1. DYNAMIC LOADING IN THE LONGWALL FACE
-An Experience of Chinese Long wall face. SCCL
1 2
M.S. VENKATA RAMAIAH MD. SURESH KUMAR
F 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 roof
conditions. It becomes inevitable to work at greater depth under hard roof to make the
technology success and economically viable in future. Earlier there were longwall faces
being worked under sand stone roof Condition in Balrampur, Kumda, Churcha,Kottadi and
Gdk9inc,10inc of Sccl with only limited success.To encounter the strata control
problems,induced blasting from surface was practiced in Balrampur and Kumda mines but
the economics could not be justified.A scientific and systematic approach in strata control
and up keeping the maintenance of hydraulics and the Powered Supports are the only
solutions 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 longwall
panels have been extracted. All these panels were worked under coal roof in middle section
of 9m thick Top seam. The 11th panel i.e. panel No.21 was worked in top section with sand
stone roof as contact roof. This paper deals with the dynamic loading and related strata
control problems faced during its operation and the experience of sand stone roof
management.
1. INTRODUCTION
The underground mining in India lagging far behind in expectations and economics. It is not
economically viable due to wage cost and production cost. Due to this reason, the opencast
mining is dominating over underground mining, resulting in closure of many underground
mines in the country. Different technologies have been tried so far but the tale of failure
continues.
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 with
longwall mechanization. Presently the depth of operation of longwall faces reached beyond
300m and it is necessary to go beyond 400m to work longwall faces under the hard roof
conditions to extract good quality of coal.
A scientific and systematic approach in strata control and up keeping the maintenance of
hydraulics and the Powered Supports are the only solutions 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 longwall
panels have been extracted. All these panels were worked under coal roof in middle section
of 9m thick Top seam. The 11th panel i.e. panel No.21 has been developed in top section
with 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 coal
3. 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 STRATA
Three boreholes drilled from surface for the purpose of monitoring of caving of different rock
beds through 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 = CLnt0.5
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 roof rocks were delineated and distinguished as e
follos:
• Immediate roof -bed 1&2
• Intermediate roof -bed 3&4
• Massive and main roof -bed 5
• Parting plane -bed 6 of clay band
The rock formation encountered in all the three boreholes found to be holding similarity in
terms 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.CLAY
5. STRATA MONITORING AND INSTRUMENTATIONS
Intense strata monitoring studies have been conducted to asses the loading pattern of power
supports, Face and gate road way convergence, caving behavior of different rock beds, the
rate of leg closures during the time of failure of cantilever beam.
Further to have advanced monitoring Bore hole extensometer was installed from installation
chamber. The data collected from MPBEx was utilised to determine main roof, main fall, the
max caving height and the sequence of failure of different rock beds.
6. PROBLEMS FACED DURING FACE OPERATION- dynamic loading
and severe leg closures
The main fall took place at the face retreat of 40.6m with an area of exposure
6957 sqm .During main weighting there were 90 legs had undergone bleeding at 38.7
Mpa.The pressure increase in the legs varies from 28-38.7Mpa against the setting pressure of
25 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 still
the main weighting.
Absolutely there was no abnormalities in the strata behavior and the sand stone
roof collapsed upto the height of 25 m behind the power supports. Breaker line did not extend
ahead 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 circuits
were carefully monitored and rectified.The condition of Legs,leg NRVs,Bleedvalves,control
Dynamic loading in the Long wall face 3

4. blocks were critically assessed and the bye passing circuits were repaired.After main fall
periodic 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 loading
and 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 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 upto rear shield in the other part of face .The reasons may be
• Change in Weight modulus of elasticity of the rock and
Dynamic 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 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.
• 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 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 upthrow and downthrow faults in the face,the piston heights were
reduced to 500mm in the weighting zone of c60-c90 to have uniformity in the
Dynamic loading in the Long wall face 5

6. floor horizon.The same situation was continuing for one week before the start of
weightingThe 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 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 set
pressure of 38.7Mpa (i.e.30 to 34Mpa) which in turn reduced the support
resistance enormously.
Date Weighting PRS become No. legs No. of Bleed valves
Zone solid attn.bleed Premature
pressure
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 49
D. SLOW RATE OF RETREAT
Due 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
Dynamic loading in the Long wall face 6

7. with the face could able to be retreated without any such strata control
problems.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 shift
21.9.04 0.5 1 2.5 4 1.9 Power off Power problem BSL pan
Track bar welding set broken
Main belt
22.9.04 0.5 1.5 Nil 2 1.1 Afc Gear box Trunk belt problem Trunk
Motor transport belt lockout
23.9.04 0.5 2.5 0.5 3.5 1.4 Trunk Bunker jam Trunk belt
belt problem
24.9.04 Nil 1.5 1 2.5 1.5 Shearer Coal evacuation 25 supports
problem problem in Surface solid
Bunker
8. 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
can not be possible to retreat the face at guaranteed faster rate of retreat due to
ageing of equipment and other constraints.Then it was decided to attempt
1) 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 cost
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.
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 RUN
Apart from induced blasting, it was holistically decided to maintain the
hydraulic run of power supports in range of 0.8 to1.0m at any given time of
face 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.
CONCLUSION
1. 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 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.
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", Dec'04.
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, Nov'03.
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