Standing support alternatives 


in western United States longwalls
 

T.M. BARCZAK AND S.C. TADOLINI 

Abstract 


Mines in the western United States 

have had limited choises for standing 

support. Wood crib were often too 

soft andunstable. The introduction of 

the Can support in the early 1990s 

provided an effective alternative, and 

it remains the dominant form of tailgate support. Water-filled prestressing 

cells are now used to cap the Can and 

preload it to provide an active roof 

load. The cluster Prop, consisting of 

three timber wedge props bundled together, provides more capacity than 

an equivalent sized Can support and 

improves transport efficiency. However, the Clusler Prop is less stable 

and does not maintain a consistent 

load throughout its loading profile. 

Pumpable roof support are another 

alternative support, but they have not 

been proven in high deformation environments. This paper compares the

performance characteristics of these 

various support systems. 

Introduction 

Gate roads and longwall recovery rooms require proper support to 

ensure successful longwall operations regardless of geographic region. 

But various circumstances make this 

particularly difficult for western U.S. 

longwalls. Nowhere in mining is the 

compatibility of secondary supports 

more critical than in western U.S. 

mines, where convergence is often an 

order of magnitude greater than in 

most eastern U.S. mines. In particular, the stiffness of the support must be compatible with 

the ground reaction, to avoid premature support failure 

or unintentional damage to the roof and floor when the 

support is too stiff and catastrophic failure of the roof 

when the support is too soft (Fig. 1).

Historically, timber cribs and posts were the dominant form of secondary support. However, the demands 

placed on these supports by longwall mining, coupled with 

the low-strength. short supply and increasing cost of mine 

timber in western U.S. mines. have necessitated the development of alternative support

systems. Beginning in the mid-1990s. 

the National Institute for Occupational Safety and Health (NIOSH)

in conjunction with various support 

manufacturers promoted the development of innovative support systems. A report published in the 

Proceedings of the 15th International 

Conference on Ground Control in 

Mining documented these initial 

support developments (Barczak and 

Molinda, 1996). 

The first alternative to the wood 

crib in western U.S. mines was the

Confined Core Crib (3C) support

(Fig. 2), The 3C support was developed by John Frederick at Southern 1 Utah Fuel Company (SUFCO)

1

Mention of any company name or 

. product does not constitute endorsement by the National Institute for Occutional Safety and Health. 

 and 

was successfully employed for years 

in the SUFCO mines (Frederick, 

1994). The 3C support is a corrugated 

steel container filled with minus 76­

mm (3-in.) pumice rock that was 

readily available at the Utah mine 

site. The steel container provides 

confinement to the pumice rock as 

it was loaded by the convergence of

the mine entry. Initially, the 3C is a 

very soft support due to the void ratio of the pumice fill. However, despite this soft response, the support 

performed adequately in the twoentry yield pillar tailgates in the 

SUFCO mines where moderate floor 

heave, occurring prior to the longwall 

panel front abutment. caused sufficient convergence to compact the fill 

material and thereby stiffen the support for improved performance as 

the face approached. The 3C support was the predecessor of the Can support. The Can support, as shown in Fig.

3, has replaced wood cribbing and continues to be the 

dominant form of standing support in western mines today. 

The Propsetter support (Fig. 4) was developed in the 

mid-1990s by Strata Products. It has become one of the 

most commonly used longwall supports in eastern U.S. 

mines. But its use in western U.S. mines has been limited, in part, by its limited yield capability and stability in 

the plus 3-m (10-ft) mining height applications. 

This paper discusses enhancements to the Can support technology 

and recent support innovations that 

provide alternatives to the Can support for western tailgate applications. 

Specifically, the use of a new prestressing technology for Can supports will maximize its stiffness and 

help to offset the deficiencies caused 

by topping the Can with timbers. A

new timber support called the Cluster Prop was developed by Strata 

Products USA. It has been successfully used in several eastern U.S. 

mines as an alternative to the Can 

support. A comparison of the performance capabilities of the Cluster 

Prop with the Can support is included. Pumpable roof supports is 
FIGURE 1 

The compatibility of standing support with the loading conditions is 

essential for western longwall 

operations.

another technology that has performed well in eastern 

U.S. longwall tailgates. It has recently been introduced 

to western U.S. mines. However, the evaluution is incomplete on how well these supports can perform in high 

deformation environments. 

Improving the performance of the Can support

The Can support remains a fundamentally sound support design with an excellent performance record. No 

other support now on the market can match the stability 

and high yield performance of the 

Can support. I t has performed well 

with both high mining heights and 

high deformation environments that 

include 0.6- to 0.9-m (2 to 3 ft) of 

floor heave that produces large lateral displacements of the base of the 

Can relative to the roof contact (Fig. 

5).The Can is installed by a machine 

that eliminates much of the material 

handling required for the support installation. It has been shown to dramatically reduce material handling 

injuries compared to wood crib construction. From a design perspective. 

it has only one significant drawback; 

it has to be topped off to establish 

roof contact. Normally. this is done 

with conventional wood crib blocks. 

A closer examination of the impuct of the crib blocks on the performance of the Can reveals just how 

important this material is to preserving the performance potcntial of the 

Can.The first point to understand is 

that the wood crib blocks will alwavs 

rcduce the stiffness of the support. 

The wood crib blocks stacked on top 

of the support act in series with the 

Can. The equivalent stiffness of the 

complete system can be theoretically 

determined from 

KCan KWoodCrib Ksystem (1) KCan KWoodCrib 

Hence, if the wood cribbing on top of the Can had 

the same stiffness as the Can itself. then the overall stiffness of the support system would be reduced by 50 percent. In other words. the Can would be twice as stiff if it 

was not topped off with wood cribbing, assuming the two 

components had equal stiffness. 

Table 1 shows the estimated initial stiffness of a 0.91­

m- (3-ft-) diam. 1.83-m- (6-ft-) tall Can topped off with 

two to four layers of a 9-point or 16-point wood-crib structure made from 0.2- x 0.2- x0.9-m (8 x8 x36-in.) Lodgepole pine timbers.The stiffness values shown in this table 

have been derived from full-scale tests of these structures 

conducted in the NIOSH Mine Roof Simulator. As seen 

from this table. the initial stiffness of a 0.91-m- (3-ft-) diam 

Can is reduced from 143 t/cm (400 st/in.) to 35 t/cm (99 

st/in.) if the Can is topped off with a 9-point crib structure that is four layers tall (0.81 or 32 in.). 

Table 1 

Impact of wood cribbing on overall stiffness of the 

Can support system. 

Wood crib System stiffness 

8 x 8 x 24 in st/in. (t/cm) 

No wood crib 400 (143) 

9 pt - 2 layers 138 (49) 

9 pt - 3 layers 116 (41) 

9 pt - 4 layers 99 (35) 

16 pt - 2 layers 178 (64) 

16 pt - 3 layers 167 (60) 

16 pt - 4 layers 145 (52) 

A full wood timber contact layer should always be 

used on the top of the Can. even if fewer timbers are 

used above the immediate laver. Without a full immediate contuct layer. the crib blocks will punch into the Can 

and initial capacity of the Can will be reduced to that 

provided by the contact area of the timbers instead of 

the full contact area of the Can. In addition. the yield 

capacity of the cribbing structure should always be greater 

than the yield capacity of the Can. The NIOSH Support 

Technology Optimization Program (STOP) software can 

be used to estimate the yicld capacity of the wood crib structure in comparison to the Can (Barczak. 2000). 

For example, if a 9-point crib would 

be constructed on top of a 0.91-m­

(3-ft-) diam Can, the 9-point crib 

would limit the capacity of the Can 

through the first 152 mm (6 in.) of 

convergence. Only after the wood 

crib has gone though several inches 

of plastic deformation (assuming it 

remains stable during this transformation) is it likely to gain sufficient 

stiffness to transfer load fully to the 

Can and cause the Can to vield. Another design consideration is the size 

of the Can for a particular scam 

height. The recommended height-todiameter aspect ratio for application 

of the Can support is 5 to1. For example, in a 3-m (10-ft) scam height. 

the minimum Can diameter would 

be 0.6 m (2 ft). 

Recently, a new prestressing 

technology was introduced that Can 

eliminate a lot of these issues. Thinwalled steel diaphragms that can be 

inflated with water have been designed for prestressing a wide variety of roof support products. 

including the Can (Barczak et al., 

2004).The prestressing units (PSUs) 

designed for the Can support arc 

generally square in shape and fabricated in different sized to accommodate different sized Can supports 

(Fig. 6). 
FIGURE 2 

Confined Core Crib (3C) support 

was the first viable alternative to 

wood cribbing in western 

longwall tailgates. 

FIGURE 3 

The Can support has raplaced wood cribbing and is now 

the dominant secondary support used in western tailgates. 

The PSU can be expanded up to 

about 152 mm (6 in.) after it is 

placed on top of the Can to close the 

gap between the Can and the mine 

roof. If the Can support can he sized 

to within 152 mm (6 in.) of the installation height, then wood crihbing 

can be eliminated. In this configuration (no wood cribbing). the PSU 

can be inflated with sufficient pressure to cause any size Can to yield. 

This eliminates all the stiffness reductions typically associated with 

wood cribbing on top of the Can and 

allows the full capacity of the Can 

to be applied immediately to the 

mine roof and floor on installation. 

If a moderate amount of timber 

is used in conjunction with the PSU, 

i.e., a full timber contact laver or 

header board, then the PSU will still 

have sufficient inflation expansion 

to generate yield loading in most applications. If the PSU is used without any timber, it should be placed 

on the closed end of the Can. Tests 

have shown that the welded seam 

sections on the open end of the Can 

will puncture the PSU on occasion 

when the Can deforms.The PSU can 

also be placed on the floor and the 

Can set on top of the PSU.There will 

still be plenty of pressure to lift the 

Can and prcload thc support in this 

configuration. 

These PSUs can also be 

equipped with a hydraulic yield 

valve to control the load development on the Can or avoid over-pressurization of the PSU to prevent 

premature rupture. The yield valve is a simple springloaded system that is incorporated internally into the inlet check valve. If the PSU were not properly sized. the 

Can would yield before the PSU would vidd.In this case, 

a yielding PSU may not he needed. Forlong-term applications of a year or more, it might be desirable to have 

the PSU yield prior to the Can yielding. This will help to 

reduce the risk of premature failure of the PSU due to 

corrosion. 

If an oversized PSU is used (Fig. 6), which would be 

typical for Can applications, the PSU should be inflated 

with sufficient pressure to cause the PSU to fully balloon around the Can support. If this is not done, then the 

PSU will reduce the initial stiffness of the support as it is 

reshaped from the roof loading.The amount of pressure 

it takes to fully reshape the PSU during the inflation depends to some degree on the thickness and size of the 

PSU. But, as a rule, a minimum setting pressure of 1.4 

MPa (200 psi) will be adequate in most cases. The reshaping can be visually seen and the operator will not 

have a problem in knowing that the unit is properly inflated. 

The key point is that the unit should be used to 

preload the Can and not just to fill the gap between the 

Can and mine roof. The operator

should also be aware that the preload

is governed by the larger roof contact area and not the area of the Can.

This may be important if the operator is trying to achieve a certain 

amount of preload on the support. 

Some caution should be exercised in 

selecting the size of the PSU. If the 

PSU is too large, folds may occur in 

the ballooned areas around the Can. 

These folds can reduce the rigidity of 

the PSU and substantially reduce its 

stiffness to the point where the PSU 

and not the Can is controlling the 

stiffness. The folds will also increase 

the probability of premature failure 

of the PSU. 

Although the Can has considerable capacity to absorb lateral displacements induced by floor heave, 

a study of this performance conducted by NIOSH through full-scale 

testing in the Mine Roof Simulator 

showed that this capability can be 

enhanced by proper fabrication of 

the Can. Cans in excess of 1.83 m (6 

ft) in height are constructed in two 

or more sections that are welded together at horizontal seams.The welds 

tend to create a stress concentration 

due to the annealing that occurs during the welding. The stress concentration can cause a fold to occur near 

the weld during the early phases of 

the loading cycle (Fig. 7). This hinge 

action will cause the Can to shed load 

throughout the loading cycle once the 

joint forms and horizontal displacements occur. The amount of load 

shedding can vary depending on how much rotation is 

occurring at the joint. But it can reach as high as 36 to 45 

t (40 to 50 st) on a 610-mm- (24-in.-) diam Can. The load 

shedding is much less likely to occur if the deformation 

is confined to the top and bottom areas of the Can. 
FIGURE 4 

The Propsetter support has provided a yielding timber 

prop, and, although it is one of the most commonly used 

suppons in eastern longwall tailgates, its application

in western mines is limited due to its yield capability

and stability in the higher mining heights. 

FIGURE 5 

The Can is the most stable support presently, available. 

It can accommodate large roof-to-floor convergence as 

well as large lateral displacements. 

FIGURE 6 

A water·filled prestressing unit (PSU) is now available 

for topping off the CAN and applying a substantial active loading to the mine roof and floor. 

Based on this observation, Can constructions where 

pieces are added on both ends of a center, main body, 

section will providc optimum performance. Figure 8

shows a 0.61-m (24-in.) Can that has gone through 0.64 

m (25 in.) of both vertical and horizontal displacement 

without failure or instability that results in significant load 

shedding. Lateral displacements larger than that caused 

the Can to rip open. 

In summary, the key to using the Can support successfully is to preserve the stiffness capability of the Can 

by avoiding too much wood cribbing on top. Because the 

wood acts in series with the Can, any amount of wood 

will degrade the initial stiffness of the Can, but the softer 

the crib structure used on top of the Can, the softer the 

Can response prior to yield will become. The hydraulic 

PSU technology can overcome this problem by 

preloading the Can to its yield rating so that this load is 

immediately applied to the mine roof and floor upon installation of the support. 

Cluster Props as alternative tailgate support

An alternative to the Can support is the Cluster Prop. 

The Cluster Prop, as shown in Fig. 9, is based on the 

Propsetter support, which relies on the wedges cut into 

the bottom section of the timber post to control yielding 

of the prop. Three Wedge Props are bound together with 

three strong steel bands positioned at the top, middle and 

bottom section of the props. This allows the three props 

to perform in unison and provide a stiff. high capacity, 

yet yielding support system. Currently, two Wedge Prop 

sizes, 216-mm- (8.5-in.-) diam and 254-mm- (10-in.-) diam. 

are used in the Cluster Prop design in western U.S. mi nes. 

The recommended operating ranges for the 216-mm (8.5­

in.) Cluster Prop is 1.83 to 2.74 m (6 to 9 ft). The recommended operating range for the 254-mm (10-in.) Cluster 

Prop is 1.83 to 3.66 m (6 to 12 ft). Each size is available 

from the manufacturer, Strata Products USA, in 152-mm­

(6-in.-) length increments. 

The performance curves fort he 216-mm (8.5-in.) and 

254-mm (10-in.) Cluster Props as measured during fullscale testing in the NIOSH Mine Roof Simulator are 

shown in Fig. 10. It includes the 0.61, 0.76. and 0.91 m 

(24,30. and 36-in.) diameter Can performance. These conclusions are drawn from the performance curves. 

The 216-mm (8.5-in.) Cluster Prop is rated at an average yield capacity of 136 t (150 st) and the 216 mm (10­

in.) Cluster Prop at 204 t (225 st). Overall. the equivalent 

sized Cluster Prop provides more support capacity than 

that of the Can support. 

The Cluster Prop will be slightly stiffer than the Can 

as a passive support, particularly when a lot of wood cribbing is used in conjunction with the Can. The 254 mm 

(10-in.) Cluster Prop is stiffer than the 216-mm (8.5-in.) 
FIGURE 7 

Buckling of Can caused by welding of sections together 

cancreate a hinge point during lateral displace ment 

that m ay cause load sheddingduring yielding. 

FIGURE 8

The limit of latteral displacement is shnown where 635 mm (25 in.) of lateral displacement occurred prior to the Can ripping open. 

250 

200 

150 

100 

50 

0 

0 5 10 15 20 25 

VERTICAL DISPLACEMENT, inches 

SUPPORT LOAD, kips 

Cluster Prop due to its larger cross-sectional area. 

The yield behavior of the Can is more controlled with 

less load shedding and is more consistent than ,that of 

the Cluster Prop. This is because the Cluster Prop relies 

on deformation of wood that can be erratic, while the 

Can relies on the confinement and folding of the steel 

container to control the post yield load. 

The Can has larger yield capability than the Cluster

Prop.The Can will yield through 50 percent strain. The 

Cluster Prop has been shown through full-scale testing 

to yield up to 0.51 m (20 in.) without headboards or 

footboards. provided the wedge props are yielding in 

unison. The yield capability of the Cluster Prop is less 

consistent and can be impacted by the loading conditions. 

It will be maximized when the props yield in unison and 

will be minimized if the props act independently. Hence. 

the steel strapping must maintain sufficient confinement 

to provide uniform behavior. 

I nstallation issues cnn also be critical to optimizing 

the performancc of the Cluster Prop. The Cluster Prop 

will perform best when used with no more than a rugged. 

102-mm- (4-in.-) thick footboard or headboard, and it can 

be used without a headboard or footboard. Like the Can 

support, any additional material in series with the prop 

will soften the overall response of the support. Its yield 

range is limited to the yielding of the Wedge Props. Attempting to extend the yield range by adding soft crib 

block timbers on top of the support (Fig. 11) is not recommended.The woou crib timbers, in addition to softening the support response. create a hinge point that can 

severely degrade the stability of the support. particulurly 

in floor heave conditions where the base of the prop is 

moved laterally from the roof contact. 

The Cluster Prop can be installed with either end up. 

Traditionally, the prop is installed with the wedged end 

of the props down, but from a performance perspective 

this orientation does not matter. If used without a 

footboard or headboard, it may make more sense to install the wedged end against the roof if the floor is too 

soft to handle the bearing load when installed with the 

wedged end on the floor. Because the prop performance 

will be optimized when the wedge formation and crushing of the individual props act in unison, the orientntion 

of the props with respect to the lateral movement or any condition that 

is likely to cause uneven loading of 

the individual props may help to determine how the prop is installed. As 

a rule, the Cluster Prop should be 

installed as vertically as is practically 

possible, although a 5 tilt was determined not to have a dramatic effect 

on thc prop performance during fullscale laboratory testing. 

Cluster Prop, like the Can 

support, is installed with a mechanical aid. Unlike the Can system that 

uses a hydraulically powered clamp. 

the Prophandler is simply a mechanical clamp and a static tray from 

which the Cluster Props are slid into 

position. One advantage or Cluster 

Prop over the Can suppmt is an im- provement in haulage productivity 

because more units of equivalent capacity can be transported on the same supply car due to their smaller size 

and physical shape. Up to 100 of the 254-mm (10-in.) Cluster Props can be transported in the same load compared 

to 35 Cans that are 0.91-m- (36-in.-) diam. 

Pumpable roof support technologies

Pumpable roof support technologies have advanced 

with new developments in the late 1990s. Heitech (part 

of Heintzmann Corp.) has led the development of a twocomponent, quick-setting grout system that can be 

pumped for more than 4.8 km (3 miles) through a surface horehole, into a fabric bag hung from the mine roof. 

The bag not only provides a structure to form the support, but also provides confinement to the fractured grout 

once its peak capacity is exceeded. The support can be 

sized to satisfy specific loading conditions, 0.61-m- (24­

in.-) and 0.76-m-(30-in.-) diam are the two standard sizes 

used in eastern U.S. tailgates. 

The performance curves for these two supports are 

shown in Fig. 12. As seen in these figures, the pumpable 
FlGURE 9 

The Cluster Prop consists of three Wedge Props bundled 

together as shown in this longwall gate-road application. 

FIGURE 10 

Comparison of Cluster Prop to the Can support performance. 

350 

300 

250 

200 

150 

100 

50 

0 

0 2 4 6 8 10 12 14 16 18 

DISPLACEMENT, inches 

SUPPORT LOADING, tons 

Cluster Prop 8.5 inch 

Cluster Prop 10 inch 

Can 24 inch dia 

Can 30 inch dia 

Can 36 inch dia 

20 22 24 

support provides a very stiff response,considerably stiffer 

than the Can support. But unlike the Can, which uses an 

air-entrained material that can be volumetrically crushed 

and a steel container that can sustain its peak support 

capacity during yielding, the pumpablc support sheds 

considerable load during its post peak hehavior. This is 

because the fabric bag does not have the rigidity of the 

steel Can container and cannot provide sufficient confinement to prevent this load shedding. A residual load 

of about 91t (100 st) can be maintained through several 

inches, but the pumpahle crib is never going to have the 

yield capability of a Can support. These supports have 

been successfully employed in a few eastern U.S.longwall 

tailgates and have performed very well for these applications. 

Successful trials of this support technology have also 

been conducted in bleeder entries in at least one western 

U.S. longwall with nearly a foot of floor heave. But the 

floor material moved around the support and did not induce much (probably less than 51 mm or 2 in.) deformation within the support. It is still unknown as to whether 

this technology can perform well in a high deformation 

environment where the convergence cannot be controlled 

by the support capacity. Hence, its application potential 

in yield pillar tailgates or areas of excessive floor heave 

remain unknown. 

Summary and conclusions 

Secondary support in western United States mines 

has evolved as Can supports replaced conventional wood 

cribbing.The Can continues to be an effective design with 

a very successful performance record.The key is to maintain the performance capability of the Can by proper 

capping practice, which historically has been done with 

crib timbers. Poor practice in this regard can significantly 

soften the support response and degrade its stability as 

well. 

Recently, a water-filled diaphragm was developed as 

a prestressing unit (PSU) for Can applications. The PSU 

can eliminate the need for wood cribbing if the Can is 

sized to within 152 mm (6 in.) of the installation height. 

The PSU can also preload the Can to its yield capacity 

and provide a substantial active load to the mine roof 

and floor. Prestressing secures the Can in place and prevents it from being dislodged, unintentionally by pillar 

sloughage or other events, once it is set in place. The PSU 

can also be used to control load development by utilizing a yield valve 

in the PSU. 

Other support concepts have 

been introduced in the past two or 

three years that can provide an alternative to the Can. These include 

the Cluster Prop and the pumpable 

roof support. The Cluster Prop is a

timber support that uses three 

Wedge Props to provide a support 

system with equivalent capacity to 

the Can. It is slightly stiffer than the 

Can prior to yielding and can yield 

through about 508 mm (20 in.) of 

convergence. But its yield load is less 

controlled than that provided by the 

Can support. Its primary advantage 

is improved haulage productivity 

because more units of equivalent 

capacity can be transported on the 

same supply car due to their smaller 

size and shape. 
FIGURE 11 

Adding crib timbers to the top of a Cluster Prop is not 

advisable. As shown in this figure, this can create a 

hinge point that can severely degrade the stability of the 

support in high deformation environments. 

FIGURE 12 

Performance assessment of Pumpable Roof Supports from Heitech showing 

load-displacement curves for 610- and 762-mm- (24- and 30- in.-) diam supports typically used in longwall tailgate applications. 

300 

250 

200 

150 

100 

50 

0 

0 1 2 3 4 5 6 7 8 9 

VERTICAL DISPLACEMENT, inches 

Heintzmann Pumpable Crib - 30 in dia 

Heintzmann Pumpable Crib - 24 in dia 

SUPPORT LOAD, tons 

 Pumpable roofsupports continue to perform well in 

eastern U.S. longwall tailgates and at least one successful 

trial has been achieved in a western U.S. longwall bleeder 

entry application. The pumpable roof support is significantly stiffer than the Can prior to yielding. But the fabric bag does not have sufficient rigidity to sustain the peak 

loading, and this is the primary performance difference 

between the pumpable roof support and the Can. The 

pumpable roof support's capability to perform in a high 

deformation environment, where the convergence cannot be controlled by the support capacity, is doubtful but 

extensive trials in such applications have not yet been 

done. 

Other advancements have been made in support 

applications for longwall support recovery. Here too, the 

Can in combination with the hydraulic PSUs, have been 

successfully employed. Other supports that show promise include the Rocprop and Omni Prop. The Rocprop 

has some proven history in this area in both eastern and 

western U.S. mines. while the Omni Prop has just recently 

been introduced to the U.S. market following a successful history in South African mines. Both of these products provide a relatively small, easily transported support 

that can be quickly set and provide a substantial active 

roof load. They also provide very controlled yielding. 

These performance characteristics make them well suited 

for longwall recovery operations. They are often used in 

conjunction with roof beams to help control the span in 

the immediate working area during shield recovery. 

Because of these innovations in support technology, 

there are now a variety of support systems to assist in 

gate road and shield recovery ground control. Each system has its advantages and disadvantages. There is not 

an ideal roof support for all conditions. However, these 

new support systems provide mine operators with several alternatives to the conventional wood cribs that have 

historically been used for secondary roof support and the 

more recent Can support applications. Performance data 

for these support systems, presented in this paper, will 

help to cnsure their safe utilization. 

References 

Barczak, T.M., 2000, "Optimizing secondary roof support with the NIOSH Support Technology Optimization Program 

(STOP)." Proceedings, 19th International 

Conference on Ground Control in Mining 

Morgantown, WV, August 1-3, pp. 74-84. 

Barczak,T,M.., and Molinda, G.M., 1996, 

"Innovative secondary support technologies 

for western mines." Proceedings, 15th International Conference on Ground Control in 

Mining. Golden, CO. August 13-15, pp. 349­

364. 

Barczak, T.M,. Tadolini, S.C. and 

McKelvey. P 2004. "Hydraulic prestressing 

units: An innovation in ronf suppurt technol· 

ogy. Proceedings , 23rd International Conference in Ground Contro l in Mining. 

Morgantown, WV. August 3-5, pp. 286-294. 

Frederick, J.R,. 1994. "The 3C support: 

A survivable alternative to wood cribbing," 

Longwall USA. June 7·9, pp, 11-21.