operators as

these machines were not provided with any canopy. Hence it is essential to

provide substantially strong canopy in such machines to protect the

operators.

Fall of side

(i) Fall of Side contributes 9 % of total accidents and 16 % of total below

ground accidents in last 10 yrs and there is decreasing trend. The number of

fatal accidents due to fall of side has come down from 12 to 04. In the year

2006, Fall of Side contributed 5 % of total accidents and 9% of below ground

accidents.

(ii) 42% cases accident due to fall of side occurred in bord and pillar

development districts and in 58% cases accident due to fall of side occurred in

depillaring district. This reveals the fact that stability of the pillars are quite

vulnerable in depillaring districts and attention is needed to maintain proper

manner of extraction to reduce the problems of instability of the pillars or ribs

or support of the working areas in depillaring district.

(iii) 60% accidents due to side fall occurred where thickness of fall were up to

0.30 metre and 100% accidents occurred where thickness of fall were up to

1.0 metres. This highlights the fact that outer core of the pillars are not very

stable due to various factors like weathering, formation of cracks due to

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blasting etc. and this outer layer has a tendency of spalling and causing side

fall. Hence stability of the sides of the pillars is very important and if needed,

sides of the pillars should be reinforced by side bolts with or without wire

mesh and plastering or shotcreting. Sometimes the sides may be

strengthened by brick walls also.

(iv)11% accident occurred within ½ hour after blasting, 22% accidents

occurred within 2 hours after blasting and 78% accidents occurred beyond 2

hours after blasting. Hence this may be noted that occurrence of side fall is a

time dependant phenomena.

(v) It is also a fact that supporting of sides are not given due attention in most

of the cases and with time, the condition of sides further deteriorates;

whereas comparatively more attention is paid for supporting the exposed

roof.

(vi) 84% accidents occurred during loading/shoveling/cleaning,

dressing/support (conventional) operations. However, only loading / shoveling

accounts for 61% of the accidents due to fall of sides, which is very high

figure. This may be due to the fact that the manual loaders are exposed to the

danger of side fall while cleaning or shoveling coal from the sides of gallery

which are not properly dressed or supported beforehand.

(vii) In 72% cases loader/mazdoor/miners were involved and in 20% cases

support personnel including dresser and roof bolter/driller were involved.

6.0 Future Projection of Coal Production

6.1 Future increase in underground activities

Though the present contribution from underground mining is only 18% of the

total production of the country, the activity in underground coal mining is sure

to multiply in the future. The percentage of coal reserve amenable to

opencast mining is decreasing very fast with the increase in depth of cover.

Winning of coal by opencast method will not be an economic option in the

years to come because of high stripping ratio. More over, quality of coal is a

major concern for the coal producer internationally because of the

environmental issues. Cleaner coal is the talk of the day and at the same time

, in the open market situation, quality of coal is an important parameter to be

considered from market point of view. As we all know, quality of coal by

opencast is quite inferior to underground coal because of its difficulty in

selective mining and mixing of dirts and rocks due to use of HEMM, sales

realization is poor and is sure to affect the economics to a great extent in the

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near future. It is also well accepted that coal will still continue to be the prime

energy source of the country, demand of coal will also continue to be very

high. Hence the gap between the demand and supply will have to be bridged

by increased underground coal production. It is estimated that the quantity of

underground production has to be brought up to 200 mt from the existing

figure of about 60mt by the end of this decade and obviously the activity of

underground mining will assume a large proportion of the total coal mining

activity of the country.

6.2 Future Underground Coal Production Technology:

The following three basic options available for increasing the share of

underground coal production in the years to come:

The traditional method of conventional bord & pillar system will still

continue for quite a longer period because of the socio-political issues related

to employment and scarcity of fund for mechanization.

With the increased strata control problem due to greater depth of

mining in future, and, for bulk production, productivity with increased safety,

thrust is to be put on long wall mining.

Intermediate mechanization using SDL / LHD and Continuous miner

with shuttle car combination may be the most suitable techno-economic

option for increasing the underground mining production in the relatively not

so deep deposits.

7.0 Problem and shortcomings in the present roof bolting system in

Indian Coal Mines

Roof bolting as the principal means of support started gaining ground in

Indian coal mining industry after 1990 following Paul Committee

recommendations. During the last one and a half decade, some progress had

been made in the area. However, problems and shortcomings remained in the

system which need to be addressed now. The application of roof bolting or

rock reinforcement technique in Indian coal mines had largely been restricted

to development areas at shallow depths, where stress level was low and

consequent strata movement could be described as “minimum”. The

performance of low capacity reinforcement systems, by and large, was

satisfactory, which essentially provides scat protection against small scale

slabbing of the immediate roof and controls delamination of the immediate

roof strata.

Generally it was observed that:

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(a) Roof bolting was applied in 76% districts mostly without assessing the

support requirement on the basis of scientific studies, leading to either under

designing or over designing of support system.

(b) Monitoring of support performance did not receive due attention. In all

the cases, the percentage testing of bolts for their anchorage capacity was

very low.

(c) Hardly any studies were conducted to monitor the strata behaviour

which is essential to understand the mechanism of roof bolting/ roof

reinforcement systems under particular geo-mechanical regime.

To sum up, it could be inferred that the progress or absorption of `Roof

Bolting systems designed on the basis of scientific studies’ in Indian

underground environment was poor and incomplete largely due to lack of a

comprehensive approach. This deficiency may have serious consequences

from the point of view of safety.

In order to understand the dimension of problems in proper perspective, a

detailed investigation into a roof fall accident which took place in the

development district of a coal mine where roof bolts were used as a primary

means of support were taken up. The accident resulted in killing four persons

and seriously injuring five. The findings of the study were,

(i) Assessment of installed support system: Support of roof in the

galleries and at the junction (accident site) was grossly deficient. Only

about 25% and 15% supports were provided at galleries and the

junction, respectively.

(ii) Support accessories: 15 mm diameter, roof bolt were used in place of

20-22 mm diameter MS/Tor steel rods. The hole diameter was 20-

22mm larger than the bolt’s diameter whereas the said value should

have been between 8-12mm. This larger annular space in the hole

may cause increase in grout consumption and `Sheath effect’ i.e. poor

mixing of the grout constituent resulting in ‘poor` anchorage.

(iii) Cement Capsules: The infrastructure provided for the manufacture of

the cement capsule was not adequate. There was no mechanism to

monitor the quality aspects of the (a) ingredients/chemicals used in the

capsules and (b) prepared cement capsules.

(iv) Installation of roof bolts: The roof bolts were not installed in a

systematic manner. The spacing between the holes in a row and the

distance between rows were not maintained. Moreover, the holes were

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drilled in different direction with widely varied angle of inclination.

Bearing plates were also not provided in the roof bolts.

As far as systematic installation of roof bolts was concerned, the

enquiry revealed a distinct lack of understanding by the supervisors

and support personnel engaged in the process of roof bolting at the

mine. Training of the officers/supervisors and support personnel

before and during the introduction of roof support by bolting was

deficient. The details of installation of roof bolts could not be found

and a system of recording and monitoring, in this regard was absent.

(v) Assessment of roof bolting system: As a part of the study, laboratory

and field tests were carried out, whose findings are summarized below:

At the accident site, the results of testing point to the fact that although the

bolts had a setting time of more than 72 hours, the anchorage capacity varied

widely between 0.0 tonne and 5.4 tonnes. Further field tests conducted in the

development district of the mine revealed that:

No anchorage development after 2 hours setting (old seized

capsules) with 15mm diameter roof bolts.

Anchorage developed after 2 hours, 8 hours & more than 24

hours setting (new cement capsules) with 22mm diameter roof bolts, were of

the order of 1.0T, 2.5T and 6.0T only.

Though the study detailed above was undertaken at one mine where a major

roof fall accident took place in a roof bolted horizon, the problems highlighted

during the study remain representative of the whole industry barring some

specific places where the system has been established.

Suitable drilling equipment for proper drilling of bore holes to install roof bolts

in coal mine roof rock has remained a problem in Indian coal mines. In many

places coal drills are in use for drilling holes in such rocks. Though coal drills

can be used in coal roof, drilling in sandstone roof with hand held coal drills

pose major problems. In countries where roof bolting is practiced with some

success, pneumatic or hydraulic drills are mainly used.

8.0 Recommendations of National Conference on Safety on Supports:

The menace caused due to fall of roof and sides because of inefficient and

inadequate strata control mechanism is well recognized over the decades and

the matter had been / is being discussed at various for a. National Conference

on Safety in Mines, being the highest tri-partite forum of the country to

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discuss major safety issues and for making policies / strategies for improving

the safety status in mines, had also discussed the issue of strata control in

four out of the nine conferences held so far. Recommendations of these safety

conferences have been instrumental in formulation of statutory guidelines.

9.0 Thrust Areas

From the foregoing analysis of accidents due to fall of roof and sides, the

following observations are found to be critical:

Roof fall accident

(i) Belowground accident contributed 59% of total accident and accident due

to fall of roof contributed 25% of total accident and 42% of total belowground

accident.

(ii) 42% of accident due to fall of roof occurred within 5 metre and 58%

accident due to fall of roof occurred within 10 metre of face.

(iii) In 42% cases, persons engaged in loading operation were involved and in

40% cases, support personnel including dressers are involved.

(iv) In 40% accident fallen roof strata was sandstone. In 59% accident,

thickness of fall was up to 0.3 metre and in 86% cases, thickness of fall was

up to 1 metre.

(v) Conventional support gets dislodged by blasting thereby requiring re-fixing

after each blast, resulting exposure of loaders who are required to clean the

floor to facilitate re-fixing of dislodge support, support crew, dresser and

supervisors below unsupported roof. Conventional timber and steel supports

offer passive resistance to the falling roof, whereas roof bolting remains

essentially an active means of roof support preventing de-lamination of

layered roof rocks,

Side fall accident

(i) Accident due to fall of sides contributed 9% of total accident and 16%

of total belowground accident.

(ii) Out of 58% of belowground accidents caused due to fall of roof and

side, fall of sides account for 16%, which is 28% of the combined causes of

roof and side fall.

(iii) It is also observed that accidents due to side fall in B&P depillaring

district (58%) is more than that of development district. Many of such

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accidents take place due to failure of ribs while extraction or excessive front

abutment pressure on the pillars.

In view of the above the following thrust areas have been identified to reduce

the potentiality of the hazards due to fall of roof & sides:

A. Use of Roof bolts as a primary means of roof support: It is suggested

that for supporting the freshly exposed roof, roof bolts shall be used as a

primary means of support. Use of roof bolts only as support system to support

freshly exposed roof will reduce exposure of persons below freshly exposed

roof. It is essential to inculcate a culture of no operation at the face till the

roof is supported by roof bolts up to 0.6 m from the face. However, while

implementing roof bolting, the following issues need special attention:

(i) The support system primarily with roof bolts shall be designed based on

scientific observations of roof rock properties / behaviour. Horizon of

prominent parting plane or plane of weakness above the working section

should be identified to decide the length of bolts.

(ii) There must be well laid mechanism to ensure supply of proper quality of

roof bolts, grouting materials (resin / cement capsules), bearing plate, nuts &

bolts etc.

(iii) At the same time quality check of installed roof bolts are also equally

important. It is observed that at many places, suitable anchorage testing

machines are not available for testing of efficacy of the roof bolts as per the

guidelines. It is need less to mention that efficacy of the entire strata control

system is based on the efficacy of installation of the roof bolts.

(iv)Considering the advantage and popularity of resin capsules world over, it is

important to consider use of resin grout in place of cement grout, in difficult

strata conditions to start with. Based on the experience, use of resin capsules

in place of cement capsules may be considered in all conditions.

(v)The other critical area is the proper understanding of the principles and

procedures of roof bolting by the workers at grass root levels, particularly the

persons engaged in roof bolting. Their proper understanding will help in

proper implementation. Hence it is suggested to arrange workshops / training

programme etc. on actual practice of roof bolting for the support persons and

supervisors.

B. Stability of sides of pillars or galleries:

From the analysis of accidents due to fall of roof and sides, it is observed that

about 28% of the accidents due to fall of roof & sides are caused due to fall of

sides only. It is primarily because comparatively much less attention is paid

for stability of sides compared to that of roof. Except in highly disturbed areas

where side spalling takes place regularly, not much of attention is paid on the

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stability of sides though its contribution to total accidents is quite significant,

i.e. 9% of total accidents and 16% of total belowground accidents.

In view of the above, in order to reduce the accidents due to fall of roof &

sides, it will be imperative on the operators to pay adequate attention towards

the stability of sides also. This may be ensured by properly dressing the

weak / loose sides, stabilizing weak sides by side bolts with or without wire

meshes, plastering, guiniting, shotcreting or brick walling as required. Further

it is also essential to maintain proper line of extraction in depillaring districts

to avoid undue accumulation of stresses.

C. Establishment of strata control cell:

The condition of strata and the stress environment around any working place

is always dynamic in nature. No two working place is having identical strata

condition. Hence any single readymade solution for strata control is not

feasible. It is essential to assess the roof condition of the working places at

regular intervals by scientific methods. It is observed that in the history of a

mine, RMR has been determined for once and the same data is being used for

designing the support system across the length and breadth of mine. This

may lead to wrong estimation of roof condition.

Monitoring of the effectiveness of roof bolts and strata condition in the active

working areas are also critically important because effective monitoring helps

in taking critical decisions like modification of SSR, withdrawal of work persons

in the event of any danger from fall of roof and sides. Now state of the art

monitoring system through instrumented rock bolts, tell-tale, multipoint bore

hole extensometer, convergence indicator, load cells etc. are available for

continuous monitoring the roof behaviour. Depending on the condition of roof,

rate of extraction and the degree of exposure, suitable monitoring schemes,

need to be developed and implemented. Hence to give a constant backup

technical support to the practicing managers, it is essential to establish

suitable strata control cell at Corporate level and also for a class or group of

mines. Need for setting of strata control units in the mining companies was

recommended in fifth conference. Unfortunately, the large PSUs are yet to

establish any such strata control cell. It is very much essential to have such

strata control cell in all the companies rendering the required technical

services and guidelines to the field mining engineers. Such strata control cell

should be manned by adequate number of technical personnel headed by a

senior official not below the rank of Chief General Manager at Corporate level

and an official not below the rank of Dy.Chief Mining Engineer at area level to

assist mine managers. Suitable training gallery for practical training of

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workers and supervisors regarding application of different strata control

devices may be established.

D. Use of suitable roof bolting machines

From the analysis of roof fall accidents, the following critical observations

were also made:

(i) In 40% accidents, nature of fallen roof was sandstone.

(ii) Implementation of proper roof bolting system suffered from the

disadvantages of non-availability of suitable drilling machines and bolting

accessories.

(iii) In 33% accidents due to fall of roof support personnel were involved.

From the above, the necessity of suitable or fit for use roof bolting machines is

strongly felt. Roof bolting machines will provide suitable drilling system

capable of drilling holes in hard strata. The drilling machine should be capable

of proper churning of the grout materials like resin or cement for effective

interaction between the bolt and the surface of drill holes. This will help in

improving the efficacy of the bolts. The bolting machine should be able to be

operated from a distance or it should be provided with protective canopy so

that safety of drillers is ensured during drilling operation.

F. Introduction of risk assessment for strata control problems:

Risk assessment exercise may be carried out for assessing the risk involved in

a particular mine or work place with respect to strata control problem and the

control mechanisms may be identified. Safety management through risk

assessment may be carried out in every mine to continuously assess the risk

and implement the required control actions. This approach will help in

(i) increasing commitment of all the work persons,

(ii) casting specific responsibility for implementation of control actions, and

(iii) continuously evaluating / assessing the risk reduction process.

10.00 Issues for consideration:

In view of the above considerations the Conference may like to deliberate

upon the following issues for appropriate recommendations:

I. To assist mine managers with regard to formulation of Systematic

Support Rules and for its implementation, suitable strata control cell should

be set up at Corporate level and Area level for a group of mines in each coal

company within a period of one year. Such cells shall be manned by

adequate number of technical personnel headed by a senior official not

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below the rank of Chief General Manager at Corporate level and Dy. Chief

Mining Engineer at Area level.

II. Roof bolting shall be used as a primary means of support for freshly

exposed roof in development as well as depillaring districts. For the roof

category “Poor”, having value of RMR of 40 or less or where there is

excessive seepage of water from the roof strata, roof bolts exclusively with

resin capsules should be used to ensure adequate and immediate

reinforcement of the strata.

III. Due emphasis should also be given to support the sides while framing

Systematic Support Rules.

IV. To ensure proper drilling for roof bolting in all types of roof strata,

suitable, fit-for-use roof bolting machines should be introduced in all mines

within a period of one year. Such machines should be capable of being

operated from a distance or be provided with suitable canopy to protect the

drillers/roof bolters during drilling or bolting operations.

V. Suitable steps are to be taken by the mining companies to inculcate a

culture of “no work at face” till the roof is supported by roof bolts up to at

least 0.6 metre from the face.

VI. Risk assessment exercises are to be carried out for each working district

for assessing the risk from the hazard of roof & side falls and also for

identifying the control mechanism with specific responsibility for

implementation. This exercise should be carried out, at regular intervals to

assess the reduction of risk level and evolving the control mechanism

continuously.

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