sufficient load carrying capacity to achieve equilibrium of the
mine roof and floor. Since it is this very downward movement
of the roof that we are trying to control, the most important
design parameter for standing supports is the stiffness of the
support system. Stiffness is simply a measure of how quickly a
support develops its load carrying capability in response to
convergence of the mine roof and floor. Stiffer supports develop
equivalent load carrying capacity with less displacement than
softer support systems.

While the stiffness of the support is the primary design
ter, it is not the only issue to consider in the support
application, Another important design parameter is the load
carrying stability of the support. More specifically, it is
important to know how well the support can sustain its load
carrying capability as a function of convergence. Stiff supports,
such as the non-yielding concrete supports (figure 5), which
develop load carrying capacity quickly but fail at little
convergence are not practical in many longwall tailgate
applications. In order to keep such supports from failing
prematurely, a large number of supports must be installed, such
that the roof loading is sufficiently shared among several
supports, while achieving a high enough load density to keep the
convergence below tlie failure point for any one support.

The ideal support is one that the support stiffhess can be
controlled through the support design so that the support capacity
at a given displacement can be engineered to match the ground
reaction behavior. Furthermore, the ideal support would be able
to maintain this capacity through a wide range of displacements
without shedding load or failing prematurely, and therefore
provide a margin of safety in the event thet ground conditions
‘worsen unexpectedly.

MEASUREMENT OF THE GROUND REACTION
CURVE AT A WESTERN PENNSYLVANIA MINE

Studies were made at a longwall mine operating in the
Pittsburgh coal seam in western Pennsylvania. The mine has
historically used a staggered double row of 4-point wood cribs
ona4 ft spacing between supports in adjacent rows (figure 8) for
longwall tailgate support. Due to uncertainties and inconsistent
timber qualities, the mine also employed concrete cribs
constructed from normal mine ventilation stopping blocks which
were also employed in staggered double row arrangement as
shown in figure 9. Since the stiffness of these two supports are
significantly different (figures 2 and 5), and the supports were
employed ina similar arrangement, a ground reaction curve was
determined for this particular mine by measurement of the load
on individual supports and the associated roof-to-floor
convergence in the vicinity of the supports. The results are
shown in figure 10.

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‘Average Load on Wooden cribs = 40 Tons
Support Load Density = 4X40 o ons/SAe
YY ABAD 0625 Tons/Sa. Ft

Figure 8. Arrangement of 4-point wood cribs in

a western Pennsylvania mine.
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Average Loed on Concrete Crive = 65 Tons

Support Loed Density = 4x65 — fons.
AGS = 1.35 Tons. Ft

Figure 9. Arrangement of stopping block concrete crib
support in a western Pennsylvania mine.


‘The Propsetter support had the lowest margin of GRC safety
at 1.79, but even this system is conservatively applied in that the
convergence was limited to 1.6 inches. Conditions outby the
face were excellent as shown in figure 12a and relatively good
inby the face (figure 12b). Some of the Propsetter supports well
inby the face mostly in the intersection areas (figure 12c),
appeared to be in a state of post-yield deformation where
“brushing” (yielding) caused the props to tilt from a vertical
orientation, which is normal for this load condition. It does not
mean that the prop is shedding load. Another possibility is that
the props were being dislodged or moved laterally by flushing of
the gob material, floor heave, and/or lateral displacements of the
roof relative to the floor by the cantilevered roof beam. Despite
these occasional abnormalities in the support condition, the
Propsetter was able to maintain an effective air way beyond the
first open cross cut inby the face. It was also reported by the
mine that 5 or 6 Propsetters were dislodged from the mine roof
and floor outby the face. Since convergence was minimal outby
the face, the cause of these props “falling over” was never
definitively determined. The same props were reinstalled and
performed weil throughout the duration of the test area.

Figure 12b. Inby area supported by Propsetter support.

oo)
Figure i2c. First open cross cut intersection inby the face
suppotted with Propsetter supports.

The Heintzmann ACS support had the most limited yield
capability of the four alternative support technologies used at this
mine. The ACS also sheds load rather quickly after reaching its
peak loading capability at about 2.2 inches (figure 7). However,
the installed spacing provided the highest support safety factor
(2.5), meaning the loads were kept well below the peak capacity
of the support. Likewise the installed load density limited the
convergence to 1.2 inches, which is considerably less than the
yield point of 2.2 inches. Hence, this is a good example of how
stiff support with limited yield capability can provide effective
ground control in a longwall tailgate, provided that a sufficient
number of supports are installed per unit area to establish a high
enough load density to minimize the ground movement,
Figure 13 shows the condition of the entry both outby and inby
the face in the area supported by ACS’s. Similar to the
Propsetter support, a few of the ACS props were tilted inby the
face, but continued to provide support capability in this condition
without becoming unstable.

Figure 13a, Outby area supported by ACS supports.


DESIGN METHODOLOGY
FOR LONGWALL TAILGATES

‘The concept examined for the western Pennsylvania mine can
be generalized and employed at any mine to optimize secondary
support design and application. The first step in the design
Process is to determine the ground reaction behavior. This can
be done by installing at least two, and preferably three, support
systems of varying support system stiffness, and measuring their
loading and corresponding convergence of the mine roof and
floor. It is important that these supports cover a wide range of
stiffness to provide a full picture of the ground reaction
‘behavior. As part of this process, an effort should also be made
to determine the critical roof-to-floor convergence where roof
failure occurs, since this will be a critical design value for the
support design, However, in order to do this, a low density
support system must be installed that will allow considerable
convergence to occur. Since this poses a risk of inadequate
ground control, precautions should be taken. One possibility is
to set additional wood cribs through this trial zone with the top
layer removed such that they could be reinstalled to provide
additional support if necessary, or in the worst case provide
ground support after the roof deforms (5-6 inches) to the cribs if
they cannot be topped off.

Altering the support system stiffness can be done several
ways. One way is to utilize the same support and same spacing
down the entry, but increase the number of rows of support
actoss the mine entry from one to two to three, which would
proportionally increase the system stiffness by the same factors.
Another way is to keep the spacing and number of rows of
support constant, but use supports of varying stiffness. This will
eliminate the impact of both span and roof coverage issues,
which can be limiting factors in the support placement (load
deformation) strategies. For example, using conventional wood
cribs, the support construction could be varied from a 4-point to
a 9-point to a 16-point crib. The support load density would
increase in direct proportion to the increase in support stiffness,

’ Adjusting the support spacing down the entry can also be
considered. The support load density is proportionally increased
as the spacing is decreased. Care should be taken to avoid
excessive spacing that will cause span related problems. A good
tule of thumb is that the support spacing should not exceed half
the entry width. The load bearing area (quality and extent of the
contact area) of the support also an important factor to consider.
The pressure exerted by any support should not exceed the
strength of the mine roof or floor.

Ideally, the loads should be measured underground on the
various support systems. Support load measurements are
typically made through a hydraulic flat jack. This will be more
difficult to do on some support designs than others. Supports
with a single contact area, such as concrete cribs are easier to
work with than something like a conventional wood crib which
has multiple load paths. Theoretically, if loads cannot be
measured, they can be estimated directly from laboratory load-
displacément data provided convergence: (support displacement)
Measurements are made. However, this estimate may not be
accurate for materials such as wood where creep can occur and

distort the approximation of load that produced the measured
displacement, Measurements of convergence are essential to this
design methodology. Roof-to-floor convergence measurements
can be made in several ways, but it is important that these
measurements correlate to displacements induced in the support
structure,

Readings of load vs displacement (convergence) should be
made under the most severe load conditions that occur. For most
longwall mines, this will be at the back of the tailgate shield. If
it is critical that the tailgate be kept open inby the face for
ventilation reasons, then ideally the load - displacement readings
should be made inby the face. Of course, this may not be as easy
to do, but it is important to realize that the design methodology
assumes worst case load conditions, arid safety factors will need
to be employed if meastirements are taken under less severe load -
conditions. Another good rule of thumb to follow is that the face
should be retreated a distance approximating the width of the
face before the full ground reaction behavior is established. In
other words, if the face width is 1,000 feet, then the face should
be retreated at least 1,000 feet before the ground reaction
behavior is measured. Of course, the first requirement for any
tailgate design is proper pillar design. While in theory a ground
reaction behavior can be determined for any pillar design and
roof geology, the methodology proposed here assumes that the
pillar design falls above the ALPS design line for a given
CMRR (3).

The load-displacement data is then used to generate a plot of
support load density as a function of convergence (ground
reaction curve). Each support type with different stiffness
represents one data point on the ground reaction curve. The
support load density is determined as the measured load in the
support at the observed convergence times the number of
supports per unit area of mine entry (equation 5). For a single
tow of supports employed ona constant center-to-center spacing,
the support load density can be determined by equation 6.

Suppor Load Density — Number Supports Support Load.)

‘Area Of Support Coverage

Support Load

Support Load Density Co f5-Center Spacing Enuy Wiath ©

Once the ground reaction curve is developed, the center-to-
center spacing (down the mine entry) of alternative support
systems employed in a single row arrangement required to
achieve ground control (equilibrium) at a desired convergence
can be determined from equation 7, The center-to-center spacing
of a double row of supports is simply twice that of a single row

arrangement,


e + “INTERSECTION GROUND REACTION CURVE
5 2 =z "ENTRY GROUND REACTION CURVE

brs =_

g Concrete “.

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Bye

a mo

§t2 t

Bs ~

g =

s .

a “Flees |
as | | a

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1 2 3 4 5s
‘CONVERGENCE (DISPLACEMENT}, inches

Figure 10, Measured ground reaction curve for western
Pennsylvania coal mine.

First, it is important to note where these measurements were
obtained. Since, the abutment loading changes dramatically in
longwall mining as the face approaches, the ground reaction
curve should always be established at the most severe load
condition, which generally will be just behind the tailgate shield.
In this case, the mine wanted to maintain sufficient control of the
tailgate entry to maintain a ventilation air way back to the next
open cross cut. Hence, measurements of support loading were
obtained to distances of 50 to 100 ft inby the face. For reasons
previously explained, a different ground reaction was measured
through the intersections compared to the entries (figure 10).
These measurements were also made under the deepest cover,
again to establish the worst load condition.

The following analysis applies, to the entries at positions
where there was no influence of the crosscuts. The load on the
4-point wood cribs was estimated at 40 tons with a roof-to-floor
convergence of 4 inches. Qualitatively, it was noted that the
integrity of the immediate roof was showing signs of
deterioration at this convergence, which suggests that 4 inches is
approaching the critical convergence where failure of the roof is
inevitable. Some of the wood cribs in the area were also
showing signs of premature failure, probably due to poor timber
quality. Conversely, the convergence in the concrete crib
supported area was only 0.5 inch and the measured load on the
cribs was 62 pet greater at 65 tons per crib. These support loads
are converted into a support system load density of 0.625 tons
er square foot for the wood cribs (equation 1) and 1.35 tons per
Square foot for the concrete stopping block cribs (equation 2).

4 Cribs x 40 tons/Crib

LOAD DENSITY (Wood Cribs) = left x 16f

u

0.625 tons/sq.ft. (1)

4 Cribs_x_65 tons/Crib

LOAD DENSITY (Concrete Cribs) = 16h x DR

= 1.35 tons/ sq.ft. Ry

‘These two data points are then used to establish the ground
reaction curve for the tailgate entry inby the longwall face as
depicted in figure 10. Since measurements were made on only
these two support systems, a linear approximation to the ground
reaction curve is made using these data as end points with a
straight line connecting between them. This curve can then be
used to determine the support load density required to control the
convergence inby the longwall face from 0,5 to 4.0 inches, For
example, if the goal is to limit the convergence to 2 inches, then
drawing a line from 2 inches vertically upward until it intersects
the ground reaction curve, and then drawing a line horizontally
to the y axis, reveals that a support load density of 1.04 tons per
sq ft must be provided. An algebraic solution to the problem can
also be found by determining the slope and y-intercept for the
ground reaction curve. Once the algebraic equation for the line
is determined, the support load density at any displacement can
be calculated.

Load Density (Convergence) =-0.2071 x Convergence + 1.45

Load Density (2 in convergence) = -0.20171 x 2.0 + 1.45
= L.04tons / sqft 3)

‘The next requirement is to transform the required support load
density into a support system design. Support load density is
determined primarily by two factors: (1) the stiffness of each
support and (2) the spacing of the supports. Continuing with our
example, if we want to increase the support load density of the
4-point wood crib support system from its current 0.625 tons per
sq. ft. to 1.04 tons per sq. ft. in order to reduce the convergence
in the entry from 4 inches to 2 inches, then we would need to
decrease the spacing of the wood cribs from the current 4 ft
spacing, but the question is by how much? The required center-
to-center spacing to provide a support load density of 1.04 tons/
sq. ft. can be determined by first identifying the load capacity of
awood crib at 2 inches of convergence, which is found from the
performance data developed from the laboratory tests conducted
in the Mine Roof Simulator (figure 2). As shown in figure 2, the
capacity of a 4-point wood crib is 27 tons at 2 inches of
displacement. The spacing of a single row of cribs is then
determined by dividing this capacity by the product of the
required support load density and the entry width. As the
following analysis shows, the required spacing of 4 point wood
cribs to achieve a 1.04 tons per sq. ft. support load density is
1.6 ft. Such a tight spacing can only be achieved through a
staggered double row arrangement, where the center-to-center
spacing in each row is 3.2 ft.

. . Capacity (2 inches)
Spacing (2 inches convergence) = Toad Density x Eniy wih

27

= _ 2 16h 4)
1.04 x 16 r@


Table 1. Assessment of standing alternative support technologies utilized in study at a western Pennsylvania, mine.

Installed Installed Load | Cony. control} Safety Factors | Observed Roof Condition
Support System .

Spacing, ft | Density, tons/ft” inches GRC! |Support”} Outby face } Inby face
|4-Point Wood Crib 8 RR) 0.625 4.0 1.00 18 Good Marginal
Concrete Stopping Block | 3 ft(DR) 1.35 0.5 2.16(0) | 1.0 Good Marginal
Crib >
Heintzmann ACS 5.0 (DR) 1.20 1.24 1.92 2.5 Excellent Good
HeiTech Pumpable 9.2 (SR) 1.35 0.5 2.16 13 Excellent | Excellent
Concrete Crib
Strata Products Propsetter | _ 4.0 (DR) 112 1.6 1.79 17 Excellent Good
Burrell Mining Products | 7.0 (DR) 1.19 1.25 1.90 18 Excellent | Excellent
\Can Support

‘Ground Reaction Curve safety factor is determined from equation 8 as the ratio of the installed support load density to the minimum

allowable support load density.

Support safety factor is determine from equation 9 as the ratio of the peak loading capability of the support to the load developed at

the installed spacing.

SDR stands for double row of cribs. All double rows of supports were installed in a staggered fashion. The spacing here'refers to the
spacing of one row of supports. With the staggered arrangement, the spacing between adjacent supports of both rows is half of that
of the individual row (see figure 8 or 9). SR refers to a single row arrangement.

“The roof condition was good prior to the failure of the support. Hence, the installed support load density actually dropped to zero
once the support failed and accounted for the deterioration in the integrity of the roof.

The HeiTech pumpable support had the highest load density
at 1.35 tons per square foct which limited convergence to
approximately 0.5 inch. Conditions both inby and outby the
face as shown in figure 11 were excellent with the Heitech
pumpable support system. However, it should be noted that the
Heitech support also had the lowest support safety factor (1.3) of
the four alternative support systems utilized, meaning that load
development approached the peak loading capability of the

* support. If the maximum loading capability of the support was
exceeded due to unexpected additional roof loading or variability

in the peak strength of the support, the convergence would
increase to approximately 4 inches at the installed spacing based
on the residual support capacity of approximately 90 tons. Since
this is the critical convergence for this mine at which point roof
conditions deteriorate significantly, it is critical that the peak
pumpable support capacity not be exceeded through the zone
where it is desired to maintain full roof control, which means
that the spacing must be properly maintained during installation
$0 as not to overload the support past failure.

Figure 11b. Inby area supported by pumpable crib.



Design Methodology for Standing Secondary Roof Support
in Longwall Tailgates

Thomas P. Mucho, Branch Chief, Disaster Prevention and Response
Thomas M. Barczak, Research Physicist
Dennis R. Dotinar, Mining Engineer
National Institute for Occupational Safety and Health
Pittsburgh Research Laboratory
Pittsburgh, PA

Joe Bower, Northern Region Manager
Heintzmann Corporation
Cedar Bluff, VA

ABSTRACT

Maintaining ground stability in the gate roads, particularly the
tailgate, has always been critical to the success of longwall
mining, both in terms of safety and productivity. Several new
support technologies have been developed in recent years to
replace conventional wood and concrete cribbing for secondary
roof support. Since their performance characteristics are unique,
the best practices that have been developed with conventional
wood cribbing may not be applicable for these alternative
support technologies. Therefore, with so many options to
consider and the importance of achieving adequate ground
control at minimal cost, the trial and error approach to longwall
gate road support is no longer prudent. This paper discusses a
design methodology for standing secondary tailgate supports.
This design technique requires in-mine measurements of tailgate
support loading and convergence to establish a tailgate ground

| reaction behavior based on the support and strata interaction.
The methodology uses the performance characteristics generated
in the National Institute of Occupational Safety and Health
(NIOSH) Mine Roof Simulator (MRS) to match the stiffness and
Joad characteristics of various supports to the measured ground
reaction behavior. It can be used to determine the appropriate
application of alternative roof support systems or to design in-
mine trials such that a fair and equitable comparison of different
support systems can be mae. A case study ofthe methodology
ata i

including a comparison of four alternative support technologies

to conventional wood and concrete cribbing historically used at

this particular mine.

INTRODUCTION

The longwall tailgate support system must flawlessly control
the tailgate ground conditions. Safety considerations,
especially the limited escape routes from  longwall face,
demand that entry be a ucgotiabie iravelway. The
location of the face electrics, support equipment, and belt line in
the head gate eniries dictate that the tailgate may be the only

James J. Bryja, Mine Manager
Cyprus Emeraid Resources Corporation
Waynesburg, PA

option for mine workers to escape from the face in the event of
an emergency. A recent example is the longwall gob fire which
occurred at a mine in Utah this past year (1998) in which several
miners evacuated through the tailgate entry to safety. In addition
to the emergency travelway requirements, inadequate tailgate
support that results in poor ground control and blocked tailgates
due to roof falls can severely retard or halt production. The
heavy reliance by mines on the longwall production for survival
dictates that loss of production for protracted time periods cannot
be tolerated. Ventilation is another issue that depends on proper
tailgate support. As the panel lengths continue to increase,
excessive closure or restriction of the tailgate entry by the
deformation and/or density of the standing support can be
problematic and potentially unacceptable. In gassy mines, it also
may be required that the tailgate be kept open inby the longwall
face in order to establish effective bleeder ventilation of the
tailgate area. Another important issue to consider is the material
handling aspects of tailgate supports. Therefore, the onus is on
the mine engineers to design a support system that maintains
adequate control of the tailgate ground conditions at all times
and with minimal ventilation resistance and material handling
considerations.

Historically, the importance of ground control has led to very
conservative applications of tailgate support. Most mines use
conventional wood crib structures. When properly designed,
conventional wood cribs provide effective ground control in
etiries, and in the past have been cheap
enough that mines could afford to use a high density of cribs at
relatively little cost. However, increasing timber costs,
inconsistent timber quality that led to poor crib performance, and
inadequate supplies of timber for western mines have reduced
the advantages of conventional wood cribbing, and have
encouraged many mines to consider other options for tailgate
support. All of these factors have prompted the support
manufacturers to develop innovative support technologies as
alternatives to conventional wood cribbing.

‘Today, there are severai aiternative support technologies that
have been developed by various support manufacturers and


Table 2. Recommended placement and safety factors for altemative support technologies.

[Center to center support spacing & individual support capacities at convergences of 05,1, 2, and 4 Inches,
0.5 INCH TINCH 2 INCHES 4 INCHES
SUPPORT SYSTEM @D=135) (LD = 1.24) (LD =1.04) (LD = 0.63)
SF =2.16) (SF = 2.0) (SF = 1.7) (SF = 1.0)
‘Load ‘Space Space ‘Space ‘Space
(tons) (ft) ey (ft) ft)
Point cribs 07 09 16 39
[9-Point cribs. 06 28 52 14
[24-inch Link-N-Lock 05 23 55 14
[27-inch Link-N-Lock 05 25 6t 126
[36-inch Link-N-Lock 06 32 80 16.1
06 13 25 $7
block cribs 30 NA NA N/A
ISFR Donut Cribs 40 141 NA NA
[SER Block (2 per layer) crib 40 106 NA NA
88 iz 67 89
19 33 34 32
18 23 61 36

Note: N/A indicates that the support fails prior to the designated convergence and does not have sufficient post failure (residual) capacity

to be considered for appli
center-to-center spacing. Likewise, SF refers to the GRC Safety

tion in this condition. LD designates the support load density of a single row of cribs on the designated

Factor as computed by equation 8 fora single row of cribs. If two rows

of supports are utilized, the designated spacing at a specific displacement would be reduced by a factor of 2 and the safety factor would

be increased by a factor of 2.

OTHER CONSIDERATIONS IN TAILGATE SUPPORT
SELECTION AND APPLICATION OF THE.
PROPOSED DESIGN METHODOLOGY

While the primary consideration in support design is
obviously the prevention of roof falls through proper ground
control, there are other factors to consider. These include: (1) the
cost of the support, (2) material Kiandling requirements and ease
of installation, and (3) the impact of the support structure on
ventilation. These issues are beyond the scope of this paper, but
will be addressed in a paper at the upcoming Longwall USA
conference in Pittsburgh this September (1999).

While this paper is focused on standing roof support
applications, several mines have explored the application of
intrinsic secondary support such as trusses to replace
conventional standing support in longwall tailgates (4). Two
Points need to be made in reference to the truss support
consideration. First, it should be noted that the design
methodology proposed in this paper applies only to standing roof
support. While some of the basic rock mechanics principles used
here may apply to intrinsic support, the support mechanisms are
different and these have not been examined in this study.

Another caveat of the design methodology pertains to the
application in yield pillar gate roads. While in theory a ground
reaction behavior can be established for yield pillar systems, the
mechanisms of ground behavior and support interaction are
different. In particular, the yield pillar system is a high
deformation environment by design. Secondary support should
ideally allow the ground to yield in accordance with the pillar
deformations and not interfere to the point where the secondary
support develops sufficient capacity to damage the roof while it
is yielding. Hence, a stiff, high density support may not be
desirable in this environment, and the important secondary
support design consideration may well be the stability and yield
capability of the support.

CONCLUSIONS

Several alternative technologies to conventional wood and
concrete cribbing have been developed in recent years. These
new support technologies provide improvements in supporting
capability as well as material handling advantages. However,
since their supporting characteristics are all different, a design
methodology must be developed in order for mines to safely
employ these technologies and to maximize their benefits
without increasing the overall cost of support. Conservative
applications or trial and error assessments are no longer
practical nor prudent for state-of-the-art longwall mines,


Figure 13b. Inby area supported by ACS supports.

‘The Burrell Can support installation had a GRC safety factor
(1.92) almost identical to that of the Heitzmann ACS supported
area. The 1.92 GRC safety factor means that the installed load
density is almost twice that needed to prevent roof failures from
occurring. The entry conditions, both inby and outby the face
were excellent with the Can support as shown in figure 14. Inby,
the conditions were slightly better than in the ACS area. This
improvement is attributed to the larger surface coverage and
improved stability of the Can support compared to the ACS.

Figure 14b. inby area supported by.the Can Support.

USING THE GROUND REACTION CURVE TO
OPTIMIZE THE USE OF ALTERNATIVE SUPPORTS

‘Once the ground reaction curveis determined, applications of
any other support technology can be defined by strategic
employment strategies relative to the ground reaction curve.
Table 2 shows alternative placement strategies for these and
other alternative support systems for this particular mine site
based on the measured ground reaction behavior with
conventional wood and concrete support systems.

Examining table 2 reveals that other support technologies
could be favorably used in this application. One example is the
Link-N-Lock crib support developed by Strata Products. A 24-
inch Link-N-Lock crib could be installed in a staggered double
row with a center-to-center spacing of 11 feet per row (5.5 feet
between cribs in opposite rows) and limit the convergence to 2
inches. Another alternative would be a single row of 36-inch
Link-N-Lock cribs on an 8-foot center-to-center spacing, which
also would limit the convergence to 2 inches. In contrast, a
single row of 9-point cribs would have to be installed on a 5.2
foot center-to-center spacing to provide equivalent ground
control capability. .

Three other points can be made by examining the data in
table 2. First, the spacing of stiff, high capacity support systems
can become excessive at large displacements. It is important
to remember that the ground reaction behavior was measured in
the immediate vicinity of secondary support. It is assumed that
the support loading is sufficiently transferred to control the roof
and floor between the supports. Obviously, there are some
limitations to this capability, which largely depend on the
strength of the immediate roof. Generally, stronger roofs can
span greater distances between supports than weaker roof.
Currently, this capability is best obtained from empirical data
within the mine, but a good rule of thumb to follow in the
absence of this information is that the span between supports
should not exceed half the entry width, particularly in weak roof
conditions such as those observed in the Pittsburgh coal seam,
Using this criteria, it is seen from table 2 that 9-point cribs and
Link-N-Lock cribs must be employed at a load density greater
than 0.63 tons per sq ft to avoid an excessive spacing where
failure might occur between the cribs. Surface contro! of the
immediate roof is another issue. Surface control refers to failure
of the immediate skin of the roof. This is different from the
excessive spacing issue discussed above in that there is no major
failure of the roof rock. If surface control is necessary to prevent
flaking of the roof skin between cribs, then methods such as wire
meshing can be effectively employed as a control measure.

Second, it is seen from table 2 that stiff concrete supports
must be installed at high enough support load density to limit
the convergence to less than 2 inches, otherwise the supports will
fail prematurely, resulting in no support capability and unstable
ground conditions after failure of the support. Thirdly, it is seen
that required load density to limit convergence below 1 inch is
not practical with the passive wood crib supports including the
stiffer Link-N-Lock cribs. Wood is just too soft to generate
meaningful loads at such small displacements.


where Spacing(displacement) = Center-to-center spacing in
feet of a single row of supports;

Capacity (displacement) = Individual support capacity
in tons at a specified displacement (obtained from
laboratory performance data) equal to the desired
convergence control;

Support load density = Support load density in tons per
sq. ft. at the required convergence (obtained from
ground reaction curve);

Entry Width = Width of the entry in feet.

This design methodology is a valuable tool in optimizing the
utilization of standing secondary roof support technology.
However, as previously described it is still up to the mining
engineer to decide how close to the critical convergence he/she
wants fo operate based on their knowledge of the particular
ground conditions. A margin of safety is provided by designing
for a convergence that is less than the critical convergence
(minimal acceptable support load density). In order to make
equivalent comparisons of alternative support systems, a safety
factor can be quantified by comparing the design support load
density to the minimal acceptable support load density which
will be representative of the maximum allowable (critical)
convergence. This is referred to as the ground reaction curve

(GRC) safety factor (equation 8),
_ Design Load Density
GRC Safety Factor = Ti imal Acceptable Load Density ©

Another factor to consider is whether the support is being
fully loaded and how much reserve capacity is left in the support
at the design load. In the event that load conditions worsen
beyond expectations, this reserve support capacity may be
needed to support the mine roof. If the support characteristic is
such that the support sheds load quickly after reaching its peak
load, such as the non-yielding concrete supports (figure 5), then
consideration must be given to avoid designing nzar the peak
loading capability of the support. A safety factor for the support
can be defined based on the support loading at the required
support load density in relation to the peak loading capability of
the support (equation 9). Hence a safety factor of 1 indicates that
there is no reserve capacity available and a safety factor of 2
indicates that the support is loaded to only 50 pet of its
maximum support capacity.

Peak Load Capability

Safety Factor (Support) = 77> at Installed Load Density

(9)

ALTERNATIVE SUPPORT STUDIES

AT THE WESTERN PENNSYLVANIA MINE

Four different standing support systems and one cribless
system in addition to conventional 4-point wood crib supports
and concrete stopping block supports were installed in the
Iongwall tailgate entry of this mine. The alternative standing

supports were: (1) Heintamann Corporation’s Alternative Crib
Supports (ACS’s); (2) HeiTech Corporation’s Pumpable Crib
Supports; (3) Strata Product’s Propsetters; and (4) Burrell
Mining Products “The Can”. Cable trusses were used in the
cribless area. The alternative supports were assigned sections of
the tailgate in between the standard wood cribbing. This was
done to ensure that there was no interaction between support
installations, thereby allowing a fair and equal evatuation to be
made (i.¢., normal cover, no excessive toof or floor damage,
normal geology).

Table 1 shows the installed spacing of the alternative support
systems and the typical 4-point wood crib system and concrete
stopping block crib system. The support load density of the
alternative supports is calculated by matching system
performance to the ground reaction curve. Essentially, this
requires working backwards through the design methodology.
‘The following steps can be used.

. Pick an arbitrary convergence within the bounds of the
ground reaction curve.

2. Determine the required support load density that matches the
ground reaction curve for this convergence.

w

. Identify the individual support capacity at this displacement
from the laboratory performance data at this convergence.

4, Determine the support load density from equation 6.

a

. If the support load density is greater (falls above the curve)
than the required support load density - pick a lower
convergence, or if the support load density is less (falls below
the curve) than the required support load density - pick a
higher convergence, and repeat steps 1 through 4 until the
support load density matches the ground reaction curve.

An analysis of table 1 reveals that all four alternative support
systems were installed with sufficient support load density to
control the convergence well below the critical level of 4.0
inches that the 4-point wood cribs provided. The high safety
factor utilized in these alternative support applications was to
provide a margin of safety in anticipation ofa tailgate horizontal
stress concentration on the next panel.


documented in figures 2 through 7, grouped by the following
description of the support type: (a) conventional wood cribbing
(figure 2), (b) engineered wood crib supports (figure 3), (c)
conventional and engineered timber post supports (figure 4), (4)
non-yielding concrete supports (figure 5), (4) deformable
concrete supports (figure 6), and (e) yielding steel supports
(igure 7).
CONVENTIONAL WOOD CRIB SUPPORTS
MOSH SAFETY STRUCTURES TESTING LABORATORY

40 T T 1

350 t : [arr Onq
3200 Hl
200 {
3 200
§ 130 ee

5 100

nl fey

a i |

. 2 4 8 8 ©
DISPLACEMENT, Inches

Figure 2. Load-displacement performance data for
conventional wood crib supports.

ENGINEERED CRIB SUPPORTS (STRATA PRODUCTS USA)
‘NIOSH SAFETY STRUCTURES TESTING LABORATORY

bead T T T
vw rt Lewis}
2x0 oo Eons
g 20 eae ee ico
3 200 Eo Sie,
El 4 : :
& : ——__1___
3” a erate oT]
50 | $M 08) UNL = Link MHLock orb Lf
i" :
o 2 4 6 8 10 12

DISPLACEMENT, inches

Figure 3. Load displacement performance data
for engineered wood crib supports.

‘CONVENTIONAL AND ENGINEERED TIMBER POSTS:
NIOSH SAFETY STRUCTURES TESTING LABORATORY

wo
| ee

&

jo jess ____Ca |

8 50 t 4

Fol WO

100 + i

i = a
50H A "65 IN OIA Propsetiar
VA

DISPLACEMENT, inches.

Figure 4. Load-displacement performance data for
conventional and engineered timber props.

NON-YIELDING CONCRETE SUPPORTS:
NIOSH SAFETY STRUCTURES TESTING LABORATORY

a :
tre [srcomrene] |
won| [_aesom |
I Wh | !
| aretrne
‘ee
Ef

| 4

i
i ee
° L NX

L
¢ 28S
DISPLACEMENT, In

Figure 5. Load-displacement performance data for
non-yielding concrete support.

8

a

‘SUPPORT LOAD. kins

aa

DEFORMAGLE CONCRETE SUPPORTS:
NIOSH SAFETY STRUCTURES TESTING LABORATORY

S00 TNO T T T T
450 PUMPABLE CRIC_
| es es me i pan

Sod | a en
golf ee
ied prs tt | racy
od ha
elem

offer

- 1 2 3 4 s 6 7 8 e 0

DISPLACEMENT, inches

Figure 6, Load-displacement data for
deformable concrete supports.

HEINTZMANN ACS PROPS
NIOSH SAFETY STRUCTURES TESTING LABORATORY

FIAT DITA PANHEADPLATE | [FLAT HEADPLATE:
260 | exbeuate| | ULED WITH coNcRETE] _| WHTH CRIB TIMBERS

200 | -
FURTFIATE

‘ware
100 HaROWOOD.

YW

a0 10 20 30 40 50
DISPLACEMENT, inches.

Figure 7. Load-displacement performance data for yielding
sieel supports.

SUPPORT LOAD, kips

‘As previously indicated, all secondary supports must be
joaded in compression to produce the required capacity to
achieve equilibrium of the mine roof. In other words, the roof
has to move down before the standing support develops


inevitable. Otherwise, the secondary support will be required to
carry the entire dead weight of the detached rock mass to prevent
aroof fall from occurring. This embodies a fundamental concept
in rock mechanics known as the "ground reaction curve"(/).

In longwall mining, the tailgate entry is subjected to three
phases of loading and equilibrium. Each will have a distinct
ground reaction curve. The first phase occurs on development
where the mine opening is created and the primary support (roof
bolts) are installed. Relatively little ground movement occurs
during this phase since the development loads are small and the
primary support is sufficient to provide equilibrium. The next
phase is adjacent panel mining. The future tailgate is subjected
to side abutment loading, and while secondary support is
typically installed to ensure that equilibrium of the rock mass is
obtained, the convergence is typically minimized by the load
density of the support. The final phase of tailgate behavior is
where the active tailgate is subjected to front abutment loading
from the panel exuaction. It is this phase where the secondary
supports play their most important role in preserving the stability
of the entry. A hypothetical tailgate ground reaction curve is
shown in figure 1. It should be noted that the ground reaction
curve will be a function of the several factors in addition to the
load density of the support system. These include the geology,
roof spans, vertical as well as horizontal stress around the
opening, and some time dependent factors such as creep. Hence,
the ground reaction curve is generally unique to a specific mine
and can change within the mine as these factors change. From
the perspective of secondary support design, it is important that
the ground reaction curve be examined under the worst case load
conditions where ground control is required. Since the ground
reaction is dependent upon roof span, a different ground reaction
behavior will typically be observed in intersections compared to
the non-intersection areas of the entry. Hence, the support
design must be altered for the intersections to accommodate this
difference in ground reaction.

Abutment Load

round Reaction

Load

Major Rock Faiture

Stitt Support
acceptabte | unacceptabte

mpport —re

React

‘Soft Support
Roof To Floor Convergence

- Figurel. Ground reaction curve concept.

Itis seen from the hypothetical tailgate ground reaction curve
(figure 1) that if the goal is to prevent convergence completely,
then the full abutment load must be resisted by the support
system. This resistance for all practical purposes canniot be
obtained by the installation of secondary support since the
required capacity would need to be equivalent to that of all the
coal that was removed by the mining process. The support
capacity required to achieve equilibrium is reduced as the
deformation increases, since the roof is shedding load to other
mine structures as it deforms. In other words, by allowing the
roof to deform and shed some load to the coal pillars and
longwall panel, less support capacity is required since the roof
load is decreased. Hence, the lowest required support capacity
would be one that is developed just before the critical roof
deformation occurs where failure of the immediate roof is fast
approaching. However, designing to this lower limit of support
capacity leaves no margin of error in the event that load
conditions worsen. Also, it can be seen that if a support system
is too soft (develops load carrying capacity too slowly),
equilibrium of the mine roof will never be achieved and failure
of the roof will be inevitable.

In summary, since most standing supports are passive
supports, convergence must take place before sufficient support
loads are developed to provide equilibrium. While some
supports have active loading capability, the magnitude of active
loading is not sufficient to achieve equilibrium or the active
loading cannot be maintained indefinitely. The supports must be
loaded in compression to provide the required load resisting
forces to achieve equilibrium of the rock mass, The amount of
convergence required to produce equilibrium is then a function
of the stiffness of the support system. Equilibrium will be
achieved at less displacement for a stiffer support system since
the support resistance to roof loading will develop quicker (at
less displacement) compared to a softer support system. Hence,
as the support load resistance (load density) of a support system
increases, the convergence at which equilibrium is attained will
decrease. If too much convergence is permitted to occur by
utilizing too soft of a support system, failure of the rock mass
(mine roof) will be inevitable. Hence, the goal of support design
is to provide sufficient support stiffness to ensure that the
required support capacity to achieve equilibrium of the rock
mass occurs before the rock mass deforms to the point of failure.
‘However, a prudent mine engineer would ensure that sufficient
support capacity is developed long before the critical
convergence is reached. Since minimizing convergence is
achieved by increasing the support capacity (load density) and
generally the cost of support, the goal of optimizing the support
selection is not to install more support than is necessary to
provide a reasonable margin of safety to prevent roof failure.

Understanding the Performance Characteristics of the
Support

The load-displacement characteristics of numerous roof
support technologies have been determined at the NIOSH Safety
Structures Testing Laboratory through full scale testing in the
unique Mine Roof Simulator load frame (2). The load-
displacement response of these various Support systems are


safety tested at the NIOSH Safety structures Testing Laboratory,
whereby their performance characteristics have been determined.
With increasing pressures to reduce support costs while ensuring
the safety of the mine workers, mine operators more than ever
before are looking for ways to optimize longwall tailgate

. support. Since the performance characteristics of these supports
are unique, the best practices that have been developed largely
through trial and error with conventional wood cribbing may not
be applicable for these alternative support technologics. In
addition, these new supports have ‘limitations, which if not
properly recognized, can lead to poor support application and
inadequate tailgate ground control.

Therefore, with so many options to consider and the
importance of achieving proper ground control at minimal cost,
the trial and error approach to longwall tailgate support can be
costly and indecisive. This paper proposes a design philosophy
for longwall tailgate secondary support whereby the ground
reaction behavior, as a function of the support load density and
stiffness of the support system, is determined by measurement of
underground support loading and roof-to-floor convergence. The
goal of support design is then to optimize the use of the support
by designing to the ground reaction curve, and controlling the
convergence to acceptable limits that will ensure stability of the
mine roof. This approach will allow mine operators to maximize
the use of alternative support systems while ensuring the safety
of the mine workers by avoiding risky and time consuming trial-
and-error assessments of support technologies. In addition, it
will provide MSHA with a means to assess various support
systems on an equivalent basis when approving roof control
plans. A case study is included in the paper which relates the use
of the proposed design methodology to a trial of alternative
support technologies at a western Pennsylvania longwall mine
operating in the Pittsburgh coal searn.

TERMINOLOGY

Secondary Support - Secondary support is that which is
intentionally added to assist the primary support (roof bolts) in
controlling the mine roof when it is known that additional roof
loading wilt occur. In longwall mining, it is installed in advance
of the abutment loading. Secondary support is not to be
confused with supplemental support which is additional support,
that is installed beyond the primary and secondary support either
for insurance purposes or in response to unanticipated poor
ground conditions.

Ground Reaction Curve - A concept of how the ground reacts
to the presence of a newly created opening. Specifically, as the
ground deforms and sheds load to other structures, there will be
a proportional decrease in roof loading and required support
capacity to maintain equilibrium of the mine roof and floor.

Critical Convergence - In relation to the ground reaction curve,
the critical convergence is where failure of the ground is
inevitable, and the full weight of the failed rock mass above the
mine entry must be supported by the secondary roof system to

prevent a roof fall from occurring. The goal of secondary
support design is to prevent this convergence from occurring.

Support Load Density - The load carrying capacity of an
installed support system per unit arca of exposed ground (tons/sq
ft) at a particular ground deformation.

Minimal Acceptable Support Load Density - Lowest load
density of support that should be provided since a lower support
load density will allow convergence greater than the critical
convergence and thereby allow failure of the roof rock to occur
that may lead to a roof fall.

‘Support Density - Term typically used to refer to the number of
supports per unit area, Should not be confused with support load
density, which is the capacity of the support system per unit area
as a function of convergence.

Support Stiffness - A measure of how quickly a support
develops its load casrying capacity in relation to convergence.
For an individual support, stiffness can be determined from the
slope of the load-deformation performance curve. “Softer”
supports have a flatter slope than “stiffer supports” when plotted
to the same load-displacement scale. Softer supports require
more convergence to develop an equivalent load carrying
capacity than stiffer supports.

‘Support System Stiffness - The resistance to load of a group of
supports. The system stiffness is the sum of the individual
support stiffness. Hence, a double row of supports would have
twice the system stiffness of a single row of the same supports.

KEY FACTORS TO CONSIDER IN.
GATE ROAD SUPPORT

A design philosophy for standing supports must be based on
the interaction of the support with the surrounding rock mass.
The question that needs to be addressed to formulate a design
methodology is to determine to what extent the support system
is controlling the ground. To do this it is necessary to understand
both the ground behavior and the characteristics of the individual
support and the support system.

Understanding the Function of the Support System

Obviously, the support system is employed to prevent roof
falls. How this is accomplished is the important issue. While
secondary supports provide the last means of support in the event
there is roof failure above the bolted horizon, the primary
function of the secondary support system is to assist the primary
support system in maintaining the integrity of the immediate
roof. As the ground deforms by the creation of an opening due
to mining, it gradually sheds load to the surrounding mine
structures, which in the case of longwall mining are the gate road
pillars and longwall panel. Secondary support must be placed in
sufficient time and develop sufficient capacity to bring
deformation of the ground into equilibrium before a critical
deformation is reached at which point failure of the ground is


‘The design methodology proposed in this paper and examined
through a field trial at a western Pennsylvania coal mine
embodies a fundamental concept of rock mechanics, that being
the "ground reaction curve”. Measurement of the ground
reaction curve at the westem Pennsylvania coal mine indicated
that at this mine operating in the Pittsburgh coal seam, the
support capacity had a significant impacton the ground behavior
in the longwall tailgate. Increasing the support load density by
a factor of 2 from 0.625 tons per square foot to 1.25 tons per
square foot decreased the convergence in the entry from 4 inches
to] inch. Conventional 4 point wood cribs installed in a double
row with an 8 ft center-to-center spacing in a staggered
arrangement (4 ft center-to-center spacing among adjacent rows)
resulted in marginal ground control. Conerete cribs constructed
from concrete stopping blocks reduced convergence to
0.5 inch, but some failed at this deformation resulting in
localized poor ground control due to the support failure.

Four alternative standing support technologies were installed
at the western Pennsylvania mine: (1) The Can Support (Burrell
Mining Products), (2) the Alternative Crib Support (Heintzmann
Corporation), (3) Pumpable concrete support (HeiTech
Corporation), and (4) the Propsetter (Strata Products USA).
These alternative support technologies were installed at a support
load density ranging from 1.12 tons per square foot to 1.35 tons
per square foot, providing ground control safety factors of 1.79
to 2.16. Ground conditions for all these support applications
were generally very good, which is consistent with the measured
ground reaction behavior and installed support density.

‘The NIOSH Wood Crib Performance Model is currently
being upgraded to a windows format. Included in this new
version will be the addition of the performance characteristics of
alternative support technologies such as those discussed in this
paper. In addition, a subroutine will be provided that
incorporates the longwall tailgate design methodology proposed
in this paper. This program will facilitate the use of this design
methodology and will allow mines to optimize the use of any
support technology once a ground reaction curve for that
particular mine is identified.

REFERENCES

- Deere, D.U., Peck, R. B., Monsees, J. E., and Schmidt, B.
Design of Tunnel Lining and Support Systems. Highway
Research Record, Washington, D.C., No. 339, 1970, pp 26-
33.

2. Barczak, T. M. Assesment of Wood and Alternative
Materials for Secondary Roof Support Construction. 13
Conference on Ground Control in Mining, Morgantown, W.
VA, August 1994, pp 190-201. :

oa

Mark, C., Chase F. E., Molinda, G.M. Design of Longwall
Gate Entry Systems Using Roof Classification. New
Technology for Longwail Ground Control. USBM Special
Publication 01-94, 1994, pp 5-17.

4. Mucho T. P. In Focus - Cable Bolts: a New Support. Falls
Church, Va. US Department of Labor, Mine Safety and
Health Administration, Holmes Safety Association Bulletin,
March 5, 1998.