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

1.0      Introduction:


         Accidents due to movemen...
Figure 1(a): Comparison of fatal accidents due to fall of roof and sides and
other causes in coal mines since 1997 to 2006...
(iii)       Though there is a general decreasing trend in fatal accidents due to roof and
                      side fall,...
From Table 2 and Figure 2, it can be observed that accidents due to fall of roof occurred
   in almost same proportion in ...
From Figure 3, it may be observed that though SSR has been framed in almost all the
mines where accidents due to fall of r...
However, the location of this plane of weakness varies from mines to mines and from
   place to place. Hence it is essenti...
From the above it is observed that in 40% roof fall accident cases nature of fallen strata
was sandstone. It is contrary t...
critical operations during which people are exposed to the hazard of roof fall and steps
     are to be taken to evolve su...
From Figure 9, it is observed that in 42% cases loader/mazdoor/miner were involved and
in 40% cases support personnel incl...
Figure 11: Distribution of roof fall accidents by depth of cover



                        Distribution of Roof Fall acci...
were not provided with any canopy. Hence it is essential to provide substantially strong
      canopy in such machines to ...
4.2     Distribution of Side fall accidents by distance from face (2002-06)


 Figure 13: Distribution of accidents due to...
From Figure 14 it is observed that 60% accidents occurred where thickness of fall were
up to 0.30 metre and 100% accidents...
From    Figure    16,    it   is   revealed   that   84%     accidents   occurred    during
     loading/shoveling/cleanin...
4.7    Analysis of Side Fall accidents by depth of cover


Figure 18: Distribution of side fall accidents by depth of cove...
Fall of roof


(i)        Fall of Roof contributes 25 % of total accidents and 42 % of total below ground
           accid...
(ix)     However, the location of this plane of weakness varies from mines to mines and
         from place to place. Henc...
(xvii) During the period of 2002 – 2006, in 50% of the six accidents due to fall of roof
           in semi-mechanised wor...
(vi)        84% accidents occurred during loading/shoveling/cleaning, dressing/support
                 (conventional) ope...
•   With the increased strata control problem due to greater depth of mining in future,
          and, for bulk production...
accident resulted in killing four persons and seriously injuring five. The findings of the
study were,


(i)     Assessmen...
•   No anchorage development after 2 hours setting (old seized capsules) with 15mm
          diameter roof bolts.
      • ...
(iii) In 42% cases, persons engaged in loading operation were involved and in 40%
cases, support personnel including dress...
(iii)   At the same time quality check of installed roof bolts are also equally important.
It is observed that at many pla...
Monitoring of the effectiveness of roof bolts and strata condition in the active working
areas are also critically importa...
Risk assessment exercise may be carried out for assessing the risk involved in a
         particular mine or work place wi...
************




               B-65
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Reduction of risk from roof and side fall in Indian coal mines

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

  1. 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 B-39
  2. 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. B-40
  3. 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% B-41
  4. 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% B-42
  5. 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. B-43
  6. 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 B-44
  7. 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 B-45
  8. 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% B-46
  9. 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. B-47
  10. 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 B-48
  11. 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% B-49
  12. 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% B-50
  13. 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% B-51
  14. 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% B-52
  15. 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. B-53
  16. 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. B-54
  17. 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. B-55
  18. 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. B-56
  19. 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. B-57
  20. 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 B-58
  21. 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: B-59
  22. 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. B-60
  23. 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. B-61
  24. 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. B-62
  25. 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: B-63
  26. 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. B-64
  27. 27. ************ B-65

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