Compiled accident statistics in Indian coal mines identified “Fall of Roof & Fall of side” as a major causes of mine accidents.
With the continuous efforts made by all the concerned during the recent past changed the direction of the curve of accidents taking place due to these reasons to downward.
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.
Comparison of fatal accidents due to fall of roof & sides and other causes in coal mines since 1997 to 2006. Belowground accidents due to fall of roof and fall of sides
* Provisional Year Fall of roof Fall of sides Total BG Accidents Total accidents in 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
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. Method 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 Total Board & Pillar Development 19 21 16 11 10 13 07 09 10 04 119 Depillaring 18 14 16 16 16 10 11 13 06 06 126 Long wall & Others 01 01 01 03 04 00 00 03 00 01 14 Total 38 36 33 30 30 22 18 25 16 11 259 Long wall & Others 5% Depillaring 49% Board & Pillar Development 46%
It is revealed from above table 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. Year No. of accidents due to fall of roof No. of SSR framed No. of SSR not framed 2002 22 20 0 2003 18 13 1 2004 26 20 0 2005 18 15 0 2006 13 9 0 Total 97 77 1
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.
Status of Support at accident place (Roof Bolt and Conventional support) (2002-2006) Not Supported 49% Supported 51%
This clearly indicates that in Indian coal measure rock, the roof rock up to 1 meter above the working section is the most critical one and steps are to be taken to take care of the roof up to this horizon.
However, the location of this plane of weakness varies from mines to mines and from place to place. Hence it is essential to identify this horizon by suitable scientific method and design the support system accordingly.
Distribution of Fall of Roof accidents by Thickness of Fall 0.31 - 1.00 m 27% 0.16 - 0.30 m 32% Not Applicable 4% 1.01 m & Above 10% 0.00 - 0.15 m 27%
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.
Distribution of Fatal Roof Fall Accidents by nature of Fallen Strata Data Not Available 4% Coal/Shale/Sandstone 2% Shale 17% Coal & Shale 8% Coal & Sandstone 0% Sandstone 40% Shale & Sandstone 9% Coal 20%
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 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.
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.
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.
Though the work place was supported with roof bolts, such small thickness of fall has caused fatal injury to the operators as these machines were not provided with any canopy. Hence it is essential to provide substantially strong canopy in such machines to protect the operators.
Year Total Roof fall accident SDL/LHD Accidents/Fatality Size of Fall (m) Type of support Remark 2002 23 2 (2) (i)1.8*1.6*0.2, (ii)0.6*0.4*0.4 Roof bolt Canopy could protect operator 2003 17 1 (1) 18*4.5*2.25 Mixed support 2004 26 1 (2) Main fall extended into working Mixed support 2005 16 1 (1) 5.0*4.5*1.2-1.5 Mixed support 2006 11 1 (1) 0.8*0.75*0.37 Roof bolt Canopy could protect operator Total 93 6 (7)
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.
Distribution of Side Fall Accidents by Method of Working (2002-2006) Longwall & Others 0% Depillaring 58% Board & Pillar Development 42%
100% accidents occurred where thickness of fall were up to 1.0 meter.
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.
Distribution of side fall accidents by Thickness of Fall (2002-2006) 0.31 - 1.00 m 40% 0.16 - 0.30 m 44% 1.01 m & Above 0% 0.00 - 0.15 m 16%
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.
Distribution of Side Fall accidents by Time Elapsed in hours after blasting 2.01 & Above (78%) 1.01 - 2.00 (0%) 0.51 - 1.00 (11%) 0.00 - 0.50 (11%)
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.
Distribution of Side Fall accidents (Operation wise) Face Drilling 4% Drilling/Roof Bolting 0% Dressing 12% Reduction of Rib 0% Tramming/Travelling 8% Inspection 0% Repairing & Maintenance 0% Others 4% Loading/Shoveling/ Cleaning 61% Supporting (Conventional) 11%
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.
Distribution of side fall accidents (Designation wise) Contractor Worker 4% Supervisory Staff 0% Roof Bolter/Driller 4% Trammer 0% SDL/LHD/RH Operator 4% Dresser 8% Loader/Mazdoor/ Miner 72% Support Person 8%
No specific trend is available. 33 % accidents have occurred in the depth range of 0-100m and 200-300 m.
The number of mines at greater depth is very few and hence the influence of depth on the stability of sides of pillars is not well established in the current analysis, though the influence of depth of cover on the stability of sides is a well established fact.
Distribution of Side Fall accidents by depth of cover (2002-2006) 301-400 m 0% 201-300 m 33% 101-200 m 24% 0-100 m 33% 400 m & above 10%
Coal will still continue to be the prime energy source of the country. The gap between the demand and supply will have to be bridged by increased underground coal production.
Cleaner coal is the talk of the day, which is possible only through underground mines in the coming years.
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 as the percentage of coal reserves amenable to opencast mining is decreasing very fast with the increase in depth of cover.
The traditional method of conventional Bord & pillar system will still continue for quite a longer period because of the socio-political issues related to employment and scarcity of fund for mechanization.
With the increased strata control problem due to greater depth of mining in future, and, for bulk production, productivity with increased safety, thrust is to be put on long wall mining.
Intermediate mechanization using SDL / LHD and Continuous miner with shuttle car combination may be the most suitable techno-economic option for increasing the underground mining production in the relatively not so deep deposits.
Assessment of installed support system - Only about 25% and 15% supports were provided at galleries and the junction respectively.
Support accessories - The larger annular space left between bolt and sides in the hole may cause increase in grout consumption and `Sheath effect’ i.e. poor mixing of the grout constituent resulting in ‘poor` anchorage.
Cement Capsules - There was no mechanism to monitor the quality aspects of the (a) ingredients/chemicals used in the capsules and (b) prepared cement capsules.
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.
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.
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 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.
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.
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.
Due emphasis should also be given to support the sides while framing Systematic Support Rules.
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.
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 meter from the face.
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.