1. 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
Total accidents in
Year Fall of roof Fall of sides Total BG Accidents
Coal Mines
1997 38 12 94 143
1998 35 15 80 128
1999 33 11 74 127
2000 27 14 62 117
2001 30 9 67 105
2002 23 11 48 81
2003 18 5 46 83
2004 26 8 49 87
2005 18 7 49 96
2006* 13 4 44 79
* Provisional
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2. 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 Roof
25%
Fall of Side
Other Causes 9%
66%
Figure 1(b): Belowground accidents due to fall of roof and fall of sides
Comparison of Accidents in coal mines
due to Fall of Roof and Fall of Sides with Belowground Accidents
(1997-2006)
Other B/G Fall of Roof
Causes 42%
42%
Fall of Sides
16%
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. In the year 2006, Fall
of Side contributed 5% of total accidents and 9% of below ground accidents.
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3. (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 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 Total
Board & Pillar 19 21 16 11 10 13 07 09 10 04 119
Development
Depillaring 18 14 16 16 16 10 11 13 06 06 126
Long wall & 01 01 01 03 04 00 00 03 00 01 14
Others
Total 38 36 33 30 30 22 18 25 16 11 259
Figure 2: Method wise percentage of accidents due to fall of roof.
Distribution of accidents due to Fall of Roof - Method wise
(1997-2006)
Long wall &
Others
5%
Board &
Pillar
Development
46%
Depillaring
49%
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4. 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 No. of SSR framed No. of SSR not
of roof 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
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
Status of Support at accident place ( Roof Bolt and Conventional support
( 2002-2006)
Not Supported
49%
Supported
51%
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5. 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
Figure 4: Distribution of roof fall accidents by distance from face
Distribution of roof fall accidents by Distance from Face
Other places 0.00 - 5.00 m
22%
42%
20.01 m & Above
11%
10.01 to 20.00 m
5.01 - 10.00 m
9%
16%
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.
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6. 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.
Figure 5: Distribution of accidents due to fall of roof by thickness of fall
Distribution of Fall of Roof accidents by Thickness of Fall
Not Applicable
4%
1.01 m & Above
0.00 - 0.15 m
10%
27%
0.31 - 1.00 m
27%
0.16 - 0.30 m
32%
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
Distribution of Fatal Roof Fall Accidents by nature of Fallen Strata
Coal/Shale/Sandstone
Shale & Sandstone 2%
9% Data Not Available Coal
4%
20%
Coal & Sandstone
0%
Shale
17%
Sandstone
40%
Coal & 8%
Shale
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7. 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 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.
Distribution of Roof Fall accidents by Time (in hours) Elapsed after Blasting
1997-2006
0.00 - 0.50
30%
2.01 & Above
39%
1.01 - 2.00 0.51 - 1.00
19% 12%
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, drilling/roof bolting and
in 31% accidents loading/shoveling/cleaning, operations were being done. These are the
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8. 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)
Distribution of fall of roof accidents (Operation wise)
Inspection Repairing &
6% Maintenance
Reduction of Rib 1% Others
3% 8%
Loading/Shoveling
Tramming/Travelling /Cleaning
3%
31%
Face Drilling
3%
Drilling/Roof Supporting
Bolting (Conventional)
11% Dressing
24%
10%
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)
Supervisory Staff Contractor
SDL/LHD/RH 6% Worker
Operator 1%
Others
5% 4%
Trammer Loader/Mazdoor/
2% Miner
42%
Roof Bolter/Driller
8%
Dresser
7% Support Person
25%
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9. 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
Distribution of Fall of Roof Accidents by Type of Support
1997-2006
Mixed/Others
28% Conventional
41%
Roof Bolt
31%
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 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.
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10. Figure 11: Distribution of roof fall accidents by depth of cover
Distribution of Roof Fall accidents by depth of cover
(2002-2006)
400 m & above
301-400 m
1%
6%
201-300 m
19%
0-100 m
44%
101-200 m
30%
3.12 Analysis of roof fall accidents in semi-mechanised workings with SDL/
LHD
Table 4: Roof fall accidents vis-à-vis involvement of SDL / LHD operator
Total SDL/LHD Size of Fall Type of Remark
Year Roof fall Accidents/Fatality (m) support
accident
2002 23 2 (2) (i)1.8*1.6*0.2, Canopy could protect
(ii)0.6*0.4*0.4 Roof bolt operator
2003 17 1 (1) 18*4.5*2.25 Mixed
support
2004 26 1 (2) Main fall Mixed
extended into support
working
2005 16 1 (1) 5.0*4.5*1.2-1.5 Mixed
support
2006 11 1 (1) 0.8*0.75*0.37 Canopy could protect
Roof bolt 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
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11. 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.
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 & Others
0%
Board & Pillar
Development
42%
Depillaring
58%
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12. 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)
At Face
11%
More than
10m
37%
Upto 10m
52%
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
Distribution of side fall accidents by Thickness of Fall
(2002-2006)
1.01 m & Above
0%
0.00 - 0.15 m
16%
0.31 - 1.00 m
40%
0.16 - 0.30 m
44%
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13. 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
Distribution of Side Fall accidents by Time Elapsed in hours after
blasting
0.00 - 0.50 0.51 - 1.00
(11%) (11%)
1.01 - 2.00
(0%)
2.01 & Above
(78%)
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 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
Distribution of Side Fall accidents (Operation wise)
Reduction of Rib Inspection Repairing &
0% 0% Maintenance
Tramming/Travelling
0% Others
8%
4%
Face Drilling
4%
Loading/Shoveling/
Cleaning
Drilling/Roof Bolting 61%
0%
Supporting
Dressing
(Conventional)
12%
11%
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14. 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.
Figure 17: Distribution of side fall accidents – designation wise
Distribution of side fall accidents (Designation wise)
SDL/LHD/RH
Operator Supervisory Staff
4% 0% Contractor
Trammer Worker
0% 4%
Roof Bolter/Driller
4%
Dresser
8%
Support Person
8% Loader/Mazdoor/
Miner
72%
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15. 4.7 Analysis of Side Fall accidents by depth of cover
Figure 18: Distribution of side fall accidents by depth of cover
Distribution of Side Fall accidents by depth of cover
(2002-2006)
400 m & above
10%
0-100 m
301-400 m
33%
0%
201-300 m
33%
101-200 m
24%
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.
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.
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16. 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.
(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.
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17. (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.
(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.
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18. (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 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.
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19. (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 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.
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20. • 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:
(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
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21. 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 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:
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22. • 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 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.
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23. (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 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.
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24. (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 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.
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25. 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 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:
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26. 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 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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