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An experience of Loading pattern on Power supports with Sand stone roof in SCCL

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This paper was presented at the Seminar on "National Brain storming session on Mechanization of underground coal mines"- organized by SECL (Coal India) at Bilaspur.

This paper was presented at the Seminar on "National Brain storming session on Mechanization of underground coal mines"- organized by SECL (Coal India) at Bilaspur.

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    An experience of Loading pattern on Power supports with Sand stone roof in SCCL An experience of Loading pattern on Power supports with Sand stone roof in SCCL Document Transcript

    • AN EXPERIENCE OF LOADING PATTERN ON POWER SUPPORTS WITH SAND STONE ROOF IN SCCLM. S. VENKATA RAMAIAH MD. SURESH KUMARDy. General Manager Addl. Manager, THE SINGARENI COLLIERIES COMPANY LIMITED. INDIA.1. INTRODUCTION:Padmavathikhani-No.5 Incline is one of the important mines of SCCL. Mechanized longwallmining has started in 1995. 11 longwall panels have been completed and the 12the panel isunder extraction. These longwall panels were worked with 4 x 760 Te supports, the details ofwhich are given in Annexure I.Longwall mining in PVK was operated in middle section of the Top seam. The contact roofwas shaley coal roof, which collapsed with the advance of the support in the goaf without anyoverhang. Contrary to the above, the longwall panel No.21, which is the 11th panel wasworked with contact roof of sand stone.The present paper deals with the loading pattern of 4x760 Te supports and the measurestaken to complete the panel No. 21. In future SCCL is planning to work longwall panels withstone roof. The present experience can be utilized for formulating the guidelines for future.2. DETAILS OF LONGWALL PANEL NO.21:Long wall panel No.21 has been worked in Top seam at a depth of 206m and 239m with aface length of 150m and the panel length of 420m. The face was worked partly with contactroof of stone and partly with shaley coal roof because of the local geological disturbances.3. LITHOLOGY OF THE OVERLYING STRATA:The lithology of the overlying strata was studied in detail for calculation of the cavabilityindex. Borehole No. A/336 has been specially drilled to delineate the bed Inclines. The dataof this bore hole was utilized for installation of Multi Point Bore Hole Extensometer andnumerical modelling by CMRI. The borehole data is given in Annexure-II.4. STRATA MONITORING IN PANEL NO. 21The strata monitoring plan for panel No.21 has been prepared in which various instrumentswere installed, the details of which are given in Annexure-III.5. PREVIOUS EXPERIENCE IN PADMAVATHIKHANI:A review of the earlier experiences of loading pattern is summarized below:
    • 1. The panels were worked in middle section of 9m thick Top seam. 2. The immediate roof comprising of 3m shaley and shaley clay has collapsed in th goaf as soon as the supports were advanced. 3. Closure of supports which hinders the movement of shearer has never occurred earlier. 4. The main fall occurred after 6000 to 7000 m2 exposure. 5. The periodic weightings occurred at 15 to 20m face advance. 6. Load on the four legs were not uniform. Front legs were more loaded than rear legs.6. OPERATION OF THE FACE WITH CONTACT STONE ROOF IN PANEL NO.21The panel started with contact roof with sand stone, which continued for the length of 360 mand ended with coal roof in the last 60m. Initially, the first local fall occurred at the faceretreat of 20.85m where sand stone roof up to the height of 3.0m (bed 1 & 2) collapsed.Successively, the collapse extended throughout the face in three stages. Leg pressureincreased to 30.0MPa. Then the intermediate beds (bed 3 & 4) collapsed exerting pressure upto 32.0MPa over the supports.The main fall and periodic falls were experienced with total contact roof of sand stone; thedetails of local fall, main fall and subsequent periodic weighting are given in the table. NO. OF AVERAG AREA OF LEG LEGS E FACE INTE- WEIGHTING EXPOSUR DATE WEIGHTING PRESSURE ATTAINED RETREA RVAL ZONE E RANGE YIELDING T (M ) (SQ.M) PRESSURE 27.08.04 LOCAL FALL 20.85 TG to C-60, 28-30 Mpa 3,832 C-60 to C-30, C-30 to MG 06.09.04 MAJOR 37.4 C-30 to C-70 29-32 MPa 6,396 FALL 07.9.04 MAIN 40.6 40.6 C-14 to C-81 28-37.8MPa 92 6,957 12.9.04 P.W.- I 49.6 9.0 C-38 to C-39 28-37.8MPa 1 8,200 C-65 to C-69 18.9.04 P.W - II 60.0 11.0 C-27 to C-64 28-37.8MPa 1 9,900 C-72 to C-100During main weighting 90 legs have undergone bleeding and almost all the supports gotloaded throughout the face. Out of 90, 45 were front and 45 were rear legs. The mean loaddensity during main weighting and periodic weighting has been calculated for the total face,which are given the following figures. MG – 25 25 – 50 50 – 75 75 – TG 80.4 90.19 71.75 69.02Loading Patten on Power Supports 2
    • Convergence Data o 39mm cumulative convergence at 2m ahead of the face in tail gate. (12mm per day in the day of weighting at this Point.) o 23mm cumulative convergence at 7m ahead of the face in tailgate. (8mm per day in the day of weighting in this Point.) o 18mm cumulative convergence at 12m ahead of the face in tail gate. o 9mm cumulative convergence at 22m ahead of the face in tail gate.7.0 PROBLEMS FACED DURING 3RD PERIOD WEIGHTING: ♦ 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.Loading Patten on Power Supports 3
    • ♦ 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.8.0 THE VARIOUS REASONS ATTRIBUTED to such dynamicloading and severe leg closures.On careful assessment the following are the various reasons brought to height with theconcrete basis. A. Standing goaf B. Reduced hydraulic run in the pistons C. Premature Bleed valves D. Slow rate of retreat.A.STANDING GOAFThe zone where the supports experienced dynamic loading had a unbroken, Solid, cantileversandstone roof extending into goaf for a length of 20 m. Normally the immediate sand stonebeds Bed 1&2 having RQD 44-93 caving index of 175-1690 and thickness 1.3m-4.0mrespectively 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 upper beds 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 up to rear shield in theother part of face .The reasons may be• Change in Weight modulus of elasticity of the rock and• Change in the Petrography of rock formation, which would have increased the value of RQD and massiveness locally.Hence, the 20 m overhang of Bed 1&2 prevented the collapse of upper beds -Bed 3, 4 & 5atleast for a length twice of its span of overhang ie.,40 m which started exerting enormousstress over the supports.Loading Patten on Power Supports 4
    • • Therefore the Dynamism of load transfer had been initiated during the course of failure of rock mass of all the beds Bed1+Bed2+Bed3+Bed4+Bed5 simultaneously.• The presence of 2.0m thick clay band (bed6) is the one more responsible to cause sudden release of rock mass 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-10mm No.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 up-throw and down-throw faults in the face, the piston heights were reduced to500mm in the weighting zone of c60-c90 to have uniformity in the floor horizon. The sameLoading Patten on Power Supports 5
    • situation was continuing for one week before the start of weighting. The reduced run ofpistons eventually led to leg closure at faster rate that it did not give any allowance to movethe face ahead during the time of Collapse of total strata mass.C. PREMATURE BLEED VALVESThe bleed valves are of spring loaded mechanical type having rate of delivery of fluid 60 litper min. These bleed valves were being regularly checked in underground and brought tosurface for calibration and testing at approved test bench at under ground machine miningwork shop R.G II Area SCCL.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 set pressure of 38.7Mpa (i.e.30 to34Mpa) which in turn reduced the support resistance enormously. DATE WEIGHTING PRS BECOME NO. LEGS NO. OF BLEED ZONE SOLID ATTN.BLEED VALVES PRESSURE PREMATURE 23.9.04 C-43 to C-98 No support become 11 70 solid 24.9.04 C-29 to C-98 C-60to90 22 49D.SLOW RATE OF RETREATDue to some equipment break down, the face down time was increased during this particularperiod. An average of 2.5m per day was maintained upto main fall with which the face couldable to be retreated without any such strata control problems. The details of face progress islisted below:Loading Patten on Power Supports 6
    • DATE NUMBER TOTAL AVG. REASONS FOR FACE SLOW RETREAT OF SHEARS SHEAR RETREAT I II III S (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 Trunk belt Motor problem problem transport23.9.04 0.5 2.5 0.5 3.5 1.4 Trunk Coal evacuation Trunk belt belt problem problem at problem. Surface Bunker24.9.04 Nil 1.5 1 2.5 1.5 Shearer Coal evacuation 25 supports problem problem at solid Surface Bunker 9. MEASURES TAKEN TO OVER COME Though there were three Borehole lithologs so closely located at the centre of this panel to assess the rock formation, it has got only less scope to predict homogeneous formation of rock mass. Hence variation is expected within the panel also. Borehole lithology can not be taken for granted solely. More over it cannot be possible to retreat the face at guaranteed faster rate of retreat due to aging of equipments and other constraints. Then it was decided to attempt ♦ Induced blasting from underground and to avoid any chances of goaf overhang. ♦ To maintain the hydraulic run of PRS to 0.8 to 1.0m at any cost 10. INDUCED BLASTING Almost in the every maintenance shift induced blasting was done in the face. Invariably the goaf over hang was monitored regularly Wherever the cantilever span exceeds by 5.0m the blasting was resorted in that particular zone. The details of induced blasting is given in Annexure – IV. 11. HYDRAULIC RUN Apart from induced blasting, it was holistically decided to maintain the hydraulic run of power supports in range of 0.8 to 1.0m at any given time of face retreat. Hence, • Whenever small up-throw 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. Loading Patten on Power Supports 7
    • 12. NUMERICAL MODELING AND COMPARISON WITH FIELDSTUDIESNumerical modelling has been done by CMRI for this panel. After the conducting of fieldstudies, the data has been compared with the predicted numerical modeling data, the detailsof which are given below: DESCRIPTION MODELING PREDICTION ACTUAL First Local Fall 28.0m 24.6 m First Major Fall 36.0m 41.40 m Main Fall 48.0m 44.60 m13. CONCLUSIONThe experience of working longwall panel No.21 with stone roof has generated lot ofexperiences. Some of the important experiences are as follows: 1. Strata monitoring studies have to be conducted exclusively so as to maintain the equipment accordingly. 2. The intensity of loading in contact stone roof is high compared to coal roof. 3. All the four legs were loaded uniformly in stone roof. 4. Closure of supports with high convergence hinders the movement of the shearer. Hence, the hydraulic run should be carefully monitored and maintained. 5. Induced blasting is necessary to fill the goaf with caved material so as to reduce the intensity of dynamic loading and air blast. 6. During the time of weighting, face has to be worked with higher rate of retreat 7. The hydraulics including valves, bleed valves and legs should be maintained properly to operate the supports at designed capacity.14. RECOMMENDATIONS:The experience for working longwall panel No.21 with stone roof can be further disseminatedfor formulating the guidelines for working longwall with contact roof of stone roof. Theapprehension that longwall can work only with shaley coaly roof is no more valid. It isrecommended that further study based on numerical modelling calibrated with the aboveexperiences can be utilized for working longwall with hard stone roof like King seam ofKothagudem Area and No. III seam of Godavarikhani area.In Padmavathikhani project report longwall mining has to be continued in contact roof ofstone. Similarly, it is also proposed to work longwall in the above conditions of No.III seam.15. ACKNOWLEDGMENTS:The authors expressed their gratitude to the management S.C.Co.Ltd., for giving permissionto publish the above paper. The views expressed in this paper are of their own and notbelonging to the organization in which they are working.Loading Patten on Power Supports 8
    • ANNEXURE –I. DETAILS OF LONGWALL PANELS WORKED P.V.K. NO.5 INCLINE PANEL PANEL PANEL PANEL PANEL PANEL PANE PANEL PANEL PARTICULARS NO.2 NO.3 NO.4 NO.5 NO.5C NO.22 NO.1A NO.1 NO.21Panel size (m) 660x150 675x150 675x150 830x150 730x150 770x150 520x61.9 500x61.5 420x150Depth (Min/Max) 59/112 76/128 96/141 113/158 155/184 174/203 54/96 48/85 206/239Date of 21.8.96 1.7.96 11.11.96 28.7.97 05.03.99 01.06.01 10.07.03 03.02.04 07.08.0commissioningDate of sealing 2.11.96 29.7.97 18.8.98 24.2.99 16.12.00 04.02.04 12.11.03 12.08.04 15.1.06DETAILS OF MAIN FALLA. Retreat (m) 66.50 80.65 81.85 61.90 76.75 50.3 98.0 80.0 40.6B. Area exposed 10164 12420 12605 9532 11820 7740 6749 5300 6957(sq.m)Max. convergenceTail gate (mm) 71 92 92 110 40 44 15 6 28Main gate (mm) 15 52 78 83 46 40 16 18 20Max.Subsidence 2.51 1.93 1.88 2.192 0.68 0.945 1.62 2.37 0.140 (m)Width/Depth ratio 1.33:1 1.17:1 1.06:1 0.94:1 0.81:1 0.64:1 0.645:1 0.73:1 1:1.50PeriodicWeighting - - - 15- 18m 18-20m 15-20m 18-25m 15-20m 8-12m Loading Patten on Power Supports 9
    • ANNEXURE -II. BOREHOLE DATAThree boreholes drilled from surface N = constant depending on RQD%for the purpose of monitoring of L = avg.length of core in cmscaving of different rock beds using T = thickness of bed in mMulti point borehole Extensometerat the center of the panel Based on cavablity index and the30,60,175m from the face start RQD,theline.The different compositerockbeds have been studied by • Immediate roof -bed 1&2CMRI and Mine management • Intermediate roof -bed 3&4carefully.The cavablity of different • Massive and main roof -bed 5rock beds has been calculated using • Parting plane -bed 6 of clay bandthe following empirical relationship. Were delineated and distinguished.The rock formationI = CLnt0.5 encountered in all the three 5 boreholes found to be holdingWhere similarity in terms of RQD and I = cavability index caving C=Compressive strength kg/cm2index. Borehole No A/336 AVG. BED LENGTH COMPRESSIVE DEPTH FROM HEIGHT ABOVE THICK RQD BED LITHOLOGY OF STRENGTH CAVING INDEX SURFACE (M) COAL SEAM, M NESS (%) 2 NO. CORE KG/CM (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.CLAYLoading Patten on Power Supports 10
    • ANNEXURE -III 11
    • ANNEXURE –IV INDUCED BLASTINGA) Location of blasting • Charging was done by using plastic spacers• More emphasis was given to blast in mid face c-40 to c-60. • Only P1-explosives with• The over hang of less than 5.0m was instantaneous electric detonators were also blasted during periodic used in the shot holes. weighting • Atleast 15-20 shot holes (say for Time example c40-c55) used to be blasted• The zone where, if by any chance the in the maintenance shift without piston height is reduced, the goaf in affecting the production shifts by an the rear legs was induced. experienced shotfirer. • Again in the next day blasting used toB) Method of blasting be carried out from c56 –c70 in a step pattern by that time face was retreated to new position if the sand• Around 3-4 m shot holes were drilled stone overhang extended upto c70. and blasted in bed 1&2 at 45º angle between the gaps of power supports, • But during face weighting, it was near rear legs without allowing men arranged to blast the entire length of onto goaf overhang even by affecting the production.• Initially a hydraulic drill was tried.But due to constraints in • Depending upon the necessity and accommodating the machine in the length of overhang along the face, face, manual drilling was done with there used to be two drilling gang, electric drills one from main gate side other from tailgate side because the drilling• The 3-4 deep shot hole was drilled operation was only the critical and with special drill rods. time consuming. • But only one shot firer used to blast 12
    • all shot holes. If the immediate stone started dripping from the cracks of bed did not break at the first day of blasted zone and the immediate beds blasting, attempts were made to blast used to break readily. the same zone on the next day in the • Thereby the plane of weakness was new position of the face. created exactly at the induced break line.C) Effect of induced blasting • Once the immediate beds collapsed, the upper beds used to deflect from• As the induced blasting was practiced the higher origin which exerted only mainly to break the immediate roof it nominal load over the supports was noticed that some times it had • Moreover the rate of leg closures was readily broken and a groove was cut reduced drastically as the upper beds to the depth of 1-2 m . lost its direct cantilever action over• But many times the blasting effect the support canopies. 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, waterReferences:1. Mathur S.P.,(2003) " Strata control - practical considerations" Coal mining technical and management. Vol.10, Nov03.2. Dr.Samir Kumar Das (2004), “Design of Powered Supports for Longwall Faces”, In house short term course for Mining Executives, 18-23 April 2004.3. 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.4. 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.5. Report on "Numerical modeling & Strata and support behaviour investigations at panel 21 PVK-5 incline", Dec04. 13