Unstable “=
immediate ———
roof .

Coal seam

Figure 20.—Controlling cavities in roof with opposing leg
reactions using four-leg shields.

Resultant
vertical force

Rotational
tendency

<—— Center of
gravity

Rotational
tendency

Resultant
vertical force

Immediate roof

se Se

SAA __Stress concentration

at rear of canopy

Stress concentration
at toe of base

ee \mmediate floor

Figure 21,—High toe loading caused by rotational moment in
two-leg shields.

key
ast ZB 2-109 shies

GBB 4-109 shieis
20h 4
tsb 4
lo} 4
sh

yy

14-16

12-14 16-18

‘SEAM HEIGHT, 10! in

NUMBER OF FACES

one o

6-7 7-8
PANEL WIDTH, 10? ft

Mica FA

20-25 25-30

5-10

10-15 15-20
OVERBURDEN, 107 ft

Figure 22.—Selection of two-leg and four-leg shields as
function of seam height, panel width, and depth of cover.

OPTIMIZATION OF SUPPORT UTILIZATION

The primary purpose of the support system is to pro-
vide a safe working place for the personnel and equipment
to extract coal. An optimization goal in terms of support
utilization is to minimize support loading during a mining
cycle. Since ground control problems generally manifest

themselves in terms of support loading, minimizing support
loading provides effective ground control with the most
efficient use of the support system. Because the operator
docs not have control over the shield stiffness (10), an
optimum setting force must be provided to minimize


EXAMINATION OF DESIGN AND OPERATION PRACTICES
FOR LONGWALL SHIELDS

By Thomas M. Barczak’

ABSTRACT

The success of longwall mining can largely be traced to the development of powered roof support
systems. The most significant improvement in powered support design has been the shield support,
which improved kinematic stability and promoted the application of longwall mining in difficult-to-
control caving conditions where chock and frame supports were inadequate. The most obvious trend
in shield design has been an increase in shield size and capacity. This U.S. Bureau of Mines report
examines shield design and operation practices and their consequences for the utilization of high-capacity
shield support systems. An optimization goal is to minimize support loading by selecting an active shield
setting force that is compatible with strata behavior and shield loading characteristics. Shield stiffness
is an important design parameter that is often overlooked. A consequence of increasing shield capacity
by incorporating larger diameter leg cylinders is a proportional increase in shield stiffness. Setting forces
have also increased in direct proportion to the increase in shield capacity. The increased stiffness and
higher setting force cause the available capacity to be consumed more quickly, severely limiting the
ability of high-capacity supports to last longer and provide reserve capacity for difficult mining
conditions.

Research physicist, Pittsburgh Research Center, U.S. Bureau of Mines, Pittsburgh, PA.


CONCLUSIONS

The full potential of longwall mining was not realized
until powered roof supports, particularly shicld support
systems, were developed. The stability of the shield per-
mitted the application of longwall mining in conditions
where caving was difficult to control and where chock and
frame supports were inadequate.

Lemniscate-guided caving shield designs quickly re-
placed caliper designs to provide a constant tip-to-face
distance throughout the shield’s operating range, and elec-
trohydraulic control systems now provide remote and auto-
mated shield operation. The next major change in shield
design is likely to be an increase in shield width from 1.5
to 2.0 m in an effort to reduce support costs and improve
stability in thick-seam operations.

Shield capacities continue to increase. Several 700- and
800-ton shield systems are now in operation in U.S. coal
mines. Justifications for this increase in capacity in terms
of shield life and reserve capacity for difficult ground con-
ditions are being questioned in light of current design and
utilization practices. An often overlooked consequence of
the increase in shield capacity is an increase in shield
stiffness. Since longwall strata do not reach a full state
of equilibrium and much of the displacement of these stra-
ta is irresistible from the perspective of shield capacities,

the stiffness of the shield is an important design considera-
tion. Higher capacity (stiffer) supports react a larger load
to the roof-to-floor convergence, and therefore use up
their available capacity as a passive support more quickly
than lower capacity (less stiff) supports do.

A closer examination of setting forces is suggested for
the high-capacity support systems being used today. Set-
ting forces greater than the yield forces of past-generation
shields are routinely used. Since many of these old faces
were not yielding excessively, the usc of the high setting
force with the high-capacity supports is questioned. The
proportional increase in setting force with increases in
shield capacity causes the high-capacity supports to be
stressed to nearly the same degree as the low-capacity
systems they replaced and severely limits their ability to
last longer.

Powered roof supports currently represent about 70 pet
of the equipment cost required for longwall mining. The
shield design provides an effective system that can be used
in a wide variety of ground conditions and is likely to re-
main the support design for the foreseeable future. In or-
der to optimize shield design and utilization and to reduce
failures and extend shield life, the Bureau continues to in-
vestigate shield design and its interaction with mine strata.

REFERENCES

1. Peele, R. Mining Engineering Handbook. Wiley, v. 1, 34 ed.,
1941, pp. 10-5-5 to 10-5-11.

2. Mason, R. H. The First Longwalls Were Primitive Compared to
the Ones Now in Use. Coal Min. and Process., v. 13, December 1976,
pp. 59-64.

3. Combs, T. H. Longwall Productivity Continues Upward Trend,
Jumps 30 pet in Year's Time. Coal, v. 25, August 1988, pp. 40-41.

4, Barczak, T. M. Practical Considerations in Longwall Support Se-
lection. Paper in Proceedings, Ninth International Conference on
Ground Control in Mining. WV Univ., 1990, pp. 128-164.

5. Coal. Longwall Census 90. June 1990, pp. 40-49.

6. Adam, R.A., T.M, Barczak, and H. Irresberger. Strata Dynamics
in Underground Coal Mines. Paper in Proceedings: Longwall USA
Conference Papers. MacLean Hunter, 1991, pp. 285-313.

7. Barczak, T. M. Safety Evaluations of Longwall Roof Supports.
BuMines IC 9221, 1989, 17 pp.

8. Barczak, T. M., and D. C. Oyler. A Model of Shield-Strata Inter-
action and Its Impact on Active Shield Setting Requirements. Paper in
Proceedings, Tenth International Conference on Ground Control in
Mining. WV Univ, 1991, pp. 35-42.

9. Barczak, T.M. Advanced Guidelines for Performance Testing of
‘Two-Legged Longwall Shields. BuMines IC 9231, 1989, 149 pp.

10. Barczak, T. M., and D. E. Schwemmer. Stiffness Characteristics
of Longwall Shields. BuMines RI 9154, 1988, 14 pp.

11. Barczak, T. M. Selection of the Right Shield Support. Coal Min.,
vv. 96, June 1990, pp. 63-67.



DESIGN REQUIREMENTS

The selection and design of longwall roof supports are
predicated by strata deformations during the extraction of
the longwall panel. The nature of these strata deforma-
tions is largely dependent upon the thickness and stability
of the structures formed by the roof and floor geology and
the stiffness of the ground supporting elements. Most of
the roof structure is capable of maintaining a dynamic
equilibrium as the longwall face advances by transferring
its weight primarily to the solid coal in front of the face
and the compressed portion of the gob beyond the face.
The powered supports develop load in response to the de-
formation of this rock mass when deformation produces
convergence of the mine roof and floor that contacts the
shield canopy and base. The immediate strata are less
stable and cave after the supports are advanced. The
powered support must also carry the weight of immediate
strata that become separated from the overlying roof
structure and are not supported by the coal or the gob.
Both vertical and horizontal loading are induced on the
supports by the strata dynamics (see figure 12).

Figure 13 depicts the mechanics of roof behavior and
the manifestation of support loading. In general, the dis-
tribution of loading on the coal, the powered supports, and
the gob is dependent upon the relative stiffness of these
support elements (8). From the perspective of strata con-
trol, it should be recognized that the shield stiffness is at
least an order of magnitude less than the coal and the gob
stiffness, which indicates that the shield is not the dom-
inant supporting element. Load distribution among the
supporting elements must comply with conservation of en-
ergy requirements in such a way that the combined work
of the coal, the powered supports, and the gob must equal
the loading imposed by the overlying rock mass. This
means that if the coalbed is very strong and the immediate
roof is sufficiently thick to provide a gob foundation for

support of the main roof, the loading on the powered sup-
ports will be considerably less than if the coalbed is weak
and deformed, or if the gob stiffness is so low that the
gob must be highly compacted before it provides much
resistance.

The required support capacity is generally determined
by estimating the height of the caving zone, A caving zone
occurs when a portion of the immediate roof is fractured
by the overburden abutment pressure, causing that portion
to separate from the overlying rock mass and cave behind
the supports once the supports are advanced. Upon cav-
ing, these strata consume a larger volume by bulking to fill

ir
isa |
loading
=
=

Horizontal
shield

Horizontal
shield
loading

Vertical shield loading

Figure 12.Horizontal and vertical shield loading produced
from support and strata interactions.

Figure 13,—Mechanics of roof behavior and manifestation of shield loading.


Pre-shield

KEY

LZ] Post- shield

100 14 500
wn
wW
Sg Zz l2F yj ow
= sot 4 3. @ 2 400 4
uw 5s of fj ee
° =] ao
w 60F 4 a5 ob | S* 300} J
wi os 8 Zz.
FS ae | 33
2 407 1 a5 6 8 5 2007 1
4

8 5 4b | #8
@ 207 7 2° YP oof 4
: é ,#
zz

° °

UTILIZATION PRODUCTION FACE PRODUCTIVITY

Figure 3.—Contribution of shield support to longwall mining.

TRENDS IN SHIELD DESIGN AND OPERATION

Two fundamental changes in design and operation have
been made since the introduction of shields: (1) The cal-
iper design was replaced with a lemniscate-guided caving
shield that maintains a constant tip-to-face distance
throughout its operating range (see figure 4); and (2)
Electrohydraulic control systems have replaced manual
systems to permit remote and automated operation of the
shield (see figure 5). Current trends in shield design are
summarized as follows:

Capacity—Support capacity has continued to increase
throughout the history of U.S. longwall mining as shown
in figure 6 (4). Support capacities in the United States
have increased by 25 pet since 1980 to an average support
capacity at yield of 579 tons for the 96 longwalls operating
in 1990. Thirteen installations employ shields with capac-
ities greater than 700 tons, and 44 installations use shields
with capacities equal to or greater than 600 tons. The
highest capacity shield used in the United States is
900 tons (5).

Size Shields have increased in size, principally mass
and length, to accommodate higher capacities and in-
creased face spans required by the increase in the size of
haulage and extraction equipment. For example, 11-ft can-
opy lengths were typical with the 400- to 600-ton supports
used prior to 1980, and these lengths have increased to

13 ft for the higher capacity 600- to 800-ton shiclds used
in 1990. Likewise, the weights of supports have typically
increased from 12 to 15 tons during the past decade, al-
though the weights do not always correspond well with
capacity. The most recent trend in shield design is an
increase in the width of the canopy from 1.5 m to 2 m.
Since fracture developments in the roof tend to parallel
the face, roof control should not be affected by the in-
crease in shield width nor should a substantial increase in
support capacity be required. The potential benefit of
wider supports is less cost, since much of the cost is due
to the machining of the hydraulic leg cylinders and since
fewer legs per unit of supported face are required with
wider shields. The wider supports may also be more sta-
ble in thick-seam operations. Some decrease in move time
might also be realized, since fewer supports are employed
ona face. Two wide-canopy shield-supported faces are
currently operating in the United States, while there are
approximately 30 systems operating in Germany.

Setting forces. Differences of opinions in regard to set-
ting forces have developed through the evolution of long-
wall mining. The British initially favored low setting forces
(25 pet of yield capacity), while the Germans and Ameri-
cans have generally favored higher setting forces (60 to 80
pet of yicld capacity). Setting forces have increased in
proportion to the increase in yield capacity, because the


INTRODUCTION

Throughout the history of longwall mining, the design
of the roof support system has been critical to the success
of the mining operation. Early forms of longwall mining
used wood props for face support and packwalls made
from roof and floor rock to control caving of the imme-
diate roof (1).2 These systems were replaced by powered
roof supports that could be advanced easily while allowing
the strata to cave behind them. The first powered roof
supports were simple frame and chock structures. These
designs were poor in their ability to resist horizontal
displacements and moment loading caused by the strata
dynamics during the caving process. They often experi-
enced difficulty and failure (see figure 1) (2).

A major improvement in support design occurred with
the introduction of the shield support. The shield greatly
improved kinematic stability by providing a horizontal

Italic numbers in parentheses refer to items in the list of references
at the end of this report.

Figure 1.Bending of leg cylinders in chock support.

stiffness between the canopy and base that was lacking in
other support designs. This was accomplished by mechani-
cally connecting the canopy to the base in a truss-like
fashion as shown in figure 2, The success of the shield
support promoted the application of the longwall mining
method in highly faulted and massive strata conditions
where caving was difficult to control and where chock and
frame supports were inadequate. The contribution of the
shield support to longwall mining can be seen in figure 3,
which compares U.S. longwall utilization and productivity
before and after the introduction of the shield support
system in 1975, Today, longwall mining accounts for ap-
proximately one-third of the coal mined underground, and
the average annual production per face is about 1.3 million
tons. Longwall mining is employed in a wide variety of
geological conditions and is the most productive and safest
underground mining system used worldwide (3).

The primary design consideration for longwall shields
is their capacity. If load conditions were well defined, the
required capacity would be easily determined and the de-
sign of the support structure would be straightforward us-
ing standard structural engineering principles, Unfortu-
nately, load conditions in longwall mining are not well
defined, and a margin of safety is employed by providing
a capacity for expected worst-load conditions. An even
more debatable issue is the utilization of the support
capacity. Since the shield is actively set against the roof
and floor with a setting force chosen by the operator,
proper setting force is necessary to realize the most
efficient utilization of the available shield capacity,

Motivations for optimizing shield design are to mini-
mize support costs and maximize shield life and at the
same time to provide effective ground control in all mining
conditions. This report examines these motivations in re-
lation to current shicld design and operation practices in
support of the Bureau’s goal to reduce accidents and
improve ground control in underground coal mines.

Canopy

Cavin
shiel

Hydraulic

leg
cylinder

Base

Figure 2.—Horizontal stiffness provided in early shield design
by mechanically connecting canopy with base.


sockets continues to be a problem when the canopy or
base is highly flexed. Most structural failures are fatigue
related and occur after many thousands of cycles (7).

Base-lifting devices (see figure 9) have been a popular
design feature in soft floor conditions. Some of these de-
signs are poor in that they exacerbate the inherent high
toe loading of two-leg shields by promoting two-point con-
tact at the toe and rear of the base.

Control systems.—Dramatic improvements in shield con-
trol have been made with the incorporation of electrohy-
draulic control systems that automate the support function.
The most recent development is a shearer-initiated system,
whereby the shearer emits a signal (usually infrared) that
is sensed by a receiver on the shield and activates the ad-
vance cycle automatically (sce figure 10). These systems
are currently used more extensively in U.S. longwall opera-
tions than in European operations, but this technology is
certain to be a worldwide standard. One reported advan-
tage of the electrohydraulic control is its ability to provide
programmable and consistent setting forces across the
face. However, recent studies show that consistent setting

Split
caving shield

Figure 8.—Split caving shield design
stability problems.

andoned because of

Hydraulic cylinder,

Base-lifting device

Figure 9.—Base-lifting device.

forces are not routinely achieved. Examples of setting
pressure inefficiency are shown in figure 11 for two state-
of-the-art electrohydraulic U.S. longwall operations. Ap-
parently, the demand placed upon these systems is greater
than the capability of the hydraulic distribution system to
provide sufficient hydraulic fluid at the required pressure.
This problem is presently being addressed by the equip-
ment manufacturers, who are trying to avoid time-
dependent setting algorithms in which the pump pressure
is available to the shield for only a brief amount of time
following the support advance.

Emitter

Figure 10.—Shearer-initiated electrohydraulic shield control
system.

507+

TMT
KEY

Tl Desired setting 4
Pressure range

30 4

a

50—>——_1—.—1— 11

ite)

FREQUENCY, pet

40- 4

30

L
5 aA L.

10
05-1 15-2 25-3 35-4 455 35-6
SETTING PRESSURE, 10° psi

Figure 11.—Examples of setting pressure inefficiency from two
mines using electrohydraulic shield control systems.


ATT

mn

IC

9320

BUREAU OF MINES
INFORMATION CIRCULAR/1992

Examination of Design and Operation
Practices for Longwall Shields

By Thomas M. Barczak

UNITED STATES DEPARTMENT OF THE INTERIOR


An analysis of shield mechanics indicates that the verti-
cal load capacity of a shield is almost totally provided by
the hydraulic leg cylinders. The caving shield-lemniscate
assembly has very little vertical stiffness and provides less
than 5 pet of the shield capacity (0). The purpose of the
assembly is to provide horizontal stiffness and stability to
the canopy and base. Therefore, if the support capacity is
to be increased, it must be done through the leg cylinders.
Three options are available: (1) The hydraulic yield pres-
sure can be increased, (2) The size (area) of the leg cyl-
inders can be increased, and (3) The number of legs can
be increased, Historically, the design practice has been
to increase shield capacity by increasing the diameter of
the legs, while keeping the yield pressure constant at about
6,500 psi. The larger leg area then provides a correspond-
ing increase in leg force. Hydraulic setting pressures have
also remained constant at about 4,500 psi, which means
setting forces have increased in direct proportion to the
increase in shield capacity. An important consequence of
the increased leg area is a corresponding increase in shield
stiffness. For example, a 500-ton shield has a vertical
stiffness of about 400 ton/in of vertical displacement,
while an 800-ton shield has a stiffness of 600 ton/in of
displacement.

The consequences of these design practices can be seen
by examination of figures 15 and 16. Figure 15 illus-
trates setting forces and available capacity after setting for
three shields of different capacities: (1) 360 tons, repre-
sentative of 1980; (2) 500 tons, representative of 1985; and
(3) 700 tons, representative of 1990. As seen in the figure,
setting forces have increased with the increase in support
capacity to remain constant at 60 pct of the total support
capacity. Hence, while there is some increase in reserve
capacity after setting with the higher capacity shields, the
setting forces represent the majority of the shicld loading,

1,000
o KEY
S  g00 Passive load J
ut EZ Setting load 700 tons
2
= 6007 4
& 500 tons
rr
ray
& 400} 360 tons 4
&
9 d 60
200F 60: ct 7
s 66 pet y
a pet
1980 1985 1990
Figure 15.—Comparison of setting force and total shield

capacity for three shields of different capacities.

and the increase in setting forces causes all three shields
to be stressed to nearly the same level. Since structures
fail from being stressed, this analysis suggests that the
700-ton shield will not last much longer than the 360-ton
shield despite having twice the capacity.

This argument is further reinforced by examination of
load development in the support after it is actively set
against the roof and floor. After the shield is set against
the roof and floor, it acts totally as a passive support
and develops resistance through the hydraulic leg cylinders
only by displacement of the canopy relative to the base.
Figure 16 compares load development for a 500-ton shield
and an 800-ton shield for the same load condition: 0.5
in of roof convergence. Since the stiffness of the 800-ton
shield (600 ton/in) is 50 pet greater than that of the
500-ton shicld (400 ton/in), the 500-ton shield reacts
200 tons of load in response to the 0.5 in of convergence,
while the 800-ton shicld reacts 300 tons of load in response
to this convergence, With a setting force of 60 pct of the
shield capacity, the full 500 tons of capacity is used in the
500-ton shield and all but 20 tons is used in the 800-ton
shield, Hence, both supports were essentially loaded to
their full capacity and stressed to the same degree for the
identical load condition, despite the 800-ton shield’s having
300 additional tons of capacity initially. Based upon this
analysis, longer shield life and additional reserve capacity
to accommodate difficult ground conditions should not be
expected with higher capacity shields.

Differences in two-leg and four-leg shield design also
provide for different strata interactions. The primary dif-
ference is the ability of the two-leg shield to provide an

1,000

KEY
¢ (ZAsetting
2 BOOP perp Pevetones 4
ui

R 4
8 Hea capacity
= 600} +
a
3 L 4
a
* 400+ 4
bE
a 4
oO
a
a 200 4
a | |

°

500 800
SHIELD CAPACITY, tons

Figure 16.—Comparison of load development for 500-ton and
800-ton shield for same load condition of 0.5 in of convergence.


the void created by the extraction of the coal seam and the
caved roof. This typically establishes a caving height of
four to six times the mining height for weak- to moderate-
strength immediate roof geologies. More competent stra-
ta, which are less likely to be rubblized by the overburden.
pressure, tend to fail along well-defined parting planes and
to cave in larger, blocky pieces with considerably less bulk-
ing than weaker strata. Caving heights of up to 12 times
the mining height arc typical for these competent roof ge-
ologies that are frequently encountered in the United
States. As a worst-case load condition, it is then assumed
that the entire weight of the rock mass in the caving zone
must be carried by the shield support. This establishes
a conservative design capacity for the support. Figure 14
indicates the maximum caving height that can be support-
ed by the indicated shield capacity, assuming an isolated
rock mass 15 ft long and 5 ft wide with a caving angle of
30° to vertical at the rear of the support. A rock density
of 165 Ib/ft* was also assumed in these calculations.

However, gravity loading by isolated rock masses in the
immediate roof is generally not the typical load condition
imposed on the shield. Under most circumstances, the
immediate roof maintains some stability and often acts as
a disjointed cantilevered beam. Shield loading is produced
in response to deflection of the immediate roof beam and
the convergence induced by the deformation of the main
roof structure. The purpose of the support is not to
prevent these strata deformations. It is not necessary, nor
is it desirable, to utilize more support resistance than is
needed to maintain the stability of the immediate roof.
Higher-than-needed support resistance unnecessarily stres-
ses the support structure and the surrounding strata, lead-
ing to a reduction in shield life and a potential degradation
of roof and floor conditions.

These interactions of the support with the strata impose
the following design requirements for the shield:

1, Adequate capacity to provide equilibrium of the im-
mediate rock mass that is not supported by the coal face
or caved gob material;

2. Vertical stiffness and hydraulic yield capacity com-
patible with the imposed displacement of the overburden
to avoid excessive yielding of the leg cylinders;

3. Horizontal stiffness to provide resistance to hori-
zontal displacements and rotations that compromise the

1,000 r
900
800
700

600 4
500 4
400

SHIELD CAPACITY, tons

300,
20 30 40 50 60 70 80

CAVING HEIGHT, ft

Figure 14.—Maximum caving height that can be supported
with designated shield capacity assuming free block formation
directly above shield.

integrity of the immediate strata or the stability of the
support structure;

4. Sufficient lateral stiffness to maintain stability against
loading parallel to the face line that may occur for eccen-
tric contact conditions or pitching scam operations;

5. Adequate bearing areas at the roof and floor
interface to effectively distribute the leg forces and to
avoid further fracturing, compacting, or movement of the
that would compromise ground control or support

6. Ability to provide adequate roof and gob cover to
avoid the hazardous infiltration of roof material into the
working area;

7. Structural components designed to maintain elastic
response for load conditions induced by vertical and hori-
zontal strata displacements under full and partial contact
configurations established at the roof and floor. Well-
engincered mechanics should be employed to avoid critical
load conditions, such as standing the support on the toc of
the base; and

8. Ability to provide adequate ground control and to
accommodate changes in operating height of 100 pct or
greater in some cases. Changes in support capacity,
stiffness, and load distribution on the canopy and base
should be determined as a function of operating height as
part of the design and performance testing procedure (9).

CONSEQUENCES OF DESIGN AND OPERATING PRACTICES

Historically, a "bigger the better" philosophy has been
pursued in longwall shield design. As shown in figure 6,
support capacities have generally increased throughout the
history of longwall mining. Two motivations for this in-
crease in support capacity are (1) to enhance support life

and (2) to provide reserve capacity for insurance against
difficult ground conditions. However, an examination
shows recent shield design and operating practices to be
counterproductive to achieving these goals.


KEY
f —-—Canopy position

Caliper
shield

Lemniscate
shield

Figure 4.—Comparison of caliper and lemniscate-guided shield
designs illustrating tip-to-face movement of canopy.

Figure 5.—Electrohydraulic shield control system.

T : T T T
750}. Key
© 2-Ieg shield /
TOO 0 4-leg shield ‘

CAPACITY, tons
8
T

° i L L 1 L L L
1974 1976 1978 1980 1982 1984 1986 1988 1990
INSTALLATION DATE

Figure 6.

end in U.S. longwall support capacity.

hydraulic setting pressures in the leg cylinders have re-
mained constant while the size (area) of the leg cylinders
has increased to accommodate the higher yield capacities.
Optimum setting forces as a function of strata interaction
and overall support loading are not pursued by longwall
operators.

Component constructions Component constructions
have remained similar since the development of the
lemniscate linkage design; however, there have been some
design improvements incorporated in recent years.

The component that has changed the most is the cano-
py. Extensible canopy designs as shown in figure 7 are
becoming popular in Europe, where friable roof geologies
must be controlled (6). These designs have yet to be uti-
lized extensively in the United States, where more com-
petent roof geologics are typically found. Canopy tips
have been curved upward to promote tip contact. How-
ever, the force generated at the tip is typically about 10 pct
of the leg force and is mostly a function of the distance of
the tip from the leg cylinders. The distribution of loading
on the canopy and base is also dependent upon the stiff-
ness of these components and the deformation characteris-
tics of the immediate strata.

Structurally, several failures have occurred with canopy
constructions in which the front portion is fabricated as a
separate unit and welded in cantilever fashion to the rear
section of the canopy. These designs are being replaced
by designs that incorporate beams that longitudinally tra-
verse the length of the canopy. Split caving shield designs
as shown in figure 8 have also been abandoned because of
stability problems. Little has been done to change the
design of the pin joints in the caving shield, but these
remain a source of failure, particularly when the shields
are in service for long periods of time. Failure of leg

Rigid canopy

Extensible
canopy

Active joint

Figure 7.—Rigid canopy design (top) compared with extensible
canopy design (bottom). Extensible canopy design is used to
control friable immediate roof geologies.


UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT

ft foot pet percent
in inch psi pound per square inch
Ib/f@ pound per cubic foot ton/in ton per inch

m meter



support loading. Therefore, setting pressures should be
adjusted until total support loading is minimized. A less
stringent optimization goal is to avoid yielding. Again, this
can be accomplished by minimizing the setting force, since
doing so provides the maximum reserve capacity to be
utilized in response to the roof activity. If the support
resistance has no influence on strata behavior, then an
active setting force is not required. The shicld can be
raised to the point where roof contact is first established,
and then used as a passive support to develop load as
needed in response to the uncontrollable roof activity.
Only if the shield resistance influences the strata behavior
is an active setting force justified, and then the setting
force should be examined in reference to the total load
developed during the mining cycle.

An examination of strata behavior is provided to eval-
uate the interaction of the shicld with the strata. Strata
behavior from the perspective of support loading can be
classified into four general categories as illustrated in
figure 23: (1) main roof convergence, (2) periodic weight-
ing from the main roof and partial caving zone, (3) free
block formation in the immediate roof, and (4) deflection
of the immediate roof beam. An assessment of the shield
influence on these load conditions follows. Setting pres-
sure recommendations are made in reference to the cur-
rent practice of setting pressures at 60 pct of yield
pressure.

Main roof convergence-—The convergence of the main
roof is irresistible in terms of any shield capacity that
conceivably could be provided. Hence, it is concluded that
the shield has no influence on main roof convergence.
Where the main roof behavior is dominant, setting pres-
sures should be reduced to provide the necessary capacity
to accommodate the main roof convergence.

Periodic weighting —Periodic weighting is most likely to
develop in the main roof and in the partial caving zone
several tens or hundreds of feet from the coal seam. The
support does not generate sufficient force to significantly
affect the state of stress in the rock mass at this level.
Hence, it is unlikely that the magnitude of the shield
resistance will have much effect on controlling periodic
weighting. Again, reduced setting pressures are recom-
mended to maximize the available support capacity to
absorb the load intensities developed from periodic
weighting.

Formation of a free block in the immediate strata —The
front abutment pressure creates fissures in the immediate
strata and the potential for free block formation once

the strata become destressed as the coal is removed. The
shield may act to equilibrate these strata, but it is unlikely
that it will prevent formation of the free block. If it is
critical to keep this isolated block from converging to pre-
vent separation from the overlying strata, then an active
setting force equal to the weight of the block is required.
Obviously, the required setting force would change from
cycle to cycle unpredictably. Therefore, only a nominal
setting force is recommended to avoid wasting available
capacity. It should also be remembered that if separation
of the strata does not manifest itself in higher support
loading or threaten strata stability, the active setting force
is not justified.

Deflection of the immediate roof beam—Of the four
types of loading discussed, the shield resistance is most
likely to influence the deflection of the immediate roof
beam. Structural mechanics indicates that the deflection
of the immediate roof beam decreases with increased
shield stiffness. The deflection of the roof beam may also
be dependent upon the magnitude of the shield resistance,
if the support resistance acts to prevent bed separation by
binding the strata layers together or if it prevents slippage
along bedding planes by increasing the frictional restraint
between layers. The effect of these actions is to increase
the bending stiffness of the roof beam by increasing its
moment of inertia (see figure 24). The increased bending
stiffness of the roof beam reduces its deflection and
subsequent shicld loading. While a stronger roof beam is
more likely to cantilever beyond the face, it is unlikely that
the increased shicld resistance significantly influences
cantilevering beyond the supports since the roof beyond
the supports is likely to relax as the support is removed.

Shield load development is also dependent upon the
stiffness of the immediate roof and floor structure. Stiffer
strata transfer more of the overlying roof convergence into
the shicld, causing higher shield loading. Hence, if there
is debris on the canopy or under the base, it is likely that
this debris will be more compacted for higher setting
forces and will cause an increase in shicld load
development.

In summary, the shield is most likely to influence strata
behavior by increasing the bending and axial stiffness of
the immediate roof or floor through higher setting forces.
However, the increased bending stiffness is likely to reduce
shield loading during the mining cycle, while the increase
in axial stiffness increases shield loading. The benefit of
this active setting force must then be examined on the
basis of overall support loading and strata stability.


1
UNDEFORMED IMMEDIATE ROOF pte___+_J

LL
=
ol

NO FRICTIONAL INTERACTION I, :
. T :
Layers slip 2 >
_ Iz at Sli
Istip = Li + e+ 13 yo =
ASlip ‘

FULL FRICTIONAL INTERACTION
No slip 3

Ino stig’ Ly + Lot 132, A. a2 ats ap

KEY
A, ,Ag,Ag3 Layer area

dj,d2,d3, Distance from layer neutral
axis to composite neutral
axis

1,,1p,13 Moment of inertia of
each layer

NA ‘Neutral axis of composite
beam

Ino sup > Tsuip
Ano stip < Asiip

Figure 24.—Increased moment of inertia of roof beam caused by shield forces.


CONTENTS

Page
Abstract 1
Introduction 2
Trends in shield design and operation 3
Design requirements 6
Consequences of design and operating practices 7
Optimization of support utilization 10
Conclusions . eee 14
References 14
ILLUSTRATIONS
1. Bending of leg cylinders in chock supporto... 60.00.00 ceceeceeeeeeeeeceeeeeeeueeneenees 2
2. Horizontal stiffness provided in early shield design by mechanically connecting canopy with base 2
3. Contribution of shield support to longwall mining... ....... 0... cc cece veveeeeneeees 3
4. Comparison of caliper and lemniscate-guided shield designs Minstrating tip-to-face movement of canopy 4
5. Electrohydraulic shield control system 4
6. Trend in U.S. longwall support capacity 4
7. Rigid canopy design compared with extensible canopy design 4
8. Split caving shield design abandoned because of stability problems 5
9, Base-lifting device . 5
10. Shearer-initiated electrohydraulic shield control system : 5
11. Examples of setting pressure inefficiency from two mines using electrohydraulic shield control systems .. 5
12. Horizontal and vertical shield loading produced from support and strata interactions 6
13. Mechanics of roof behavior and manifestation of shield loading 6
14, Maximum caving height that can be supported with designated shield capacity assuming free block
formation directly above shield . 7
15. Comparison of setting force and tot shield capacity for three shields of different capacities eee 8
16. Comparison of load development for 500-ton and 800-ton shield for same load condition of 0.5 in of
convergence 8
17. Active horizontal force provi ed by two-leg shiel designs 9
18. Comparison of resultant force locations for two-leg and four-leg shields 9
19. Four-leg shield operation in cavity-prone conditions illustrating problems of canopy control . 9
20. Controlling cavities in roof with opposing leg reactions using four-leg shields 10
21. High toe loading caused by rotational moment in two-leg shields 10
22. Selection of two-leg and four-leg shields as function of seam height, panel width, and depth of cover 10
23. Four general categories of strata behavior pertaining to shield loading 12
24. Increased moment of inertia of roof beam caused by shield forces 13



Library of Congress Cataloging in Publication Data:

Barczak, Thomas M.
Examination of design and operation practices for longwall shields / by Thomas M.
Barczak.
p. em. — (Information circular / Bureau of Mines; 9320)
Includes bibliographical references (p. 14).
Supt. of Docs. no.: I 28.27:9320
1. Mine roof control. 2. Longwall mining. I. Title. II. Series: Information cir-

cular (United States. Bureau of Mines); 9320.

TN295.U4 [TN288] 622 S—de20[622".28] 92-8898 CIP



active horizontal force (17). Since the leg cylinder in a
two-leg shield is inclined toward the face, horizontal com-
ponents of the leg force push the canopy toward the coal
face as shown in figure 17. This induces a force into
the immediate strata that tends to maintain the strata in a
state of compression. In comparison, the legs of a four-leg
shield are inclined in opposite directions to one another
and the horizontal components of leg force cancel one
another out and do not produce an active horizontal force.
In conclusion, two-leg shields are more effective in con-
trolling highly jointed or friable immediate roof gcologies,
since the active horizontal force arrests slippage along
fracture planes and improves overall strata stability.

A reported advantage of the four-leg shield is that it
provides a resultant vertical force farther from the coal
face than does a two-leg shield (see figure 18). This is
supposed to be more effective in controlling cantilevered
strata, since the support force acts at a mechanically more
efficient location. However, laboratory and field data sug-
gest that this advantage is highly overrated for two rea-
sons. First, the difference in resultant locations and the
corresponding load profile on the canopy are too small to
make a significant difference. Second, there is typically an
imbalance in load distribution between the front and rear
legs of a four-leg shield, so that the resultant force lo-
cation is similar to that of a two-leg shield.

The unbalanced distribution of loading between the
front and rear legs also makes the four-leg shield less
effective in cavity-prone strata. As the example in fig-
ure 19 illustrates, the force in the rear legs causes the
canopy to rotate up into the cavity, which causes a loss of
roof contact at the canopy tip. This condition ultimately
results in further cavity formation and requires the front
legs to do all of the supporting work. Since the front legs
of a four-leg shield are considerably smaller than they
would be in a two-leg shield of equivalent support capacity,
the four-leg shield provides much less supporting force
than would a comparable two-leg design. If the legs are
double-acting cylinders, four-leg shiclds can be effective in
controlling cavities in the immediate roof by having one set

Unstable roof

Figure 17.—Active horizontal force provided by two-leg shield
designs.

of legs act in the opposite direction of the other as illus-
trated in figure 20. This action controls the rotation of the
canopy about one set of legs and can ensure contact load-
ing at the tip or rear of the canopy. However, opposing
leg reactions greatly diminishes the capacity of the shield
and is not an efficient utilization of the support system.
The primary disadvantage of a two-leg shield is general-
ly higher contact pressure on the canopy and base. High
toe loading, caused by the moment created by the line of
action of the resultant vertical forces acting on the canopy
and base (see figure 21), can be a problem in high-capacity
two-leg shields and should be a primary consideration in
the support design. This behavior is alleviated in four-leg
designs since the two sets of legs provide a canopy result-
ant force more coincident with the base resultant and pro-
vide for a more uniform load distribution. However, if a
two-leg shield of the required support capacity can be ob-
tained, it is the preferred support in most applications.
Figure 22 illustrates two-leg and four-leg shield utilization
as a function of seam height, panel width, and depth of

cover.

‘Support response
oy < Le
Pp > Py

\canopy
rotation

Figure 19.Four-leg shield operation in cavity-prone con-
ditions illustrating problems of canopy control.


Figure 23.—Four general categories of strata behavior pertaining to shield loading. A, Main roof convergence;
B, periodic weighting; C, detached immediate roof; D, deflection of immediate roof.