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
M. S. VENKATA RAMAIAH MD. SURESH KUMAR
Dy. General Manager Addl. Manager,
THE SINGARENI COLLIERIES COMPANY LIMITED. INDIA.
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:
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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
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
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.
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
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
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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
♦ 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.
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♦ 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
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
A. Standing goaf
B. Reduced hydraulic run in the pistons
C. Premature Bleed valves
D. Slow rate of retreat.
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.
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• 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
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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.
NO. OF BLEED
23.9.04 C-43 to C-98 No support become
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
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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
22.9.04 0.5 1.5 Nil 2 1.1 AFC Gear box
23.9.04 0.5 2.5 0.5 3.5 1.4 Trunk
24.9.04 Nil 1.5 1 2.5 1.5 Shearer
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
• 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.
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12. NUMERICAL MODELING AND COMPARISON WITH FIELD
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
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
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.
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.
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.
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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
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
• Immediate roof -bed 1&2
• Intermediate roof -bed 3&4
• Massive and main roof -bed 5
• Parting plane -bed 6 of clay
Were delineated and
distinguished.The rock formation
encountered in all the three
boreholes found to be holding
similarity in terms of RQD and
Borehole No A/336
COAL SEAM, M
FROM TO FROM TO AVG. MAX. AVG. MAX
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,
4 93 24.3 92 104 1690 1910
203.3 207 BED-3 5.3 9
CG SST, GW,
3.7 77 13.7 89 101 792 898
196.8 203.3 BED-4 9
CG TO FG SST 6.53 94 19.2 98 110 1736 1945
184.3 196.8 BED-5 15.5
CG SST, GW,
12.5 78 16.6 92 104 1893 2140
182.3 184.3 BED-6 28.0
2 - - - - - -
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
• 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
• Initially a hydraulic drill was
tried.But due to constraints in
accommodating the machine in the
face, manual drilling was done with
• The 3-4 deep shot hole was drilled
with special drill rods.
• Charging was done by using plastic
• 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
• 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
• 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
• But only one shot firer used to blast
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
• 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.
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.