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International Conference on Underground Space Technology              Technical Papers




  Selection of Powered Roof Supports – 2-Leg Shields vis-à-vis
                     4-Leg Chock Shields

                                  B Ramesh Kumar
                         CGM, Corporate Project and Planning,,
                                    U Siva Sankar
                            Under Manger, Project Planning,
                                     VNS Prasad,
                     Deputy Manager, Corporate Project and Planning
                                   SCCL, AP, India


ABSTRACT

The success of a longwall face depends to a large extent on the type and
capacity of the Powered Roof Supports. In India, different types of Powered Roof
Supports of various capacities were tried earlier, but the four legged chock
shields have been the most widely used supports. An extensive literature review
on Indian longwall mining scenario over the last few decades suggests that
majority of the downtimes and or failures were mainly due to ground control
problems and inadequate capacity and type of powered roof supports. In India
several mines Kottadih, Churcha and Dhemomain had experienced catastrophic
failures of longwall faces.


In this paper, a case study was presented, summarizing the experiences of
working Longwall faces with IFS, 4-leg chock shields under varying contact roofs,
viz; coal and sand stone. Based on the field observations, conclusions were
drawn regarding the suitability of 2 leg shields over 4-leg chock shields under
Indian geo mining conditions in general and particularly regarding longwall mines
of Singareni Collieries Company Ltd (SCCL).




Key words: Longwall, Powered Roof Support, Contact Roof




                                                                         CM - 05 - 1
17 – 19 January 2011, Bangalore, India
International Conference on Underground Space Technology            Technical Papers



Introduction
SCCL introduced mechanized longwall mining in 1983. The first few faces were
worked with 4x360 t (conventional) 4x450 t IFS supports. Failure of the support
with regard to breakage of immediate roof between canopy tip and face resulted
in formation of cavities in face and subsequent closure of chocks and face
stoppages. The cavities were more frequent and negotiation of the same with IFS
supports was even more difficult.
With the introduction of second generation longwall supports, the support
capacity was increased to 4x760 t (PVK) and 4x800 t (GDK 10A & GDK 9E),
there was a considerable improvement in strata control. Face stoppages due to
problems related to strata failures or closure of chocks were few, though not
totally eliminated.
In SCCL, majority of the longwall faces are worked under weak immediate roof
conditions. The strength of immediate roof coal/sandstone of worked Longwalls
was of weak to moderate strong. In most of the above cases, it has been
observed that the front legs were taking more load than rear legs of conventional
as well as IFS chock shield supports, due to crumbled and premature caving
nature of immediate roof [1-5].

Studies conducted and analysis of performance of powered roof supports
suggests that above supports with higher capacity are able to deal most of the
problems of strata under the prevailing geo-mining conditions. However there is
always a scope for selection of correct type of support to reduce the face
stoppages because of strata control problems, as high cost equipment installed
should generate revenue for long term financial requirements.

2.0 Trend of development of Powered Roof Supports:

Two fundamental changes in shield design have been made since the
introduction of the shield in 1975: (1) the caliper design was replaced with a
lemniscate-guided caving shield that maintains a constant tip-to-face distance
throughout its operating range; and (2) electro hydraulic control systems have


                                                                       CM - 05 - 2
17 – 19 January 2011, Bangalore, India
International Conference on Underground Space Technology              Technical Papers



replaced manual systems to permit remote and automated operation of the shield
[6, 9].


In India first mechanized powered roof support face, the new-age Longwall, was
launched in August 1978 at Moonidih Colliery. In India, 33 Longwall packages
have been deployed to date in both Coal India Ltd and SCCL in collaboration
with U.K, Russia, Poland, China and France mostly funded by GOI. The list of the
powered roof supports deployed at Indian longwall faces is as given in the
Table.1.


The basic shield structure has remained unchanged for the past 25 years,
although the structures have grown dramatically in size and capacity. Early
generations of shields experienced several structurally related failures and had to
be strengthened to prevent premature failures and provide a reasonable working
life. Through this evolution of improvements, the life expectancy has been
increased by a factor of 7 from 10,000 loading cycles in the late 1970's to 70,000
loading cycles. The support capacity has continued to increase throughout the
history of longwall mining [7].


There has been a steady increase in the use of two-leg shields in favour of four-
leg shields during the past decade, and two-leg shields are becoming the
favoured support worldwide. The overview of trend of increasing shield capacities
was as shown below Figure.1. However, a full range of roof supports are
available suitable for mining heights from 5.50 to 7.50m with support capacities in
excess of 1750 tonnes [8]. In SCCL, powered roof supports were introduced in
the late 80’s and early 90’s, where the maximum capacity was only 800 tonnes,
which was the state of the art.




                                                                         CM - 05 - 3
17 – 19 January 2011, Bangalore, India
International Conference on Underground Space Technology                             Technical Papers



Table.1: List of Powered Roof Supports deployed at Longwall faces in India
      Name of the Project                Make      Support Capacity     Working       Depth of
                                                   (tonnes) & Type     Range (m)     Working(m
                                                                                         )



    BCCL
    Moonidih                    Dowty, UK        4x280, Chock          1.24 - 1.82      400
    Moonidih                    Kopex, Poland    6x 240, Chock         1.25 - 1.98      400
    Moonidih                    Dowty, UK        4x280, Chock          1.49 - 2.90      400
    Moonidih                    MAMC, Dowty      4x325, Chock Shield   1.90 - 3.20      400
    Moonidih                    MAMC, Dowty      4x400, Shield         1.27 - 2.40      400
    Moonidih                    Jessop/Gullick   4x400, Chock Shield   0.70 - 1.65      400
    Moonidih                    Kopex, Poland    4x400, Chock Shield   2.00 - 3.50      400



    ECL
    Sheetalpur                  Gullick, UK      4x240 Chock Shield    1.40 - 2.09   420 - 450
    Dhemomain                   Gullick, UK      4x360 Chock Shield    2.02 - 3.20      300
    Dhemomain & Jhanjra         Jessop/Gullick   4x550, Chock Shield   1.70 - 3.05    40 - 100
    Jhanjra                     KM -130,USSR     2x320, Chock          2.50 - 4.10     40 - 90
    Churcha & Jhanjra,          Joy              4x680 Chock Shield    1.65 - 3.60    90 - 200
    Kottadih,                   CDFI, France     2x470 Shield          2.20 - 4.70   180 - 220
    Pathakera,                  MAMC, Dowty      6x240 Chock           1.11 - 1.74      110



    SECL
    Balrampur                   CMEI&E,China     4x650, Chock Shield   1.40 - 2.70     45 - 55
    New Kumda                   CMEI&E,China     4x450, Chock Shield   1.40 - 2.70     45 - 55
    Rajendra                    CMEI&E,China     4x450, Chock Shield   1.70 - 3.10     50 - 90



    SCCL
    GDK 7 & 9                   Gullick, UK      4x360, Chock Shield   2.10 - 3.21   100 - 350
    JK5                         Gullick, UK      4x450, Chock Shield   2.0 - 3.20    138 - 265
    VK 7                        Gullick          4x360, Chock Shield   2.0 - 3.20      93-272
    VK 7                        Gullick          4x450, Chock Shield   2.0 - 3.20      38-382
    GDK-11A                     Gullick, UK      4x430, Chock Shield   1.50 - 3.00    70 - 200
    GDK-11A                     MECO&Gullick     4x450, Chock Shield   1.50 - 3.00    70 - 200
    GDK-10A                     MAMC             4x750, Chock Shield   1.65 - 3.60      240
    GDK-9 Extn.                 MECO             4x800, Chock Shield   1.65 - 3.60      225
    PVK & GDK 9                 CME, China       4x760, Chock Shield   2.20 - 3.40    54 - 297




                                                                                        CM - 05 - 4
17 – 19 January 2011, Bangalore, India
International Conference on Underground Space Technology                   Technical Papers




            Fig.1. Historical overview of increasing shield capacities [9]


3.0 Ground control aspects of Long wall mining


The roof falls in unsupported area are mainly due to tensile failure. Generally, the
weak roofs can not sustain a tensile stress. Therefore, an important function that
supports should provide in order to secure a better or more stable roof condition
is to prevent possible roof falls at the face. In order to prevent roof falls at face, it
requires complete elimination of tensile stress in the unsupported roof between
canopy tip and the face line.
Roof stability is a function of lateral confinement, which is generated by the
support resistance and the coal seam. In general, stability of the roof strata is
highly dependent on the span-to-thickness ratio of the roof beam [10]. Roof
instability along a longwall face is generally driven by one or both of two
fundamental geo mechanisms. They are guttering type of failure of the immediate
roof and formation and subsequent opening of sub vertical shear and tensile
factures, i.e., delineation of the large intact blocks via the propagation of
weighting induced sub vertical tensile/shear fractures.




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17 – 19 January 2011, Bangalore, India
International Conference on Underground Space Technology               Technical Papers



The hydraulic legs orientation and distribution of load between front and rear legs
are two very significant factors in the design of powered support in longwall
mining [11]. The principal factors which influence the magnitude of load on
supports include setting load density, height of the caving block, distance of
fracture zone ahead of the face, the overhang of goaf, the support yield
characteristics, and the mechanical strength of the debris above the canopy and
below the support base [12].


The maximum tensile stress in the main roof occurs mostly 5m to 15m ahead of
the longwall face and the deformation of main roof appears to be not only in goaf
area, but also in the unmined area [13]. As the longwall face advances, the
position of high stresses very close to the face, moves with the face in such a
way that the entire roof over the opening is broken by vertical and horizontal
fractures [14].


A problem encountered in working under weak roofs is the break-up of the roof
over the rear half of the canopy. If the caving line moves forward of the line of
action of resultant thrust, then the back legs will tend to push upwards into the
broken roof. This will cause the front of the canopy to lower, leaving the roof over
the AFC essentially unsupported, and exacerbating roof condition, as shown in
below Figure.2 [15].




 Fig.2 Caving line moving forward of line of action of support resultant [15]




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17 – 19 January 2011, Bangalore, India
International Conference on Underground Space Technology                  Technical Papers



3.1 Effects of load distribution in front and rear legs

In India and Overseas many experiments were conducted and numerical
modeling was done to study the effects of load distribution on front and rear legs
of the chock shield. Distribution of vertical and horizontal stresses on the canopy
of the support, for different load ratios of front to rear was analyzed [11].

Vertical stress distribution on the canopy of the support is different for different
load ratios. When the load in the front leg is higher, the vertical stress distribution
on the front portion of the canopy is the largest and the horizontal force acts
towards the face. As a result, there is no tensile stress in the immediate roof of
unsupported area between the canopy tip and face line and consequently the
roof will be stable. Conversely, when the load in the front leg is smaller, the
vertical stress distribution on the front portion of the canopy is also smaller and
the horizontal force acts towards the gob resulting in development of tensile
stress in the immediate roof of unsupported area. This is the main reason for roof
falls in unsupported area. The vertical stress distribution in the immediate roof
under varying roof conditions is as shown below in Figure.3 [5].




                               Fig.3: Vertical stress Variation [5]


Horizontal stress distribution for different load ratios of rear to front legs of a
chock shield is as shown in Figure.4. When the rear to front load ratio increases,
stress in unsupported area will change from compression to tensile stress. In
other words, the stresses in the unsupported area change from compression to


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17 – 19 January 2011, Bangalore, India
International Conference on Underground Space Technology                   Technical Papers



tension, when the front leg load continues to decrease. When the ratio is 1.0, that
is, the magnitude of front and rear legs is the same; there is only a small
compressive stress in unsupported roof area. After that, as the ratio decreases
further, the stress will become tensile and the unsupported roof area is unstable.
This situation must be prevented in designing parameters of the powered roof
support under the weak roof condition.




                   Fig.4. Magnitude and type of horizontal stress [11]

From the above, in general it is suggested that

i.       A smaller load ratio, i.e., the rated load of front leg is should be larger than
         that of rear leg is preferred for the weak roof condition.
ii.      If a stronger immediate roof exists, a larger load ratio (Rear to front) is
         preferable.
iii.     If the rear leg load is much smaller than the front one, it will act like and
         can be treated as 2 leg shield. When the small load ratio is, 0 to 0.5 is
         expected, 2 leg shields are preferable.
iv.      The load ratio of rear legs and front legs must be in the range of 0 to 1,
         under weak roof conditions.




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17 – 19 January 2011, Bangalore, India
International Conference on Underground Space Technology                    Technical Papers



4.0 2 -Leg shields vs. 4- Leg Chock Shields

From a support strata interaction perspective, the two-leg shield provides an
active horizontal force toward the coal face due to inclination of the leg cylinders.
This active horizontal force improves overall strata stability by arresting slippage
along fracture planes or by prevention the expansion of highly jointed or friable
immediate roof geologies which may be further damaged by the front abutment
loading [6,7,9] (see Figure. 5. a.)
In terms of the shield loading, this increase in active horizontal loading also
translates into proportionally higher lemniscate link loading. Caving shield /
lemniscate assembly reaction to forward canopy displacement reduces active
horizontal roof loading caused horizontal component of the leg force [9].
The unbalanced distribution of loading between front and rear legs makes 4-leg
chock shields less effective in cavity prone areas. As shown in Figure.5.b, the
force in the rear legs causes the canopy to rotate up into the cavity. This
condition ultimately results in further cavity formation and requires front legs of
the supports to do all of the supporting work. since the front legs of a 4-leg chock
shield is considerably smaller than they would be in two-leg shield of equivalent
support capacity , the four leg shield provides much less support force than
would a comparable two leg design [6].




                5. a. Active horizontal force provided by 2-Leg shield, and
               Fig. 5.b. 4-leg shield operation in cavity prone conditions [6].


Another issue related to the two-leg concept is higher contact pressure on the
canopy and base. High toe loading, caused by the moment created by the line of


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17 – 19 January 2011, Bangalore, India
International Conference on Underground Space Technology                         Technical Papers



action of the resultant vertical forces acting on the canopy and base, can be a
problem in high capacity two-leg shields and should be a consideration in the
support design. Base toe pressures of 800 psi or greater can be expected on
high-capacity two-leg shields. Base toe lifting devices are now standard on most
two-leg shields to assist in the advancement of the shields particularly in soft
floor conditions. There has also been a trend toward solid base designs to
reduce floor-bearing pressures in two-leg shields [7].


Advantage of 4 leg shield is that it provides resultant vertical force farther from
the coal face than does 2-leg support. This is supposed to be more efficient in
cantilevered strata, since the support force acts at a mechanically more efficient
location. 2-leg shields inferior to 4-leg type in this mine from structural stability
point of view. However if a 2-leg shield of the same capacity is to be obtained, it
is the preferred in most applications. The other operational characteristic
differences between 2-leg and 4-leg powered supports in brief is given in
Table.2.

Table.2. Comparison of operational characteristic of 2-leg and 4 –leg
powered supports

       Parameter                             2- Leg shield                 4-Leg shield
Canopy ratio                        optimum at approx. 2 : 1         > 2:1
Canopy length                       short and compact                longer canopy design
Supporting force into               minimum distance to the          due to construction
the roof                            coal face                        larger distance
Range of adjustment                 up to approx. 3 : 1              <3:1
Travelling route                    in front of / behind the props   between the props
Handling                            very easy and quick              more complicated
Possibility of faulty                                                insufficient setting of
                                    extremely low
operation                                                            the rear props
Cycle time                          < 12 sec                         > 15 sec
Requirement of
                                    relatively small                 larger
hydraulics




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17 – 19 January 2011, Bangalore, India
International Conference on Underground Space Technology             Technical Papers



5.0 CASE STUDY

Padmavathi khani (PVK) 5 incline is located in Kothagudem area of SCCL,
Andhra Pradesh State, India. The mechanized longwall mining technology was
introduced in August, 1995 with fully mechanized longwall mining equipment
imported from M/s CME, China. Since then, ten longwall panels and two short
longwall panels have been worked to extract 5.08 Mt of coal by longwall method
of retreating with caving along strike direction with out any major strata control
problems.

5.1 Geo-mining Details


Of the three coal seams existing in the mine, the Top seam was extensively
extracted by mechanized powered roof support longwall retreating method. The
thickness of the top seam was varied from 6 to 10m, dipping at about 1 in 9 due
N680E. Top seam was the overlying seam of King and Bottom seams at this
mine. The King seam was worked in two sections, the top section by caving and
bottom section with stowing. The parting between Top seam and King Seam
varies from 40 to 48m. The longwall face of all the panels in the top seam were
laid out along the dip-rise direction. The middle section of the top seam was
extracted by longwall technology leaving shaly coal in both roof and floor. The
longwall panels were worked at varying depths ranging from 48m to 297m. All
the Longwall panels except panel 8 were underlined by king seam extracted
panels, where as panel 8 was overlain by standing pillars of adjacent No. 5B
Incline. The details of operating parameters of all longwall panels are given in
Table.3.


The equipment used in the longwall panel consisted of an AM 500 Double Ended
Ranging Drum shearer with a extraction height range of 2 to 3.5m, with
Armoured face conveyor of 800 t/hr, the beam stage loader 1000 t/hr, and the
belt conveyor 1000 t/hr capacities. The roof was supported with 4x760 tonnes


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17 – 19 January 2011, Bangalore, India
International Conference on Underground Space Technology                          Technical Papers



IFS chock shield powered roof supports provided with extension bar to support
the freshly exposed roof at the face. The specifications of the powered roof
support are given in Table.4 and the diagram of the same is as shown in
Figure.6.
 Table. 3 Details of operating parameters of all Longwall panels of PVK-5 Incline

 Name            Panel            Depth     Progres        Frequenc   Period of    Maximum
    of         Dimensio           Min. &        s             y of    Extractio    Subsiden
Longwa          ns (m)            Max.          at         Periodic   n             ce (m)
ll Panel                           (m)       Main          Weightin
                                            fall (m)        gs (m)
                                    59 &
Panel 2        660 x 150                      66.5            -       8 Months         2.54
                                    112
                                    76 &
Panel 3        675 x 150                      80.65           -       7 Months         1.94
                                    128
                                    96 &
Panel 4        675 x 150                      81.85           -       9 Months         1.88
                                    141
                                   113 &                              1 Year
Panel 5        830 x 150                      61.90        15 to 18                    2.20
                                    158                               2 Months
 Panel                             135 &
               560 x 147                      112.0        15 to 18   1 Year           0.74
  5A                                150
 Panel                             155 &                              1 Year
                730 x150                      76.75        18 to 20                    0.68
  5C                                184                               9 Months
 Panel                             174 &                              1 Year
               690 x 150                      91.50        18 to 20                    1.09
  5D                                203                               5 Months
 Panel                             174 &
               770 x 150                      50.30        15 to 20   2 Years          0.95
  22                                203
 Panel
                520 x 62         54 & 96      98.00        18 to 25   4 Months         1.65
  1A
Panel 1         500 x 62         48 & 85      80.00        15 to 20   4 Months         2.60
 Panel
                                   203 &
  21            420x150                       45.00        10 to 12   8 Months         0.835
                                    239
                                 275&29
Panel 8         420x150                       80.30        15 to 25       -            0.24
                                    7




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17 – 19 January 2011, Bangalore, India
International Conference on Underground Space Technology                 Technical Papers




                     Table 4. Specifications of chock shield support
                        1.     Support height              2.2 to 3.4m
                        2.     Support width               1.5 m
                        3.     Support length              3.87m
                        4.     Canopy ratio                2.5
                        5.     Roof coverage               6.3 sq.m
                        6.     Yield load                  760 tonnes
                        7.     Support density             110 t/sq.m
                        8.     Floor specific pressure     3.1 MPa
                        9.     Force to advance conveyor 360KN
                        10. Force to advance support       633 KN
                        11. Support weight                 20.5 tonnes




Fig.6. Schematic diagram of Powered Roof Support (4 X 760 t) (Courtesy: SCCL)




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17 – 19 January 2011, Bangalore, India
International Conference on Underground Space Technology             Technical Papers



5.2 Field observations

The experiences of working the longwalls with 4x760t chock shield supports
under varying contact roofs such as shaly coal and sandstone are studied. The
compressive strength of the immediate coal roof varies from 9.3 MPa to 11 MPa,
where as that of the sandstone varies from 16 MPa to 21 MPa. The roof above
this level is sandstone predominately, which varies from fine to coarse grained
and consists of a massive strong strata bed of approximately 12-17m thickness
within close proximity to the longwall roof. Except the Panel No.21, all other
panels were worked under coal roof, where as panel 21 had been worked partly
under stone roof and partly under coal roof due to thinning of the Top seam. The
supports were set to 65% of the yield load in coal roof and 75% in case of stone
roof conditions. The monitoring of the supports is conducted during the retreat of
the panel with the help of pressure gauges, continuous chart pressure recorders
and tape measurements. The pressure gauges are provided to all four legs of the
all the supports placed in the face. Continuous chart recorders are provided to
legs of the strategic chock shields representing various zones in the longwall
face.

Immediate shaly coal roof caves in as soon as the supports are advanced. With
coal roof, the main weighting was observed after an area of exposure of 8000 to
12,500 sq.m., and periodic weightings at an interval of 15 to 25m. While working
with sandstone roof, main weighting occurred after 7000 sq.m area of exposure
and periodic weightings at an interval of 10 to 12m.

From the analysis of strata monitoring data pertaining to longwall faces worked
under coal roof, it can be inferred that;

•   Setting pressure / increase of resistance showed that front legs were 30 to
    40% higher than rear legs. There were variations in leg operating
    characteristics and variation in pressure distribution on front and rear legs
    (see Figure.7).
•   The load ratio of rear legs and the front legs is 0.6:1.


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International Conference on Underground Space Technology               Technical Papers




•   Front legs are more loaded than rear legs because of weak immediate roof,
    which gets crumbled over rear legs due to cyclic loading effect. Due to which,
    the load exerted during the breakage process of immediate and main roof is
    transferred to the front legs. During the Breakage and lowering of the upper
    roof the crumbled roof over rear legs gets compacted and rear legs used to
    take some of load less than front legs.
•   In regard to performance of the powered supports, under weak roof rear legs
    were usually higher than front legs. Pins in rear legs were pulled and bent,
    consequently, the cave line moved, the coal fractured, head end of the front
    bar hung down and coal roof failed and coal roof failed above tip bar.
•   At the time of main and periodic weightings, the front legs used to take more
    loads than rear legs, due to the crumbled and premature caving nature of
    immediate shaly coal roof. At the same time, the rear legs are lightly loaded
    by vertical stress. During some of the major periodic weightings the
    unsupported roof between canopy tip and face line failed and led to cavity
    formation.
•   Front legs can bring their actions into full play compared with rear legs, while
    the longwall was near the borders of residual pillars and goaves of underlying
    seam.
•   In one of the observations, in panel 22, at the time of main weighting, total 32
    legs attained bleed pressures of which 23 legs are of front and eight are of
    rear.
•   Measured Mean Load Density (MMLD) to Rated Mean Load Density (RMLD)
    was observed to be around 0.60 to 0.65, meaning thereby that only 60 to 65%
    of the rated load density of 110 t/sq.m, was utilized, which could be attributed
    to proper bulking up of caved material increased the goaf compaction
    resistance.
From the field observations and analysis of chock shield leg pressures in
longwall panel with immediate sandstone roof, it can be understood that;



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17 – 19 January 2011, Bangalore, India
International Conference on Underground Space Technology                Technical Papers



§    Setting resistance / increase of resistance showed that the front legs were
     only 8 to 10% higher than rear legs. This means supports were more or less
     uniformly loaded. During main and periodic weightings some of the rear legs
     recorded higher pressures than the front legs (see Figures 8 and 9).
§    There was a considerable overhang of immediate stone roof observed
     behind the supports. The rear legs of the chock shields induced break line at
     the goaf edge.
§    During some of the major periodic weightings, all legs of the supports were
     intensively loaded, due to delineation of large intact blocks led failure of
     immediate roof between face and canopy tip, caused cavity conditions.
§    Front legs as well as rear legs can bring their actions into full play while the
     longwall face was near borders of residual pillars and goaves of underlying
     seam.
§    The ratio of MMLD to RMLD was found to be around 0.8 to 0.85 or some
     times even more; implies that almost 80 to 85% of the rated load density was
     utilized due to delayed caving and poor bulking up of immediate sandstone
     roof and also mainly due to close proximity of main roof. This indicates that at
     this geo-mining condition higher rated powered roof supports are desirable.
§    During the main weighting in panel No.21, it was observed that out of 62
     legs, which attained bleed pressures in the mid face of the longwall panel,
     31are of front and 31 are of rear legs.




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                                              27
                                                        Front

                                              25        Rear
                         Leg pressure (MPa)
                                              23


                                              21


                                              19


                                              17


                                              15
                                                   34          95           145        212        279          355   429        498
                                                                                  Average face progress (m )



                      Fig.7. Average pressure distribution between front and rear legs under
                                                                         shaly coal roof in panel No. 1



                        40
                                                         Bleed Pressure
                                                                                                                               Fron t
 Leg Pressure (MPa)




                        35                                                                                                     Rear

                        30

                        25                                                                    Set Pressure

                        20
                                              1                     21               41                61            81               101
                                                                                        s u p p o rts #



       Fig.8. Pressure distribution between front and rear legs under sandstone
                                                        roof during periodic weighting in panel No. 21




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17 – 19 January 2011, Bangalore, India
International Conference on Underground Space Technology                                       Technical Papers



                       32
                                                                                                          F ro n t
                                                                                                          R ear
                       30


                       28
  Leg Pressure(MPa)




                       26


                       24


                       22


                       20


                       18
                            0     50      100       150         200        250         300   350       400

                                                D is t a n c e F r o m B a r r ie r ( m )

            Fig.9. Average pressure distribution between front and rear legs under
                                          sandstone roof in panel No. 21

Conclusions

It is evident that high rating powered roof supports is a prerequisite for meeting
longwall support requirements under competent strata formations. However, a
detailed strata control and face powered support investigations are of paramount
importance for assessing performance of longwall face.

From the analysis of the observations under varying contact roofs such as shaly
coal and sandstone, the following conclusions and recommendations were
drawn;

Ø When selecting a support design, mining engineers should give careful
                      consideration to local conditions and requirements. The desirable type and
                      capacity of the powered roof support must be selected based on the site
                      specific geo-mining conditions.


Ø While deploying powered roof supports with foreign collaborations, sufficient
                      scientific study regarding suitability of powered roof support, under a
                      particular geo-mining condition should be conducted by both Indian

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17 – 19 January 2011, Bangalore, India
International Conference on Underground Space Technology              Technical Papers



     researchers and foreign researchers of Australia, china, and Germany, where
     longwall technology was well proven.


Ø Under immediate weak and moderate strong roof conditions, containing
     overlain massive sandstone beds, high capacity 2- leg shields of same
     capacity are desirable over 4-leg chock shields.


Acknowledgments

The authors are thankful to the SCCL management for giving permission to
publish this paper. The views expressed in this article are of the authors only and
not necessarily of the organization to which they belong.

References

1.     Zhao Honghu & Venkata Ramaiah M.S., (1996), “Strata Movement on
       Shallow fully Mechanised Longwall working Face at PVK Mine of SCCL &
       Option of Powered Support – Strata Control Observation Research at 2#
       Longwall working face PVK, SCCL”, 2nd National Conference on Ground
       Control in Mining, CMRI, pp. 123-142.
2.     Prabhakar Rao C, Veera Reddy B., (1999), "Strata Behaviour with
       Immediate Forward Support - a Case Study", Proceedings of the National
       Conference on Rock Engineering Techniques for site characterization, 6-8
       December 1999, pp. 413- 420.
3.     Venkat Ramaiah, M.S and Sudhakar Lolla (2002) “Selection of Powered
       Roof Supports for Weak coal roof”, Journal of Mines, Metals & Fuels, April,
       2002.
4.     Siva Sankar U, (2005), “Monitoring Strata Behaviour of Shallow Longwall
       panel – a case Study”, M.Tech. Thesis Unpublished, BHU, Varanasi, India.

5.     Venkata Ramaiah M.S Sastry V.R, Roshan Nair (2005), “Analysis of the
       Influence of Contact Roof zone on powered roof supports during extraction
       by Longwall using F.E.M”, IE(I), MN-335.
6.     Barczak T.M., (1992), “Examination of Design and operation of powered
       supports for longwall mining”, Bureau of Mines Information Circular, USA.
       IC.9320.
7.     Thomas M. Barczak, “Design considerations for the next generation of
       longwall shields”, NIOSH, www.cdc.gov/niosh/mining/pubs/pdfs/dcftn.pdf.


                                                                        CM - 05 - 19
17 – 19 January 2011, Bangalore, India
International Conference on Underground Space Technology           Technical Papers



8.     http://www.bucyrus.com/media/24858/roof%20support.pdf
9.     Barczak-TM (2006), “A Retrospective Assessment of Longwall Roof
       Support with a Focus on Challenging Accepted Roof Support Concepts and
       Design Premises”, Proceedings of 25th International Conference on Ground
       Control in Mining, August 1-3, 2006, Morgantown, West Virginia. pg 232-
       244
10. T. P. Medhurst (2005) “Practical Considerations in Longwall Support
    Behaviour and Ground Response” Coal Operators' Conference 2005, Pg
    49-57.
11. Peng S.S., Hsiung S.M. and Jiang Y.M. (1988), “Parameters Affecting the
    Shield support Efficiency in Longwall Mining”, 21st Century higher
    Production Coal Mining Systems Symposium, pp.122-135
12. Gupta RN and Farmer IW, (1985), “Interaction between roof and support on
    longwall faces with particular reference to support resistance”, Proceedings
    of 4th International Conference on Ground Control in Mining, West Virginia,
    pp. 58-77.
13. Zhu D, Qian M, Peng SS (1989), “ A study of Displacement Field on Main
    Roof in Longwall Mining and its Application”, Proceedings of the 30th US
    Symposium on Rock Mechanics, Rotterdam, 1989, p.146
14. Kidybinski A and Babcock C.O (1973), “Stress Distribution and Rock
    Fracture Zones on the Roof of Longwall face in Coal Mine”, Rock
    Mechanics, no.5, 1973, p.1.
15. Roberts B.H. (1990), “A review of the performance of various powered
    support types” Mining Science and Technology, 11 55-69          Elsevier
    Science Publishers B.V., Amsterdam - Printed in The Netherlands




                                                                     CM - 05 - 20
17 – 19 January 2011, Bangalore, India

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Selection of powered roof supports – 2 leg shields vis-à-vis

  • 1. International Conference on Underground Space Technology Technical Papers Selection of Powered Roof Supports – 2-Leg Shields vis-à-vis 4-Leg Chock Shields B Ramesh Kumar CGM, Corporate Project and Planning,, U Siva Sankar Under Manger, Project Planning, VNS Prasad, Deputy Manager, Corporate Project and Planning SCCL, AP, India ABSTRACT The success of a longwall face depends to a large extent on the type and capacity of the Powered Roof Supports. In India, different types of Powered Roof Supports of various capacities were tried earlier, but the four legged chock shields have been the most widely used supports. An extensive literature review on Indian longwall mining scenario over the last few decades suggests that majority of the downtimes and or failures were mainly due to ground control problems and inadequate capacity and type of powered roof supports. In India several mines Kottadih, Churcha and Dhemomain had experienced catastrophic failures of longwall faces. In this paper, a case study was presented, summarizing the experiences of working Longwall faces with IFS, 4-leg chock shields under varying contact roofs, viz; coal and sand stone. Based on the field observations, conclusions were drawn regarding the suitability of 2 leg shields over 4-leg chock shields under Indian geo mining conditions in general and particularly regarding longwall mines of Singareni Collieries Company Ltd (SCCL). Key words: Longwall, Powered Roof Support, Contact Roof CM - 05 - 1 17 – 19 January 2011, Bangalore, India
  • 2. International Conference on Underground Space Technology Technical Papers Introduction SCCL introduced mechanized longwall mining in 1983. The first few faces were worked with 4x360 t (conventional) 4x450 t IFS supports. Failure of the support with regard to breakage of immediate roof between canopy tip and face resulted in formation of cavities in face and subsequent closure of chocks and face stoppages. The cavities were more frequent and negotiation of the same with IFS supports was even more difficult. With the introduction of second generation longwall supports, the support capacity was increased to 4x760 t (PVK) and 4x800 t (GDK 10A & GDK 9E), there was a considerable improvement in strata control. Face stoppages due to problems related to strata failures or closure of chocks were few, though not totally eliminated. In SCCL, majority of the longwall faces are worked under weak immediate roof conditions. The strength of immediate roof coal/sandstone of worked Longwalls was of weak to moderate strong. In most of the above cases, it has been observed that the front legs were taking more load than rear legs of conventional as well as IFS chock shield supports, due to crumbled and premature caving nature of immediate roof [1-5]. Studies conducted and analysis of performance of powered roof supports suggests that above supports with higher capacity are able to deal most of the problems of strata under the prevailing geo-mining conditions. However there is always a scope for selection of correct type of support to reduce the face stoppages because of strata control problems, as high cost equipment installed should generate revenue for long term financial requirements. 2.0 Trend of development of Powered Roof Supports: Two fundamental changes in shield design have been made since the introduction of the shield in 1975: (1) the caliper design was replaced with a lemniscate-guided caving shield that maintains a constant tip-to-face distance throughout its operating range; and (2) electro hydraulic control systems have CM - 05 - 2 17 – 19 January 2011, Bangalore, India
  • 3. International Conference on Underground Space Technology Technical Papers replaced manual systems to permit remote and automated operation of the shield [6, 9]. In India first mechanized powered roof support face, the new-age Longwall, was launched in August 1978 at Moonidih Colliery. In India, 33 Longwall packages have been deployed to date in both Coal India Ltd and SCCL in collaboration with U.K, Russia, Poland, China and France mostly funded by GOI. The list of the powered roof supports deployed at Indian longwall faces is as given in the Table.1. The basic shield structure has remained unchanged for the past 25 years, although the structures have grown dramatically in size and capacity. Early generations of shields experienced several structurally related failures and had to be strengthened to prevent premature failures and provide a reasonable working life. Through this evolution of improvements, the life expectancy has been increased by a factor of 7 from 10,000 loading cycles in the late 1970's to 70,000 loading cycles. The support capacity has continued to increase throughout the history of longwall mining [7]. There has been a steady increase in the use of two-leg shields in favour of four- leg shields during the past decade, and two-leg shields are becoming the favoured support worldwide. The overview of trend of increasing shield capacities was as shown below Figure.1. However, a full range of roof supports are available suitable for mining heights from 5.50 to 7.50m with support capacities in excess of 1750 tonnes [8]. In SCCL, powered roof supports were introduced in the late 80’s and early 90’s, where the maximum capacity was only 800 tonnes, which was the state of the art. CM - 05 - 3 17 – 19 January 2011, Bangalore, India
  • 4. International Conference on Underground Space Technology Technical Papers Table.1: List of Powered Roof Supports deployed at Longwall faces in India Name of the Project Make Support Capacity Working Depth of (tonnes) & Type Range (m) Working(m ) BCCL Moonidih Dowty, UK 4x280, Chock 1.24 - 1.82 400 Moonidih Kopex, Poland 6x 240, Chock 1.25 - 1.98 400 Moonidih Dowty, UK 4x280, Chock 1.49 - 2.90 400 Moonidih MAMC, Dowty 4x325, Chock Shield 1.90 - 3.20 400 Moonidih MAMC, Dowty 4x400, Shield 1.27 - 2.40 400 Moonidih Jessop/Gullick 4x400, Chock Shield 0.70 - 1.65 400 Moonidih Kopex, Poland 4x400, Chock Shield 2.00 - 3.50 400 ECL Sheetalpur Gullick, UK 4x240 Chock Shield 1.40 - 2.09 420 - 450 Dhemomain Gullick, UK 4x360 Chock Shield 2.02 - 3.20 300 Dhemomain & Jhanjra Jessop/Gullick 4x550, Chock Shield 1.70 - 3.05 40 - 100 Jhanjra KM -130,USSR 2x320, Chock 2.50 - 4.10 40 - 90 Churcha & Jhanjra, Joy 4x680 Chock Shield 1.65 - 3.60 90 - 200 Kottadih, CDFI, France 2x470 Shield 2.20 - 4.70 180 - 220 Pathakera, MAMC, Dowty 6x240 Chock 1.11 - 1.74 110 SECL Balrampur CMEI&E,China 4x650, Chock Shield 1.40 - 2.70 45 - 55 New Kumda CMEI&E,China 4x450, Chock Shield 1.40 - 2.70 45 - 55 Rajendra CMEI&E,China 4x450, Chock Shield 1.70 - 3.10 50 - 90 SCCL GDK 7 & 9 Gullick, UK 4x360, Chock Shield 2.10 - 3.21 100 - 350 JK5 Gullick, UK 4x450, Chock Shield 2.0 - 3.20 138 - 265 VK 7 Gullick 4x360, Chock Shield 2.0 - 3.20 93-272 VK 7 Gullick 4x450, Chock Shield 2.0 - 3.20 38-382 GDK-11A Gullick, UK 4x430, Chock Shield 1.50 - 3.00 70 - 200 GDK-11A MECO&Gullick 4x450, Chock Shield 1.50 - 3.00 70 - 200 GDK-10A MAMC 4x750, Chock Shield 1.65 - 3.60 240 GDK-9 Extn. MECO 4x800, Chock Shield 1.65 - 3.60 225 PVK & GDK 9 CME, China 4x760, Chock Shield 2.20 - 3.40 54 - 297 CM - 05 - 4 17 – 19 January 2011, Bangalore, India
  • 5. International Conference on Underground Space Technology Technical Papers Fig.1. Historical overview of increasing shield capacities [9] 3.0 Ground control aspects of Long wall mining The roof falls in unsupported area are mainly due to tensile failure. Generally, the weak roofs can not sustain a tensile stress. Therefore, an important function that supports should provide in order to secure a better or more stable roof condition is to prevent possible roof falls at the face. In order to prevent roof falls at face, it requires complete elimination of tensile stress in the unsupported roof between canopy tip and the face line. Roof stability is a function of lateral confinement, which is generated by the support resistance and the coal seam. In general, stability of the roof strata is highly dependent on the span-to-thickness ratio of the roof beam [10]. Roof instability along a longwall face is generally driven by one or both of two fundamental geo mechanisms. They are guttering type of failure of the immediate roof and formation and subsequent opening of sub vertical shear and tensile factures, i.e., delineation of the large intact blocks via the propagation of weighting induced sub vertical tensile/shear fractures. CM - 05 - 5 17 – 19 January 2011, Bangalore, India
  • 6. International Conference on Underground Space Technology Technical Papers The hydraulic legs orientation and distribution of load between front and rear legs are two very significant factors in the design of powered support in longwall mining [11]. The principal factors which influence the magnitude of load on supports include setting load density, height of the caving block, distance of fracture zone ahead of the face, the overhang of goaf, the support yield characteristics, and the mechanical strength of the debris above the canopy and below the support base [12]. The maximum tensile stress in the main roof occurs mostly 5m to 15m ahead of the longwall face and the deformation of main roof appears to be not only in goaf area, but also in the unmined area [13]. As the longwall face advances, the position of high stresses very close to the face, moves with the face in such a way that the entire roof over the opening is broken by vertical and horizontal fractures [14]. A problem encountered in working under weak roofs is the break-up of the roof over the rear half of the canopy. If the caving line moves forward of the line of action of resultant thrust, then the back legs will tend to push upwards into the broken roof. This will cause the front of the canopy to lower, leaving the roof over the AFC essentially unsupported, and exacerbating roof condition, as shown in below Figure.2 [15]. Fig.2 Caving line moving forward of line of action of support resultant [15] CM - 05 - 6 17 – 19 January 2011, Bangalore, India
  • 7. International Conference on Underground Space Technology Technical Papers 3.1 Effects of load distribution in front and rear legs In India and Overseas many experiments were conducted and numerical modeling was done to study the effects of load distribution on front and rear legs of the chock shield. Distribution of vertical and horizontal stresses on the canopy of the support, for different load ratios of front to rear was analyzed [11]. Vertical stress distribution on the canopy of the support is different for different load ratios. When the load in the front leg is higher, the vertical stress distribution on the front portion of the canopy is the largest and the horizontal force acts towards the face. As a result, there is no tensile stress in the immediate roof of unsupported area between the canopy tip and face line and consequently the roof will be stable. Conversely, when the load in the front leg is smaller, the vertical stress distribution on the front portion of the canopy is also smaller and the horizontal force acts towards the gob resulting in development of tensile stress in the immediate roof of unsupported area. This is the main reason for roof falls in unsupported area. The vertical stress distribution in the immediate roof under varying roof conditions is as shown below in Figure.3 [5]. Fig.3: Vertical stress Variation [5] Horizontal stress distribution for different load ratios of rear to front legs of a chock shield is as shown in Figure.4. When the rear to front load ratio increases, stress in unsupported area will change from compression to tensile stress. In other words, the stresses in the unsupported area change from compression to CM - 05 - 7 17 – 19 January 2011, Bangalore, India
  • 8. International Conference on Underground Space Technology Technical Papers tension, when the front leg load continues to decrease. When the ratio is 1.0, that is, the magnitude of front and rear legs is the same; there is only a small compressive stress in unsupported roof area. After that, as the ratio decreases further, the stress will become tensile and the unsupported roof area is unstable. This situation must be prevented in designing parameters of the powered roof support under the weak roof condition. Fig.4. Magnitude and type of horizontal stress [11] From the above, in general it is suggested that i. A smaller load ratio, i.e., the rated load of front leg is should be larger than that of rear leg is preferred for the weak roof condition. ii. If a stronger immediate roof exists, a larger load ratio (Rear to front) is preferable. iii. If the rear leg load is much smaller than the front one, it will act like and can be treated as 2 leg shield. When the small load ratio is, 0 to 0.5 is expected, 2 leg shields are preferable. iv. The load ratio of rear legs and front legs must be in the range of 0 to 1, under weak roof conditions. CM - 05 - 8 17 – 19 January 2011, Bangalore, India
  • 9. International Conference on Underground Space Technology Technical Papers 4.0 2 -Leg shields vs. 4- Leg Chock Shields From a support strata interaction perspective, the two-leg shield provides an active horizontal force toward the coal face due to inclination of the leg cylinders. This active horizontal force improves overall strata stability by arresting slippage along fracture planes or by prevention the expansion of highly jointed or friable immediate roof geologies which may be further damaged by the front abutment loading [6,7,9] (see Figure. 5. a.) In terms of the shield loading, this increase in active horizontal loading also translates into proportionally higher lemniscate link loading. Caving shield / lemniscate assembly reaction to forward canopy displacement reduces active horizontal roof loading caused horizontal component of the leg force [9]. The unbalanced distribution of loading between front and rear legs makes 4-leg chock shields less effective in cavity prone areas. As shown in Figure.5.b, the force in the rear legs causes the canopy to rotate up into the cavity. This condition ultimately results in further cavity formation and requires front legs of the supports to do all of the supporting work. since the front legs of a 4-leg chock shield is considerably smaller than they would be in two-leg shield of equivalent support capacity , the four leg shield provides much less support force than would a comparable two leg design [6]. 5. a. Active horizontal force provided by 2-Leg shield, and Fig. 5.b. 4-leg shield operation in cavity prone conditions [6]. Another issue related to the two-leg concept is higher contact pressure on the canopy and base. High toe loading, caused by the moment created by the line of CM - 05 - 9 17 – 19 January 2011, Bangalore, India
  • 10. International Conference on Underground Space Technology Technical Papers action of the resultant vertical forces acting on the canopy and base, can be a problem in high capacity two-leg shields and should be a consideration in the support design. Base toe pressures of 800 psi or greater can be expected on high-capacity two-leg shields. Base toe lifting devices are now standard on most two-leg shields to assist in the advancement of the shields particularly in soft floor conditions. There has also been a trend toward solid base designs to reduce floor-bearing pressures in two-leg shields [7]. Advantage of 4 leg shield is that it provides resultant vertical force farther from the coal face than does 2-leg support. This is supposed to be more efficient in cantilevered strata, since the support force acts at a mechanically more efficient location. 2-leg shields inferior to 4-leg type in this mine from structural stability point of view. However if a 2-leg shield of the same capacity is to be obtained, it is the preferred in most applications. The other operational characteristic differences between 2-leg and 4-leg powered supports in brief is given in Table.2. Table.2. Comparison of operational characteristic of 2-leg and 4 –leg powered supports Parameter 2- Leg shield 4-Leg shield Canopy ratio optimum at approx. 2 : 1 > 2:1 Canopy length short and compact longer canopy design Supporting force into minimum distance to the due to construction the roof coal face larger distance Range of adjustment up to approx. 3 : 1 <3:1 Travelling route in front of / behind the props between the props Handling very easy and quick more complicated Possibility of faulty insufficient setting of extremely low operation the rear props Cycle time < 12 sec > 15 sec Requirement of relatively small larger hydraulics CM - 05 - 10 17 – 19 January 2011, Bangalore, India
  • 11. International Conference on Underground Space Technology Technical Papers 5.0 CASE STUDY Padmavathi khani (PVK) 5 incline is located in Kothagudem area of SCCL, Andhra Pradesh State, India. The mechanized longwall mining technology was introduced in August, 1995 with fully mechanized longwall mining equipment imported from M/s CME, China. Since then, ten longwall panels and two short longwall panels have been worked to extract 5.08 Mt of coal by longwall method of retreating with caving along strike direction with out any major strata control problems. 5.1 Geo-mining Details Of the three coal seams existing in the mine, the Top seam was extensively extracted by mechanized powered roof support longwall retreating method. The thickness of the top seam was varied from 6 to 10m, dipping at about 1 in 9 due N680E. Top seam was the overlying seam of King and Bottom seams at this mine. The King seam was worked in two sections, the top section by caving and bottom section with stowing. The parting between Top seam and King Seam varies from 40 to 48m. The longwall face of all the panels in the top seam were laid out along the dip-rise direction. The middle section of the top seam was extracted by longwall technology leaving shaly coal in both roof and floor. The longwall panels were worked at varying depths ranging from 48m to 297m. All the Longwall panels except panel 8 were underlined by king seam extracted panels, where as panel 8 was overlain by standing pillars of adjacent No. 5B Incline. The details of operating parameters of all longwall panels are given in Table.3. The equipment used in the longwall panel consisted of an AM 500 Double Ended Ranging Drum shearer with a extraction height range of 2 to 3.5m, with Armoured face conveyor of 800 t/hr, the beam stage loader 1000 t/hr, and the belt conveyor 1000 t/hr capacities. The roof was supported with 4x760 tonnes CM - 05 - 11 17 – 19 January 2011, Bangalore, India
  • 12. International Conference on Underground Space Technology Technical Papers IFS chock shield powered roof supports provided with extension bar to support the freshly exposed roof at the face. The specifications of the powered roof support are given in Table.4 and the diagram of the same is as shown in Figure.6. Table. 3 Details of operating parameters of all Longwall panels of PVK-5 Incline Name Panel Depth Progres Frequenc Period of Maximum of Dimensio Min. & s y of Extractio Subsiden Longwa ns (m) Max. at Periodic n ce (m) ll Panel (m) Main Weightin fall (m) gs (m) 59 & Panel 2 660 x 150 66.5 - 8 Months 2.54 112 76 & Panel 3 675 x 150 80.65 - 7 Months 1.94 128 96 & Panel 4 675 x 150 81.85 - 9 Months 1.88 141 113 & 1 Year Panel 5 830 x 150 61.90 15 to 18 2.20 158 2 Months Panel 135 & 560 x 147 112.0 15 to 18 1 Year 0.74 5A 150 Panel 155 & 1 Year 730 x150 76.75 18 to 20 0.68 5C 184 9 Months Panel 174 & 1 Year 690 x 150 91.50 18 to 20 1.09 5D 203 5 Months Panel 174 & 770 x 150 50.30 15 to 20 2 Years 0.95 22 203 Panel 520 x 62 54 & 96 98.00 18 to 25 4 Months 1.65 1A Panel 1 500 x 62 48 & 85 80.00 15 to 20 4 Months 2.60 Panel 203 & 21 420x150 45.00 10 to 12 8 Months 0.835 239 275&29 Panel 8 420x150 80.30 15 to 25 - 0.24 7 CM - 05 - 12 17 – 19 January 2011, Bangalore, India
  • 13. International Conference on Underground Space Technology Technical Papers Table 4. Specifications of chock shield support 1. Support height 2.2 to 3.4m 2. Support width 1.5 m 3. Support length 3.87m 4. Canopy ratio 2.5 5. Roof coverage 6.3 sq.m 6. Yield load 760 tonnes 7. Support density 110 t/sq.m 8. Floor specific pressure 3.1 MPa 9. Force to advance conveyor 360KN 10. Force to advance support 633 KN 11. Support weight 20.5 tonnes Fig.6. Schematic diagram of Powered Roof Support (4 X 760 t) (Courtesy: SCCL) CM - 05 - 13 17 – 19 January 2011, Bangalore, India
  • 14. International Conference on Underground Space Technology Technical Papers 5.2 Field observations The experiences of working the longwalls with 4x760t chock shield supports under varying contact roofs such as shaly coal and sandstone are studied. The compressive strength of the immediate coal roof varies from 9.3 MPa to 11 MPa, where as that of the sandstone varies from 16 MPa to 21 MPa. The roof above this level is sandstone predominately, which varies from fine to coarse grained and consists of a massive strong strata bed of approximately 12-17m thickness within close proximity to the longwall roof. Except the Panel No.21, all other panels were worked under coal roof, where as panel 21 had been worked partly under stone roof and partly under coal roof due to thinning of the Top seam. The supports were set to 65% of the yield load in coal roof and 75% in case of stone roof conditions. The monitoring of the supports is conducted during the retreat of the panel with the help of pressure gauges, continuous chart pressure recorders and tape measurements. The pressure gauges are provided to all four legs of the all the supports placed in the face. Continuous chart recorders are provided to legs of the strategic chock shields representing various zones in the longwall face. Immediate shaly coal roof caves in as soon as the supports are advanced. With coal roof, the main weighting was observed after an area of exposure of 8000 to 12,500 sq.m., and periodic weightings at an interval of 15 to 25m. While working with sandstone roof, main weighting occurred after 7000 sq.m area of exposure and periodic weightings at an interval of 10 to 12m. From the analysis of strata monitoring data pertaining to longwall faces worked under coal roof, it can be inferred that; • Setting pressure / increase of resistance showed that front legs were 30 to 40% higher than rear legs. There were variations in leg operating characteristics and variation in pressure distribution on front and rear legs (see Figure.7). • The load ratio of rear legs and the front legs is 0.6:1. CM - 05 - 14 17 – 19 January 2011, Bangalore, India
  • 15. International Conference on Underground Space Technology Technical Papers • Front legs are more loaded than rear legs because of weak immediate roof, which gets crumbled over rear legs due to cyclic loading effect. Due to which, the load exerted during the breakage process of immediate and main roof is transferred to the front legs. During the Breakage and lowering of the upper roof the crumbled roof over rear legs gets compacted and rear legs used to take some of load less than front legs. • In regard to performance of the powered supports, under weak roof rear legs were usually higher than front legs. Pins in rear legs were pulled and bent, consequently, the cave line moved, the coal fractured, head end of the front bar hung down and coal roof failed and coal roof failed above tip bar. • At the time of main and periodic weightings, the front legs used to take more loads than rear legs, due to the crumbled and premature caving nature of immediate shaly coal roof. At the same time, the rear legs are lightly loaded by vertical stress. During some of the major periodic weightings the unsupported roof between canopy tip and face line failed and led to cavity formation. • Front legs can bring their actions into full play compared with rear legs, while the longwall was near the borders of residual pillars and goaves of underlying seam. • In one of the observations, in panel 22, at the time of main weighting, total 32 legs attained bleed pressures of which 23 legs are of front and eight are of rear. • Measured Mean Load Density (MMLD) to Rated Mean Load Density (RMLD) was observed to be around 0.60 to 0.65, meaning thereby that only 60 to 65% of the rated load density of 110 t/sq.m, was utilized, which could be attributed to proper bulking up of caved material increased the goaf compaction resistance. From the field observations and analysis of chock shield leg pressures in longwall panel with immediate sandstone roof, it can be understood that; CM - 05 - 15 17 – 19 January 2011, Bangalore, India
  • 16. International Conference on Underground Space Technology Technical Papers § Setting resistance / increase of resistance showed that the front legs were only 8 to 10% higher than rear legs. This means supports were more or less uniformly loaded. During main and periodic weightings some of the rear legs recorded higher pressures than the front legs (see Figures 8 and 9). § There was a considerable overhang of immediate stone roof observed behind the supports. The rear legs of the chock shields induced break line at the goaf edge. § During some of the major periodic weightings, all legs of the supports were intensively loaded, due to delineation of large intact blocks led failure of immediate roof between face and canopy tip, caused cavity conditions. § Front legs as well as rear legs can bring their actions into full play while the longwall face was near borders of residual pillars and goaves of underlying seam. § The ratio of MMLD to RMLD was found to be around 0.8 to 0.85 or some times even more; implies that almost 80 to 85% of the rated load density was utilized due to delayed caving and poor bulking up of immediate sandstone roof and also mainly due to close proximity of main roof. This indicates that at this geo-mining condition higher rated powered roof supports are desirable. § During the main weighting in panel No.21, it was observed that out of 62 legs, which attained bleed pressures in the mid face of the longwall panel, 31are of front and 31 are of rear legs. CM - 05 - 16 17 – 19 January 2011, Bangalore, India
  • 17. International Conference on Underground Space Technology Technical Papers 27 Front 25 Rear Leg pressure (MPa) 23 21 19 17 15 34 95 145 212 279 355 429 498 Average face progress (m ) Fig.7. Average pressure distribution between front and rear legs under shaly coal roof in panel No. 1 40 Bleed Pressure Fron t Leg Pressure (MPa) 35 Rear 30 25 Set Pressure 20 1 21 41 61 81 101 s u p p o rts # Fig.8. Pressure distribution between front and rear legs under sandstone roof during periodic weighting in panel No. 21 CM - 05 - 17 17 – 19 January 2011, Bangalore, India
  • 18. International Conference on Underground Space Technology Technical Papers 32 F ro n t R ear 30 28 Leg Pressure(MPa) 26 24 22 20 18 0 50 100 150 200 250 300 350 400 D is t a n c e F r o m B a r r ie r ( m ) Fig.9. Average pressure distribution between front and rear legs under sandstone roof in panel No. 21 Conclusions It is evident that high rating powered roof supports is a prerequisite for meeting longwall support requirements under competent strata formations. However, a detailed strata control and face powered support investigations are of paramount importance for assessing performance of longwall face. From the analysis of the observations under varying contact roofs such as shaly coal and sandstone, the following conclusions and recommendations were drawn; Ø When selecting a support design, mining engineers should give careful consideration to local conditions and requirements. The desirable type and capacity of the powered roof support must be selected based on the site specific geo-mining conditions. Ø While deploying powered roof supports with foreign collaborations, sufficient scientific study regarding suitability of powered roof support, under a particular geo-mining condition should be conducted by both Indian CM - 05 - 18 17 – 19 January 2011, Bangalore, India
  • 19. International Conference on Underground Space Technology Technical Papers researchers and foreign researchers of Australia, china, and Germany, where longwall technology was well proven. Ø Under immediate weak and moderate strong roof conditions, containing overlain massive sandstone beds, high capacity 2- leg shields of same capacity are desirable over 4-leg chock shields. Acknowledgments The authors are thankful to the SCCL management for giving permission to publish this paper. The views expressed in this article are of the authors only and not necessarily of the organization to which they belong. References 1. Zhao Honghu & Venkata Ramaiah M.S., (1996), “Strata Movement on Shallow fully Mechanised Longwall working Face at PVK Mine of SCCL & Option of Powered Support – Strata Control Observation Research at 2# Longwall working face PVK, SCCL”, 2nd National Conference on Ground Control in Mining, CMRI, pp. 123-142. 2. Prabhakar Rao C, Veera Reddy B., (1999), "Strata Behaviour with Immediate Forward Support - a Case Study", Proceedings of the National Conference on Rock Engineering Techniques for site characterization, 6-8 December 1999, pp. 413- 420. 3. Venkat Ramaiah, M.S and Sudhakar Lolla (2002) “Selection of Powered Roof Supports for Weak coal roof”, Journal of Mines, Metals & Fuels, April, 2002. 4. Siva Sankar U, (2005), “Monitoring Strata Behaviour of Shallow Longwall panel – a case Study”, M.Tech. Thesis Unpublished, BHU, Varanasi, India. 5. Venkata Ramaiah M.S Sastry V.R, Roshan Nair (2005), “Analysis of the Influence of Contact Roof zone on powered roof supports during extraction by Longwall using F.E.M”, IE(I), MN-335. 6. Barczak T.M., (1992), “Examination of Design and operation of powered supports for longwall mining”, Bureau of Mines Information Circular, USA. IC.9320. 7. Thomas M. Barczak, “Design considerations for the next generation of longwall shields”, NIOSH, www.cdc.gov/niosh/mining/pubs/pdfs/dcftn.pdf. CM - 05 - 19 17 – 19 January 2011, Bangalore, India
  • 20. International Conference on Underground Space Technology Technical Papers 8. http://www.bucyrus.com/media/24858/roof%20support.pdf 9. Barczak-TM (2006), “A Retrospective Assessment of Longwall Roof Support with a Focus on Challenging Accepted Roof Support Concepts and Design Premises”, Proceedings of 25th International Conference on Ground Control in Mining, August 1-3, 2006, Morgantown, West Virginia. pg 232- 244 10. T. P. Medhurst (2005) “Practical Considerations in Longwall Support Behaviour and Ground Response” Coal Operators' Conference 2005, Pg 49-57. 11. Peng S.S., Hsiung S.M. and Jiang Y.M. (1988), “Parameters Affecting the Shield support Efficiency in Longwall Mining”, 21st Century higher Production Coal Mining Systems Symposium, pp.122-135 12. Gupta RN and Farmer IW, (1985), “Interaction between roof and support on longwall faces with particular reference to support resistance”, Proceedings of 4th International Conference on Ground Control in Mining, West Virginia, pp. 58-77. 13. Zhu D, Qian M, Peng SS (1989), “ A study of Displacement Field on Main Roof in Longwall Mining and its Application”, Proceedings of the 30th US Symposium on Rock Mechanics, Rotterdam, 1989, p.146 14. Kidybinski A and Babcock C.O (1973), “Stress Distribution and Rock Fracture Zones on the Roof of Longwall face in Coal Mine”, Rock Mechanics, no.5, 1973, p.1. 15. Roberts B.H. (1990), “A review of the performance of various powered support types” Mining Science and Technology, 11 55-69 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands CM - 05 - 20 17 – 19 January 2011, Bangalore, India