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Dynamic loading in the longwall face

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  • 1. DYNAMIC LOADING IN THE LONGWALL FACE -An Experience of Chinese Long wall face. SCCL 1 2 M.S. VENKATA RAMAIAH MD. SURESH KUMARF uture coal mining in the country depends mainly on the underground longwall mechanization. But the failures and economics of longwall faces could not be improved so far. Particularly the long wall panels faced major failures under hard roofconditions. It becomes inevitable to work at greater depth under hard roof to make thetechnology success and economically viable in future. Earlier there were longwall facesbeing worked under sand stone roof Condition in Balrampur, Kumda, Churcha,Kottadi andGdk9inc,10inc of Sccl with only limited success.To encounter the strata controlproblems,induced blasting from surface was practiced in Balrampur and Kumda mines butthe economics could not be justified.A scientific and systematic approach in strata controland up keeping the maintenance of hydraulics and the Powered Supports are the onlysolutions to make any longwall face successful under such hard roof conditions.In Padmavathikhani, the longwall mining was started in the year 1995. So far ten longwallpanels have been extracted. All these panels were worked under coal roof in middle sectionof 9m thick Top seam. The 11th panel i.e. panel No.21 was worked in top section with sandstone roof as contact roof. This paper deals with the dynamic loading and related stratacontrol problems faced during its operation and the experience of sand stone roofmanagement.1. INTRODUCTIONThe underground mining in India lagging far behind in expectations and economics. It is noteconomically viable due to wage cost and production cost. Due to this reason, the opencastmining is dominating over underground mining, resulting in closure of many undergroundmines in the country. Different technologies have been tried so far but the tale of failurecontinues.As the depth of operation are increasing, the opencast mining nearing its end in near future.On the other hand the present level of underground production can be enhanced only withlongwall mechanization. Presently the depth of operation of longwall faces reached beyond300m and it is necessary to go beyond 400m to work longwall faces under the hard roofconditions to extract good quality of coal.A scientific and systematic approach in strata control and up keeping the maintenance ofhydraulics and the Powered Supports are the only solutions to make any longwall facesuccessful under such hard roof conditions.In Padmavathikhani, the longwall mining was started in the year 1995. So far ten longwallpanels have been extracted. All these panels were worked under coal roof in middle sectionof 9m thick Top seam. The 11th panel i.e. panel No.21 has been developed in top sectionwith sand stone roof as contact roof.1 BE, Mtech, PHD Addl .General Manager. SCCL.2 BE, PGDCA, MBA, Addl.Manager. SCCL, Email: MDS_ROOJI@yahoo.co.in
  • 2. 2. SALIENT FEATURES OF PANEL No 21 Length of the panel : 500m Face width : 150 mt Working height : 3.0 mt. Depth of the top seam : 206m min. 239m max. Face Gradient : 1 in 8.9 No. of supports installed in the Face : 102 Roof : Sand stone Floor : coal and shally coal3. POWER ROOF SUPPORTS Make : CME China Support Designation : 4 X 760 Te Chock Shield Type of Linkage : IFS Lemniscate Minimum height : 2.2 m Maximum height : 3.4 m Support Resistance : 110 te/m²/Chock. 104te/m² (overall) Yield pressure : 38.7 MPa (760 Te) Legs : Single telescopic Setting pressure : 29 MPa (75 % of yield pressure). Support Push force : 360 KN (36.0 Te) Support Pull force : 633 KN (63.3 Te) Support travel : 600 mm Support weight : 20.05 Te/Chock Canopy length : 3.87 m. Canopy width : 1.5 m.4. LITHOLOGY OF OVERLYING STRATAThree boreholes drilled from surface for the purpose of monitoring of caving of different rockbeds through Multi point borehole Extensometer at the center of the panel 30,60,175m fromthe face start line.The different composite rockbeds have been studied by CMRI and Minemanagement carefully.The cavablity of different rock beds has been calculated using thefollowing empirical relationship. I = CLnt0.5 5Where I = cavability index C = Compressive strength kg/cm2 N = constant depending on RQD% L = avg.length of core in cms T = thickness of bed in mBased on cavablity index and the RQD,the roof rocks were delineated and distinguished as efollos: • Immediate roof -bed 1&2 • Intermediate roof -bed 3&4 • Massive and main roof -bed 5 • Parting plane -bed 6 of clay bandThe rock formation encountered in all the three boreholes found to be holding similarity interms of RQD and caving index.Dynamic loading in the Long wall face 2
  • 3. Borehole No A/336 AVG. BED LENGT COMPRESSIVE DEPTH FROM HEIGHT ABOVE THICK RQD BED LITHOLOGY H OF STRENGTH CAVING INDEX SURFACE (M) COAL SEAM, M NESS (%) NO. CORE( KG/CM2 (M ) CM.) FROM TO FROM TO AVG. MAX. AVG. MAX COAL SEAM 211 212.3 BED-1 0 1.3 CG SST, GW, 1.3 44 8.67 89 101 175 199 CG SST, GW, 207 211 BED-2 1.3 5.3 4 93 24.3 92 104 1690 1910 PEBBLE CG SST, GW, 203.3 207 BED-3 5.3 9 3.7 77 13.7 89 101 792 898 PEBBLE 15.5 196.8 203.3 BED-4 9 CG TO FG SST 6.53 94 19.2 98 110 1736 1945 3 28.0 CG SST, GW, 184.3 196.8 BED-5 15.5 12.5 78 16.6 92 104 1893 2140 2 PEBBLE 30.0 GREY AND 182.3 184.3 BED-6 28.0 2 - - - - - - 2 CARB.CLAY5. STRATA MONITORING AND INSTRUMENTATIONSIntense strata monitoring studies have been conducted to asses the loading pattern of powersupports, Face and gate road way convergence, caving behavior of different rock beds, therate of leg closures during the time of failure of cantilever beam.Further to have advanced monitoring Bore hole extensometer was installed from installationchamber. The data collected from MPBEx was utilised to determine main roof, main fall, themax caving height and the sequence of failure of different rock beds.6. PROBLEMS FACED DURING FACE OPERATION- dynamic loadingand severe leg closures The main fall took place at the face retreat of 40.6m with an area of exposure6957 sqm .During main weighting there were 90 legs had undergone bleeding at 38.7Mpa.The pressure increase in the legs varies from 28-38.7Mpa against the setting pressure of25 Mpa.Almost 75% of the supports got loaded during weighting throughout the face. Date WEIGH- Average Interval Weighting No. of Area of TING Face Zone Legs Exposure Retreat Started (Sq.m) (m) yielding 07.9.04 MAIN 40.6 40.6 C-14 to C-81 92 6,957 12.9.04 P.W.- I 49.6 9.0 C-38 to C-39 1 8,200 C-65 to C-69 18.9.04 P.W - II 60.0 11.0 C-27 to C-64 1 9,900 C-72 to C-100 The average face retreat was constantly maintained at the rate of 2.5 m/day stillthe main weighting. Absolutely there was no abnormalities in the strata behavior and the sand stoneroof collapsed upto the height of 25 m behind the power supports. Breaker line did not extendahead of the canopy.The leg closure was measured as 11mm per meter of face advance. Around four static test have been conducted ,the defects and the faulty circuitswere carefully monitored and rectified.The condition of Legs,leg NRVs,Bleedvalves,controlDynamic loading in the Long wall face 3
  • 4. blocks were critically assessed and the bye passing circuits were repaired.After main fallperiodic falls were reported at the regular interval of 10 m. During third periodic weighting at the face retreat of 67.0 m the face had undergone severe dynamic weighting.The power supports experienced heavy leg closures and Around 25 supports from c-60 to c-90 become solid towards tailgate side. The shearer could not be moved which was trapped on other end. The face become stand-still,it took around 15 days to restore the normal operation with lifting the power supports one by one by blasting off sand stone roof from underground itself.7. THE VARIOUS REASONS ATTRIBUTED to such dynamic loadingand severe leg closures.On careful assessment the following are the reasons could be brought on concrete basis. A. Standing goaf B. Reduced hydraulic run in the pistons C. Premature Bleed valves D. Slow rate of retreat.A. STANDING GOAF The zone where the supports experienced dynamic loading had a unbroken,Solid,cantilever sandstone roof extending into goaf for a length of 20 m. Normally the immediatesand stone beds Bed 1&2 having RQD 44-93 caving index of 175-1690 and thickness 1.3m-4.0m respectively found collapsing regularly behind the supports leaving only 1-2 m overhang.Thereby the collapse of beds 1&2 used to open a room to the upperbeds to converge readily.But for no reason, this immediate roof held a long cantilever in this particular zone for alength of 50m in the direction along the face.Whereas it collapsed upto rear shield in the other part of face .The reasons may be• Change in Weight modulus of elasticity of the rock andDynamic loading in the Long wall face 4
  • 5. • Change in the Petrography of rock formation, which would have increased the value of RQD and massiveness locally.Hence, the 20 moverhang of Bed1&2 prevented thecollapse of upperbeds -Bed 3, 4 & 5atleast for a lengthtwice of its span ofoverhang ie.,40 mwhich startedexerting enormousstress over thesupports. • Therefore the Dynamism of load transfer had been initiated during the course of failure of rockmass of all the beds Bed1+Bed2+Bed3+Bed4+Bed5 simultaneously. • The presence of 2.0m thick clay band (bed6) is the one more culprit to cause sudden release of rockmass in total. • Which in turn closed around 25 supports and become rigid.The intensity of load in the other part of the face within the weighting zone was relativelyless where the goaf overhang is less than 2.0m which is compared below: Description C 60 to c90 Other part in the weighting zone Leg closure 400mm-600mm 2mm-10mmNo. of legs bleeding 100 55 Goaf overhang 20.0m 1-2m Leg pressures Almost all bleed pressure 28-32MpaB. REDUCED HYDRAULIC RUN OF THE PISTONSDue to minor upthrow and downthrow faults in the face,the piston heights werereduced to 500mm in the weighting zone of c60-c90 to have uniformity in theDynamic loading in the Long wall face 5
  • 6. floor horizon.The same situation was continuing for one week before the start ofweightingThe reduced run of pistons eventually led to leg closure at faster ratethat it did not give any allowance to move the face ahead during the time ofCollapse of total strata mass.C. PREMATURE BLEED VALVESThe bleed valves are of spring loaded mechanical type having rate of delivery offluid 60 lit per min. These bleed valves were being regularly checked inunderground and brought to surface for calibration and testing at approved testbench at Ramagundem.On careful verification it was revealed that in the zone of dynamic load transfer(c-60 to c-90) around 70 Nos. of bleed valves started bleeding below the setpressure of 38.7Mpa (i.e.30 to 34Mpa) which in turn reduced the supportresistance enormously. Date Weighting PRS become No. legs No. of Bleed valves Zone solid attn.bleed Premature pressure23.9.04 C-43 to C-98 No support become 11 70 solid24.9.04 C-29 to C-98 C-60to90 22 49D. SLOW RATE OF RETREATDue some equipment break down the face down time was increased during thisparticular period.An average of 2.5m per day was maintained upto main fallDynamic loading in the Long wall face 6
  • 7. with the face could able to be retreated without any such strata controlproblems.The details face progress listed below:Date Number of Total Avg. Reasons for face slow retreat Shears Shears retreat I II III (m) I shift II shift III shift21.9.04 0.5 1 2.5 4 1.9 Power off Power problem BSL pan Track bar welding set broken Main belt22.9.04 0.5 1.5 Nil 2 1.1 Afc Gear box Trunk belt problem Trunk Motor transport belt lockout23.9.04 0.5 2.5 0.5 3.5 1.4 Trunk Bunker jam Trunk belt belt problem24.9.04 Nil 1.5 1 2.5 1.5 Shearer Coal evacuation 25 supports problem problem in Surface solid Bunker8. MEASURES TAKEN TO OVER COMEThough there were three borehole lithologs so closely located at the centre ofthis panel to assess the rock formation, it has got only less scope to predicthomogeneous formation of rock mass. Hence variation is expected within thepanel also. Borehole lithology can not be taken for granted solely.More over itcan not be possible to retreat the face at guaranteed faster rate of retreat due toageing of equipment and other constraints.Then it was decided to attempt1) Induced blasting from underground and to avoid any chances of goaf overhang.2) To maintain the hydraulic run of PRS to 0.8 to 1.0m at any costINDUCED BLASTINGAlmost in the every maintenance shift induced blasting was done in the face.Invariably the goaf over hang was monitored regularly Wherever the cantileverspan exceeds by 5.0m the blasting was resorted in that particular zone.A) Location of blasting• More emphasis was given to blast in mid face c-40 to c-60.• The over hang of less than 5.0m was also blasted during periodic weighting Time• The zone where, if by any chance the piston height is reduced, the goaf in the rear legs was induced.Dynamic loading in the Long wall face 7
  • 8. B) Method of blasting• Around 3-4 m shot holes were drilled and blasted in bed 1&2 at 45º angle between the gaps of power supports, near rear legs without allowing men onto goaf• Initially a hydraulic drill was tried.But due to constraints in accommodating the machine in the face, manual drilling was done with electric drills• The 3-4 deep shot hole was drilled with special drill rods.• Charging was done by using plastic spacers• Only P1 explosives with instantaneous electric detonators were used in the shotholes.• Atleast 15-20 shot holes (say for example c40-c55) used to be blasted in the maintenance shift without affecting the production shifts by an experienced shotfirer.• Again in the next day blasting used to be carried out from c56 –c70 in a step pattern by that time face was retreated to new position if the sand stone overhang extended upto c70.• But during face weighting ,it was arranged to blast the entire length of overhang even by affecting the production.• Depending upon the necessity and length of overhang along the face, there used to be two drilling gang, one from maingate side other from tailgate side because the drilling operation was only the critical and time consuming. But only one shotfirer used to blast all shotholes• If the immediate stone bed did not break at the first day of blasting ,attempts were made to blast the same zone on the next day in the new position of the face.C) Effect of induced blasting• As the induced blasting was practiced mainly to break the immediate roof it was noticed that some times it had readily broken and a groove was cut to the depth of 1-2 m .Dynamic loading in the Long wall face 8
  • 9. • But many times the blasting effect could not be able to break the roof. But it shattered the strata thereby cracks were developed and• During the time of upper beds and the main bed started deflecting with load transmitted over the supports, water started dripping from the cracks of blasted zone and the immediate beds used to break readily.• Thereby the plane of weakness was created exactly at the induced break line.• Once the immediate beds collapsed, the upper beds used to deflect from the higher origin which exerted only nominal load over the supports• Moreover the rate of leg closures was reduced drastically as the upper beds lost its direct cantilever action over the support canopies.HYDRAULIC RUNApart from induced blasting, it was holistically decided to maintain thehydraulic run of power supports in range of 0.8 to1.0m at any given time offace retreat.Hence,• Whenever small upthrow and down throw faults encountered in the face ,was blasted off to maintain the hydraulic run.• Also it was cut with shearer to have correct horizon without bothering the consumption of picks.• Some times the floor horizon got lifted up thereby the total height in the face was reduced due improper floor cutting.The operators thoroughly educated and with the dedicated approach it was monitored round the clock.CONCLUSION1. For the first time a longwall panel was worked under sand stone roof successfully which has brought lot of experiences,techniques to work under any hard roof condition to the team of officers and workmen.2. Scientific strata monitoring studies have been conducted extensively.Also by proper maintenance of power supports, reasonable rate of retreat and by disciplined face operations the main weighting was successfully negotiated.3. Periodic weightings were occurring at an interval of 8-12m.4. Due to 20.0m overhang of immediate sand stone beds and slow rate of retreat a dynamic loading was initiated.As the effect,around 25supports were totally closed.Simultaneously the supports were not in a position to offer required support resistance due to premature bleeding of legs.The reduction in the hydraulic run of pistons did not give any allowance to retreat the face forward.5. The face become stand still.With war footing efforts the operations were restored.6. The different likely causes were carefully studied.Accordingly emphasis was given to induced blasting from underground and to maintain sufficient hydraulic run in the pistons.Dynamic loading in the Long wall face 9
  • 10. 7. Special scheme of operation of induced blasting aimed to break the immediate sand stone beds was drawn.Though it was tedious job, it was done whole-heartedly.8. As the effect of shattering the immediate roof and maintaining sufficient hydraulic run there were lot of change in the loading pattern over the supports ie., • Periodic weightings/falls were well regularised. • MLD particularly in zone of weighting did not reach 90 te/sq.m against its support resistance of 110 te/sq.m except during third periodic weighting. • Though the supports were loaded to their yield pressure in successive periodic weightings,the rate of leg closures was observed to be very nominal.9. With these,the face did not experience any severe,dynamic loading and abnormal leg closures during its entire extraction length.10. Steps are being taken to replace all the mechanical type yield valves by ‘gas filled” type.11. The knowledge,experience and the competency gained by the longwall crew Working under sand stone roof in top seam of Padmavathikhani mine useful to work longwall panels in the bottom seam of the mine with sand stone roof to extract high quality of coal. *******************************************Acknowledgments:The authors expressed their gratitude to the management S.C.Co.Ltd., for givingpermission to publish the above paper. The views expressed in this paper are oftheir own and not belonging to the organization in which they are working.References:1. Quarterly review meeting report on "Ground Movement Assessment" of panel no.21.2. Report on "Numerical modeling & Strata and support behaviour investigations at panel 21 PVK-5 incline", Dec04.3. Sarkar SK (1998) “Mechanized Longwall Mining – The Indian Experiences”.3. Dr.Samir Kumar Das (2004), “Design of Powered Supports for Longwall Faces”, In house short term course for Mining Executives, 18-23 April 2004.4. Venkata Ramaiah M.S., (2001), “Experience of Strata Monitoring in Longwall Mining at GDK.10A Incline”, National Conference on Strata Control in Coal Mines, Godavarikhani, 25-26 November, 2001.Dynamic loading in the Long wall face 10
  • 11. 5. Venkata Ramaiah M.S. and Suresh Kumar M.D., (2004) "Experience of Strata monitoring studies in shallow depth longwall extraction by caving in Panel no. 1A & 1 of PVK-5 Incline" 3rd National seminar on rock excavation techniques at Nagpur organised by The Indian Mining and Engineering Journal Bhubaneswar chapter.6. Suresh Kumar M.D., and U.Shiva shankar (2006)” Need for working longwall under hard roof in future underground mining-An experience of negotiating main weighting in sand stone roof”-workshop on future of underground coal mining in india mechanised board&pillar or longwall”organised by JMMF.Kolkata.7. Mathur S.P.,(2003) " Strata control - practical considerations" Coal mining technical and management. Vol.10, Nov03.8. Technical Paper on Hard roof management-a tool for successful longwall caving by M.P.Dikshit and M.L.Guptha.Dynamic loading in the Long wall face 11

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