AN EXPERIENCE OF LOADING PATTERN ON POWER SUPPORTS
WITH SAND STONE ROOF IN SCCL
M. S. VENKATA RAMAIAH MD. SURESH KUMAR
Dy....
Loading Patten on Power Supports 2
1. The panels were worked in middle section of 9m thick Top seam.
2. The immediate roof...
Loading Patten on Power Supports 3
Convergence Data
o 39mm cumulative convergence at 2m ahead of the face in tail gate.
(1...
Loading Patten on Power Supports 4
♦ The face become stand-still, it took around 15 days to restore the normal operation
w...
Loading Patten on Power Supports 5
• Therefore the Dynamism of load transfer had been initiated during the course of failu...
Loading Patten on Power Supports 6
situation was continuing for one week before the start of weighting. The reduced run of...
Loading Patten on Power Supports 7
DATE NUMBER
OF SHEARS
TOTAL
SHEAR
S
AVG.
RETREAT
(M)
REASONS FOR FACE SLOW RETREAT
I II...
Loading Patten on Power Supports 8
12. NUMERICAL MODELING AND COMPARISON WITH FIELD
STUDIES
Numerical modelling has been d...
Loading Patten on Power Supports 9
ANNEXURE –I. DETAILS OF LONGWALL PANELS WORKED P.V.K. NO.5 INCLINE
PARTICULARS
PANEL
NO...
Loading Patten on Power Supports 10
ANNEXURE -II. BOREHOLE DATA
Three boreholes drilled from surface
for the purpose of mo...
11
ANNEXURE -III
12
ANNEXURE –IV INDUCED BLASTING
A) Location of blasting
• More emphasis was given to blast in
mid face c-40 to c-60.
• Th...
13
all shot holes. If the immediate stone
bed did not break at the first day of
blasting, attempts were made to blast
the ...
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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.

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

  1. 1. AN EXPERIENCE OF LOADING PATTERN ON POWER SUPPORTS WITH SAND STONE ROOF IN SCCL M. S. VENKATA RAMAIAH MD. SURESH KUMAR Dy. 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 longwall mining has started in 1995. 11 longwall panels have been completed and the 12the panel is under extraction. These longwall panels were worked with 4 x 760 Te supports, the details of which are given in Annexure I. Longwall mining in PVK was operated in middle section of the Top seam. The contact roof was shaley coal roof, which collapsed with the advance of the support in the goaf without any overhang. Contrary to the above, the longwall panel No.21, which is the 11th panel was worked with contact roof of sand stone. The present paper deals with the loading pattern of 4x760 Te supports and the measures taken to complete the panel No. 21. In future SCCL is planning to work longwall panels with stone 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 a face length of 150m and the panel length of 420m. The face was worked partly with contact roof 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 cavability index. Borehole No. A/336 has been specially drilled to delineate the bed Inclines. The data of this bore hole was utilized for installation of Multi Point Bore Hole Extensometer and numerical modelling by CMRI. The borehole data is given in Annexure-II. 4. STRATA MONITORING IN PANEL NO. 21 The strata monitoring plan for panel No.21 has been prepared in which various instruments were 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:
  2. 2. Loading Patten on Power Supports 2 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.21 The panel started with contact roof with sand stone, which continued for the length of 360 m and ended with coal roof in the last 60m. Initially, the first local fall occurred at the face retreat 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 pressure increased to 30.0MPa. Then the intermediate beds (bed 3 & 4) collapsed exerting pressure up to 32.0MPa over the supports. The main fall and periodic falls were experienced with total contact roof of sand stone; the details of local fall, main fall and subsequent periodic weighting are given in the table. DATE WEIGHTING AVERAG E FACE RETREA T (M) INTE- RVAL WEIGHTING ZONE LEG PRESSURE RANGE NO. OF LEGS ATTAINED YIELDING PRESSURE AREA OF EXPOSUR E (SQ.M) 27.08.04 LOCAL FALL 20.85 TG to C-60, C-60 to C-30, C-30 to MG 28-30 Mpa 3,832 06.09.04 MAJOR FALL 37.4 C-30 to C-70 29-32 MPa 6,396 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 C-65 to C-69 28-37.8MPa 1 8,200 18.9.04 P.W - II 60.0 11.0 C-27 to C-64 C-72 to C-100 28-37.8MPa 1 9,900 During main weighting 90 legs have undergone bleeding and almost all the supports got loaded throughout the face. Out of 90, 45 were front and 45 were rear legs. The mean load density 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 69.02
  3. 3. Loading Patten on Power Supports 3 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.
  4. 4. Loading Patten on Power Supports 4 ♦ 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 dynamic loading and severe leg closures. On careful assessment the following are the various reasons brought to height with the 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 immediate sand 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 upper beds to converge readily. But for no reason, this immediate roof held a long cantilever in this particular zone for a length of 50m in the direction along the face. Whereas it collapsed up to rear shield in the other 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 & 5 atleast for a length twice of its span of overhang ie.,40 m which started exerting enormous stress over the supports.
  5. 5. Loading Patten on Power Supports 5 • 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 relatively less 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 28-32Mpa B.REDUCED HYDRAULIC RUN OF THE PISTONS Due to minor up-throw and down-throw faults in the face, the piston heights were reduced to 500mm in the weighting zone of c60-c90 to have uniformity in the floor horizon. The same
  6. 6. Loading Patten on Power Supports 6 situation was continuing for one week before the start of weighting. The reduced run of pistons eventually led to leg closure at faster rate that it did not give any allowance to move the face ahead during the time of Collapse of total strata mass. C. PREMATURE BLEED VALVES The bleed valves are of spring loaded mechanical type having rate of delivery of fluid 60 lit per min. These bleed valves were being regularly checked in underground and brought to surface for calibration and testing at approved test bench at under ground machine mining work 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 to 34Mpa) which in turn reduced the support resistance enormously. DATE WEIGHTING ZONE PRS BECOME SOLID NO. LEGS ATTN.BLEED PRESSURE NO. OF BLEED VALVES PREMATURE 23.9.04 C-43 to C-98 No support become solid 11 70 24.9.04 C-29 to C-98 C-60to90 22 49 D.SLOW RATE OF RETREAT Due to some equipment break down, the face down time was increased during this particular period. An average of 2.5m per day was maintained upto main fall with which the face could able to be retreated without any such strata control problems. The details of face progress is listed below:
  7. 7. Loading Patten on Power Supports 7 DATE NUMBER OF SHEARS TOTAL SHEAR S AVG. RETREAT (M) REASONS FOR FACE SLOW RETREAT I II III I SHIFT II SHIFT III SHIFT 21.9.04 0.5 1 2.5 4 1.9 Power off Power problem Track bar welding Main belt BSL pan set broken 22.9.04 0.5 1.5 Nil 2 1.1 AFC Gear box Motor transport Trunk belt problem Trunk belt problem 23.9.04 0.5 2.5 0.5 3.5 1.4 Trunk belt problem Coal evacuation problem at Surface Bunker Trunk belt problem. 24.9.04 Nil 1.5 1 2.5 1.5 Shearer problem Coal evacuation problem at Surface Bunker 25 supports solid 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.
  8. 8. Loading Patten on Power Supports 8 12. NUMERICAL MODELING AND COMPARISON WITH FIELD STUDIES Numerical modelling has been done by CMRI for this panel. After the conducting of field studies, the data has been compared with the predicted numerical modeling data, the details of 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 m 13. CONCLUSION The experience of working longwall panel No.21 with stone roof has generated lot of experiences. 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 disseminated for formulating the guidelines for working longwall with contact roof of stone roof. The apprehension that longwall can work only with shaley coaly roof is no more valid. It is recommended that further study based on numerical modelling calibrated with the above experiences can be utilized for working longwall with hard stone roof like King seam of Kothagudem Area and No. III seam of Godavarikhani area. In Padmavathikhani project report longwall mining has to be continued in contact roof of stone. 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 permission to publish the above paper. The views expressed in this paper are of their own and not belonging to the organization in which they are working.
  9. 9. Loading Patten on Power Supports 9 ANNEXURE –I. DETAILS OF LONGWALL PANELS WORKED P.V.K. NO.5 INCLINE PARTICULARS PANEL NO.2 PANEL NO.3 PANEL NO.4 PANEL NO.5 PANEL NO.5C PANEL NO.22 PANE NO.1A PANEL NO.1 PANEL NO.21 Panel size (m) 660x150 675x150 675x150 830x150 730x150 770x150 520x61.9 500x61.5 420x150 Depth (Min/Max) 59/112 76/128 96/141 113/158 155/184 174/203 54/96 48/85 206/239 Date of commissioning 21.8.96 1.7.96 11.11.96 28.7.97 05.03.99 01.06.01 10.07.03 03.02.04 07.08.0 Date 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 15.1.06 DETAILS OF MAIN FALL A. Retreat (m) 66.50 80.65 81.85 61.90 76.75 50.3 98.0 80.0 40.6 B. Area exposed (sq.m) 10164 12420 12605 9532 11820 7740 6749 5300 6957 Max. convergence Tail gate (mm) 71 92 92 110 40 44 15 6 28 Main gate (mm) 15 52 78 83 46 40 16 18 20 Max.Subsidence (m) 2.51 1.93 1.88 2.192 0.68 0.945 1.62 2.37 0.140 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 1:1.50 Periodic Weighting - - - 15- 18m 18-20m 15-20m 18-25m 15-20m 8-12m
  10. 10. Loading Patten on Power Supports 10 ANNEXURE -II. BOREHOLE DATA Three boreholes drilled from surface for the purpose of monitoring of caving of different rock beds using Multi point borehole Extensometer at the center of the panel 30,60,175m from the face start line.The different composite rockbeds have been studied by CMRI and Mine management carefully.The cavablity of different rock beds has been calculated using the following empirical relationship. I = CLn t0.5 5 Where 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 m Based on cavablity index and the RQD,the • Immediate roof -bed 1&2 • Intermediate roof -bed 3&4 • Massive and main roof -bed 5 • Parting plane -bed 6 of clay band Were delineated and distinguished.The rock formation encountered in all the three boreholes found to be holding similarity in terms of RQD and caving index. Borehole No A/336 DEPTH FROM SURFACE (M) BED NO. HEIGHT ABOVE COAL SEAM, M LITHOLOGY BED THICK NESS (M ) RQD (%) AVG. LENGTH OF CORE (CM.) COMPRESSIVE STRENGTH KG/CM 2 CAVING INDEX 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 89 101 175 199 207 211 BED-2 1.3 5.3 CG SST, GW, PEBBLE 4 93 24.3 92 104 1690 1910 203.3 207 BED-3 5.3 9 CG SST, GW, PEBBLE 3.7 77 13.7 89 101 792 898 196.8 203.3 BED-4 9 15.5 3 CG TO FG SST 6.53 94 19.2 98 110 1736 1945 184.3 196.8 BED-5 15.5 28.0 2 CG SST, GW, PEBBLE 12.5 78 16.6 92 104 1893 2140 182.3 184.3 BED-6 28.0 30.0 2 GREY AND CARB.CLAY 2 - - - - - -
  11. 11. 11 ANNEXURE -III
  12. 12. 12 ANNEXURE –IV INDUCED BLASTING 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. 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 shot holes. • 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 main gate side other from tailgate side because the drilling operation was only the critical and time consuming. • But only one shot firer used to blast
  13. 13. 13 all shot holes. 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 . • 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. References: 1. Mathur S.P.,(2003) " Strata control - practical considerations" Coal mining technical and management. Vol.10, Nov'03. 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", Dec'04.

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