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
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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
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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.
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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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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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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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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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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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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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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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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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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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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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• 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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§ 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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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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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.
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